Blowing up an asteroid/comet really potentially worse? Often on television shows and in articles I see it mentioned that it'd always be bad to blow up an asteroid or comet because then the energy would just be spread out and cause even more damage.

According to some estimates I've seen around 100 tons (or more) of meteoroids hit the Earth each day. If all of this were combined together into a single asteroid, it could destroy an entire, large city.

Given that alone, it makes it seem to me that it be more logical to take the chance and blow up an asteroid and thereby trim its weight down, causing a lot more to be more easily burnt up on entry, so that when it did hit, it would cause less damage.

Is this logical at all? If my science/math/physics is incorrect, I want to understand why having more burn up by spreading out is worse compared to having it more concentrated and vastly more dangerous.

Well there are some things to consider. Initially if you could make sure that after you blow up an asteroid you will end up with numerous but small enough pieces so that they will either: one, burn up in the atmosphere or two, be headed away from Earth (and not hitting us five years later) then we are OK, and blowing up the asteroid with a missile would be a fair solution.

The problem here lies in the fact that we know little about the internal composition of asteroids in general, and presumably even less about a particular one, so it is very hard to predict exactly where the pieces of the asteroid generated by an impact are or aren't going to end up or, be headed towards or even its size.

Another scenario could be that if you effectively smashed the asteroid into small pieces that could then burn into the atmosphere, and if those pieces were coincidentally to end up being consumed by Earth's atmosphere, it would heat up provoking presumably an unpleasant day on Earth of course depending on the mass of the object.

But there is a much better solution than that Armageddon-Hollywood inspired one. It is call gravitational tethering. There is something we know, and we know very well about asteroids, and that would be their trajectory paths or orbits. Even when a new asteroid is discovered, its orbit can be computed pretty quick and with great accuracy (because we know the solar system's gravity very well). So if an asteroid is to impact Earth, it is likely that we will know with years, probably decades in advance. And so we can just send a space vehicle (called gravity tractor), with enough mass and time in advance, and place it just beside the asteroid, hence allowing us to tilt its orbit by just a tiny amount, due to the gravitational pull between the two objects. Now when you consider the effect of that tiny amount in the long run, it effectively deflects the path of the given asteroid from that of the Earth so that it won't hit us 20 or 30 years later.

And this is something we have control over, and something we can predict with great accuracy. It is the (safe) way to go.

If you are still not happy with my answer, you can listen to Neil de Grasse Tyson himself explaining it in this 5 min video.

Also check out this talk from the American Museum of Natural History on "Defending Earth from Asteroids" LINK

Further reference here.


Can a solar flare destroy every electronic item on earth? I was reading a book about the sun which suggested that a solar flare can destroy every electronic item on Earth. Is this really true and if so, how would it happen? Could we do anything to to prevent it?

Here's the how: very energetic charged particles interact with the Earth's magnetic field, and when they do so they emit electromagnetic radiation. If the energy of this radiation was high enough then when it reached wires/conductors at the Earth's surface the opposite effect would take place: ie the EM radiation would result in the flow of charged particles (a current). Such currents, if strong enough, could damage very many electronic and electric devices.

As to the possibility: it seems unlikely that such a high energy event would take place (ie to destroy all/nearly all electronic equipment), but it could be possible. We don't have sufficient data.


Determining the shape of ʻOumuamua via stellar occultation As far as I know, our current knowledge regarding the exact shape and size of ʻOumuamua is based on the analysis of its light curve and the assumption that it has a uniform albedo.

ʻOumuamua is very dark and too small to be resolved by even the largest telescopes as much more than a point of light. But so is Ultima Thule and we did manage to get a really good idea of its size and shape by observing stellar occultations caused by it.

Could the same technique be used to get a better idea of what shape ʻOumuamua really is? Or does its hyperbolic orbit render this kind of observation impractical? Have any attempts been made to perform such observations?

Observing star occultations is dependent on the object passing in front of a star.

During its short time in the inner solar system, ʻOumuamua didn't pass in front of a star (from the point of view of Earth) It is now beyond the orbit of Saturn. It became too dim for Hubble in about January 2018. There are no forecast occultations of any star (and its orbit is probably not well enough determined to predict any, given the observation of non-gravitational and not well-understood accelleration). Any occultation that did occur would only happen for a very short time and only be visible in a very narrow strip of the Earth. We can't set up large telescopes every 100m on Earth, and constantly observe all the (very dim) stars that might possibly dim further for half a second.

By contrast, Ultima Thule is a much larger object, with a well-known orbit, and the possibility of forecasting the exactly when and where an occultation of a reasonably bright star will occur, and set up observation stations on the narrow strip that is predicted.

So while an occultation could have told us about its shape, none occurred and none are expected.


Will I age slower in space or on other planets? I came across few  videos which says Astronauts in ISS will have 0.07 seconds extra life.

So if life is possible on mars, than what is the time period of life for us humans? Is it the same as that of earth or different?

For example, if I spend 500 days on mars, that will be equal to spending around 515 days on earth. So is it that I'm gaining 15 days of life? I mean, can a human life span increase?

If a human average life span is 60-80 years on earth, on mars will the average life span of a human be 62-82 years of earth years? Or will it be 60-80 years of Mars years?

I don't know how much this makes sense but as a new learner just got a thought on space time.

If a human average life span is 60-80 years on earth, on mars will the average life span of a human be 62-82 years of earth years? Or will it be 60-80 years of Mars years?

Relativity is about the relative state of the thing observing and the thing they observe.

Locally (e.g. on Mars) things do not go slower for you, but someone observing (e.g. from Earth) sees time passing more slowly for you.

Locally your life on Mars is not lengthened.  Measured from Earth, more time will pass on Earth (as measured by someone local to Earth) than passes on Mars (as measured by someone on Earth observing Mars).

This is more complicated than it might seem at first glance because it works both ways.  You (on Mars) see me (on Earth) moving, so from your point of view you also see my time frame slow down relative to yours.  From an everyday human point of view that seems like a paradox, as we would not expect both of us to see the other's person's time pass more slowly than ours, but relativity does work that way.

There's a question (or two) on Physics SE dealing with the "why" that happens and why it's not a mistake or a fault with the theory.  Here is one.

It's even more complex when you have planets in orbits around a star, because that uses General Relativity and there are even more subtleties and additional effects on the rate of time we each measure for the other.  Gravitational time dilation does not work both ways - that depends on the strength of the gravitational field (from everything) so it's different for each observer.  Wikipedia's Time Dilation page is a reasonable place to start reading from, I think.  Note that general relativity is considerably more complex mathematically than special relativity.

We have measured these effects - they are real, but very small in human experience of them.


Who invented the blink comparator? The Wikipedia page for the blink comparator fails to mention who invented it. Many other pages extol the importance of the device, but we cannot find any mention of an inventor.

Wikipedia does mention that Ben Mayer invented the The Projection Blink Comparator (PROBLICOM), which is a low-cost improvement on the blink comparator. However, he did not invent the original device.

It would appear to have been developed by Carl Pulfrich working for Zeiss in 1904.

Alternatively Max Wolf in 1900 again working with the Zeiss company.

Looks like the idea was Wolf's and the realisation Pulfrich's. From the second link we have:

  Wolf was a codeveloper of the stereo comparator together with Carl
  Pulfrich from the Zeiss company. The  stereo comparator consists of a
  pair of microscopes arranged so that one can see simultaneously two
  photographic plates of the same region taken at diff erent times. Wolf
  seems to have experimented with such techniques as early as 1892, but
  without success. When Pulfrich approached him to adapt the technique
  from geodesy to astronomy, Wolf was delighted. A steady exchange of
  letters followed. Wolf and Pulfrich then worked together to analyze
  the rapidly growing accumulation of photographic plates. Tragically,
  Pulfrich lost one eye in 1906, preventing him from using the
  stereographic tool from then on.


Do supernovae produce powerful gravitational waves? I would think that the amount of energy produced by one supernova is much more than a pair of colliding black holes, no? Wouldn't that much material and energy output from the explosion generate a powerful gravitational wave?

Actually the peak luminosity for the first binary black hole merger LIGO detected was $3.6 \times 10^{49} \,\text{Watts}$ (source). Whereas the brightest supernova ever recorded was ASASSN-15lh which had a peak luminosity of $2 \times 10^{38} \,\text{Watts}$ (source). In fact the energy radiated by the binary black hole merger during the $0.2 \,\text{s}$ it took to merge was over 10 times the combined luminosity of the observable universe (source).
This translates to the total energy emitted to be $5.4 \times 10^{47} \,\text{J}$ for the black hole merger and $(1.1 \pm 0.2) \times 10^{45} \,\text{J}$ for the ASASSN-15lh supernova. So as you can see, the binary black hole merger released around 100 times more energy than the brightest known supernova.
In relation to the gravitational waves emitted by supernovae, their wave-forms are not as well understood as the "chirps" you get from merging black hole binaries, however they are still expected to be strong enough to be detected in the local universe.


How could I determine what areas of the Milky Way will be visible? There is a new moon on Saturday, and the forecast in my area of Winnipeg, Manitoba, Canada calls for clear skies on that night.

I would like to take a widefield astrophotograph.

I have no idea what areas of the milky way will be visible, and where they will be located.

What are some steps I could take to determine this?

Are there different dense (photogenic) portions of the milkyway visible at different times of the year?

Are there certain constellations associated with the dense portions of the milky way that I should be looking for?

I would like to further develop my knowledge of astronomy, for the purpose of widefield milkyway shots.

Have you tried to use Stellarium?
You can insert the date of the observation and see how the sky will look like from your location.
 There is also setting to make the Milky Way be seen more intense.


mirror image of the earth in the past For example, if there was an object somewhere in the room, say 100 light years away, which would have to be very large and have the surface of a mirror, you could theoretically point the telescope at it and you would see the earth as it is was back then. Of course you would have to do all the details for years, the telescope would have to be much stronger and bigger, and something like this would have to exist and be discovered, but theoretically it could work that way. Your opinions would interest me very much!
Thank you

though the discovery of such a polished large body is odd, even if it is there it would not be much useful because it will then show us the historic times of 19th century which archaeologists themselves can easily discover using evidences.


How should I find a telescope for research time? I'm an undergrad in physics and astronomy deciding to look into applying for time on a research telescope, and I'm not sure how to proceed with locating/finding telescopes that I can apply for time on. Specifically, I'm interested in looking at the chemical composition of protostellar disk(s). Any applicable information would be nice.

As an undergraduate, you are a member of a university. Your first step then should be to talk to your Professors.

As a guide to what might be expected, look at the University of Arizona telescope proposal page They accept proposals from members of Arizona University. You have to have a clear aim and plan. This is why you need to talk to your professors first.


Is it dangerous to look at an L-type brown dwarf from too close? Are some L-type brown dwarves bright enough so that it would be dangerous to the eyes if one looked at them from habitable distance or closer? If not, are the weakest M-type stars undangerous to look at from habitable distance too? T-type brown dwarves are dim enough that there shouldn't be any danger, and Y-type ones are invisible.

A typical L-type brown dwarf is about 1200-2200 K in surface temperature and is about the size of Jupiter. Using the Stefan-Boltzmann law, we can deduce that the hottest brown dwarfs have a luminosity of$$\Big(\dfrac{2200}{5778}\Big)^4 \cdot \Big(\dfrac{1R_J}{1R_\odot}\Big)^2 = 0.00021224 L_\odot$$
According to Wikipedia, the dimmest apparent magnitude that causes pain to look at is $-25$. We can use the luminosity formula for apparent magnitude and distance calculate how far away an observer must be to experience this magnitude: $$0.00021224 = 0.0813   \text{ ly}^{-2}d^2\cdot10^{-0.4\cdot(-25)}$$
Solving the equation gives $d=0.03232 \text{ AU}$, or $67.62 $ times the brown dwarf's radius. Using this useful HZ calculator and plugging in $T_{eff}=2200$ and $L=0.00021224$, we get the distance must be between $0.015$ and $0.032$ AU, which is inside that limit. This range is inside the limit for $-25$ apparent magnitude, so no, looking at brown dwarfs at their habitable zone is not guaranteed to be safe.


What's the slit and slot on EIS(EUV Imaging Spectrometer) instrument of Hinode? There are two slits and two slots on EIS, are they just four types of grating?

You can go back and reference the paper published on this telescope. Specifically it is Culhane et al. 2007. This paper talks about the overall telescope, however the paper describing the optics is Korendyke et al. (2006). Unfortunately this optics paper is behind a paywall.

Within the Culhane paper, in section 3, the describe the slit mechanism as

  Exposure times are controlled by a rotating shutter while a slit exchange mechanism can allow selection of four possible apertures — two spectral slits and two spectral imaging slots.

That's about as much detail as they give, except to note in table 1 that the slit and slot widths are 1, 2, 40, and 266 arcsec.

In the Korendyke optics paper, they give a bit more detail.

  Light passing through the slit is dispersed and stigmatically reimaged by the toroidal grating onto two 1024 x 2048 pixel CCD detectors. The focus of the grating can be remotely adjusted with a mechanism. In flight, the mirror will be rotated in ~0.125 arcsec increments about the Y axis [solar north–south (N-S)] to sample different structures of the solar surface with the slit. High-resolution spectroheliograms (raster images) are formed by steadily moving the solar image in fine increments on the spectrograph slit and taking re-peated exposures. An interchange mechanism allows selection among two slits (1 and 2 arc sec wide) and two slots (40 or 266 arc sec wide). The slot observations of the solar disk produce spectrally pure images
  of large areas of the disk in bright solar emission lines with each exposure. The slot images exhibit a modest spatial blur along the dispersion direction.

For completion, they also include the diagram below of the optics.

From reading all of this, it appears to me that the various slit/slot widths are designed for taking different types of images/spectra, either of very focused, single-instance areas (such as with the slits) or else of large regions (using the slots).


Which wavelength is the most quiet, for a ground-based radio telescope? This might seem like a backwards question, but I'm interested in what wavelength to select in a (hypothetical) ground-based radio telescope observation to expect detecting as little as possible! :)

It should still be reachable from space, i.e. a wavelength with a high atmospheric absorption is probably a bad candidate.

I guess this could be answered by looking at a broad averaged spectrum collected by a radio telescope, and comparing against an atmospheric absorption spectrum chart.

You may be looking for the "water hole". See http://www.setileague.org/general/waterhol.htm.


If the Sun got larger, but maintained its luminosity, would the Earth get hotter or colder? A recent question If the Sun were bigger but colder, Earth would be hotter or colder? asked - if the Sun got bigger and cooler, would the Earth heat up or cool down. I think the answer to that is mainly that it depends on the final luminosity.

However, what I want to know here (hypothetically), is if the Sun got larger and it's effective temperature decreased such that it's luminosity was unchanged; how would that affect the equilibrium temperature of the Earth? I suspect the answer may involve the wavelength dependence of the albedo, emissivity and atmospheric absorption of the Earth.

Another, less hypothetical, way of asking this is, if you put an Earth-like planet at different distances from stars with a variety of temperatures, such that the total flux incident at the top of the atmosphere was identical, how would the temperatures of those planets compare?

To a first approximation if the Sun got bigger (by a relatively small factor, say a few times) but maintained its present luminosity, as the Earth would still be intercepting the same total energy per unit time the temperature would stay the same. Constant luminosity gives a constant intensity at the Earth's distance from the Sun (intensity is the energy crossing unit area in unit time) which would mean that the total solar energy intercepted by the Earth would remain the same.

However in this scenario the colour of the sun would change and we would get secondary effects due to the reflectivity of the Earth being frequency dependent so the fraction of the incident energy reflected rather than absorbed would change, which would result in a change in temperature. Which direction this would go in is difficult to say as an increase in temperature should result in more cloud which would increase reflectivity ...


Stargazing (Tenerife) - different quality today and yesterday I'm on the Tenerife island and (in the mountains above the clouds - 2300) the day before yesterday, I could clearly see the Milky Way and stars (1 am). So I went to the same place yesterday and the quality was much worse (at midnight).
I'm curious why there was such a difference and what parameters could cause this difference.

bigger moon?
different moon position?
sand from Sahara?

It's El Teide park cerca 2300 meters above the sea.
Is it worth to try again today? What parameters should I look up? It's like 2 hours of driving so I would like to be able to estimate the results in advance.

If you want to see the milky way, you really need the moon to have set. The moon sets about 50 minutes later each day, so you will need to go later.  On the 28th of July, the moon sets at about 2 am and on the 29th it sets at about nearly 3am.  If the moon is up that will significantly affect the background sky brightness.  But when the moon is down, Tenerife has dark enough skies to see not only the milky way, but also the zodiacal light.
The other parameter is high cloud.  Although you are above low lying stratus, High cirrus or cirrostratus cloud can still obscure your view. Sand might also have an effect, but the reason that you can't see the milky way is primarily because the moon is up. So go back at 2am or later, until dawn at about 6am.


If we "turned universe upside-down" and changed all matter to anti-matter instantly ...what would be the immediate consequences?

Or putting it better, if (from big-bang or whathever), the universe were always made of what we call Anti-matter what would it be?

The scientists of this universe would call 'Matter' 'Anti-Matter'.

Will the universe looks like exactly the same? By "looks", I mean the laws of Physics, the constitution of the universe with its superclusters of galaxies containing clusters containing galaxies and so on...

It would look very similar, but tiny differences are suspected, see e.g. this LHC experiment.

Here a list of more antimatter experiments at CERN.

See also CP violation on Wikipedia hinting towards tiny differences between matter and antimatter.

Immediate consequence: The CP violation parameter of the CKM matrix in this Wikipedia article would be different, because of different quark generation probabilities.

If you also reverse time, you'll get to the CPT theorem (Charge, Parity, Time). Matter and antimatter are thought to be symmetric in this sense.

Or to state it with simpler words: Antimatter behaves like matter travelling backward in time.


Red dwarf variation in Luminosity I recently saw this video. The creator of the video says that one of the reasons for non-existence of life on planets around red dwarfs might be that these stars can increase luminosity roughly by a factor of 2 in a matter of seconds which will boil off water from planets. He also said something about Helium/Hydrogen convection belts which are responsible for this. Can someone please explain this phenomenon?

The creator seems to be referring to flare stars. Flares may be magnetic in origin, like various manifestations of the Sun's magnetic field. These flares can be quit luminous across the electromagnetic spectrum, including in the x-ray wavelengths.

Some red dwarfs are flare stars, though flare activity may be largely restricted to a small part of a red dwarf's lifetime. However, during this period, life on a planet orbiting close to a red dwarf could be in serious danger.

The reference to convection may be that red dwarfs are generally fully convective, unlike other stars. Magnetic fields in stars are generated via the dynamo effect; in red dwarfs, full convectivity can play a huge role in generating strong magnetic fields, and thus stellar flares.


Do space objects stay in orbit once they get sucked in? Can they escape? Think of my question with respect to 2020 CD$_3$ minimoon that's been recently discovered.
If an object gets caught into a planet's orbit, can it ever escape it?
Obviously it may escape on the first pass without making any full revolution.
But if one or two revolutions took place, can it ever escape the planet's orbit on its own momentum it came in with?

If it can get captured, it can escape: we know this because the law of gravity is time-symmetric. If there is a trajectory that leads to a capture, then by running time backwards, there is a trajectory that leads to escape.
What happens with small objects like 2020CD3, is that they pass into the space between the Earth and moon, and it is the interaction between the Earth and Moon (and sun) that slows them down and allows them to enter into a long looping orbit.
But this orbit is too close to the moon to be really stable.  Usually they go around in a complex orbit that changes each time, as they are affected by the gravity of the Earth, moon, (and sun).  Usually after a few orbits they will have another interaction with the moon, but this time it will speed the object up, and eject it from the Earth's gravity well.  Interactions that lead to a permanent capture are rare.
There are no objects in Earth orbit that have been captured.  Asteroids are rather rare in the inner solar system, and the moon probably does a good job of ejecting any that do get into an orbit around the Earth.  There are two captured asteroids around Mars, and many of Jupiter and Saturn's moons may have started out as asteroids.  Triton is also probably a captured body. So it is possible for a body to get into a stable orbit from capture, but it is rare.
There is no sucking effect.  Instead the orbit of the asteroid around the sun gets tangled up in the gravity field of the Earth and moon for a while, but eventually it will almost always escape.


Orbital parameters of the Sun What are the orbital parameters of the Sun such as orbit velocity etc in it's orbit around the Solar System's center of mass? Consider the Sun pointlike or alternatively when talkin about the Sun's movement I mean it's center of mass.
Do not tell me that the Sun is stationary because the planets' masses can be neglected. I do no want such oversimplifications.

Since the Solar system is a multi-body system (with $N>2$ bodies of significant mass), the orbits of its constituents are not exact Keplerian orbits.
To lowest order, each planet orbits the Sun (or rather the centre of mass of all interior planets) on a Keplerian orbit, but the interactions with the planets as well as the fact that the centre of mass of the interior is not fixed lead to deviations of the true orbit from this simplification. These deviations can be treated either numerically or via perturbation theory, but are non-trivial functions of time.
The same holds for the Sun: to lowest order one can neglect all planets but Jupiter (which is more then twice as massive as all the remaining planets combined), when the Sun follows an elliptic orbit with semi-major axis of about 0.005AU (smaller than that of Jupiter by their mass ratio). This is of the same order as the radius of the Sun, i.e. the barycentre of the Solar system hardly leaves the Sun. However, as above, the pull by all other planets lead to a deviation from this simple model. Again, these deviations are non-trivial.


How much helium has been created since the Big Bang? When I try to find figures for the amount of helium today and just after the Big Bang, I get similar figures.  I was expecting that all of the stars since then would have created more helium.  Is the amount below our ability to estimate?  Is helium being consumed at a similar or greater rate to produce heavier elements?

Alternatively, can we measure/ estimate a drop in the hydrogen?  I presume that little hydrogen is being created.  I am unaware of any common nuclear process which creates lone protons.  (Please correct me if appropriate.)

Clarification: The Wikipedia Big Bang article mentions helium being created after a few minutes in the Big Bang nucleosynthesis.  So, it is helium created after this point that I am asking about.  I am presuming that negligible helium was created between this point and the first stars much later.  (Again, please correct me if I am wrong.)

We can estimate the answer by considering the "mass to light ratio" for baryons, and assuming that ratio hasn't changed too much over the time that there has been galaxies, say for ten billion years.  The Wiki (https://en.wikipedia.org/wiki/Mass-to-light_ratio) on the mass to light ratio says this ratio for the universe as a whole is about 100 times what it is for the Sun right now, but that includes dark matter.  If we restrict to baryon mass-to-light, that would be more like 16 times the Sun right now.  The Sun's main-sequence lifetime is about ten billion years, conveniently, and in that time it will convert perhaps 20% of its hydrogen, so a mass-to-light ratio 16 times larger would convert only about 1% of its mass from hydrogen to helium.  So that says roughly 1% of all the hydrogen in the universe has already been converted to helium.

Since it started out 75% hydrogen and 25% helium by mass, now it should be more like 74% hydrogen and 26% helium by mass.  So although the estimate is crude, I think it's fair to say we are just now beginning to see a trace of an effect, which would be hard to see after only 1 billion years, and will be quite obvious after 100 billion years.


What is main radiative mechanism in Gunn-Peterson effect? I'm having a hard time understanding the Gunn-Peterson effect and the Ly-alpha line and I've written a couple of questions below.

The trough we see in the spectra of quasars is the Gunn-Peterson trough, right? Does this trough have to start from the Ly-alpha line?

If neutral hydrogen along the line of sight to the distant quasar is few, do we expect the Ly-alpha forest to be sparse?

What is the main radiative mechanism in the Gunn-Peterson effect?

The Gunn-Peterson trough is a feature in the spectrum of a (background) source near the Lyman $\alpha$ line, caused by a high density of intervening, neutral hydrogen (HI).

The background source may be a quasar, but could also be e.g. a gamma-ray burst or just a regular galaxy with a strong Ly$\alpha$ line.

Absorption vs. scattering

The cross section of hydrogen for Ly$\alpha$ photons is quite large, so the mean free path of the photons is very small. Because Ly$\alpha$ excites hydrogen to the first excited level, the electron can only fall back to the ground state again, emitted another Ly$\alpha$ photon, in another direction. Hence, hydrogen doens't absorb Ly$\alpha$ photons, but rather scatters them.

Inside the (host) galaxy, the HI density is so large that Ly$\alpha$ is all the time scattered both into and out of the LOS. However, when the photons escape the galaxy and enters the circum- and intergalactic medium (IGM), the density decreases. Along the LOS to the galaxy, a photon hitting a hydrogen atom is "lost", and no new photons are added to the observed flux, so effectively, it becomes an absorption process rather than a scattering process.

The figure below (from Laursen 2010) illustrates this phenomenon: the three photons are initially emitted in random directions, but end up traveling toward the observer on the right. On their way through the IGM, two of them are scattered out of the line of sight (LOS), and the probability of a photon from another galaxy being scattered into the LOS is negligible.

Reionization

In the early days of the (post-recombination) Universe, hydrogen was mainly neutral, but as star formation emerged, these stars started to reionize the IGM. This process started already at $z\simeq15$–$20$, when the Universe was ~200 million years old , but until $z\simeq6$, when the Universe was ~a billion years old, there was still so much HI left that Ly$\alpha$ photons scatter all the time in the IGM.

Blue wavelengths are eventually redshifted to Lyman $\alpha$

Now when light leaves a galaxy, it gets redshifted due to the expansion of the Universe. That means that after a while, light blueward of the Ly$\alpha$ line is redshifted to become Ly$\alpha$, and may hence scatter. As the spectrum travels through the IGM, it redshifts continuously, so that every wavelength of the "original" Ly$\alpha$ line eventually becomes Ly$\alpha$.

If the HI density in the IGM is high enough, that means that everything blueward of the Ly$\alpha$ line is wiped out. This is the Gunn-Peterson trough. If the density is low, but with some neutral clouds dispersed, then an absorption line occurs exactly at the wavelength that, at the location of the cloud, happens to have been redshifted to Ly$\alpha$. When there are many such clouds around, you'll gets lots of absorption lines, forming the so-called Lyman $\alpha$ forest. The fewer the amount of such clouds, the more sparse the forest will be.

The Gunn-Peterson trough doesn't have to start at Ly$\alpha$, but since this is the strongest transition, it is most distinct here. But you will often see a corresponding absorption at Ly$\beta$ (and even Ly$\delta$ and Ly$\gamma$, as well as the stronger metal lines like MgII and CIV). This is visible in the figure below (from Becker et al. 2015)

You see that immediately blueward of Ly$\alpha$, everything is wiped. As you go along the spectrum toward the blue, that part corresponds to later and later epochs, because is takes longer time to redshift that part into Ly$\alpha$. Hence, eventually the Universe becomes less and less neutral, and the absorption is no longer complete.


Which direction does the Earth face as it travels through the Galaxy According to the Helical model, as the Sun orbits around the galaxy, the planets that orbit the sun are travelling perpendicular to the path of the Sun.  This is better shown in this video:

https://www.youtube.com/watch?v=0jHsq36_NTU

My question is, which pole on Earth faces in the direction that the Sun moves?  The South Pole or the North Pole?

My question is, which pole on Earth faces in the direction that the Sun moves? The South Pole or the North Pole?

The North pole is in the hemisphere of the direction of motion of the solar system through the galaxy.

The direction in which the Sun in traveling relative to surrounding stars is called the solar apex and is located at (RA)  18h 28m 0s (dec) 30°N, which is a point in the northern hemisphere.


Spotting the International Space Station (ISS) Recently, I visited the NASA website which provides sighting locations to see the International Space Station (ISS). As we all know, we can even see ISS with the naked eye in a clear sky as a passing star without any blinking.

But unfortunately, I don't know how to measure the angle on the sky to see it. On the official NASA website there's given height (altitude) and direction as 21° and 11 above N respectively.

Here is the link: NASA - Spot The Station

How do I trace ISS?

The official site you point to has 3 pieces of location information:
1) the max height angle
2) the point it appears
3) the point it disappears
The only one of these that is especially accurate is the max height angle, and that isn't the number you want to use to try and see it. However, passes with a bigger number for the max height angle will be easier to see.

To find the ISS, get the reference to the point it "Appears" - that is the "11 above N" you have described. Now, notice that the reference to the compass direction is simply N or NNE or SW or whatever. This is not a precise directional reference, but it doesn't need to be, as if you're looking towards that direction, you shouldn't have any problem seeing ISS appear. It isn't clear to me if the website is using True North or Magnetic North for the direction; Magnetic North might be easier for people with a simple compass to find, but usually these directions will be relative to True North, which is not where a compass points. From a quick check using my own location, it appears that they have used True North, which is maybe a bit unfortunate. If you don't know where True North is for your location, you can look it up, and then you have to remember to adjust the reading off your compass by the offset for your location.

Notice that the description is "11 above N". This means 11 degrees above the N direction. The reason for it appearing some distance above the horizon is because it is coming up out of the shadow of the Earth, which is being cast some distance above the horizon.

To quickly and easily estimate angular distances, use the trick of observing your hand at arms length. Different parts give different angular dimensions. It is a pretty fair estimate, because no matter how big or small you are, the relative size of your hands and the length of your arms is probably about the same as everyone else... not good enough to use as a survey instrument, but good enough to guesstimate locations on the sky.

The second position description is the "Disappears" location. That is where the ISS will again drop into the shadow of the Earth. Similarly, it is described as degrees above a True North compass direction.

You can figure out (approx) the path the ISS will take by drawing an imaginary line between these two points. However, don't draw that imaginary line so it goes over your head, note the "Max Height" angle again: this is the highest point on the line that you draw between the "Appears" and "Disappears" spots.

Just give it a go, especially one with a pass that starts around 30mins or so after sunset, and that has a high "Max Height" angle. It won't be hard to see.


How many celestial bodies can I see with a 90x telescope? I am a 9th class student so I'm a beginner at using a telescope, hence please help me.

How many bodies can I see using a 90x telescope?
How will I know when and where I should point a telescope to see planets?

The key issue for a telescope is not magnification but light gathering capability - which is (crudely) the size of the main aperture. Thus a 90x magification on a very large (wide) telescope would let you see a very large number of things (if you are in an area where the sky is dark), but 90x on a small telescope would let you see a number of interesting things (the Moon, planets, some nebulae and star clusters) but not relatively faint objects.

Small, cheaper, telescopes are still worth buying if you want to dabble, seeing the Moon through a telescope is always amazing and a small telescope can show lots, for instance.

Also, for a small telescope don't go for big magnifications - 90x would be fine for a low cost 50mm refractor for instance.

As for where the planets are - Venus, Mars, Jupiter, Saturn are generally easy to spot if you look up their positions online and know your way round the constellations. Because the planets are not, as stars effectively are, points at infinite distance, but small disks they (on most nights) do not twinkle as the stars do.


What radio frequency ranges are most beneficial for astronomy? I cannot get the idea of citizen radio astronomy out of my head, and choosing an antenna (design) heavily depends on the desired frequency range.
The Arecibo telescope as my gold standard operated 300 MHz to 10 GHz. Jovian Whistlers as nice solar source are observable at around 20 MHz. Then, there also some frequency bands which are commercially not used for the sake of astronomy, as depicted e.g. in a presentation:

I am now trying to figure out which frequency bands are important for astronomy for which purposes. My dream answer would contain a table with columns frequency range, benefits for astronomy, complications (like terrestrial use of the frequencies), and other remarks.
Related

Jupiter FM - What are practical and inexpensive ways for the amateur detection of signals from Jupiter, especially of the transit of her moons?

Radio astronomy involves a wide range of frequencies, covering the range from $\sim$10 MHZ to $\sim$100 GHz.$^{\dagger}$ With four orders of magnitude to work with, the most valuable band depends strongly on what sort of object you're observing, as well as what receivers are actually available on a specific telescope. Given the diversity of sources out there, it's difficult to narrow down the list to any small number of important bands, so this answer is far from comprehensive - my apologies to any extremely high-frequency spectral line observers out there. A truly comprehensive answer would be several pages long, so I'll forego that in favor of a briefer overview.
Spectral lines
The most well-known radio line is the famous hydrogen line arising from the spin-flip transition. Its rest frequency is approximately 1420 MHz, but as is the case with all spectral lines, extragalactic HI sources will show the line at significantly lower frequencies due to cosmological redshift. The same holds true for clouds within the Milky Way due to their motion relative to us, albeit to a much weaker extent. Neutral hydrogen is an important tool both locally (e.g. mapping the structure of the Milky Way) and for high-redshift sources (e.g. providing a handy tool for probing the early universe around the time of reionization).
There are four major hydroxyl radical lines between 1612 MHz and 1720 MHz, including the 1665 MHz line that led to the first detection of masers. They remain one of the key molecules detected in masing sources, and hydroxyl megamasers remain the dominant type of megamaser currently known. Notable, the hydroxyl lines lie on the opposing side of the water hole from the hydrogen line.
Carbon monoxide (12CO) has rotational transitions at multiples of 115 GHz, with the most notable lines being those at 115 GHz and 230 GHz (13CO also has a line near 110 GHz). They are quite useful for probing molecular clouds, meaning that CO surveys can reveal structures that might be absent on HI maps.
These are just a few of the key spectral lines in the radio spectrum; a 1991 National Academy of Sciences report came up with a far more comprehensive list of spectral lines of varying priorities. Formaldehyde, water (two more key maser molecules), methanol, ammonia, methine, and deuterium are all included, along with many others. Almost all lie between 1 GHz and 300 GHz (only two orders of magnitude!). An even more detailed list can be found here.
Continuum observing
Some broadband sources may be observed across several bands, and observations may take advantage of existing receivers often used for other purposes. For instance, the Arecibo L-Band Feed Array, or ALFA, included the 1.4 GHz hydrogen line and has been important for neutral hydrogen surveys (e.g. ALFALFA), but it turned out that its capabilities were also quite useful for pulsar searching (e.g. PALFA). Therefore, continuum observers might use some of the bands mentioned above, depending on available receivers and backends.
That said, different types of continuum emission peak at different frequencies, so we can certainly divide the spectrum up into chunks. The Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses has some nice figures illustrating this on pages 13 and 14 of chapter 4. I won't reproduce them because I'm not sure about copyright status, but the gist is the following:

Pulsars are the lowest-frequency continuum sources, observable below a few GHz. The 406-410 MHz band is often used, but again, 1.4 GHz receivers are also commonly used due to their availability. As pulse profiles sometimes show different features at different frequencies, it can be quite useful to observe the same pulsar in different bands.
Supernova remnants (SNR) produce radio emission through synchrotron emission from relativistic electrons, and are actually visible across quite a wide range of frequencies (with synchrotron emission also in other parts of the electromagnetic spectrum). I'm not as familiar with radio SNR observations.
The same holds true for radio galaxies, which are primarily detectable through synchrotron emission across a similarly large frequency range.
The cosmic microwave background is a blackbody; at its current temperature of 2.7 K, it peaks close to 300 GHz, which again places it near the boundary of radio astronomy.

Protected bands and unusable bands
Many spectral lines (e.g. the HI line) lie in protected frequency bands in many jurisdictions, with widths which ideally take into account redshifts or satellite lines. Some key continuum bands (e.g. 406-410 MHz) have also received protection. Unfortunately, human activities are not the only thing that can make some portions of the spectrum unusable. For example, pressure-broadened oxygen lines in the 50-70 GHz range can be problematic. Water vapor, in addition to the strong line at 22 GHz, also produces strong lines at 557 GHz, 752 GHz and 970 GHz.${\ddagger}$

$^{\dagger}$Stricly speaking, there are indeed observations in the THz range, but at this point, the boundary between radio astronomy and infrared astronomy becomes a bit shaky.
$^{\ddagger}$See Essential Radio Astronomy by Condon & Ransom.


How bright would Sagittarius A* be from Earth if it became an Active Galactic Nucleus? Essentially I'd like to know how bright our central black hole could appear from earth in relation to full moon brightness. Assuming maximum energy output (like when it's gobbling up an entire globular cluster). I've read that the light from an AGN could actually support photosynthesis on planets within 1100 light years, so I'm curious if it would have any affect on Earth life 25000 light years away.

https://aasnova.org/2019/05/29/do-active-galactic-nuclei-help-or-hurt-life/

If Sagittarius A* were to ignite and feed on a massive accretion disk, blasting the massive jets we call quasars, it wouldnt be noticed by anyone here on Earth unless they were looking for it.  The relativistic jets accelerate out of the poles of the Supermassive Black Hole (SMBH) shooting out of the plane of the Milky Way, not through the disk structure which would cause massive damage to the galaxy. And with the naked eye, relativistic jets wouldnt be visible to us through all the gas and dust of the Milky Way, combined with the 26000 LY away from the galactic center we are, we really wouldnt even know the quasar came to life, and the Moon would still be the brightest heavenly body at night.

For more information, check out the Wikipedia article about quasars.


Extinction and reddening dataset Where can I find a dataset containing the interstellar reddening and extinction values for all globular clusters, preferably in the HST/WFC3 system.

Note that they are related via the reddening law, i.e.
\begin{equation*}R_\lambda = \frac{A_\lambda}{E(B-V)}\end{equation*}
where $A_\lambda$ is the extinction at wavelenght $\lambda$, and $E(B-V)$ is the  color excess for $B-V$. In literature is commonly used $E(B-V)$, while in practice is usually needed $A_\lambda$, to switch back and forth from apparent magnitude $m_\lambda$ to absolute magnitude $M_\lambda$ with
\begin{equation*}m_\lambda=M_\lambda+A_\lambda+(m-M)_0\end{equation*}
using the absolute distance modulus $(m-M)_0$.

Back to your question, classical references for the color excess of galactic globular clusters are the Harris catalog and the Schlegel maps (I would prefer the second). A nice solution is the Python module dustmaps (Green 2018); in the documentation you find all the available maps (here).

Finding $R_\lambda$ is more tricky. You can search in the literature for tabulated values (e.g. Tables 14 and 15 in Sirianni et al. 2005), otherwise a rough-but-still-working approach is to take a $R_\lambda$ vs. $\lambda$ plot (e.g. the one in Fitzpatrick 1999) and to extrapolate by-hand $R_\lambda$ at the effective wavelength of your filter. Note that usually are presented several reddening laws, distinguished by $R_V$. This is because they're not universal and depends on the environment; a reasonable choice for GGCs is $R_V=3.1$ .


Can orbital migration cause a planet to crash into the sun? If the planets move out of their current orbits into other orbits over long periods of time could this cause a planet to crash into the sun? Maybe the term crash is a little dramatic, could the orbit shrink to the point that the planet intersects with or is vectored into the sun?

Definitely. In the early solar system, there were probably several more planets than there are today. Gravitational interactions between the planets caused some to be flung into the sun, while others were sent spiralling into the sun until it settled out into the more or less stable configuration we have today. With the Kepler spacecraft and other exoplanet finding programs, we've come to realize just how big a factor migration can be. The best example of this is what are called 'hot Jupiters'. These are gas giant planets up to several times the mass of Jupiter that are in orbits that would be well within Mercury's orbit in our own solar system. These planets could not have formed where they are now, as when their parent stars ignited fusion they would have blown away all the gas in their immediate vicinity. This means that these planets must have migrated all the way in towards their sun. For each one we see in a stable orbit tight around its star or spiralling its way in, there were probably several more that collided with their parents. We just see the 'lucky' ones.

Just to assuage any fears: This sort of thing mostly happens on such large scales in the first few million years of a solar system's history. A major migration isn't going to happen in our system. Our planets' orbits aren't entirely stable, but they'll last longer than the Sun will.


Galactic Equator zenith transit I need to find the two times of zenith passage of the Galactic Equator for a given Earth Lat/Lon on any day between 3500 BC and 1500 AD.  Since the Galactic Equator is a line and not a point, I try to use PyEphem to create a zenith point for my geographic location (see code below), then find the Galactic coordinates of that point.  When the Galactic latitude of this zenith point is zero, that's the time of zenith passage of the Galactic Equator.

However, I think I need my zenith point to be in Equatorial coordinates (not Ecliptic) to accomodate the geographic location and the date/time, but I can't figure out how to make that work.

Here is my Python script based on PyEphem:

import ephem
from math import *

D = (input("Enter date (yyyy/mm/dd): "))
d = ephem.Date(D)
print(d)

location = ephem.Observer()
location.lon = '-87.0'
location.lat = '20.0'
location.elevation = 12
location.pressure = 1013.0
import csv
my_list = []
with open(r'C:\Users\Mark\Desktop\PYTHON\gzt.csv', 'w', newline='') as f:
        for T in range(1,1440):
                dm = ephem.Date(d + ephem.minute*T)
                location.date = dm
                ra, dec = location.radec_of('0', '90')
                ec = ephem.Ecliptic(ra, dec, epoch = d)
                eq = ephem.Equatorial(ec)
                #print(eq.ra, eq.dec)
                z = ephem.FixedBody()
                z._ra = eq.ra
                z._dec =eq.dec
                #z._epoch = d
                z.compute()
                #print(z.ra, z.dec)
                zg = ephem.Galactic(z.a_ra, z.a_dec)
                dmt = (('%s') % (dm))
                #print(dmt, 'Zlat = ', zg.lat)
                AZ = (('%s  %s') % (dmt, zg.lat))
                my_list = [AZ]
                writer = csv.writer(f, dialect='excel-tab')
                writer.writerow(my_list)

If I eliminate ec and z, and do this:

    eq = ephem.Equatorial(ra, dec, epoch = d)
    zg = ephem.Galactic(eq)

the crossings I find in gzt.csv agree with Stellarium within a minute of time.

If you're open to another package by the same developer, skyfield.almanac can help with such calculations.
In this case you would implement a function which takes a time argument and returns an integer indicating which side of the galactic equator the zenith is on.
Then you would pass this function as an argument to find_discrete.


Do magnetic fields affect planetary rings? Planetary rings seem to generally form on the ecliptic plane because of the rotation of the planet, but I was wondering what kind of effect the magnetic field of the planet might have on the ring system. Would it help keep the ring system together, disrupt it, or have little to no effect? What if the magnetic poles were on or near the ecliptic?

(I will assume that these planets orbit a star.) Planetary rings are formed by the gravitational capture of small objects, mostly ice and dust. These particles can be magnetic and in such cases would be affected by the planetary magnetic field. It is also to be pointed out that the magnetic field is not really uniform, but let us assume that it is. The presence of a magnetic field would be important for the sustenance of a ring, but the stability of the ring is an entirely different issue. If the ring does consist of magnetically sensitive materials, then large enough fluctuations in the magnetic field can in principle cause the ring to become unstable. So depending on the answer of stability, I choose to say yes, it does depend on the magnetic field. To calculate the stability would be a complex n-body problem but can be averaged over.


the more concentrated the mass, the more powerful is gravity? the more concentrated the mass, the more powerful is gravity? or does it depend exclusively on the total mass? example: 1 $\small\mathsf{cm^3}$ material A = 1000 kg, 20 $\small\mathsf{cm^3}$ material B = 2000 kg
which of the two generates more powerful gravitational force?

I apologize for my English, I am not a native speaker, and I used google translate

To simplify things, I will assume that we have a spherical body and that we are considering Newtonian Physics.

If we have two bodies of the same mass but different radius, as long as we are outside of both bodies, and at the same distance from their centres, the force of gravity will be the same:
$$
F = G \frac{m M_i}{r_i^2}
$$
where $m$ is "our" mass, $M_i$ is the mass of body $i$ and $r_i$ is the distance between our centre and the centre of the body, and $6.67408 × 10^{-11} m^3 kg^{-1} s^{-2}$ is the Gravitational Constant.

So, there is no dependence on the density of the body, only on its total mass.

However, if you are thinking about the gravitational acceleration $g$ at the surface of different planets, not only the masses may differ, but also the distance to their centre. Note that $g$ is given by:
$$
g = G \frac{M_i}{r_i^2}
$$

So, computing for Earth we have that $M_{Earth} = 5.972 \times 10^{24} kg$ and $r_{Earth} = 6.371 \times 10^6 m$, which gives, $g_{Earth} \approx 9.820 m s^{-2}$, slightly different from the standard gravity: $9.80665 m s^{-2}$.

The same computation for Mars, which has $M_{Mars} = 6.39 \times 10^{23} kg$ and $r_{Mars} = 3.3895 \times 10^6 m$, gives $g_{Mars} \approx 3.71 m s^{-2}$. So, since Mars has, roughly, $1/10$ of Earth's mass and $1/2$ of the radius, the acceleration of it's gravity is around $40\%$ of Earth's acceleration.


What is the farthest blueshifted object to our galaxy? Is it also roughly the limit where gravity will be able to overcome space expansion? In other words, we can only hope to get to there, or at least see them, in the very far future?

My impression is that, aside from the very rare galaxy that is moving towards us, only stuff inside the local galactic gravitational field can be blueshifted.  Written perhaps more clearly,  from spaceanswers ,

  Andromeda is not the only galaxy to be moving towards us. With the
  help of galaxy surveys, astronomers have found that around 100
  galaxies are moving towards us. Compared to the numbers of galaxies
  that we know of (hundreds of billions), blue-shifted galaxies are
  seemingly quite rare. Those that are moving towards us are either part
  of our Local Group, which means that we are gravitationally connected
  to each other, or they are found in the Virgo Cluster which everything
  in our Local Group is moving towards. The galaxies M90, M86 and M98
  are all in the Virgo Cluster and all show blue shifts.


How much volume of gases could be added to Mars atmosphere that the planet gravity can retain? This article Mars atmosphere states that

  Today Mars atmosphere is very thin, but it was not always like this. Around 3.5 billion years ago, the red planet had a thick enough atmosphere for liquid water to run smoothly on its surface

Suppose you could increase Mars atmosphere volume by melting frozen CO2 or whatever. How much volume of gases could be added to Mars atmosphere that the planet gravity can retain?

There are hard limits on what types of gas that Mars can retain based on its temperature and mass
(Graph of what gasses an astronomical body can retain).

Volume wise, it's not clear. Currently Mars is still losing its atmosphere, so it can't even retain that amount of atmosphere. But if you continually added gas to Mars, there isn't an end point where the planet could no longer hold the atmosphere being added. It just increases the pressure of the atmosphere at the surface and only slightly extends the height of the atmosphere. And after it stopped being added, Mars would slowly lose the atmosphere again.


Could undiscovered smaller black holes within galaxies be an explanation for dark matter? From what I've read, given the speed of stars rotating within a galaxy and the known mass, the stars' speed is too fast to stay in orbit. Dark matter was proposed to explain this but is it possible that there are undiscovered smaller black holes with no orbiting stars?

MACHOs (massive compact halo objects) are thought to make up a portion of the dark mass and portion of these are thought to be black holes, but there are several very solid reasons why black holes not generally seen as a serious alternative explanation to non-baryonic dark matter:

1) Observed elemental abundances are not consistent with the amount of baryonic matter being produced in the early Universe to explain the dark mass

2) Microlensing searches for MACHOs of the size of black holes formed by stellar evolution rules them out as making a significant amount of dark matter in the Milky Way.

3) Black holes are observable through accretion. For black holes to make a significant amount of dark matter they would have to be in isolated locations within the galaxy so as not to be visible. However most dark matter must be distributed evenly throughout the Milk Way to explain the rotation curve.


Is it possible that only one hemisphere of a planet has an atmosphere? Suppose there is a tidally locked planet orbiting a star. The planet's surface consists of a global ocean, that is, liquid water. At the inner hemisphere the temperature is so high that the water is constantly boiling, creating the atmosphere of water vapor. But the vapor does not reach the dark side of the planet, and precipitates around the terminator line. The further side has no atmosphere and an icy surface.

Is this setup possible? Can it be such thing that one side of a planet has liquid (and boiling) water surface while the other side has no atmosphere?

Is it possible at all that a part of an ocean was boiling while the other one was icy?

That situation would lead to heat transfer between the cold and the hot zones just like it happens here at the Earth.

There is a paper on this that may help you:

  [...] heat transport from the day side to the night side of the
  planets efficiently reduces temperature contrasts [...]

Source: http://arxiv.org/abs/1001.5117


Aperture photometry uncertainties Suppose I have a 2D data array with number of counts in each pixel (i.e., this is the image array). Suppose I have another 1-1 mapped same-shape data array that gives the 1-sigma Gaussian standard deviation in the number of counts in each pixel in the image (i.e., this is the error array).

If I do circular aperture photometry, I just calculate -2.5*log10(summed counts within aperture) + magnitudeZeropoint.

On the other hand, for the magnitude uncertainty on my aperture magnitude, I read that I'm supposed to do a quadrature sum of the errors within my aperture, then compute the fractional flux uncertainty as the ratio of my quadrature-summed uncertainty divided by my measured counts from the image, and then magnitude error = 2.5*log10(1 + fractional flux uncertainty).

How come the uncertainties have to be quadrature summed ($\sqrt{\Sigma \sigma_{x,y}^2}$), instead of just added up like I do with the image pixel values? The quadrature sum results in a smaller errorbar but is that realistic? Also, I emphasize that my data arrays are in units of counts (i.e., ADU's), not electrons.

Because the uncertainties in the number of counts detected in each pixel are considered to be independent. That means some of them are higher than the "true value" (the count rate you would measure if you observed for an infinitely long time), whilst others are lower. To some extent these independent uncertainties will cancel, and the net result is that for normally distributed errors the correct protocol is what you have described.

Perhaps a good way of seeing this is that suppose you took 100 independent measurements of the same thing, each with its own, roughly equal uncertainty. If I asked what the uncertainty in the average was, you wouldn't just add up all the errors and divide by 100 because that would give an uncertainty that was identical to that of an individual measurement. Instead you would do the quadrature sum of the uncertainties and divide by 100, which reduces the uncertainty in the average by a factor of $\sqrt{100}$.


First Confirmed Visual Observation of Gravitational Lensing Is it possible to identify the first direct observation of the gravitational lensing?

What object was lensed, and where did the energy detected fall on the electromagnetic spectrum?

The first identified gravitationally lensed object is known as the Twin Quasar.

The Twin Quasar (also known as Twin QSO, Double Quasar, SBS 0957+561, TXS 0957+561, Q0957+561 or QSO 0957+561 A/B), was discovered in 1979 and was the first identified gravitationally lensed object. It is a quasar that appears as two images, a result from gravitational lensing caused by the galaxy YGKOW G1 that is located in the line of sight between Earth and the quasar.

You can see it right in the middle of this picture:

The Twin Quasar's two images are separated by 6 arcseconds. Both images have an apparent magnitude of 17, with the A component having 16.7 and the B component having 16.5. There is a 417 ± 3-day time lag between the two images.

The quasars QSO 0957+561A/B were discovered in early 1979 by an Anglo-American team around Dennis Walsh, Robert Carswell and Ray Weyman, with the aid of the 2.1 m Telescope at Kitt Peak National Observatory in Arizona, United States.
[...]
Critics however identified a difference in appearance between the two quasars in radio frequency images. In mid 1979 a team led by David Roberts at the VLA (Very Large Array) near Socorro, New Mexico/USA discovered a relativistic jet emerging from quasar A with no corresponding equivalent in quasar B. Furthermore, the distance between the two images, 6 arcseconds, was too great to have been produced by the gravitational effect of the galaxy G1, a galaxy identified near quasar B.
Young et al. discovered that galaxy G1 is part of a galaxy cluster which increases the gravitational deflection and can explain the observed distance between the images. Finally, a team led by Marc V. Gorenstein observed essentially identical relativistic jets on very small scales from both A and B in 1983 using VLBI (Very Long Baseline Interferometry). The difference between the large-scale radio images is attributed to the special geometry needed for gravitational lensing, which is satisfied by the quasar but not by all of the extended jet emission seen by the VLA near image A.


How do you measure something that has already left the observable universe? They say that the observable universe is only 5% of the entire equation. The rest of it consists of dark matter and energy. I believe that they are trying to find a way to see something that cannot be seen due to being outside of the observable universe, sort of like how galaxies redshift and slowly face away of out existence.

tl;dr  does dark matter/energy increase every year?

What comes after red shift? Black? If so, then does that mean the universe is mostly black because 95% of matter has "Blackshifted"?

Did I post a question correctly?

First off, you need to understand what "Redshift" is.  Light that is redshifted means that its wavelength has increased, making it appear more red.  This can come from a variety of sources, including Doppler shifting.  What we're more concerned with here is "cosmological redshift".  As light travels through space, its wavelength increases due to the expansion of space.  We can use a measurement of how much the light has redshifted to calculate the distance to its source.  There is nothing "beyond" redshift, because redshift is just a lengthening of the wavelength.  And nothing is happening to the objects themselves, they just appear to get redder to us.

Next, the definition of the "observable universe".  Because light travels at a finite speed, it takes a certain amount of time to reach us.  The universe had a beginning in the big bang, meaning that light has only had a certain amount of time to travel.  Therefore we can only see things out to a certain distance.  Light coming from objects further than that has not had enough time to reach us yet, so we can't see them.  That's what we mean by the "observable universe".

Galaxies do not "phase out of existence" because of redshift.  As we look outwards, we're looking at older and older objects.  Because the light takes a certain amount of time to reach us, we're essentially looking back in time.  As time passes, however, we can see more and more because light from further away objects has more time to reach us.

Dark matter and dark energy cannot be directly observed because they do not interact with light.  They do not absorb it, emit it, reflect it, or anything else.  However, they are still within the observable universe, because the observable universe is basically just the size of the universe.  There are many different theories as to what dark matter and dark energy actually are, but they are not ordinary matter that has been "blackshifted".


How does diffraction occur in a reflecting telescope? It's understandable how diffraction occurs in refracting telescopes, because the lens itself is a small aperture for light to pass through.

But what about reflecting telescopes? Since light is reflected, and a mirror is not exactly an 'aperture'? How can light diffract in a reflecting telescope?

Diffraction has nothing to do with whether you are using a lens or a mirror in your telescope. Diffraction occurs for the same reason in reflecting telescopes as it does in refracting telescopes: it is passing through an aperture.

There is no threshold aperture diameter for which you have diffraction. No matter how big the aperture, you will have diffraction. No matter the shape of the aperture, you will also have diffraction. This happens on any aperture.

However, the bigger the aperture, the smaller the diffraction pattern, or Airy disc. In
page 19 of the document you mention in the comments, there is a relation between the size of the aperture and the size of the Airy disc.

To clarify what is said in the first paragraph of page 18

  If the telescope is replaced by one with a narrower objective, the images of the two stars would overlap too much and the observer would not be able to see them as separate stars.

If the two stars are replaced with two stars that are close enough together, the images of the two stars would overlap too much and in this case the observer would not be able to see them as separate stars either.

As a side note, refracting telescopes aren't necessarily smaller than reflecting telescopes. There are refracting telescopes with a lens more than a meter in diameter.  A lot bigger than most amateur reflecting telescopes !


Why do we keep orbiting through the Perseid meteors? I understand that the meteors originated from the the comet Swift–Tuttle. Presumably they left the comet with some velocity relative to the solar system. As comets follow elliptical orbits presumably the meteors are generally also following an elliptical orbit. And as the earth follows a circular orbit, why do we keep meeting them every year despite being on different orbits?

This is probably best understood by picture.

Comet Swift-Tuttle is both on a highly elliptical orbit, and a highly inclined orbit, though the inclination of the orbit doesn't matter to your specific question, but it helps with the overall picture.   What really matters for Earth getting a meteor shower is that the two orbital paths intersect.

What makes the periodic meteor shower is the dust trail, that follows (and precedes) the comet (not to be confused with dust tail - that's something else).   Any solid debris that gets blown off the comet at low velocity enters an orbit with a very similar orbital path to the comet and over hundreds or thousands of orbits, as the comet effectively gets broken up each time it passes the sun, the comet generates a dust or debris trail that largely follows it's entire orbit, at least, that's where the greatest concentration of particles is.   Not all meteor showers are periodic, but all periodic meteor showers happen in this way with a semi permanent orbiting ring of dust that the Earth flies through once a year.

Dust trail (generally not visible, too spread out, unless heated by a close pass to the sun)

Dust tail (can be visible), only exists as the comet passes close to the sun.

Every short period comet, unless it's very young, has a debris or dust trail that roughly spans it's entire orbit though the trail is most dense near the comet.  In fact, the density of the dust trail can tell us how long the comet has been on it's current orbit.   Most short period comets were deflected into their current orbits within the last 100,000 years or so.  Short period comets have relatively short life-spans.

Long period comets or near-parabolic comets are different because their orbits are much longer, though they can still generate trails during a pass close to the sun, they tend to be much more spread out given the enormous size of their orbits.

Because the Earth's orbit intersects Swift-Tuttle's debris trail once a year and the debris trail stays mostly in the same place, we get the Perseid meteor shower about the same time every year, and the meteors always come from the same direction in the sky.

Not every short period comet has an orbit that intersects with the Earth's orbit, which is why not all comets are associated with meteor showers, in fact, the majority aren't.   3 dimensions are hard to draw in 2-D pictures, but Pluto and Neptune can be used as an example.  It's often said that they cross orbits, but they actually don't.  Pluto gets closer to the sun than Neptune, but only when Pluto is near it's perihelion, at which point it's well below (or well above, depending on orientation), Neptune's orbital plane.

An interesting sidebar to this is that comets that give us meteor showers are also comets that are potential earth impacters.


Which force makes planets to continuously revolve around their parent star? We know that planets revolve around their parent star (or specifically Barycenter), they maintain a specific speed of revolution so that they don't end up smashing into their planet star. Which force or energy do that, so that they maintain their speed? If we have infinite amount of time, would planets smash into their parent star ?

Newton's First Law of Motion states that "an object in motion stays in motion with the same speed ... unless acted upon by an unbalanced force". On earth, an object in motion will slow down and stop due to air resistance (the unbalanced force), but in space, there is no atmosphere to cause air resistance, so the object will maintain its speed.


Is there any planet or black hole where we could experience time dilation like in the movie Interstellar? As we seen in Interstellar movie Copper goes to a planet where he experienced time dilation which was 1 hour of staying on that planet will cost the 7 years on the earth. So, is there any planet or black hole where time will nearly behave like this? Because the planets in our solar system will show very little differences.

You can definitely scratch out any planets, as their gravitational field will not be strong enough. Even a very dense neutron star wouldn’t give much effect, as far as I know.
As for black holes, well, basically any black hole will give this effect. However, those are too far to reach with our current means.
Safe travels!


Since when do astronomers have the notion that space is void? Since when do astronomers have conjectured that space is void, and not full of air like our immediate environment?
I am more interested about how long the notion has been around and how influential rather than when and how it has been scientifically demonstrated.
Background for the question: I heard a conference about philosophy of science explaining how Galileo revolutionized physics by postulating principles that where true, but defying immediate observation, like "all objects fall with the same acceleration" when in fact heavier ones tend to fall faster due to air resistance, or "undisturbed objects move in a straight line at constant speed" when such a trajectory can't be observed on Earth because of gravity and frictions.
I was wondering how he came to have these notions, then thought he might have been inspired by the motion of planets because he knew them very well and they move in a void so their movement is, as far as a XVI century astronomer is concerned, undisturbed.
But that would imply he knew space is actually void of air, which is not intuitive and requires observation technology he did not have access to (planes, weather balloon, sounding rockets...). So i was wondering since when this fact have been established or at least conjectured.

(This is a mash up of the answers in Who was the first to postulate that space was a vacuum? and What was the first astronomical measurement which demonstrated that “the Earth is surrounded by vacuum”? )
The notion that earth is surrounded by vaccum has been around since at least the atomist philosophers like Democritus (c. 450 BC), albeit without demonstration outside of speculation it was open to debate.
Galileo himself had the notion that air is not weightless, which implies that the amount of air weighting on our bodies is limited, suggesting there is vacuum above it.
His disciple Torricelli invented the barometer, allowing him to estimate the amount of air above our head and demonstrate that space around the Earth is a vacuum.
Conclusion, Galileo most certainly had a clear notion that the planets are moving in a vacuum (which does not mean it is what inspired his laws of motion), and it was firmly established shortly after his death.


Lightning in Jupiter's "string of pearls?" Is there any reason there couldn't be lightning in the storms that make up Jupiter's "string of pearls?"
Addendum:I'm writing a science fiction novel, with a ship passing by Jupiter and lightning is seen in one of those storms. I don't want to be in the position where someone says, "No way, there wouldn't be lightning in one of those storms."

According to the wikipedia page, there are two preconditions necessary for lightning:

Firstly, a sufficiently high potential difference between two regions
of space must exist, and secondly, a high-resistance medium must
obstruct the free, unimpeded equalization of the opposite charges. The
atmosphere provides the electrical insulation, or barrier, that
prevents free equalization between charged regions of opposite
polarity

Since the "string of pearls" are massive counterclockwise rotating storms, one would presume that high potential differences would exist in these turbulent regions. So the only remaining reason for not having lightning would be if there was an electrically conductive atmosphere.  But there are numerous examples of lightning strikes in storms in Jupiter, so, unless the atmosphere at the Pearls is somehow different than the rest of Jupiter, one would expect lightning there as well. Indeed, "storms on Jupiter are always associated with lightning".
From NASA's website, here is a diagram of one of the mechanisms thought to be responsible for lightning on Jupiter:

In conclusion, if you want a reason not to have lightning at the string of pearls, there would need to be little or no turbulence or an an electrically conductive atmosphere.  Since neither of these conditions is the case, one could expect lightning at these storms.


Current limitations in radio-astronomical spectrometry analysis of local interstellar cloud's hyperfine structures I've been trying to wrap my head around capabilities of current radio-astronomical spectrometry technology to isolate not too distant tenuous sources, say, chemical composition and density of the the interstellar medium right in the neighborhood of our Solar system. In essence, what is our local interstellar cloud made out of and how much of it is there:

We've had much success so far in isolating distant radio wave sources and analyzing their hyperfine, i.e. molecular level composition. Same goes even for interstellar medium composition down to molecular ions of diazenylium (N2H+) that is surrogate to the presence of unobservable, nonpolar N2.

For example, P. Caselli et al. from the Harvard-Smithsonian Center for Astrophysics presented a paper (PDF) on J = 1 → 0 transition of N2H+ at 93 GHz toward the quiescent low-mass cloud core L1512 in Taurus, back in 1995. That is 18 years ago!

What I don't understand is, what's preventing us, with current technology, from isolating local low-mass formations in the radio wave spectra from its background noise and analyzing its hyperfine structure through radio-astronomical spectrometry? Is the signal to noise ratio simply unworkable and the density of the local cloud too thin to filter it out from its background? Or have such observations actually already been done, and we already have molecular data on the local cloud that I'm not aware of?

The first thing to notice is that the Local Interstellar Cloud, in which the Sun is evolving right now, is a fairly diffuse region, with a typical density of about one to a few particles per cubic centimeters. Clouds with such low density are actually mostly atomic; as you can see on this plot (Snow & McCall 2006, adapted from Neufeld et al. 2005):

It is then quite challenging to observe molecules in this region.

Good news is, there is not only molecular lines to give informations about the physical properties (temperature, density, ionization, velocity structure and morphology) of the Local Interstellar Cloud. In particular, ultraviolet is a perfectly suited wavelength range to sudy this region in absorption, and there are some quite extensive studies of the subject (see for example Paper I, Paper II, Paper III and Paper IV of Redfield et al.).


Eccentricity of Exoplanets The field of astrobiology specifically discovering other planets that revolve around the sun-like star (also known as Exoplanets) has risen since 1990, but certain orbits of exoplanets are observed to have a much wider range in eccentricity than exhibited by the planets in our solar system. What's the reason behind this?

Orbital eccentricities and inclinations are damped to zero during the protoplanetary disc phase. This is because the gaseous parts of the protoplanetary disc are a great sink for excess angular momentum.
After the gas disc dissipates, the formed planets start to interact dynamically as they do not necessarily form in stable configurations. This can lead to the exchange of angular momentum. A giant planet with small initial eccentricity can hereby pump a lot of eccentricity into less massive planets, as the angular momentum $L=rmv$ is conserved.
So 'classical' thinking involves the exchange of angular momentum between planets after the gas disc dissipates as the origin of exoplanetary eccentricities.
A modern caveat to this is that there are mechanisms to incline parts of the inner gas disc during the planet formation phase w.r.t. the rest of the disc. As eccentricity and inclination are intrinsically linked, those inclined sub-discs may form inclined planets, which later lead to strong eccentricity pumping.


How much solar energy concentrates at Jupiter's focal point via gravitational lensing? How much solar energy concentrates at Jupiter's focal point via gravitational lensing?

Solar energy - from our Sun at Jupiter's focal distance?    A negligible amount.    Gravitational lensing doesn't have a focal point so much as a focal region that begins roughly at a point, and that point (which really shouldn't be called a focal point) can be calculated, ProfRob gives the formula here.
Jupiter's gravitational lensing distance is so far (about 10 times the Sun's or about 5500 astronomical units), that Jupiter essentially appears 1/10th the size of the Sun from that distance, so it doesn't really work at focusing the Sun's rays.  You still get a lot more light direct from the Sun, which at that distance, would just be the brightest dot in the sky, with Jupiter blocking more light than it lenses in the observer's direction.   You get more light from the Sun when Jupiter isn't blocking the sun at that distance.
Gravitational lensing works best when the lensing object is either very massive, so it pulls in light from a sizable area, or if it's larger than the object that's being studied.  That is, You could use Jupiter's gravitational lensing to study an exoplanet, which is incredibly tiny to our observation light years away, but the gravitational bending of light by Jupiter gives a very distorted image of the incredibly tiny/distant object, but that distorted image would have a great deal more information.  In that scenario, Jupiter would focus much more light from the very distant, very dim object, but only because the object is effectively so small by our observation.
Maybe somebody here knows how to run the math for gravitational lensing brightness, but it varies on how pin-point the object you want to get a better look at is, so it's not a simple one line formula.
With the case of the Sun and Jupiter, the Sun being larger, it doesn't really work.   A black hole could image something larger than itself, but a black hole bends a lot more light than Jupiter does and it's so called "focal point" begins much closer.
A magnifying glass, which focuses sunlight to a point (and this only works because the sun is so distant that the rays of light are essentially parallel), that focused sunlight can never be hotter than the surface of the Sun itself.   That's a law (xkcd did a nice bit about that)
The focused (but very blurred) image that a planet makes at it's gravitational lensing point wouldn't come close to hot enough to cook ants, if that's your goal.   A better way to think about it would be looking through a telescope, but the image is spread out into a ring but not that bright.   Maybe with a perfect scenario, like a black hole or neutron star and a star in the distance, you might get some brightness, but still much less concentration than a magnifying glass can give you because it doesn't focus at a point, it's more of a range.
If you make your question more specific, somebody might be able to give you a more exact mathematical answer.


Is a wavy path possible for a shooting star? I was camping recently (May 2014), and observed several shooting stars (very fast, short lived), a few satellites (very slow, long lived) and lots of aircraft (flashing lights) in the night sky.

All of the shooting stars appeared normal, as very fast streaks across the sky, which disappeared within a fraction of a second. All except one. One started fairly straight as normal, then took a completely unexpected wavy path (like releasing a balloon to fly around the room) towards the end of its flight. Then it winked out. What caused this?

Note, I was completely sober at the time, the sky was clear, and I believe this could not have been an aircraft or satellite because of its tremendous speed. Apart from this 'meteor', there was nothing else in the sky which could be interpreted as a firework.

Could this really have been a meteor, and what could have made it take this path? If not a meteor, what else could it have been?

(Note: I have tagged this 'UFO' because I'm not sure I can identify this object, not because I think it's an alien space craft).

I saw a zig zag meteorite in Bukuru, Northern Nigeria, when I was about 13 in 1959. That started from small to increasing swings, then went out near the horizon. For many years I puzzled about this, but rarely mentioned it, because no one would believe it. Years later I attended an astronomy lecture at college in London. I cornered the astronomer with this sighting and he said we know what this is. A flat dish shaped meteorite enters the atmosphere shallowly & flies in a circular motion that gets wider as it approaches the earth then extinguishes. Seeing this from the horizon, it appears to zigzag down. He told me that I was very very lucky to witness this meteor show, and I am glad that others have seen it too. Adios, UFO folks.


Discrepancy between my results of Great Conjunction 2020 and Wikipedia Using python-skyfield to calculate the upcoming conjunction of Jupiter and Saturn. I'm off a few minutes with the times on Wikipedia.
Wikipedia Great conjunction times (1800 to 2100)
Using Right Ascension:

Dec 21,2020 13:22:00 UTC - Wikipedia.
Dec 21,2020 13:34:33 UTC - My Calculation.

Using Ecliptic Longitude:

Dec 21,2020 18:37:31 UTC - Wikipedia
Dec 21,2020 18:20:40 UTC - My Calculation.

from skyfield.api import load, tau, pi
from skyfield.almanac import find_discrete

planets = load('de421.bsp')
sun = planets['sun']
earth = planets['earth']
jupiter = planets['jupiter barycenter']
saturn = planets['saturn barycenter']

ts = load.timescale(builtin=True)

def longitude_difference(t):
    e = earth.at(t)
    j = e.observe(jupiter).apparent()
    s = e.observe(saturn).apparent()
    _, lon1, _ = s.ecliptic_latlon()
    _, lon2, _ = j.ecliptic_latlon()
    return (lon1.degrees - lon2.degrees) > 0

def longitude_difference1(t):
    e = earth.at(t)
    j = e.observe(jupiter).apparent()
    s = e.observe(saturn).apparent()

    jRa, _, _ = j.radec()
    sRa, _, _ = s.radec()
    return (sRa._degrees - jRa._degrees) > 0

longitude_difference.rough_period = 300.0
longitude_difference1.rough_period = 300.0

print()
print("Great conjunction in ecliptic longitude:")
t, b = find_discrete(ts.utc(2020), ts.utc(2021), longitude_difference)
for ti in t:
    print(t.utc_jpl())

print()
print("Great conjunction in right ascension:")
t, b = find_discrete(ts.utc(2020), ts.utc(2021), longitude_difference1)
for ti in t:
    print(t.utc_jpl())

Can anyone verify the correct UTC time?  Is there a bug in my code?

There are no bugs in your codes. But you can make it more accurate with the following corrections (additions):

Using new ephemeris series (de430 or de440) most recent one is de440.
Use True equinox of True epoch of the date. Adding epoch='date' as below:

.radec(epoch='date')
.ecliptic_latlon(epoch='date')

use the following codes for loading timescale instead of ts = load.timescale(builtin=True).

url = load.build_url('finals2000A.all')
with load.open(url) as f:
    finals_data = iers.parse_x_y_dut1_from_finals_all(f)

ts = load.timescale()
iers.install_polar_motion_table(ts, finals_data)

These codes load the updated values of ΔT and also apply polar motion. Of course, because your calculations are geocentric, the polar motion does not affect the result.
You can reach the following result by the corrections and some extra calculations for other attributes.
JUPITER -- SATURN Conjunction 2020:

A.D. 2020-Dec-21 18:20:37.3129 UT
Angular Separation: +  0° 06ʹ 06.38154ʺ
JUPITER is -  0° 06ʹ 06.98608ʺ of ('Below', 'Right') of SATURN in direction of (+  0° 00ʹ 02.56845ʺ, N)


Is this a potential planetary setup for the Alpha Centauri System? I originally posted this on the World Building SE, but I was suggested to post it here for a better answer.
I am attempting to create a [semi]plausible star system in Alpha Centauri for a series. The system was originally intended to have 1 habitable planet per star, but after my first couple of attempts I noticed that it might be possible to get at least 3 planets to be habitable. The latest attempt actually shows 5 potentially habitable planets (2 of which are a binary[That is another SE question]). Ultimately, I would like this to be the case as it allows a much more diverse universe for the series.

Attempt:

aCen A Planets

aCen B Planets

I don't have access to, or knowledge how to use, any form of software like Universe Sandbox. I have read several reports on planet orbits and I think I did an "ok" job with this.

Here are some papers that I have looked at for this:
http://adsabs.harvard.edu/full/1997AJ....113.1445W
https://arxiv.org/pdf/1801.06131
https://core.ac.uk/download/pdf/25201586.pdf

My main concern is that the planets might orbit too closely. I have attempted to find a formula (that I could understand) that could aid in spacing the planets. The nearest I could manage is using the Mutual Hill Radii. There are conflicting reports where one says that 10 - 12 MHR (Delta-H) is good or a tightly packed system. Earth and Venus have around a 25 MHR value. One of the reports that I linked mentioned up to 25 MHR for aCenA, but it also shows several other numbers and after trying to comprehend everything my brain reached orbital velocity.

Question: Is the planetary spacing stable enough to host planets on Gigayear timescales?
They don't necessarily need to be able to have spawned life, but they should be able to support life with little to no human intervention.

Note and Bonus, aCen V is a binary planet that orbits with a Semi-Major axis of 750,589km with an eccentricity of 0.01204. (There is an error in the image in Yellow that shows 148.623 and Eccentricity of 0.0910). The inclination should be 0 as both planets should be on the same plane.
Last note: The Semi-Major Axis distance is in Megameters (1 million meters).

The only way to answer the question about stability is to do the integration, because this problem does not have an analytic solution. There are approximate solutions for the stability of two-planet systems (although these are based on a somewhat weaker constraint that allows the outermost object to escape to infinity) but they do not necessarily generalise to more planets. Furthermore, planetary systems tend to exhibit chaotic behaviour so you aren't going to get definitive answers even with an integration, because errors get introduced by the limited precision at which the calculations can be done.

You also need to bear in mind that the parameters you've listed in the table do not give sufficient information to set up an integration:

You need the argument of pericentre ($\omega$) and the longitude of the ascending node ($\Omega$), which together with the inclination form a set of Euler angles describing the orbit's orientation in 3D space.
You need the mean anomaly ($M$) or equivalent (e.g. mean longitude, $\lambda$) to describe where the objects are along their orbits.
You need to specify at what epoch these parameters are specified for.

The integration is going to be further complicated by the fact that it isn't sufficient to just simulate Alpha Centauri AB, you also have to take into account Proxima, which may perturb the orbits of the AB pair on gigayear timescales, and is also on such a wide orbit that you probably need to take the galactic tide into account and the unknown history of stellar encounters along the system's path through the galaxy.


What is it exactly about these flares of infrared light from Sgr A* that "confirms" it is a supermassive black hole? CNET.com's SCI-TECH
Scientists confirm a 'supermassive black hole' at the heart of our galaxy
It's "mind-boggling", they say.

This links to Astronomy.com's Scientists finally confirm the Milky Way has a supermassive black hole
This links to ESO's eso1835 — Science Release Most Detailed Observations of Material Orbiting close to a Black Hole; ESO’s GRAVITY instrument confirms black hole status of the Milky Way centre

FYI, the GRAVITY instrument measures infrared light, not gravity.

What is it exactly about these flares of infrared light from Sgr A* that confirms it is a supermassive black hole?

Certainly there is plenty of evidence, the orbits of stars nearby for one. But this use of the word confirm seems strong and frequent enough to suggest the deal has been done, the ink is dry, and it's a black hole without question and that until now that wasn't so.

I've shamelessly absconded the following images from this answer to the question What is the evidence for a supermassive black hole at the center of Milky Way?, which may need a new answer!

Note, the size of these stellar orbits are compared to those of Sedna, Eris, Pluto and Neptune in the bottom right corner (same scale)!

Source

We have reasonably good measurements of the mass of Sagittarius A*, thanks to measurements of the movements of stars like S0-2 over several decades. It's been well-established that the mass of the central object is $M\approx4\times10^6M_{\odot}$; this alone is fairly good evidence for a supermassive black hole, and we can constrain the size of the object with the measurements (Ghez et al. 2008). Models of the orbits of other stars have further improved these results.

The recently-published paper (Abuter et al. 2018) uses a similar technique. The motion of the flares is well-described by the rotation of a "hot spot" of gas in the inner reaches of the accretion disk, with the flares arising from magnetic recombination or some similar event. In particular, it lies near the innermost stable prograde circular orbit (ISCO) - just outside it, actually (the orbital radius can be found because we know the mass of the object and the period of the orbit, with the best-fit model of the latter being $40\pm 8$ minutes). This both constrains the object's size and even qualitatively provides support for the black hole model, as we might expect to see such events around a supermassive black hole.

Looking at those articles, I think the use of the word "confirm" is inaccurate. As far as I can tell, the term is only used by the project leader, Reinhard Genzel, in that ESO statement - and it's not a claim repeated in the paper. The team describes their results as "strong support" for the supermassive black hole model, and say that their findings are "consistent with" that theory. As scientists should be, they're cautious. The results don't definitely confirm that Sagittarius A* corresponds to a supermassive black hole; they're simply additional (excellent) evidence for it.


Calculate telescope orientation based on RA, DEC and Lat/Long I'm parsing FITS files for a project based on data from a telescope. These files include 'DEC', 'RA' and lat long values. I understand roughly the concept of celestial coordinates and I assume that these values are sufficient to calculate the direction that the telescope is facing and the angle that it's at but my trigonometry is not up to the task.

I am using astropy for the project so any answers which reference that would be great.

PyEphem will get you Alt, Az from RA, Dec, Lat, Lng, Date, Time, and Elevation
http://rhodesmill.org/pyephem/


Mass-magnitude relation of evolved stars In short: what mass is appropriate for absolute magnitude $M_V \sim -7$?

Some context: I'm processing the observational data (supernova light curve) and curve fitting gives me about $0.4M_\bigodot$ for ejecta mass. However, possible progenitor candidate has $M_V \sim -7$, which, I suppose, points to quite a large star. My background in astronomy is not so good (I was mostly doing theoretical physics, namely nonlinear optics and GR before I got to supernovae), so I'm doubting correctness of either my estimates or progenitor candidate parameters.

A star with absolute magnitude of -7 is big and powerful, It would be among the most powerful stars in the galaxy.

The star would be near the end of its life, so comparing with other aging stars:

Aldebaran is slightly more massive than the sun. It is a red giant (or orange giant according to some sources) It has an absolute magnitude of about -0.6. Aldebaran is too small to undergo a supernova.

Betelgeuse is much bigger. Estimates of its mass put it at about 11 solar masses, but it could be bigger. It has a magnitude of about -6 (it is variable and there is some uncertainty on its distance).

Eta Carinae is one of the most powerful stars in this part of the galaxy. It is also variable, but has an absolute magnitude of -8—-9, and mass of 100-200 suns

AG Carinae is a LBV with a mass of 50 suns and a minimum absolute magnitude of about -8, but becoming much brighter during outbursts. (as bright as -11)

So an evolved star with a magnitude of -7 could be supposed to be between the LBV stars and Betelgeuse, with a mass of around 15-20 suns. Stars do get very unstable this late in life, and there could be a great deal of uncertainty.

Others here know more about supernovae mechanics, and may be able to explain any discrepancy between the supposed stellar mass, and the mass of the ejecta.