The battle over Gliese 581 and Gliese 667C? In 2014, Robertson et al. made a case that stellar activity is masquerading as planets (here and here) for these well-known systems. In both cases, they claim that the only real planets discovered so far are planets "b" and "c". In 2015, Anglada-Escude et al. questioned the methods of Robertson et al. (here). Also in 2015, Brewer et al. suggested that there is evidence for multiple planets at Gliese 581 here (note, I only have access to the abstract - fulltext is here). Is there a direction that a majority of the professional community is leaning?

Going by multiple online exoplanet catalogues and a quick scan through the papers listed on the SIMBAD pages for the host stars:

Gliese 581

At the time of writing, the Extrasolar Planets Encyclopaedia (http://exoplanet.eu), the NASA Exoplanet Archive (https://exoplanetarchive.ipac.caltech.edu) and the Exoplanet Orbit Database (http://exoplanets.org) are in agreement and list only three confirmed planets for this system: in order of increasing orbital period these are e, b and c. It's not just Robertson et al. claiming that d is an activity signal, see for example Hatzes (2016). The most recent RV study of the system I have seen is Trifonov et al. (2018) who give a 3-planet solution, not sure whether they searched for evidence of a fourth.

Gliese 581 is known to host a debris disc with an inner edge around 25 AU (Lestrade et al., 2012), much further out than c (or indeed any of the claimed planets beyond c), so I wouldn't be too surprised if there are additional planets to be found.

Gliese 667 C

This one's a bit less settled because it doesn't seem to have attracted so many studies. Extrasolar Planets Encyclopaedia lists 6 confirmed planets, the NASA Exoplanet Archive lists 5 (they don't have planet d), while the Exoplanet Orbit Database only lists 2 (b and c). Again it is not just Robertson et al. claiming that there are only two planets, there's also the study by Feroz & Hobson (2014). The identification of planet d as being caused by the stellar rotation is supported by Suárez Mascareño et al. (2017) which is probably why this one was removed from the NASA list. Planet e is at roughly 2/3 of the rotation period and so is also considered suspicious. There doesn't seem to be nearly as much discussion about the outer candidates f and g.

Hope that helps as a summary. More data would definitely be useful for these systems if there is going to be any consensus, particularly for Gliese 667 C.

Update (June 2019): the recent study posted on the arXiv by Tuomi et al. brings back planet d, unfortunately they don't discuss this case in detail. Confusingly they also decided to swap the designations of planets d and e (at least they did in their tables, they kept the old name elsewhere in their text). They also include the 91-day planet d at Gliese 667C, also unfortunately without a detailed discussion.


Exposure time for spectroscopy compared to photometry For a report I'm writing, I'm interested in comparing the exposure time required for spectroscopy versus photometry for a given magnitude. I found the figures on pages 7 and 8 of this lecture particularly informative. I tried emailing the author, but his email address is inactive. Where can I get similar information?

Your question is a bit unclear. Compare in what sense?

The problem boils down to that you have a fixed number of photons in a wavelength band and you can choose, with the appropriate filters or dispersion elements, how many "bins" to divide these photons into.

To accumulate the same total number of photons with the same telescope will take the same amount of time, except putting more optical elements in the way will reduce the efficiency of the instrument. Filters are more efficient than spectrographs by factors of a few to an order of magnitude.

However a further factor to consider, is that although in principle we could divide the light up as finely as we want, there may be static sources of noise that are independent of the signal. In the case of a CCD detector, this means the read noise.

If you want read noise not to limit the observations, then the product of the bin size and exposure time needs to be large enough that the square root of the accumulated counts (including any sky background) in a bin exceeds the read noise.

An example might help. Suppose I have a star that I observe for 10s through a V-band filter with a width of 100 nm and it produces 1000 detected photons on a CCD, where the star image effectively illuminates 10 pixels, each of which has a read noise of 10 counts. The total read noise is $10\sqrt{10} = \sqrt{1000}$ counts (the square root of the total variance) and equals the Poisson noise from the source plus sky. Ignoring the sky, the signal to noise ratio is $1000/\sqrt{1000+1000}=22$.

Now suppose I do slitless spectroscopy, passing the starlight through a diffraction grating, splitting the light into 1 nm bins. My signal strength in each bin is divided by 100, but the read noise is approximately the same. To obtain a spectrum with a similar signal to noise ratio you would need to expose for 100 times longer, i.e.1000 s (probably longer because a diffraction grating is not as efficient as a filter).

So there is no general answer to your question, it depends on the spectrograph efficiency, the "resolution" of the spectrograph, the relative contributions of source, sky and read noise and the signal to noise ratio desired.

A simple rule of thumb to get an exposure that is just limited by read noise for an imaging filter of wavelength width $w$ and a spectrograph with a resolution element $\Delta \lambda$, would be to observe for a factor of $w/\Delta \lambda$ longer, with an extra factor of a few to account for poorer spectrograph efficiency.

Edit:
To test my ideas I go to the Isaac Newton group of telescopes' signal to noise calculator.http://catserver.ing.iac.es/signal/

I try observing a 20th magnitude star for 10s with a V filter on the WHT prime focus, assuming 1 arcsec seeing and no moon. This gives me a SNR of 46.

To get the same SNR in a low resolution spectrum withe ISIS spectrograph and 158R grating (lowest resolution available) takes 15,000 s.

Does this stack up with what I said above? Each pixel in the spectrum is 0.16 nm. If the V band filter is about 100nm wide, my crude formula above suggested an increase in exposure time of 100/0.16 = 625. But the instrument throughput is only 33% (compared with essentially 100% for the filter) so we have to add another factor 3, to get a required exposure that is 1875 times as long as for the photometry. Pretty close!


In rocky planets,does fast rotation cause flatting or low flatting imply slow rotation? As far as I know, Venus and Mercury have 0 flatting, but Mars and Earth have detectable flatting, and Venus and Mercury are both rotating slowly. I'm confused as to the relation between rotational speed and flatting.

Does rotation cause a rocky planet to be flat? Or zero flatting implies Venus and Mercury rotate slowly? Which is correct?

The flattening of a planet is a function of both its spin rate and its structure. But for a series of planets of homologous structure the flattening depends on the spin rate. the faster it spins the greater the flattening (for spin rates typical of planets any way). So if a planet is spinning slowly it will display little polar flattening, similarly if it does not show flattening it should be spinning slowly.


Feasibility of Grand Tack Hypothesis Something I've been wondering lately is how much the grand tack hypothesis stands up to present scrutiny. While the grand tack proposes that Jupiter and Saturn were caught in a 3:2 mean-motion resonance which results in migration outwards to roughly close to their present distances, simulations of dynamics of planetary migration seem to indicate that a 1:2 MMR is much more likely to occur, which most likely does not result in migration outwards.
Also, a much more recent hypothesis suggests that Jupiter formed much further out than the present location and then migrated in over a period of ~700,000 years, which would explain the distribution of Trojans in Jupiter's orbit. The starting scenario of this hypothesis is completely different from the grand tack which states that Jupiter formed around 3.5 AU distance. It also says absolutely nothing about Jupiter and Saturn migrating further inwards and then back out due to resonance.
Can the grand tack hypothesis be reconciled in any way from these newer studies?

What you have to keep in mind is that the Grand Tack was devised as a mechanism to generate a truncated planetesimal disc between 0.4 and ~2 AU during the protoplanetary disc phase, in order to explain the small masses of Mars and Mercury (Walsh et al., 2011).
The smallness of Mars and Mercury might be explained differently by pebble accretion (Lambrechts et al., 2019), which wasn't fully worked out yet during that time.
A direct test of whether Jupiter/Saturn indeed migrated during their growth phases, is their Trojan number asymmetry, which is the basis of the paper by Pirani et al., which you quoted. They find that the Trojan-to-Greek numbers should be equal only in the case of in-situ formation, but they are not.
The observed asymmetry is consistent with migration inwards during Jupiter's growth. However, from their work, no clear statements can be made on

where Jupiter's core formed;
how fast the inward migration proceeded; or
whether there was a later outward migration.

Furthermore, the migration rates required for the Grand Tack to operate might be wrong. Recent work coming from people working on migration rates (McNally et al., 2019, Lega et al., 2020) seem to indicate that migration in low-viscosity discs works very differently than what was envisioned in earlier decades. Outwards migration, or stopping, is then easier for giant planets.
Summarizing, many of the puzzle pieces on why the Grand Tack was envisioned and how it worked are currently under active investigation. This is not an issue I expect to be solved soon, as certainty into any of those models would additionally require Trojan asymmetry numbers for Neptune and Uranus, and those are very challenging.


What would happen if we stepped on the Sun? I mean in intrinsic detail. What would happen to our bodies besides "burst into flames"?

Well first thing's first: You would disintegrate. At the temperature of the Sun, most of the molecules that make up our bodies could not even survive, that is why we would not only fry and die, we would really disintegrate (all the molecules breaking apart, leaving only loose atoms).

Let's now pretend heat doesn't exist. This is what would happen. First, we must remember that there is no solid surface to the Sun just like there is no surface to Uranus.

As soon as you reached the Sun itself, you would sink. The sun's density is less than 1 — and 1 is the density of water. So you would be sucked inside kind of. Except that there are convection currents. If we happen to be just above one, we might be kept close to the surface.

But, eventually, the eddies would entrain us sideways until we came to a "downdraft" that would take us downwards.

Anyway, let's talk about gamma rays now. Gamma rays are a form of light.

So even though heat is out of the way, all of these light rays are now trying to tear up whatever remaining molecules we have. This could trigger nuclear reactions.

Eventually, you (not me, I'm out of there) would cycle between two layers, in a convection cell moving you up and down, above and below the depth where the density of the gas is the same as the density of your body.

So I guess there's one sure thing about all of this.

You're going to need some sunscreen.


Software for simulating a night sky Not sure if this is the right StackExchange for this, but seems appropriate.

Is there any off-the-shelf software that can display a star field (view of the night sky) as if seen from a particular location and looking in a particular direction?  Ideally, I'd like to be able to specify the location (Lat/Lon coordinates) and the direction in which the virtual camera is pointing, and get an image of the starfield.

The ability to modify the parameters of the virtual camera would be of additional benefit.

Free software would be best, but paid may be ok as well, depending on the price.

Yes. Stellarium is a very nice package that meets your needs. I use it on my mac and it works great. There are versions for Windows and Linux as well. It is a free package. You can enter your location and see the night sky in any direction. It has zoom capability and you can rotate sky to look in any direction. Their website describes it as a planetarium for your computer. It has a database of over 600,000 stars. The website is here


A neutron star without a flat surface and an atmosphere? In the Kurzgesagt video Atoms As Big As Mountains — Neutron Stars Explained they state that neutron stars have an atmosphere that reaches about 10 centimeters above the surface.

How is this possible? Wouldn't the strong surface gravity pull all of the material onto the surface of the star?

Further, Kurzgesagt also states that the neutron star has a surface variation of about 5mm. Still, wouldn't gravity pull it flat? Of is the gravity not strong enough to do so?

There is a number of issues with the question, but let me sketch out some kind of answer, so you get something out of it.

The atmosphere of a neutron star is a topic that's a bit speculative. Estimates vary a lot. Regardless, neutron stars can have an atmosphere - sure, gravity is huge, but they are also extremely hot. Some molecules are bound to jump up a bit.

X-ray measurements have shown the atmosphere of a certain neutron star to be 10 cm thick. That's not 10 meters, but it's better than nothing. Other neutron stars surely have different kinds of atmospheres, some thicker, others thinner. It looks like this is a matter for further research.

https://spacemath.gsfc.nasa.gov/weekly/6Page78.pdf

You say the relief on the surface could be up to 5 mm height differential. Earth has mountains 8.8 km tall, and ocean holes 11 km deep - that's a 20 km differential, more or less. That's 4 milion times greater than 5 mm. Keep in mind, the surface material on a neutron star also has different properties from the normal stuff on Earth, different composition, and it's under extreme compression and extreme magnetic fields, which give it different mechanical properties. So 5 mm is quite believable. However, that is an extreme, and most surface should be smooth as glass. Any surface irregularity should be unstable in the long run.

Depending on the model, in some cases, extremely hot neutron stars might have surfaces that are more like a fluid than a solid. Emphasis on "might".

None of the above is set in stone. Neutron star physics is an area of active and intense research.

I recommend the book called Dragon's Egg by physicist Robert L. Forward. It's sci-fi, but the author was a bona fide scientist with extensive work in the field of gravitational wave detection. In his own words, Dragon's Egg is "a textbook on neutron star physics disguised as a novel". It's fun to read, too, one of my favorite hard sci-fi novels.


Blackhole Finding Techniques I know of two methods for finding exoplanets: the transit method and the radial velocity method. These two methods work as follows:

Transit: we observe stars and watch for when a planet obstructs the light from the star.
Radial Velocity: detecting the planet by observing the motion of the star and using Kepler's law.

Are these methods also used for discovering black-holes? What are the pros and cons of these methods, I think that the con of transit is that it's difficult to see the black hole. A pro is that it doesn't matter how many stars there are around the black hole.

With the Radial Velocity method, a pro is that it's easy to detect but requires exactly one companion star (I used this source).

We detect black holes mostly by their effects on nearby matter. The "radial velocity" method is important after x-ray observations suggest a likely black-hole.

Matter in orbit around a black hole can form an accretion disc. This disc will become very hot, and glow brightly in X-ray radiation. If we see X-ray radiation coming from a star we look at the radial velocity of the star, and use this to infer the mass of the orbiting object. Since a neutron star cannot exist at more than 3 solar masses, if the object is more than that it must be a black hole.

Compared to planets, black holes are rare. Only about 20 candidates are known in the Milky way.


Can astronomical radio sources be used as a verifiable randomness beacon? A randomness beacon is a source of random data that is broadcast to multiple parties. Users listening to the beacon receive the same sequence of random strings and no one can predict the values in advance. This is a construct that has many applications in cryptography and distributed communications. Such random sequences have been generated using atmospheric noise, quantum measurements, and financial data (*).

Using astronomical radio sources (such as meteor bursts, solar noise, planetary observations, CMBR, etc.), is it possible for multiple independent observers spread around the globe, using relatively inexpensive equipment, to detect the same unpredictable cosmic signals, and agree on the time and properties of those signals (to within some margin of error)?

If this were feasible, you could create a publicly-verifiable randomness beacon that didn't require trusting a central authority to generate the sequence in an unbiased way. Instead, observers can certify the random values independently using their own local measurements.

(*) usenix dot org /legacy/events/evtwote10/tech/full_papers/Clark.pdf

I see two problems.  1) Any particular source in the sky is above the horizon for a limited region of the Earth at a given time, so one would need to relay the signal around the globe.  2) It is hard to think of an astronomical source that is truly random all of the time.  Astronomical sources are physical systems reacting to internal and environmental changes that are causal, so they all tend to be partially quasiperiodic to some degree or become temporarily nearly periodic.  So it depends on just how random you need them to be, but I doubt any astronomical source is dependable for this purpose.


How is a Galaxy formed? How is a galaxy formed? I know that the center of our galaxy is considered to be Sagittarius A* and it's surrounded by a lot of stars (also a lot of neutron stars).

But what makes this happen? I mean, why are all those stars in that right formation, like other galaxies, rather than just being random clouds of stars in the universe?

Well this is not a question that can be answered in a few sentences!

In short

We still do not know, how exactly the galaxies formed and have the shapes they possess. The large-scale structure (LSS) in the Universe as we see them today are a consequence of tiny primordial density fluctuations that arose right after the
Big Bang.
The reason for these tiny density fluctuations is believed to be quantum in nature.

So, these tiny fluctuations in the early past lead to agglomeration of gas and dust clouds, leading to certain areas becoming denser. These denser areas slowed down the expansion of the Universe, allowing the gas to accumalate into small protogalactic clouds. Gravity in these clouds casued the gas and dust to collapse, and in turn form stars. These stars burned quickly and became globular clusters (while gravity was still collapsing the dust and gas).

Also, according to the $\Lambda$CDM model, the structures form in a "bottom-up" fashion, i.e. small structures forming first (stars and galaxies) followed by large structures (galaxy clusters). This is exactly what we are observing today thanks to surveys that are probing high redshift ranges.


In relation to Meteor Showers, what is meant by Lambda Omicron? I see for example, the L.O. of the Quadrantids is 283.16 and Lyrids is 32.32.

src: https://en.wikipedia.org/wiki/List_of_meteor_showers

Not $\lambda \omicron$ or $\Lambda. \mathrm{O}.$ but $\lambda_⊙{}^\circ$.

This is the solar ecliptic longitude, the position of the sun relative to the vernal equinox, in degrees clockwise. This varies from 0 at the spring equinox, over 360 degrees. The Solar longitude is a way of giving the date of a meteor shower.

The effects of leap days mean that the date of the peak of a meteor shower will vary from year to year by a few days. However, the solar longitude at a meteor's shower's peak should remain the same from year to year. That is any variation in the solar longitude peak represents a real astronomical variation in the shower, not just an artefact of how we structure the calendar.


Techniques for locating origin of gravitational waves The gravity wave detections (GW150914 and GW151226) have both been ambiguous in their exact position, the LIGO papers (GW150914 and GW151226) give no precise information about the location of the origin of the waves.

In the case of GW150914, the approximate location is in the area known as the Magellanic Clouds. In case of GW151226, the parsecs are known, but no precise location. There are two detectors (in Hanford and Livingstone,USA) that made gravitational wave detection possible, does one need a third detector(or more) to pinpoint the position of the origin of these waves?

In other words can triangulation from three detectors pinpoint the location  or are there other techniques for pinpointing the origin of gravitational waves?

Short answer
Three detectors would improve the ability to pinpoint the direction of the gravitational waves drastically.
Long answer
Finding the direction of the source of a gravitational wave, as with most things in science, is harder than it might initially seem. I'd like to point out that the process used in determining direction is not precisely a form of triangulation (even if it had 3+ detectors) because triangulation involves determining precise position from knowing distances to the object from 3 or more locations. That's not really what's happening here.
I'll outline the basics for determining the direction and how that method might be improved with more detectors, but you can also read this excellent source.
Time of Arrival
The main way we can determine direction is time of arrival at the various detectors. The currently active detectors are located some 10 milli-lightseconds apart. That means one of them is going to detect the wave first, then sometime later, the other one detects it. Given that the wave has to be traveling at the speed of light, you can determine the direction it was traveling from such that it traversed the distance between the two detectors in the time difference measured.
But, this presents a problem because this method has degeneracies with only 2 detectors. Really, all you can calculate is the angle of the direction of the wave with respect to the line connecting your two detectors. That only gives you a cone on the sky of where the wave could've come from and you can't pinpoint the azimuthal angle without more information. This concept can be seen in the image below showing the probable direction. You can see they've limited it further, but their shape still takes part of a circle (the projection of that cone on the sky).

What if you added another detector?
As stated above, you have a degeneracy with only 2 detectors, but adding a third would break that degeneracy. You'd turn your cone into a point and you'd know much more precisely where the source came from. More detectors would allow you to pinpoint that location even more precisely, but only 3 are necessary to remove all degeneracies.
Now, of course I've been describing how it would work "in theory". In practice, observations are usually much more messy. Any measurement comes with error, so even with 3 detectors you're not going to be able to pinpoint the location exactly. You'll more likely be able to pinpoint it to a small patch in the sky. That's where other direction detection techniques can be used to refine that location.
Detector Sensitivity
The way in which the experiment works is that a mass is set up which can length contract under the influence of gravitational waves. This tiny contraction of space-time (and thus the mass) by the passing is measurable with precise lasers and interferometry. However, the lasers can only measure the contraction of the mass in one direction. The degree to which the mass contracts in that direction depends on the direction in which the wave is hitting it. This is an oversimplification, but if the modulation of the wave (that is the direction in which it goes "up" and "down") is aligned perfectly with the direction in which the contractions are being measured, you measure the full effects of the wave. If the wave comes in at some angle to that, you're still measuring the effects, but you'll only be measuring some part of the effect, reduced by the sine of the angle.
This process of wave direction detection requires at least 2 detectors, but more is better for beating down the noise.


Is there any way a planet could form independent of a star? I just recently heard that there are currently no theoretical models that support planet formation without a star, but I can't remember where. So, I thought I would request all of your expertises. Thank you much. I wanted to confirm whether this is true or not and to know if there are any theories that could account for planet formation in interstellar or intergalactic space.

Well you need to see the related question brown dwarfs and planets , because the answer to your question depends on how you define a planet.

If you demand that a "planet" has a rocky core then it seems very unlikely that a planet could form in isolation away from a parent star. The parent star is needed in order to differentiate the rocky material from the gas and allow it to condense.

On the other hand, if you wish to define a planet as simply an object below a certain mass (say the deuterium burning threshold at 13 Jupiter masses) then it seems very likely that such an object could form in isolation. They would be entirely gaseous, but there would be little to distinguish them from brown dwarfs at only slightly higher masses.

At present there are plenty of candidate "free-floating planetary mass" objects.
For example see Joergens et al. (2014); Liu et al. (2013); Zapatero-Osorio et al. (2000). Unless we have our understanding of the physics completely wrong, then it is likely that at least some of these are lower than 13 Jupiter masses. However, their origin remains unclear. It is possible they could all have formed around stars and then subsequently been ejected, but the presence of significant numbers of these objects in young star forming regions and the lack of $\sim$10 Jupiter-mass objects orbiting stars, suggests that there is an alternative formation scenario that can produce such objects in isolation.

What could these formation scenarios be? These low-mass objects could just be an extension to lower masses of the fragementation process that forms stars; they could be ejected embryos that started their lives in multiple systems; they could be "failed" stellar cores that could not accrete more gas because of photoevaporation by nearby massive stars; or they could form by gravitational instability around stars with unusually massive disks and be ejected by a close encounter with another star. These possibilities are reviewed by Whitworth et al. (2006) and Chabrier et al. (2014), and are all still thought plausible to some extent.


Is it thought that the majority of Earth size or larger terrrestrial planets have retained most of their original atmospheres? I used to think that most terrestrial planet atmospheres were secondary (from out gassing). However, after reading this https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140017487.pdf and noticing that the ratio of CO2 to N2 is far higher on Venus than that just expected from out gassing, I have done a 180.

I'm thinking that Earth has a primarily secondary atmosphere because of the Moon creating event and smaller planets like Mars just aren't massive enough to retain original nitrogen, etc.

Would this cast a shadow on the idea of early extreme stellar activity ripping off atmospheres of close in planets?

Too long for a comment, so . . .

It's important to remember that the field of primordial atmospheres has a good bit of uncertainty to it.  I've tried to read up on it as a layman and there's a good deal of contradicting theories and not much certainty.  Your first article addresses this uncertainty several times.

  The reason I mention the ratio of CO2 to N2 is because I read
  here, for example, that out gassing produces the molecules in
  about a 4.6:1 ratio.

Could you pull the quote to that 4.6:1 ratio.  I didn't see it in that article and that sounds much too accurate for an estimate in this field.   4.6 to 1 suggests about a 2% range of uncertainty.   I find that hard to believe.   Certainly a ratio with that kind of accuracy should cite a respected research study.

  And noticing that the ratio of CO2 to N2 is far higher on Venus than
  that just expected from out gassing

What ratio is expected from outgassing?  Your 2nd article mentions a neat and tidy percentages:

  The composition of outgassing is similar for Venus, Earth and Mars and
  is composed of 58% H2O, 23% CO2, 13% SO2, 5% N2 and traces of noble
  gases (Ne, Ar, Kr). The latter evolution of this outgassing is driven
  primarily by the surface temperature and chemistry of the planet.

But I call BS, cause one, this doesn't site a source and some basic google-research on Earth volcanic activity suggests the numbers are far from that neat and tidy - see charts below.

Source

and

Source

The 2nd source, above, doesn't even mention Nitrogen (N2), Wikipedia does, though they don't give percentages beyond 60% for water, 10-40% for CO2.

Earth's volcanic gas today is probably not a good measuring stick because of 3 plus billion years of life (some nitrogen fixing, lots of long chain carbon molecules) and some 4 billion years of plate tectonics.

And to my knowledge, a sample of volcanic gas from Venus has never been studied.  In fact, there are no known active volcanoes on Venus at all.   Source.   Outgassing can perhaps be estimated by atmospheric content, but I'm skeptical on the accuracy of those numbers they gave.  I think your first article from NASA far more reliable.   They quote a ton of sources and several times in the article, indicate legitimate uncertainty.

What I took away from the NASA article:

Yes, when the solar system was very young, the t-tauri phase of the sun blew most of the atmosphere off the 4 inner planets (or 3 Earth-like if you want to ignore Mercury, which the first article does).  This is backed up in part by the lack of Xe-129 which should form as a gas based on the 15 million year half life of I-129.   If the atmospheres weren't blown off, we should see a certain percentage Xe-129 in the atmospheres of Earth/Mars/Venus.   The percentage is much too low, suggesting the blow-off did happen over the first, perhaps 100 million years, continuing well after the end of the T-tauri phase, when the sun was still very active with solar flares.

The giant impact that is thought to have created the Moon (about 30 million years after planet formation) may have blown off a chunk of Earth's atmosphere as well, but the Solar wind probably did too.

Where the primordial atmosphere came from? Whether out gassing or comets (or a combination of both), I don't think that's a resolved question.  What seems true, based on the first article, is that Venus's water evaporated and it lost it's water by a combination of photo-chemistry and it's higher temperature and perhaps some jeans escape.   No water on it's surface may have slowed down any hope it had for plate tectonics and that reduced the rate that it outgassed.    (a general summary of the NASA article).  To get a better idea of Venus history, they'd like some rocky samples, but they don't have any.

Earth maintained it's oceans, perhaps because it had more water to begin with, or a lower temperature or combination of the two.  The lunar impact may have sped up Earth's outgassing as it liquified the entire crust for thousands, perhaps as long as a couple million years.  It's unclear if Venus ever had it's crust liquified.

But after Earth cooled after the giant impact, it had enough water for oceans and oceans made it possible to form carbonate rock and remove much of the CO2 from it's atmosphere.  Life assisted with the removal of CO2 and fixed some nitrogen too.  Ammonia also dissolves in water, so much of the nitrogen that came to Earth by comets could have been stored in the oceans, while ammonia that came to Venus by comet would have been vaporized as gas and light enough where some of it would inevitably be lost to jeans escape and subject to photo-chemical dissociation, and, a weaker magnetic field.

Earth and Venus have similarish amounts of CO2, it's just that Earth has most of it's CO2 stored in carbonate rock.    Earth and Venus may have similar amounts of Nitrogen too, though Venus atmosphere has about 4 times as much as Earth's atmosphere, currently, that doesn't take into account Nitrogen in the oceans and fixed or bonded within the Earth.

I don't think Venus "extra" CO2 is a mystery because it didn't have oceans long enough to pull the CO2 out of it's atmosphere.  We also don't know enough about the materials that formed Venus might have been different than those that formed Earth, and we don't know that much about the process that followed, so I don't think it's fair way to say it has too high a CO2 to N2 ratio.

It's entirely possible that the giant impact did blow away a portion of Earth's atmosphere, leaving it somewhat atmosphere depleted compared to Venus, but your NASA article points out that there's some uncertainty on how much atmosphere the Earth had after the giant impact leading to uncertainty on how quickly the Earth cooled.

It could, I think, work in the other direction, where the enormous heat of impact and liquification of the crust may have enabled more atmosphere to outgas, giving Earth a thicker atmosphere early on but post impact, where, the Venus equivalent of that gas might still be trapped inside Venus.

There's a lot about this subject that remains unresolved.   This is just my takeaway from the articles.   Take this answer with a grain of salt.


Ptolemy’s understanding of the planet’s whereabouts I imagine that Ptolemy’s epicycles were performed as real circles - around equants - in two dimensions, e.g that he was able not only to give the angles to planets and the Sun as seen from the Earth, but actually able to point out the location in space in a projection in two dimensions or otherwise, of all then known planets in the system.
He would then have had the same type of knowledge as Kepler had, at least in principle, albeit with lower accuracy.
Is that actually the case?

I suppose you mean in three dimensions, but anyhow…
Yes, Ptolemy did know that planets move also in latitude, not only in longitude, and thus the Almagest includes sections for how to predict the latitude of a planet for a given moment. In fact, Book XIII of the Almagest is entirely written about this specific subject of the latitude of the planets.


How are the photos of the Milky Way taken? We are inside the Milky Way, right? So, how are we able to take photos of the Milky Way?

We are situated inside the galactic disk. The first image below, showing our approximate location, is a fabrication rather than a photograph. Such a photo would require a space probe to be tens of thousands of light years above the plane of the disk, which is patently infeasible with current technology. We don't even know exactly how many spiral arms our galaxy has!

But from where we are, we can still see a great density of gas and dust stretching 360 degrees across our sky, as you look along the galactic plane. These "inside views" are indeed real photographs - much the same as if you go outside on a dark night, well away from the city lights, you yourself will see its faint, dusty outline much like in the second image (which I can assure you is real).


UK groups that investigate extraterrestrial life What are some UK based groups that research astrobiology, especially the chance of extraterrestrial intelligence?

Extraterrestial intelligence is a hard research topic as the search for evidence is not straight forward in any way. The search for intelligent life as far as I know is limited to observations in radio and for that you want to see if any SETI members are UK based.

However if you're more into astrobiology in general you might wanna look up the UK Centre for Astrobiology which is based in Edinburgh. I don't know any of the faculty but I do know that they are hosting a meeting for early career scientists in 2017. There are also some smaller research groups based in the UK but you might wanna check out the specific research aim for these (e.g. the group at Imperial College in London is focused on Mars) as it might not be the area within astrobiology that interests you the most.

There is also Mark Burchell who is a professor at the University of Kent. His group did some exciting work on Panspermia (the idea of life travelling through space) that I happened to read earlier this year. Note that this is focused on microorganisms so this is probably as far from "intelligence" that it gets.

But you can try to look up departments at different universities. You probably want to look under astronomy or sometimes physical or Earth science as the research aims of some groups put them more in e.g. Earth science than actual astronomy. Sometimes you can also find smaller groups under biology or life science.


Does creation of galaxy depend on black holes? I've read many times that every galaxy contains a black hole at the center. So is the creation of a galaxy linked to black holes?

Most of the galaxies are believed to host a super massive black hole. The black hole in return has a dramatic impact on the evolution of the galaxy, an effect usually called "AGN feedback". Quoting W. Ishibashi, A. C. Fabian:

  Active Galactic Nuclei (AGN), regions of extremely high luminosity at the centers of some galaxies, are thought to radiate due to accretion onto a supermassive black hole.  This emitted radiation, or feedback, sends energy into its environment and influences the evolution of the host galaxy.  The details of this feedback are not well understood.  We do, however, observe several relationships between properties of the host galaxy and the central black hole — the most prominent of which is the M-sigma relation.  This empirical correlation states that the black hole mass (M) scales as the fourth power of the velocity dispersion (sigma) of stars in the galactic bulge.  Clearly, there is some connection between the evolution of the black hole and that of its host galaxy.

The topic of understanding how the supermassive black hole form in the first place, evolve and interact with their galaxy is still in debate among the scientific community, so it is impossible to fully answer your question yet.


What is the period of Jupiter being visible in the sky? For more than 3 months now, we can enjoy seeing Jupiter in the sky, and this will be the case until end of December.
This means that Jupiter move slowly in the sky every day, but it makes me wonder when we'll be able to see it again after December 2020?
Or, put it differently: how long does it takes for Jupiter to do a full "rotation" in the sky?
Thank you !

You can make a ballpark estimate of Jupiter's synodic period if you know that Jupiter orbits the sun once every twelve years, while Earth orbits in the same direction in one year.  So when your Earth calendar has completed a year, Jupiter has moved approximately one-twelfth of the way around its orbit, and Earth has to move approximately another month to catch up.  That gives you a synodic period of about thirteen months, consistent with the more careful calculation by Mike G.
The ballpark estimate doesn't work very well if the periods are too similar (e.g. for Mars or Venus), but it's fine for the outer planets.
As to when you can see it again, Jupiter's next solar conjunction is on 2021-01-28.  So if it disappears into your evening twilight at the end of December, you should expect to find it in the morning twilight around the beginning of March.


What is the "shockwave" of a supernova? A common notion is that supernovae create shock waves in interstellar gas, and can compress them to star forming densities.
How is that shockwave transmitted? Is it by ejected (sublight) particles, or is the radiation strong enough to move several solar masses of gas?

As one might expect,  wikipedia is our friend:

the resulting supernova explosion expels much or all of the stellar
material with velocities as much as 10% the speed of light (or
approximately 30,000 km/s). These speeds are highly supersonic, so a
strong shock wave forms ahead of the ejecta.


Encode place and date in star pattern Disclaimer: I'm an astronomy noob

You sometimes see in (B?) movies, that they infer dates from sky patterns.
I was asking myself if you can encode a place and date in a mostly meaningful pattern? How would you do it? Would it be possible?

I was looking at 1 and maybe the relation between the visible planets?
(Moon, Mars, Saturn)

Why: I want to make something special for my girlfriend (who likes "stars"), the date is special to us.

The position of stars in the sky has been used by navigators for a long time. For instance, astrolabs give you the time based on your position (or vice versa). And until GPS came along, airline pilots learned how to determine where they were using the stars. This comes with a few caveats however.

Determining the date and place from just looking at the sky is ambiguous. Since the Earth rotates, the position of stars and planets at a given place and time is indistinguishable from their position a bit later (say a couple hours) and a bit further West (how many miles further West depends on your latitude).
If you know what epoch you are in, you can determine the latitude by looking at the position of Polaris above the horizon. But to know longitude, you must also know the date and time.

Because of precession and nutation, the star that is aligned with the axis of rotation of the Earth will change over long periods of time (roughly 25 000 years). So the position Polaris isn't even enough to know your latitude if you don't know what millennium you are in.


What might a CN filter be in the context of comet watching? Is it showing dust, or gas, or something else? In this post on the website for the Comet Wirtanen Observing Campaign at wirtanen.astro.umd.edu, there's mention of a CN filter being used to observe the comet, and this cool GIF.

What exactly is a CN filter? What wavelengths does it pass? Where can I read more about it? Why is it showing a spiral shape of stuff being emitted from comet 46P? Is this comet dust, gas, something else?

GIF:

A CN filter shows abundances of cyanogen, a molecule with the structure $(\mathrm{CN})_2$. Cyanogen emission lines at 387 nm allow us to study gas in the coma of a comet, telling us more about its structure, properties and outgassing. Other gases, like $\mathrm{OH}$, $\mathrm{NH}$ and $\mathrm{C}_2$, also give away a lot of information about a comet, but the often-strong cyanogen lines are a good tool.

The mechanism behind cyanogen production from parent gases isn't entirely clear, although a number of theories have been proposed. For instance, cyanogen could be released by icy grains undergoing sublimation, or by the decomposition of more complex molecules.


Has celestial navigation been materially impacted by the imperfect nature of celestial reference frames over time? In this video on inertial reference frames, it is mentioned that the stars are humanity's best inertial reference frame: the earth experiences a subtle acceleration relative to the sun due to the earth's own orbit and rotation; therefore the earth is only roughly an inertial frame. But, the stars are relatively fixed, they say.

However, the stars in galaxies move relative to each other--I assume many with some sort of rotational velocity and therefore acceleration--and galaxies themselves are all slowly moving away from one another, perhaps even with centrifugal acceleration of their own.

Has this acceleration been great enough that humanity has needed to update navigational tools that rely on the assumption that stars are an inertial reference frame?

Is it possible that every observable object in the universe is in fact accelerating relative to an unobservable reference frame?

The best inertial frame is that which is comoving with the cosmic microwave background. There is a notable dipole in the microwave background, the result of the Solar system's motion relative to it. The invariance of this dipole provides evidence that the frame of the barycentre of the solar system is also a very good inertial frame.

This is of no significance to navigation, either by the stars, or among the planets.

Within the solar system, the sun's gravitation influence is dominant. While the sun is orbiting with the galaxy, a space probe is also orbiting the galaxy, and the variation in gravitation due to the galaxy is extremely small. Galactic tides are several orders less significant than photon pressure, various quadrupole effects or the influence of other bodies.


Has the great Andromeda Galaxy ever collided with any galaxies? We know that the Andromeda Galaxy is moving towards the Milky Way and vice versa, so there will be a collision. Because of the vast space between them, the  stars as they are defined will not themselves come into contact until much later if at any time at all.

My question is whether Andromeda has had a collision prior to this 4 billion year inevitability.

Galaxies grow through cosmic time by accretion of the surrounding matter. Some of its mass increase happens through smooth accretion of gas, but much also happens through merging with small clumps of dark matter, gas, and stars, called satellite galaxies. This is called "minor merging".

If merging galaxies are similar in size, it's called major merging. Most large galaxies have gone through of the order of one major merger through its life (Man et la. 2014), but whether or not it happened for Andromeda, we cannot know for certain.

However, if you take a look at Andromeda, you'll see that it has quite a large bulge, i.e. the central, reddish part of the galaxy. This region is dominated by stars with a more "chaotic" velocity patterns. That is, their paths aren't "rotation-dominated" (lie in the disk), but more "dispersion-dominated" (leave the disk). To achieve such velocities, the system has to be disturbed by infalling matter, so a large bulge is probably a signature of a major merger. Investigating the velocity patterns of Andromeda, Dorman et al. (2015) find that it has had a much more violent history than the Milky Way, and in fact models by Davidge et al. (2012) and simulations by Hammer et al. (2010) of the history of Andromeda suggest that 8-9 billion years ago, Andromeda went through a major merger.

Note however that a large bulge can also be explained, at least in cosmological simulations, by a more smooth and "cold" accretion, i.e. gas that doesn't get shock-heated (Dekel et al.)


Stability of orbital resonance Recently I've become more interested in the topic of the stability of planetary systems. I have been reading about it and it seems that orbital resonances play an important role in the stability of Solar System (as well as in the moons of Jupiter and other planets).

I first thought that the orbital resonance is somehow more stable and thus we have several cases in the Solar System.

But as I kept reading, I found out that in the asteroid belt there were some gaps precisely where the resonances happen, so the resonances are actually instable for asteroids.

I then thought that some resonances are stable while others are instable, but some of the resonances that makes gaps in the asteroid belt are actually present in the Solar System between planets, so I am completely lost.

Why resonances are sometimes stable and sometimes instable? What am I missing?
Maybe I am misunderstanding something because it doesn't make sense to me. Any help will be welcome.

I am not very familiar with orbital dynamics (so please correct me if I'm wrong). I was told that, for instance in the case of the mean motion resonances that cause the majority of the Kirkwood gaps in the asteroid belt, not only the ratio of the periods, but also the timing is important.

Let's take Pluto as an example, which is in 2:3 resonance with Neptune. Although Pluto crosses Neptune's orbit, both bodies will never get closer to each other than a certain threshold. That is, because the timing between their orbits was preventing close encounters in the first place, which is then enforced by the resonance.

If the timing was different, close encounters would be possible, thus destabilizing the orbit of the smaller body. In the case of mean motion resonances, there is an algebraic expression that allows to investigate the stability of a resonance (see, e.g., here https://en.wikipedia.org/wiki/Resonant_trans-Neptunian_object [toward a formal definition]).


Have there been studies of "old photons" to see just how constant things like Planck constant has been? The question Are photons aged? and answers therein have got me thinking:

I vaguely remember hearing something about experiments where "old photons" were collected by large telescopes from very distant objects, and either their energies or their frequencies were compared to their wavelengths to check the (linear or inverse) some fundamental constants, perhaps those used in:

$$E = hf = h \frac{c}{\lambda}$$

Questions:

Am I misremembering, or has this been done?
What were the results and to what level of precision was non-variation constrained?

You can not check if a dimensional constant has changed because you can always reverse that change by a smart change of coordinates (system of units). Despite that, since the current Physics assumes the immutability of certain constants, you can verify this assumption by testing the change of an adimensional constant.

One of the most common adimensional constants that is used to search for constant variability if the fine-structure constant (a fundamental physical constant characterizing the strength of the electromagnetic interaction between elementary charged particles)
$$\alpha=\dfrac{1}{4\pi\epsilon_0}\dfrac{e^2}{\hbar c}\,.$$

The Wikipedia pages about Time-variation of fundamental constants and fine-structure constant have nice explanations about this.


Cosmic microwave background radiation If the big bang is true, after the emission of light from the hydrogen plasma, the universe was still expanding. Why would we expect to see uniform radiation if earth very well could have formed outside of this hydrogen plasma. Then we would expect to see bits of background radiation coming from one direction. People sometimes answer this question by saying that the big bang happened everywhere and that where the earth is right now was a haze of hydrogen plasma 13.7 billion years ago. I do not see how one can assume this as since the universe expands we could have formed outside of the plasma and therefore should not expect to detect uniform background radiation.

We could not have formed "outside" the expanding universe, as there would not be any substance to form our galaxy, and solar system -including our Sun and planet.


Do Roche limits apply to black holes? Consider black hole A, a super massive black hole at the center of the galaxy. Orbiting it is black hole B, a much less massive black hole.

If some passing body were to modify black hole B's orbit such that it fell within the Roche limit of black hole A, what would happen to black hole B?

If it were to turn into a ring, would the black hole matter re-inflate since it wouldn't be under such high gravity? Do black holes even respond to Roche limits like regular matter does?

It's different. Black holes are not objects, like planets or stars. Rather, they are powerful distortions of spacetime, maintained by the concentration of mass/energy therein (which itself is kept there prisoner by the spacetime distortion - a vicious cycle broken only slowly by the Hawking radiation). As such, they cannot be "ripped apart", since there is nothing to be ripped apart there - just a giant warped tangle of spacetime.

Instead, two black holes could merge, if they get close enough and lose enough mutual orbital motion.

merging black holes

The result is a single, larger black hole.


What were the challenges for the ancients to observe the orbit of the Moon (instead of Mars)? Astrophysics can be said to have been founded by Johannes Kepler around the year 1600. He based his break-through science on data of the position of Mars in the sky and disproved the ancient ideas about circular orbits and epicycles.

But why wasn't this done far earlier, by using observations of the Moon? Wasn't it pretty obvious to a careful astrologer a thousand years ago, that the Moon does not have a circular orbit and does not describe epi-cycles? It is the easiest celestial object to observe, visible both night and day. Moon calendars may have been designed tens of thousands of years ago, there's no lack of observational data. Kepler instead used a few oppositions of Mars which take place only once every two+ years. Since the Moon is the one object which does orbit Earth, in a geocentric world view it should've been the perfect test of circular and epi-cyclical theories about its orbit. Its nearness causes a daily parallax between moonrise and moonset, but that wouldn't be beyond a genius like Kepler or many mathematical astrologers before him.

What made the orbit of the Moon difficult for the ancients to understand?

Wasn't it pretty obvious to a careful astrologer a thousand years ago, that the Moon does not have a circular orbit and does not describe epi-cycles?

The ancient Greek model of the motion of planetary bodies remained unchallenged for almost two millennia, so obviously not.

Hipparchus' model did a fairly good job dealing with the elliptical motion of the Moon; it did even better with the planets. The Moon's motion is tough to model because of perturbations by the Sun, Venus, and Jupiter. Ptolemy discovered what would eventually be called evection, the largest of the perturbations caused by the Sun. There was one problem with Ptolemy's model: It had the Moon swinging in and out by a huge amount. If Ptolemy's model was correct, we would see the Moon changing in diameter by a factor of two over the course of a bit over half an orbit. Copernicus much later came up with a scheme that fixed this problem and still relied on those old concepts of deferents, equants, epicycles, etc.

While Newton pointed the way to describing the Moon's orbit, it wouldn't be until 200 years after Newton's death that a decent (one that matches observations) model of the Moon's orbit was developed.


Will the conditions that allow life to evolve still be around in a trillion years? Some billions of years ago, systems like the Earth couldn't emerge and therefore life couldn't arise, because not enough of the heavy elements had formed yet.

Assuming there'll neither be a big crunch nor a rip, stars will still be forming in a trillion years.

So the sun is one of the earlier stars in the lifetime of the universe.

It seems weird to be living at the very start of the era of the universe in which life can evolve.
If a conscious being was chosen at random, the odds of it being on one of the first planets that allow life to evolve seems very slim!

So I'm curious about whether some evidence indicates that life will be less likely to emerge later in the universe.

Will stars of a very late generation (the sun being a 3rd generation star)) be similar to the sun, keeping the possibility open for earth-like planets to form?

According to the 1997 paper "A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects" (arXiv version) by Fred C Adams and Gregory Laughlin, which was the basis for their book "The Five Ages of the Universe", the stelliferous era (the time when star formation is ongoing) is likely to last until ~1014 years after the Big Bang. Their estimated gas depletion timescale for the galaxy is 1012–1014 years. They do note that for the stars born after ~1012 years, metallicity effects are likely to result in significantly shorter stellar lifetimes.

So it seems likely that there will still be conditions to allow life to survive in a trillion years time, unless you happen to be using the long-scale trillion.


If Jupiter were to absorb all other planets in the solar system, would it reach degeneracy pressure? Jupiter's mass is about the maximum a planet can be before it starts to fuse hydrogen in its core and undergo a massive transformation.

Total mass of all solar system objects in question is: here.

Jupiter is 1 Jupiter large, while the summation of all else is 0.405 Jupiter large.

Given the mass of Jupiter and the mass of all other planets, if Jupiter were to somehow collide and "absorb" all mass from every planet in the current solar system, would it turn into a star?

I guess that you can distill this question down to:

If Jupiter were 1.405x more massive, would it be a star?

Note: Still kind of new to these concepts, please explain misconceptions I may have.

No, hydrogen burning doesn't start until 82 $m_{\rm Jup}$ and deuterium burning starts at 13 $m_{\rm Jup}$, so evidently 1.4 $m_{\rm Jup}$ is not enough for this experiment.

Your misconception is that

  Jupiter's mass is about the maximum a planet can be before it starts to fuse hydrogen in its core and undergo a massive transformation.

which is simply not true.


Where can I find a catalog of all stars in the Milky Way? Is there a catalog of all known stars or brightest stars in our galaxy? Preferably with some sort of galactic coordinates instead of just night sky coordinates (right ascension, etc.) I'm trying to make a model of the milky way, explorable with an oculus rift.

Hipparcos, the predecessor to Gaia, has a dataset (http://cdsarc.u-strasbg.fr/viz-bin/Cat?I/239) with 3D positions for 100,000 stars. While we have much larger datasets of galactic stars, such as SDSS, finding the distances to stars is much harder. Parallax is the most precise technique for finding distances. Even for Gaia, we will only be able to measure distances to 10% precisions.

There is a "galactic coordinate system", but that stil has the sun at the center and the line between the sun and the galactic center as its reference. There are calculators that can convert RA and Dec to galactic coordinates (e.g. http://python4astronomers.github.io/astropy/coordinates.html). You can use that and the distance to place them in space.


How can a supernova affect black hole in a binary system? Suppose in a binary stars system there is a dying star and a companion black hole as they orbit around each other. My question is what will possibly happen to the black hole if the partner goes supernova? I understand that the kinetic energy from the supernova cannot destroy the black hole.

The likely result would either be a black hole-black hole binary system; a neutron star-black hole binary system, or the black hole and the compact remnant from the second supernova explosion would go their separate ways at reasonably high speeds.

You cannot disrupt a black hole in this way. In fact all that will happen to the original black hole is that it will likely get a bit more massive from accreting some of the supernova ejecta.


How much larger would a star have to be to cause thermonuclear reactions if it was made out of mostly rock like Earth, instead of gases? I assume it would have to be considerably larger since rock isn't as flammable as gases are? But it's also more dense to begin with..

Do we know of any suns that are made like this?

Is it even possible to have a thermonuclear reaction with solid rock?

It really depends what you mean by "rock". At the temperatures and pressures at the cores of stars (and at which nuclear fusion reactions are possible), "rocks" as I suspect you are thinking of, do not exist.

Thermonuclear reactions do not occur because the gas is "flammable", they occur because the kinetic energies of the nuclei in the gas (at these temperatures, atoms are fully ionised) are sufficient to get them close enough together for nuclear fusion to take place.

"Rocks" are made of atoms of silicon and oxygen (for example) in the form if silicates. But these are dissociated at fairly low temperatures compared to the centres of stars. Oxygen and Silicon thermonuclear fusion ignition requires temperatures in excess of $10^9$ K, and these temperatures are only reached late in the lives of stars of mass $>8 M_{\odot}$.

It is possible to have stars with solid cores. This is thought to be the fate of white dwarf stars as they cool. Most white dwarfs are made of a mixture of carbon and oxygen and this "crystallises" once the core of the white dwarf cools below about a few million degrees.

Ordinarily, the cores of white dwarfs are inert as far as nuclear reactions are concerned, because the temperatures are too cool. However, if the white dwarf is massive enough (or mass is added to it), then the central densities climb, and for white dwarfs of mass $\sim 1.38M_{\odot}$ it is thought that the densities become high enough ($\sim 10^{13}$ kg/m$^3$) to start nuclear fusion of carbon via the zero point energy oscillations in the crystalline material (so-called pyconuclear reactions). Such reactions in degenerate matter are highly explosive and might result in the detonation of the whole star in a type Ia supernova.


If our galaxy is mostly empty space then why does it look so solid? This question has probably been asked quite a few times, but I'm having trouble finding anything due to the fact that this question is very specific in nature. So I'm asking again.
We know that outer space is mostly empty and stars are very far apart from each other. So much so that when two galaxies collide, the stars within them will just fly past each other instead of crashing. But when I look at pictures of the Milky Way, (some of them are real pictures of the center of the Milky Way taken from earth, while others are renderings) it looks so bright and so solid, like theres so much stuff in there. It becomes hard to believe how it can be that it's mostly empty.
Can anyone please explain what's going in there?
Thanks.

You are mostly empty space.  Every atom in your body is very tiny compared to the relatively vast spaces between it and its nearest neighbors.  And the same goes for every "solid" object you ever saw.
But when you look at yourself in a mirror, the number of atoms you see is so incredibly vast that you don't see the spaces between them but instead think that yhour body is a solid object.
And probably the wavelengths of visible light are too long to fit between atoms and so get bounced back from the surface of your body.
That is not the case with the galaxy, since the spaces between stars in the galaxy are wide enough for all wavelengths of light to pass through instead of being reflected.
But if you look at our galaxy from a distance of about one million light years, the average separation between stars of about 6 light years will be a very small angle.  I believe that at one million light years distance, one arc second will be about 4.848 light years wide.

The maximum angular resolution of the human eye is 28 arc seconds or 0.47 arc minutes,[19] this gives an angular resolution of 0.008 degrees, and at a distance of 1 km corresponds to 136 mm. This is equal to 0.94 arc minutes per line pair (one white and one black line), or 0.016 degrees. For a pixel pair (one white and one black pixel) this gives a pixel density of 128 pixels per degree (PPD).

https://en.wikipedia.org/wiki/Visual_acuity#Physiology[1]
So at a distance of one million light years the smallest detail a human could see would be 28 arc seconds, or 135.7 light years wide.  So such a smallest possible thing you could see would be a square about 136 by 136 light years wide, and possibly containing about 350 stars in a single layer and all the vast interstellar spaces between them.
Furthermore, galaxies are not a single layer deep.  Depending on the size of a galaxy, the distribution of stars within it, and the angle from which it is viewed, a viewer might see  tens, hundreds, or thousands of layers of stars one behind the other.  The stars are not arranged in any sort of regular lattice pattern, but orbit around the center of the galaxy in separate orbits and their relative positions slowly change, thus making their relative positions random.
Thus the empty spaces between stars in the outermost layer of a galaxy will show the light of stars farther behnd them.
So when you look at galaxy from a distance of one million light years, the smallest part of the galaxy you can see will have the light of hundreds or probably thousands of stars blended together with the black space between them to make a pale grayish light.
The farthest objects which humans can see with the unaided eye are the Andomeda Galaxy M31 about 2,540,000 light years from Earth, and the Triangulum Galaxy M33 about 3,200,000 light years from Earth.  Since they are several times a million light years away, the smallest part of them which can be seen with the unaided eye will combine the light of even more thousands of stars together.
Astronomers have photographed galaxies tens of millions, hundreds of millions, billions, and tens of billions of light years distant.  With astronomical telescopes they can detect some individual stars in galaxies tens of millions of light years away.
But any photograph which shows an entire galaxy will show the light of millions or billions of stars blurred together into a blob of light.


Asteroid apparent magnitude I'm writing an SF story in which a main belt asteroid is diverted into a collision course trajectory with earth (long story), and unknown to us here on earth.

Its H magnitude is 12. As it nears earth, I would assume it gets brighter in V--apparent magnitude. Considering all the eyes on the sky these days, how bright, apparent mag V, would it get before being discovered here on earth? 9 or 10 seem reasonable?

And if you could give its distance in AU, at discovery, that would be super (trying to find out how much time we have left ;) )!

That's a big asteroid!

Recent discoveries of objects of this size are rare, one example was 2013UQ4. We believe that all objects of this size in the main asteroid belt have been discovered. (The discovery of 2013US10, which appeared to be main belt object turned out to be an error, the combination of observations of two different objects) This object has a highly inclined and retrograde orbit. It turned out to be a comet not an asteroid.

For context US10 it was at magnitude 19 when discovered. It is unlikely that a large body would remain undiscovered until it was at mag. 10 or 9, especially if it was in the main belt. A very small object that is rapidly brightening could reach these magnitudes before discovery, but not a 10-20km sized object that your story implies.


Why was helium hydride (HeH+) the universe's first molecule? The abstract of the new Nature paper Astrophysical detection of the helium hydride ion HeH+ tell us that infrared spectroscopy from SOFIA detected

...rotational ground-state transition of HeH+ at a wavelength of 149.1 micrometres in the planetary nebula NGC 7027.

It also says:

In this metal-free and low-density environment, neutral helium atoms formed the Universe’s first molecular bond in the helium hydride ion HeH+ through radiative association with protons. As recombination progressed, the destruction of HeH+ created a path to the formation of molecular hydrogen.

Question: Why was helium hydride (HeH+) the universe's first molecule? Why was it the hydrogen helium cation rather than the dihydrogen cation?

At the risk of self-plagiarism: (https://physics.stackexchange.com/a/532568/43351 with a bit added).
Molecular chemistry in the early universe requires species with bound electrons. Helium hydride is the first molecule to form because neutral helium atoms, formed about 120,000 years after the big bang, could combine with plentiful protons; but it was another 260,000 years until significant numbers of neutral hydrogen atoms formed, and it is only once these are present that there is a route to forming H$_2$.
Details:
Helium has a much higher ionisation energy than hydrogen and therefore starts to recombine at higher temperatures (about 7000 K at redshifts of $\sim 2500$, compared with 3000 K and a redshift of 1100 for hydrogen). Thus in the primordial gas of hydrogen and helium, it is the helium that recombines first. There is therefore a period of time in the early universe, $120,000 < \tau < 380,000$ years, in which almost all the hydrogen is ionised, but most of the helium is in the form of atoms.
The two react to form helium hydride
$${\rm He} + {\rm H}^{+} \rightarrow {\rm He H^+}$$
As you might expect, the concentration of this molecule is low, because the temperatures were still high enough to easily radiatively disassociate it - about 1 part in $10^{21}$ at $z \sim 2000$ (Stancil et al. 1998; Galli & Palla 2013).
This is the first molecule to be produced with any important level of abundance. Note, this appears to be a somewhat arbitrary definition, since it is also claimed (e.g. Lepp et al. 2002), that He$_{2}^{+}$ formed the first molecular bond, via He$^+ +$ He, but was too weakly bound to survive in any concentration (the concentration peaks about 100 times lower than HeH$^+$ according to Galli & Palla 2013).
It is also possible to form small quantities of $H_2$ at $z> 2000$ via the reaction of a hydrogen atom with another in an excited state: H + H$^* \rightarrow$ H$_2$; but of course, although the hydrogen molecule is much more strongly bound than HeH$+$ (the dissociation energy of $H_2$ is 4.5 eV, versus about 1.8 eV for HeH$^+$), there is very little atomic hydrogen present and this reaction requires not one, but two hydrogen atoms to get together. It is not until H atoms recombine in quantity some 260,000 years later that hydrogen molecules are formed in various gas phase processes and H$_2$ becomes the dominant molecular species.
The dihydrogen cation, H$_2^{+}$ cannot form directly before either helium hydride or atomic hydrogen are formed as precursors. e.g.
$$ {\rm HeH}^{+} + {\rm H} \rightarrow {\rm H}_2^{+} + {\rm He}$$
$$  {\rm H} + {\rm H}^+ \rightarrow {\rm H}_2^{+}$$
and thus H$_2^{+}$ forms after HeH$^+$.


What starting circumstances have to be for "grand tack" hypothesis to work, and why they are lacking in other planet systems(as far as we know)? I recently read a bit about "grand tack" hypothesis. It was extremely interesting and it seems to reasonably well explain many features of our solar system. At the same time it supposedly is consistent with our knowledge of other planet systems, which I don't understand. What exactly happened in solar system and at which point , that is so rare out in the other systems? Combination of Jupiter/Saturn showing up at the right places and the right time? Isn't it common that the biggest gas giants appear somewhere over the edge of frost line?

We do not yet know whether our solar system is a common or rare outcome of the planetary process.

The statistical properties of exoplanetary systems are severely affected by observational biases. It is far easier to detect close-in giant planets; objects like Saturn, at Saturn-like distances from their host star, have not been found due to the limitations of the observations.

It is therefore far too early to say whether the Grand Tack model may or may not be applicable to a wide variety of exoplanetary systems. What we do know, is that something drastically different happened in the case of those systems that have a Jupiter-sized planet within an astronomical unit or so of their parent star; but these are found in only a few per cent of stars.

As an addendum, it is also the case, that despite taking 10+ years of data to establish them, there are now many cases of Jupiter-like planets in Jupiter-like orbits, though they are probably not that common. The overall conclusion at present is that the solar system does not look that unusual, although the lack of close-in planets larger than the Earth is notable (see Martin & Livio 2015).


Table of dates for planet retrograde motion Is there a table which shows the dates of the planets retrograde motion? I want to know at which date a certain planet will start the retrograde motion.

EDIT: It turns out calsky.com will do these calculations for several years out, including other calculations commonly found in almanacs. Sample page:

https://www.calsky.com/cs.cgi?cha=7&sec=6&sub=3

The best place to start on this site isn't the home page, but rather the TOC:

https://www.calsky.com/cs.cgi/TOC?&f=

I now consider my answer to be better than @JohnHoltz's :)

Note: I saw @JohnHoltz's answer while I was typing out my answer, so some of the below may be redundant. Upvote/checkmark his answer, not mine.

This isn't really an answer, but too long for a comment:

I was disappointed how difficult it was to find this information, given how easy it is to calculate and how many other astronomical phenomena are listed in various places on the Internet.
When a planet starts or ends retrograde motion, it is temporarily stationary before it "turns around". Thus, some astronomical sources will say "Jupiter stationary" instead of "Jupiter starts retrograde motion".
Based on the above I google'd "jupiter stationary" "mars stationary" (as quoted; google tries to correct one of the stationary's to stationery, so I had to click one more time to do the search correctly), and finally found something: http://aa.usno.navy.mil/publications/docs/ap.php
The publication doesn't mention retrograde, but it does mention when a given planet is stationary and provides a brief description of each planet's position in the sky for the given year.

Because this seems like a fairly easy calculation to make, I may do it myself and update this answer.


Was there ever a meteorite with a measurable gravity? Did we ever find a meteorite (a meteoroid that has fallen down to Earth) whose mass, density and gravity were high enough to demonstrate their gravity similar to the Cavendish experiment, so that something was attracted to it?

Was there ever a meteorite with measurable gravity?

At risk of stating the obvious:   Yes.  All mass, as far as every test I know about, has measured gravity.  No test has ever measured no gravity within the range of accuracy, so even low mass objects can have their gravity measured.
Rubble pile asteroids couldn't form if they didn't exert gravity.  Asteroids have been observed in orbiting pairs, so there's little question that they exert measurable gravity, if you can get close enough and have good enough equiptment to one to measure it.

Did we ever find a meteorite (a meteoroid that has fallen down to Earth) whose mass, density and gravity were high enough to demonstrate
their gravity similar to the Cavendish experiment?

Cavendish measured gravity using 158 kg lead balls and some sensitive equiptment.   These larger lead balls were stationary sources of perpendicular gravity on two smaller, 0.73 kg lead balls as the suspended objects on a wire upon which the gravity acted.   By the twisting of the suspension wire, and from a distance, so his mass wouldn't affect the experiment, Cavendish measured gravity.
A 158 kg lead ball is about 15 cm in radius and the smaller balls about 2.5 cm.  The distance between them (and I'm assuming this was center to center as that's standard in physical notations of this type, is listed as about 9 inches or 29.5 cm.    That means, surface to surface the distance was about 12 cm, which doesn't change the equation, but you wouldn't want to calculate gravity with two surfaces inside each other, even if the math lets you get away with that, so, I thought I'd mention it.
Some quick math and 29.5 cm, 158 KG, the gravitational force is easy to calculate. F=Gm/r^2, which comes to about 2.7/10,000,000 meters per second squared about 35 million times weaker than earth gravity.   A meteor could certainly have that force of gravity pretty easily.  While less dense than lead, greater overall could accomplish 1/35 million G forces easily enough.
What does that mean?  1/35 million G forces?   Lets call that amount of gravity one Cavendish - for obvious reasons.
Have you ever walked up a flight of stairs, one step at a time?  By just taking one step up a flight of stairs, you've reduced the gravitational force acting on you by one Cavendish.  A bit more than one, actually.   By taking an elevator up one floor works out to about 33 Cavendishes drop in gravity, for every 3 meters.    That's how gravity works.   Mass increases (roughly speaking) with the cube of the radius.  Gravity falls off with the square of the distance, so larger objects not denser objects, usually win in the gravity contest.   There are exceptions if there's enough variation in density, obviously.  Earth, for example has higher surface gravity than Uranus and Mercury has very nearly equal surface gravity to Mars, but as a general rule, when the overall mass is much different, density matters less.   The Sun and the Moon appear about the same size to our eyes and as a result have similar tidal forces, but the Sun exerts hundreds of times more gravity on the Earth than the Moon does.
The weakness of this force indicates how sensitive Cavendish' experiment was for it's time.
How did Cavendish know that there wasn't a mountain range in the distance that would exert equal pull on his test, or maybe some part of the Earth that was heavier on one side of where he lived than the other side?
Lets imagine that there was a mountain 12 km from Cavendish's laboratory, lets pretend this mountain was a 1.5 km high and weighs about 4 trillion kg.  That mountain would exert nearly 7 times the gravitational pull on Cavendish's test balls that the his 158 kg stationary lead balls did, so, did Cavendish need to find a flat region of equal gravitational pull in all directions?
Actually, the answer is no, because a mountain in the distance would have generated a gravitational field that would have been part of the initial conditions and he was measuring the change from the initial conditions.
And even if somebody had decided to mess up Cavendish's experiment by putting a mountain 12 miles from his house during his experiment, the effect would have been nearly equal on the two balls so it wouldn't have made much difference. It would just have been a tidal force, and a very small specific force on the torque of the wire that Cavendish was interested in.   Cavendish's experiment only works under very specific conditions, one 158 kg weight pulling both suspended balls on the wire in the same clockwise direction so the twist can be measured.  That doesn't work with a single large mass. It's the wrong kind of setup.
But to answer your question, forces greater than 1 Cavendish are easy for any relatively large body and a meteor could do that.  Bennu, which you mentioned in a comment exerts 6 micro-G, or about 22 Cavendishes.  It's radius is about 250 meters, so within about 1100 or 1200 meters, an object that size would exert 1 Cavendish of gravitational force.
Ofcourse, if Bennu was to actually strike the Earth, it would likely decrease, not increase gravity at the spot it hits because it would eject material over a wide area, leaving a crater.  Likewise, the effect would be very small, but on the edge of the crater, probably several Cavendish's worth of decreased gravity.
Perpendicular forces like this happen on Earth.  As land rises and falls.  As glaciers shift, the perpendicular gravitational force near those massive objects changes and this is most noticeable on the ocean.  The gravity of the Greenland ice sheet or the Antarctic ice sheet is measurable on sea level and calculating sea level rise if those ice sheets melt (which will take some time), but accurate calculation has to take into account the gravitational pull of the mass of glacial ice on the ocean's sea level.
The mass of a meteorite is generally quite small in the grand scheme of things and it's gravitational force on a nearby blade of grass, for example, while perhaps several Cavendishes, is negligible for all practical purposes, but measurable if you have sensitive enough equiptment.  You can now tell your friend when you drive past a mountain range that your car is experiencing a force greater than the force Cavendish first use to measure the gravitational constant and they probably won't know what you're talking about.
And if you're talking a much smaller, mostly Iron meteorite fragment, then you need to be quite close.  The largest meteorite fragment or solid piece is the Hoba, about 60,000 KG.  You'd need to be about 4 meters from it's center to experience 1 Cavendish of gravitational force.
It's fun to think that walking past a very dense, very heavy object, that you might feel it's gravity, but most things aren't nearly massive enough.  You don't feel the gravitational pull of a mountain range when you walk past it because the pull is negligible and mountain ranges are much more massive than most asteroids that are likely to hit the Earth.   Mountain ranges also are generally made up of lighter rock than the surrounding area, so there's also that, but even if they weren't, you still couldn't feel the gravitational pull of even the largest string of mountains.
Hope that wasn't too long.  I got answering and it came out longer than probably necessary.


Telescope magnification compared to distance from an object I'm looking at Saturn through a modest telescope at 50x. What point in space, expressed as the distance from Saturn, let's say it's 750 million miles from Earth, would I need to view Saturn with the naked eye and see it at the same apparent size as viewed from Earth at 50x. How do I estimate this in general for other objects as well. Thanks!

Just divide the actual distance by the magnification.  In your example that would give 750 million /50, which is 15 million miles.  (Actually saturn is currently over 800 million miles from earth).

The reason this works is that a magnification of M means the object is M times its apparent linear size, which requires being M times closer.  This is a slight approximation, but is fine for distant objects.


What caused the comet ISON to disintegrate? So curious to know why comet ISON disintegrated? Well, it went near the Sun but was the Sun the only reason it died? Or is there any other reason for it's fading away?

ISON slingshot past the Sun close enough (around one Solar diameter) for its surface temperature pointed towards it to reach 6,000 °C and it wasn't a fast spinner either to help this temperature average out along its total surface. In a nutshell, it melted to its nucleus into a hot gaseous plasma of materials it was composed of long before it reached perihelion. These materials still followed more or less same ballistic trajectory prior to perihelion, it merely elongated substantially due to strong gravitational influence of the Sun.

As it was approaching perihelion, the Sun's tidal forces broke this plasma into a wave-like pattern, increasing its surface area and exposing its materials to solar winds (and possibly hit by one CME, Coronal Mass Ejection that was detected near comet ISON on the perihelion day), changing orbit of different mass and size particles differently.

This was apparent past perihelion when the comet failed to create tails pointing away from the direction of solar winds like comets with a nucleus do, but instead started splitting into two  masses inclined roughly 90° to each other, suggesting different mass particles separated and the comet was indeed torn apart by Sun's gravity.

Since most of its materials thus gained angular momentum different enough to negate its own gravity, and they weren't bound to each other as they would if it remained solid, it entered a spin of death, slowly evaporated and dissipated entirely. No nucleus was observed to survive the perihelion, end even Hubble wasn't successful in finding any of its remains, further proving this fate of the comet ISON.

This is also additionally described with the help of ISON perihelion videos from various spacecraft in this Emily Lakdawalla's blog post, and since a picture tells a thousand words, here's my capture of a frame from SDO (Solar Dynamics Observatory) that matches the time, estimated size and distance, suggesting it could indeed be comet ISON being torn apart by tidal forces on its perihelion:

   A frame from SDO/AIA - 4500 203/11/28 18:00:07 (ISON pre-perihelion time). Click on image to download full frame JPeG.

This SDO frame (SDO/AIA - 4500 203/11/28 18:00:07) seems to have been missed by all other astronomers that were following the demise of ISON, likely because SDO was just positioning itself for ISON's perihelion at the time and this isn't a complete frame. So, in a sense, this is Astronomy Stack Exchange exclusive. :)

Update: Turns out it was a reflection / filter defect after all. I would like to point out that I never claimed this SDO/AIA frame shows comet ISON, merely that it seemed to me that it might. Position did seem odd, but not being equipped with all required SDO data, I had no way of confirming that. Clearly, I'm not experienced in interpreting SDO data, let alone a qualified expert, but my perseverance did pay off regardless and Dean Pesnell of the SDO team kindly looked into this and wrote an eye-opening explanation for this SDO/AIA frame in his SDO blog post Was Comet ISON Seen in AIA 4500? A polite debunking of my suspicions, an interesting read, and another win for science over one layman's speculation. Kudos to Dean Pesnell and the NASA SDO on Twitter!


How often and over what period is Earth’s rotation averaged to compute UT1? I understand that UT1 (and for that matter UT0, UT2, etc.) are based on averages of actual earth rotation, and serve as a form of mean solar time. However it’s not clear to me when these averages are taken and over what range? Certainly the average is taken over at least a year; but beyond that I can’t tell. Daily over the year? Yearly over the year; over several years?

How often and over what period is Earth’s rotation averaged to compute UT1?

Conceptually, the average is over an average year. Practically, that's not what's done anymore.

Sidereal time is much more readily measured than is solar time. Radio astronomy can measure the position of remote quasars much, much more precisely than they can measure the position of the Sun. The Sun is a big fiery blob with a not quite-well defined surface. Quasars are pinpoints. Very long baseline interferometry makes the observations of those quasars extremely precise.

The equation of the equinoxes converts between apparent and mean sidereal time. A very simple relation converts between mean sidereal and mean solar time. This makes variations in UT1 observable on a rapid basis. Sub-arc second astronomy depends on these rapid updates. IERS Bulletin A is updated daily, with the data for the most recent week or so subject to subsequent small updates.

Update

Universal Time is not based on apparent solar time. Time is now effectively divorced from the the apparent revolution of the Earth with respect to the Sun. It took several thousands of years to make this divorce final. (And the divorce is still not quite final. There's the matter of leap seconds. There's a proposal to eliminate leap seconds that has been shelved/postponed multiple times since 2003. A vote is scheduled for 2015.)

The ancient Egyptians divided daytime into 12 equal parts and nighttime into 12 equal parts. Daytime and nighttime hours were of different lengths, and these lengths varied over the course of a year. Hipparchus (190 – 120 BCE) suggested dividing one day into 24 equal parts, the basis for our modern hour. Ptolemy, using information already garnered by ancient Babylonians, asked "what day"? The length of a solar day (local noon to local noon) varies over the course of a year thanks to what we now call the equation of time.

This was of mostly academic interest until Christopher Huygens' development of the pendulum clock in 1656. Pendulum clocks tick at a rate proportional to the mean solar day rather than the apparent solar day. Those early pendulum clocks weren't particularly accurate. They had to be reset on a regular basis. The equation of time was central in determining the time to which those clocks needed to be set.

By the mid 1800s it was becoming apparent that even a mean solar day was not that good a basis for defining time. The clock kept by the solar system didn't agree with a clock kept by the Earth's mean rotation. A clock based on the solar system was more accurate and more consistent than was a clock based on the Earth's rotation.

By 1900, the divorce between Earth rotation and time was well on its way to being finalized. The basis for time was switched from the Earth's rotation to the clock kept by the solar system, with the second based on the length of a year rather than the length of a day. About 150 years of observations of the length of the mean solar day went into that circa 1900 definition of the second.

In 1967, time was further divorced from the clock kept by the solar system. By that time, atomic clocks had become extremely accurate devices, better than observations of the solar system. The International Atomic Time second was defined to be consistent with that circa 1900 definition of a second, which in turn was defined to be consistent with that ~150 years of observations of the length of an apparent solar day, centered on about 1821.

That 1821-based definition of a second as 1/86,400 of a day is no longer correct. A day is now about 86400.002 seconds long, and it will grow longer with passing millennia. This leads to the need to insert leap seconds to keep UTC in synchronization with the Earth's mean rotation. There is considerable debate with regard to whether even that should be done. There is a serious proposal to abandon leap seconds. This is now scheduled for a vote in the upcoming year (2015).


How far are we from the edge of the Universe? Following the Big Bang the Universe continues to expand, presumably and roughly equally in all directions. It is understood that the Big Bang occurred 13.798 ± 0.037 billion years ago.

Is there any way for us to know how far we are away from the nearest edge of the expansion front of the Big Bang?

In the standard models of the Big Bang, there is no such thing as the "edge of the expansion front". The universe, as far as we can see and as far as standard cosmology assumes, is homogeneous and isotropic on the large scale, so there is no edge or anything analogous to an the shock front of an explosion.

According to the seven-year WMAP results [pdf], the proper distance to the surface of last scattering that emitted the cosmic microwave backgrounds is approximately $46.0\,\mathrm{Gly}$, which is basically as far as we can actually see. The true cosmological horizon slightly more distant that this, closer to $46.6\,\mathrm{Gly}$. What's beyond the horizon is not known.

Another cosmologically significant distance scale is the Hubble radius, at which the galaxies comoving with the Hubble flow recede from us at the speed of light. It is $c/H_0 = 13.9\pm0.3\,\mathrm{Gly}$.


How could a hobbyist astronomer determine apparent magnitude of a star? Apparent magnitude is a rather complex way to determine the brightness of a star. Quoting the introduction text from the linked to Wikipedia page:

  The apparent magnitude (m) of a celestial body is a measure of its
  brightness as seen by an observer on Earth, adjusted to the value it
  would have in the absence of the atmosphere. The brighter the object
  appears, the lower the value of its magnitude. Generally the visible
  spectrum (vmag) is used as a basis for the apparent magnitude, but
  other regions of the spectrum, such as the near-infrared J-band, are
  also used. In the visible spectrum Sirius is the brightest star in the
  night sky, whereas in the near-infrared J-band, Betelgeuse is the
  brightest.

While it is of course a useful measure also for the slightly more casual, non-scientific observer to help determine the observed star from its neighboring cluster, or identification in general, I always wondered if there is a way to measure apparent magnitude with enthusiast-class equipment, and what would these procedures be?

                              Apparent magnitude scale and observational limits (Source: ESA Science)

Also interesting would be a description of what level of precision and enthusiast can get with such equipment while measuring apparent magnitude of a distant star. If you need specifics to answer the question, such as precise available equipment or subject of observations, please feel free to choose at will any that would broadly match the capabilities of enthusiast-grade equipment.

The easiest way to determine the magnitude of a given star is probably to use the Pogson relation. The idea is to determine the magnitude of a star knowing the magnitude of a reference star; it is thus quite easy, using a well-known reference as Vega or Sirius.

The Pogson relation is given by:

$$m_1-m_2=-2.5\ log\ \left({\frac{E_1}{E_2}}\right)$$

where $m_1$ and $m_2$ are the magnitudes of star 1 and star 2 (your reference star), and $E_1$ and $E_2$ brightness (that could be in arbitrary units, which is a good news for you).

With a CCD detector the brightness is easy to determine since it's just the flux of a pixel. You don't have to convert it in physical units (as Watt per square meter) since you have a ratio $\left({\frac{E_1}{E_2}}\right)$, so the raw intensity is enough to determine the magnitude.

As for any other measurement with a CCD camera, you have of course to determine the bias, dark and flat-field and to reduce your data before estimating any magnitude. The precision will depend on the number of observations performed and, as always, it is better to perform several short exposure images than a single long exposure one.


How can space expand in accelerating speed when dark energy is constant? I came to know that dark energy is constant.As mentioned in the Friedmann equations

How can a constant energy cause an accelerating rate of expansion in the universe?

This is a very good question and it comes down to what is meant by "accelerating expansion".

In cosmology we define something called the scale factor $a$. By definition at the present time $a=1$, billions of years ago when linear distances on a cosmic scale (e.g. the distance to a faraway galaxy) were half what they are now $a$ was equal to $0.5$ and many years in the future when cosmic linear distances have increased to twice their current size the $a$ will equal $2$.

$a$ is a function of cosmological time and we also define its 1st derivative and 2nd derivatives, with respect to cosmological time, $\dot{a}$ and $\ddot{a}$.

$\dot{a}$ is a measure of the Universe's rate of expansion and so when $\dot{a}$ is increasing with time we say that the expansion of the Universe is accelerating. $\ddot{a}$ is a measure of the change of the rate of the Universe's expansion so equivalently we say if  $\ddot{a}$ is positive then the expansion of the Universe is accelerating.

However note we have defined $a$ at the present time to be $1$, but the present time is entirely arbitrary. This means that $a$, $\dot{a}$ and $\ddot{a}$ depend on when we define $a$ to be equal to $1$. To get round this can instead think of the Hubble Parameter $H = \frac{\dot{a}}{a}$ as a better measure of the rate of expansion as it does not depend on when $a=1$.

In a de Sitter Universe (spatially flat, empty, positive cosmological constant) the Friedmann equations reduce to:

$$ \frac{\ddot{a}}{a} = H^2 = \frac{\Lambda c^2}{3}$$

From this we can see that $H$ and $\ddot{a}$ are both positive constants. So in one sense (the sense of $H$) the rate of expansion of the Universe is constant, but in another sense (in the sense of $\dot{a}$) the rate of expansion is increasing. So we can see a link between constant energy desnity (the energy density of the cosmological constant is constant) and a constant rate of expansion as long as we define the rate of expansion in a certain way.

In the currently-favoured cosmological model dark energy is a cosmological constant and we are currently in an era when dark energy dominates the dynamics of the Universe. (Looking at the full Friedmann equations) the contribution of matter reduces $\ddot{a}$, but as the cosmological constant dominates it is still positive and we say that the Universe's acceleration is expanding. Looking at $H$ however we say that, because the energy density of matter decreases with time asymptotically to zero, $H$ is decreasing asymptotically to the value given above for the de Sitter Universe.


What does this figure imply about the luminosity of pre-main-sequence stars? Check out Figure 1 in http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1955PASP...67..154H&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf .

It seems to show that the luminosity of a pre-main-sequence star depends primarily on the mass of the star, and that this basic relation continues to hold, to reasonable accuracy, into the main sequence.  Since there is no fusion in a pre-main-sequence star, is it correct to conclude that the relationship between the mass and luminosity of a pre-main-sequence star does not hinge on fusion physics?

It shows that the mass of the (proto)star is the major factor determining bolometric magnitude.

The brightness of the proto-star increases a little as it contracts and becomes significantly hotter (and the radius decreases), The star moves toward the left of the diagram. As fusion in the core becomes significant the star reaches the maximum point on the curve, this stabilises the core and the star settles into the main sequence, it cools slightly and becomes slightly less bright.


How big are the rocky/icy cores of Jovian planets? The size of Earth? Or bigger? I'd think they'd have to be pretty big to attract all that gas when the planets were being formed.

edit I'm writing a short story and the setting consists of late eighties/early nineties type of technology and knowledge of astronomy, so a rough estimate as an answer is fine

10-30 Earth masses is an estimate from this abstract, but 12-45 is a more accurate answer.