Will the Earth ever be tidally locked to the Moon? From my basic understating,

Momentum is being transfered from the Earth's rotation to the Moon's orbit by tidal friction. The Earth's rotation slows down and the Moon receedes from the Earth as it moves into a higher orbit. This will continue until the Earth's rotational period is equal to the orbital period of the Moon, i.e the Earth is tidally locked to the Moon.

Assuming I have the above correct - and please correct me if I don't - will there, realistically, be enough time for tidal locking to  occur before the sun expands and engulfs the Earth? Or is there another reason the Earth will never be locked towards the Moon?

As the moon orbits Earth, tidal forces slow down the Earth's rotation by 2 milliseconds per century. Eventually, in tens of billions of years, the Earth and Moon would achieve a double tidal lock, where both are stuck with one side facing the other as they orbit the Earth-Moon barycenter. In 7.5 billion years, the Sun will expand past the Earth's current orbit, but the Earth may drift out further, preventing it from being vaporized.

However, this is beside the point, because in about one billion years, all of Earth's water will have boiled away, meaning that there would be no more ocean tides, and thus the Earth-Moon system will likely never achieve a double tidal lock.

References:

Will the Moon ever leave Earth’s orbit? - Giles Sparrow, Space Answers
Harvesting Lunar Eccentricity? - Terry R. McConnell, Syracuse University
When Will Earth Lock to the Moon? - Fraser Cain, Universe Today
The Sun Will Eventually Engulf Earth--Maybe - David Appell, Scientific American
Distant future of the Sun and Earth revisited - K.-P. Schröder and Robert Connon Smith, MNRAS
When will Earth lose its oceans? - CNRS, Science Daily


collimating AstroMaster 114 I've owned my AstroMaster 114EQ for about a year now, and feel like I know my way around pretty well, but I'm only just becoming aware that collimation is recommended. The user manual has instructions on basic collimation. Does this look sufficient? Or should I invest in a celestron 94183 collimation cap, a celestron collimation eyepiece, or something fancy like an Orion LaserMate Deluxe II? The telescope itself cost less than $200 so I can't help but wonder if the fancier collimators are overkill. I've watched this longish video and one must apparently perform some disassembly on the telescope to user the laser collimator. This doesn't seem like the easy 'couple of minutes' adjustment I've seen described.

I'm also confused about the process of adjusting the secondary mirror. If anyone has good instructions about my specific model, I would greatly appreciate it. I'd really like to get it tuned up now that Mars is looking so good.

With a tube length of 46 cm and a focal length of 100 cm, probably the AstroMaster 114EQ is not a classical Newtonian but uses a fast (e.g. f/4) spherical primary mirror and a corrector lens built into the base of the focuser to reduce spherical aberration and increase the effective focal length (e.g. to f/8.8).
This is a variant of the Jones-Bird design of some custom telescopes.
There is some doubt about how well the corrector actually works in budget-priced telescopes.

Most collimation instructions (e.g. Seronik or Carlin) assume a classical Newtonian design.
Jones-Bird collimation is similar in theory but more complicated in practice.
In particular, the corrector hinders the use of a laser device.
Try a collimation cap; anything that fits a 1.25 inch focuser is fine.
You can even get decent results with a homemade one.
If you decide to invest in a Cheshire sight tube, it can be used with any Newtonian reflector.


If there are neutron stars, would most stars be considered "proton stars"? For example, the Sun is a giant sphere of positively charged plasma. About 72% of the elements in it are protium. This means that these protium atoms would have had to lose their electrons, right? If so, the majority of the Sun would be protons. So just as there are neutron stars, would stars like our Sun be considered "proton stars"?

This may be a stupid question, but what I'm really getting at is: are stars mostly protons? Is what I described how it all works?

Protium is a proton + an electron.

Under enormously high pressure, it's energetically favorable for electrons to merge with protons and become neutrons - see here.

  are stars mostly protons

By mass, yes, at least before they get too old.

The mass of the universe is more complicated, but anything solid that we think of as matter is made of atoms, which are by mass, mostly protons and neutrons (you can break it up further than that if you like, but that's best for another question).

Most hydrogen has no neutrons so any hydrogen rich object (the sun, most young stars, gas giant planets) are by mass, mostly protons.  That's no longer true when a star gets close to the end of it's life and has burned much of it's hydrogen.

Jupiter, by mass is roughly about 80% protons.  The sun, because it's been turning hydrogen into helium for about 4.5 billion years, is roughly about 67% protons by mass.   The Earth, mostly other elements, Oxygen, Silicon, Iron, etc, is a about 50% proton by mass.

  Would stars like our Sun be considered "proton stars"?

I suppose you could use that term, but I don't see any benefit to it.    It's not too different than calling the sun a "hydrogen" star.  All stars start out as hydrogen stars.


What fills the new space created during expansion? Does it remain void and pushes even more or will virtual particles eventually create normal matter?

It's not the case that something needs to be put in there, it's just that space itself is expanding. So if the bit you are watching expand is void then it will still be void, only bigger - you don't get new space, you just get bigger space.


How can the universe be infinite? I've heard from renowned astrophysicists that we don't yet know whether or not the Universe is infinite. How is that possible regarding the big bang theory is accepted (as they all do)? Are they referring to the existence of other Universes when they say it could be infinite, or what?

What I mean is, even if it is capable of expanding, if everything originated at the big bang how can that space turn to be infinite in a finite time - the age of the universe.

In the standard ΛCDM model of the Big Bang, the universe is infinite and has always been such. The Big Bang singularity happened everywhere, in the sense that far back enough in time, the density diverges to infinity at every place.

But this is just a particular model--it assumes that the universe if spatially flat and is globally homogeneous and isotropic. There are extended models in which it is not exactly flat, and so could be finite even if it is still homogeneous and isotropic (if the curvature is even slightly positive). And of course we don't actually know whether it is homogeneous and isotropic at scales much larger than we actually see. Some inflationary models imply that it isn't.

To clarify: the ΛCDM model uses the assumes a spatially flat FLRW solution of general relativity, in which space is the Euclidean $3$-space The Euclidean $3$-space is the only flat homogeneous and isotropic $3$-manifold, so there no way to make it finite without violating at least one of those modeling assumption (e.g., a flat torus could have the same form for the metric, but would not be globally isotropic).


local stellar mapping data I've been playing around with Near Star Map, and I think I kind of want something more sophisticated, so for all stars within 100ly/30pc, is the following known?

velocity (it would be nice to have a vector field showing how fast in which direction stars are moving)
magnetic field intensity
estimated age
local ecliptic plane (and as much as that's meaningful for three-star systems with Poincare-befuddling orbital dynamics)

The short answer is no.

We don't even think the census of stars within 30pc is complete, let along have all the other information you want.

For all bright stars (say V<10) and close, there are decent parallaxes and proper motions in the Hipparcos and Tycho catalogues, so you can compile tangential velocities for these. A lot of these will also have radial velocities, but there is no recent uniform catalogue of these. Things like the General catalogues of radial velocities are getting a bit old. The Hipparcos input catalogue might be a reasonable source.

Magnetic fields have been measured on relatively few stars and usually those which are much younger and more active than the Sun.

There is no way of directly measuring stellar ages. This is one of the big problems in astronomy. Estimates can be made for stars in the field but they are rarely more precise than 20-30% and their accuracy, especially for older stars is open to question. I know of no uniform compilation that would help.

If you were to restrict your sample to even closer to the Earth, then you might find that the RECONS database for stars closer than 10pc might be of use.


Could the earth's atmosphere have been partially stripped by a passing rogue planet? I am involved in the fight between palaeontologists and aerodynamicists on the theoretical flight capabilities of the largest pterosaurs (who reached their climax in the Cretaceous and then disappeared at the K/T extinction event).

Is it possible that the earth's atmosphere was reduced at this time by a passing rogue planet or other sufficiently large body?

If so, the atmosphere would have been thicker.

Using the principle of Occam's razor, this one single hypothesis solves the problem of flight in the Mesozoic; the problem of how the dinosaurs could carry such weight; why they all had tails that would be much more use in a thick medium such as water...

The K/T extinction has attracted too many theories about what was 'added' to life on earth such as the comet/asteroid/Deccan Traps theories etc. No-one has looked at what might have been taken away from earth to cause the same effect!

If the earth's atmosphere had been much thicker (far higher pressure/density at the surface) the dinosaurs would have had a much easier time. Losing a huge amount of their atmosphere would kill them off because their 'effective weight' would have been doubled almost overnight.

Very interesting question. I'm okay with this question being here since you specifically ask for the effect of rogue planets on Earth's atmosphere. Thinking "astronomically" could be useful in coming to conclusions about whether or not your idea is even a possibility.

When I saw this I instantly thought of ice cores, since trapped gas bubbles could tell you about the composition of the atmosphere. Unfortunately ice cores go nowhere near that far back to be able to tell you about the composition of the atmosphere during the Mesozoic.

Now for the answer:

I can't really speak to the title of your question effectively, since I am unaware of any research involving rogue planets and atmospheric stripping, however, from an astronomy point of view, here is one other thing I would like to point out that may work to reduce the thickness of the atmosphere with time.

A small fraction of the gasses of the atmosphere exceed the escape velocity of Earth at any given time. Over time, many of these gasses get bombarded by high energy photons (as well as other particles) and fly out of Earth's gravitational potential well. If there are no other sources of these lighter gasses then there is a net loss.

Temperature does play a key role here (as the above plot suggests), but regardless, this process works to reduce the amount of atoms and molecules in the atmosphere (and it affects smaller mass atoms/molecules more so than heavier ones - this is why the Martian atmosphere is primarily carbon dioxide. Essentially it wasn't massive enough to be able to hold on to its lighter gasses, and over time the vast majority of them escaped).

If you had temperature information going back as far as the Mesozoic era, you could in theory run the clock backwards by running a simulation of the evolution of the atmosphere. I don't know how helpful this would really be (or how accurate), since there are potentially other things like volcanic outgassing and the bi-products of living things to complicate the situation. Quite honestly, I know little about these topics and so I couldn't tell you whether there is a net loss or gain of atmospheric gasses.

As far as rogue planets are concerned, I would find it rather unlikely that such a massive object would still be lurking around the solar system between about 252 to 66 million years ago. The early solar system was certainly more chaotic than it is now (for example, Giant impact hypothesis: $\sim 4.5$ Gya; the collision which caused Uranus' tilt: a few billion years ago), however it is not out of the realm of possibility.


Can the WISE telescope detect black holes? Black holes are hot, aren't they? With its infrared scan, could the WISE telescope also detect a black hole? The hypothetical planet beyond the Kuiper belt could actually be a primordial black hole. The WISE telescope might already have imaged the object but could it have done it also if it is a black hole?

Black holes are black. They are only observed directly by telescopes if they are accreting matter. Any radiation observed comes from the matter surrounding the black hole. Generally speaking, the smaller the black hole, the hotter the accreted material becomes. For something of planetary mass, one might expect X-rays and Gamma rays from accreting material.

A non-accreting black hole would produce no signature at any wavelength and there is very little material to be accreted in the outer solar system, except perhaps a small possibility of seeing X-rays if something like an Oort cloud object interacts with the black hole.

It is possible that such a black hole could be detected via its lensing effect on light coming from distant stars, but since the black hole would be very small as seen from Earth, this is unlikely, and WISE would not be the instrument to do that.

EDIT: Just to prevent any confusion. "Naked" black holes are hypothesisd (but not yet observed) to produce Hawking radiation. Planet X, if it exists, is thought to be about ten times the mass of the Earth. Such a black hole would have an event horizon about 20 cm in diameter and would emit Hawking radiation with a luminosity of $10^{-20}$ Watts at a blackbody temperature of around 0.002 K.  This is not going to emit any detectable infrared radiation.


What is the closest an asteroid or comet has passed the Sun and survived? After SOHO began watching the Sun from space with its coronagraphs 24/7 it was discovered that there are comets passing very close to the Sun quite regularly, and while some are destroyed, some are seen to emerge and continue on.
From these observations and from all other comet and asteroid observations there should be quite a collection of objects whose orbits take them very close to the Sun.
Is there any way to find out What is the closest an asteroid or comet has passed the Sun and survived?

Here is a GIF I made for this answer also used in this answer. In addition to some comets you can see Venus on its approach to occultation by the Sun in 2016. Seeing the Pleiades so close to the Sun is exciting as well!

These LASCO C3 images from SOHO were downloaded sohodata.nascom.nasa.gov/cgi-bin/data_query. The square frame is about 15.9 degrees wide.

Wikipedia offers some useful info on sungrazers:

  As of October 21, 2017, there are 1495 known comets that come within ~12 solar radii (~0.055 AU).[2] This accounts for nearly one third of all comets.[3] Most of these objects vaporize during their close approach, but a comet with a nucleus radius larger than 2–3 km is likely to survive the perihelion passage with a final radius of ~1 km.

https://en.wikipedia.org/wiki/Sungrazing_comet

There's a bit about pyroxenes sublimating off at 7 solar radii, but no mention of whether any such object made it around the sun. Pyroxenes generally melt at around 1000°C.
An Iron/Nickel body melting at 1538°C/1455°C might well make it through a 7 solar radii passage.


Is Earth's rotation Slowing down or Increasing Recently read that Earth is rotating faster than earlier1. However so far it is understood that earth's rotation is slowing down and to conserve the angular momentum Moon is going away from the Earth. What is true?
1for example see answers and comments at What effect does climate change have on the Earth's rotation? in Earth Science SE, and the 2015 Smithsonian article Is Global Warming Changing How Fast the Earth Spins?

Earth's rotation is slowing down, but it's a little bit irregular, which is why leap seconds can't be predicted in advance. There's provision for negative leap seconds, but they've never been used.
You can get the current list of leap seconds, maintained by The Internet Engineering Task Force (IETF), at https://www.ietf.org/timezones/data/leap-seconds.list
The decision to insert a leap second is made by the International Earth Rotation and Reference Systems Service (IERS), who coordinate the operations that gather the actual Earth rotation data. There is a wealth of information on their very extensive website.

Measuring the irregularities of the Earth's rotation
The variability of the earth-rotation vector relative to the body of the planet or in inertial space is caused by the gravitational torque exerted by the Moon, Sun and planets, displacements of matter in different parts of the planet and other excitation mechanisms. The observed oscillations can be interpreted in terms of mantle elasticity, earth flattening, structure and properties of the core-mantle boundary, rheology of the core, underground water, oceanic variability, and atmospheric variability on time scales of weather or climate. The understanding of the coupling between the various layers of our planet is also a key aspect of this research.
Several space geodesy techniques contribute to the permanent monitoring of the earth's rotation by IERS. For all these techniques, the IERS applications are only one part of their contribution to the study of planet earth and of the rest of the universe.
The measurements of the earth's rotation are under the form of time series of the so-called Earth Orientation Parameters (EOP). Universal time (UT1),
polar motion and the celestial motion of the pole (precession/nutation) are determined by VLBI.
The satellite-geodesy techniques, GPS, SLR and DORIS, determine polar motion and the rapid variations of universal time.
The satellite-geodesy programs used in the IERS give access to the time variations of the earth's gravity field, reflecting the evolution of the earth's shape, as well as the redistribution of masses in the planet. They have also detected changes in the location of the centre of mass of the earth relative to the crust. This makes it possible to investigate global phenomena such as mass redistributions in the atmosphere, oceans and solid earth.
Universal time and polar motion are available daily with an accuracy of 0.5 mas [milliarcseconds] and celestial pole motion are available every five to seven days at the same level of accuracy - this estimation of accuracy includes both short term and long term noise. Sub-daily variations in Universal time and polar motion are also measured on a campaign basis. Past data, going back to the 17th century in some cases, are also available.
EOP series are provided by the IERS in bulletins and as permanently updated series (long term earth orientation data).


Mass limit of planetary ring Is there a limit to how massive a planet's ring can be?

If there is, how does it depend on the mass of the planet, presence/absence of moons, distance to the star or other parameters?

A couple of points on this.  While faint rings can exist outside the Roche Limit (like Saturn's G and E rings) - photo is too big to link but click here.  rings of any significant mass are only going to reside inside the Roche Limit, so the practical limit of the mass of a planet's ring is about the mass of the largest moon a planet is likely to have, roughly speaking.

A ring around a planet can form in a couple of ways, by giant impact, by a moon passing inside the Roche limit and breaking apart or by accretion, like how Enceladus feeds Saturn's faint e-ring.

Also, rings don't have to be particularly massive to be impressive.  The mass of Saturn's rings estimated here (and there's some uncertainty to that estimate, but close enough for our purposes)

  Based on Voyager observations, the total mass of the rings was
  estimated to be about 3 x 10^19 kg. This is a small fraction of the
  total mass of Saturn (about 50 ppb) and is just a little less than the
  moon Mimas

So the mass of Saturn's rings is the mass of a pretty small moon, a little under 200 km in radius, but that wasn't your question, so, moving on.

A planet with rings 200 times the size of Saturn (40,000 times the surface area) was observed and if we assume the same density as Saturn's rings (which is a very bad assumption to make, but this is just an approximation), 1.2 x 10^24, or about twice the mass of Mars.   Now we're getting somewhere, but there's a few points to make on our friend J1407B

First, it may not be a planet at all.  It's mass estimate is 10-40 jupiters which suggests it's in the brown dwarf category.   2nd, it's a young solar system, just 16 million years old, so the ring system may still be in the process of forming a moon system and not be a permanent ring at all.  Source.   Also, same article, it mentions that parts of the ring system block 95% of the sun's light, so it's a far denser system than Saturn and there's a moon in there thought to be 0.8 times the mass of earth, but I still don't think that should counts since it may be an accretion disk remaining from the formation of that solar-system.

If we set the planetary mass limit to be about 13 Jupiter masses, which is roughly the cut-off between heavy Jupiter and brown dwarf, and we take some estimates of the size of moons to planets, during formation, it's unlikely that moons can be more than maybe a ratio of one to several hundred, due to limitations in angular momentum and likely size of formation.   In our solar system, the largest moon to planet ratio, not counting giant impacts or captures is about 1 to 4,200 (Saturn to Titan).  Jupiter to Ganymede is 1 to about 12,800 and Tritan to Neptune (and Tritan may be a captured moon), about 1 to 4,800.   If we assume a heavy Jupiter of about 13 Jupiter masses and a moon in a decaying orbit that breaks up with a mass ratio of 1 to 400 - and needless to say, that's very ballpark, then this theoretical estimate works out to a practical limit around a heavy jupiter to have a moon system of roughly 10 earth masses.  (1/400th the mass of a 13 jupiter mass planet).

Now, there are creative ways to increase it, like, lets say there are two moons, perhaps in Trojan points to each other, and they both spiral in gradually.   Or, lets say a passing object of greater mass flies past, but the problem with a passing object is that, due to escape velocity speed, such an object, if it passes within the Roche Limit, it would have a highly elliptical orbit, which wouldn't lend itself to a ring system, since the momentum of the elliptical orbit would only pass through and not stay inside the Roche sphere.  What you need for a good sized ring system to form is a slowly decaying circular orbit, not a recent capture of a large passing object.

See Shoemaker Levy 9 which passed through but quickly went back outside Jupiter's Roche limit.

Now if we imagine a scenario of a double planet, which is probably a rare scenario but not impossible, and over time due to proximity and tidal forces, the two planets form a decaying circular orbit around each other, in this theoretical, lets say we have two heavy Jupiter orbiting each other, one of 12 Jupiter masses and the other of 6 Jupiter masses (purely theoretical) and the 6 Jupiter mass planet begins to break up as it enters the Roche limit of the 12 Jupiter mass one.   There's 2 problems with assuming you'd get a ring with half the mass of the planet.   One is, hydrogen and helium gas are likely at least 80% of the mass of the planet that's breaking apart, perhaps 90% and hydrogen and helium don't easily bind into ice particles, so over time, much of that gas is likely to get blown away by the solar wind.  You're left with a rocky and icy mass of quite a bit less than 6 Jupiter masses, probably less than 1.  The other problem, with an orbiting object that large, it doesn't break apart all at once, but it's denser core stays together and spirals closer to the planet while the lighter ices around the surface which do begin to form the ring, can get spun off and flung either into the planet or flung outside the Roche limit, as the denser core of the smaller of the two planet moved slowly closer, there would be a vacuum cleaner effect wrecking havoc on the very same ring that planet was forming.  For smaller moons, this effect can be seen around saturn as tiny moons create tiny breaks in Saturns' rings.  With a much more massive core of the planet that was forming a moon, you'd get orbital chaos and it wouldn't be a good scenario to form a ring, even with all that available ring material.   You'd still probably end up with a ring, but I think only a small percentage of the original smaller planet's mass would survive the process.

A similar problem arises with giant impacts.  If the impact is too large, the planet breaks apart.   If it's not quite that large, like the giant impact on earth was thought to be, 4.4 billion years ago, then debris is blown outside the Roche Limit and forms a moon, in our case 1/81st the mass of the earth, though at the time it might have been a bit more, maybe 1/72-74 or so, and there were probably two moons initially not one, but I digress.   The point is, to have debris blown off the planet but staying inside the Roche limit so it forms a ring, not a moon, you need a smaller impact than the one that formed our moon, and a smaller impact implies less debris, so with Rocky worlds, what does that mean, 1/100th?   1/200th?  In there somewhere.  With gas giants the impact method is even worse due to the outer layer of gas giants being so much hydrogen that you don't get good material for a dense permanent ring structure from giant impact.

So, there are practical limits to the size of a ring system you're likely to see (if we ignore young solar-systems still information like J1407b.   It's a very ballpark just barely educated guess, but I think the practical limit for a ring system is probably pretty tiny compared to the mass of the planet, like a ratio of 1 to 200 or so, which if you have a planet the mass of 13 Jupiters, that's still quite a bit of mass.   If we use the 1 to 200 ratio and 13 jupiters, that's the mass of 20 earths.  I have a very hard time seeing how a ring system around a planet could get much more massive than that.

Now, in theory, like, lets say we want to build a ring system, and we design this enormous snow blower and we blow ice crystals around a planet, just for fun.   A ring system would probably remain stable well beyond that mass.   I don't have the means to calculate where instability might come into play, but if you build your own ring system around a planet, you might be able to get the ring system as massive as 1/10th the mass of the planet, maybe a bit more before some kind of gravitational instability took over.   I don't think you're ever likely to see that in a natural situation, but theoretically I think it could be done.

Now, to answer your questions:

  does it depend on the mass of the planet?

absolutely.  The mass of the planet (and it's density) determines the size of it's Roche limit, which defines how far the ring system can extend.   As pointed out above, planets can have rings beyond the Roche limit but only faint ones.   Dense/Thick rings only form inside the Roche Limit and the more massive the planet, the larger the Roche Limit.

  presence/absence of moons?

Small moons don't matter much.  Saturn has small moons inside it's rings and they create small breaks in it's ring system.  Large moons, particularly if they are close to the planet, can create gravitational disruptions and wouldn't be good for a permanent ring system.

  distance to the star or other parameters?

Beyond the frost line is best, cause that's where ices don't melt.  Ices make up more mass of our solar system and probably most solar systems than rocky material and they break apart more easily, so statistically speaking, a ring system should do better far away from the sun, in our case, at least Jupiter Distance.   Jupiter's enormous magnetic field might also not be long term ring friendly, so in addition to far from the sun you'd want a planet without too strong a magnetic field that's shooting high velocity charged particles through the ring.  Here's a fun article about how Saturn's magnetic field erodes it's ring.  Jupiter would melt any ice ring it had much faster than Saturn does.

Hope that wasn't too long and it's more my trying to work it out than a definitive answer, but until we get much better telescopes, there might not be a definitive answer to this one.


Find constellation over Earth coordinates on a specific date-time I would like some help in finding the exact constellations, or some sort of visualization of the stars over an specific location an time. The idea is to find out the closest star or group of stars on the vertical of that position in that exact moment.

I'm also aware than "vertical" can have some different interpretations, but a nice approximation should be good enough.

I'm not an expert in physics nor astronomy, but I think providing the coordinates and the date/time should be enough for an approximation.
According to google maps, the coordinates are:

39.467062 -0.377381

No idea what format that is, I'm familiarized with degree/minutes/seconds but I don't know exactly how to translate those. The time would be:

  March 25, 2015, 09:40 (Spain)

, which would be 08:40 in UTC, correct me if I'm wrong.

Any help would be appreciated, if you can't provide a direct answer, at least point me to some website form or program (Linux preferably) I can use to find this out.

In case someone is wandering, this involves a girl and a tattoo, so please bear in mind this is something I'm going to be carrying for the rest of my life :)

Thanks!

Ahh got it. The coordinates are given as latitude, longitude.

Fortunately, the longitude hardly matters (depending what precision you need), but the latitude does. A star at the zenith will have a declination equal to the latitude on Earth.

As far as Right Ascension goes, well around the March equinox then stars with 12 hours in Right Ascension will reach their highest point (culmination) at "local midnight".

So one would expect roughly speaking for stars at around 21 hours, +40 would be overhead at 9h40m later.

Checking out Stellarium for that location and time it seems that this is pretty much spot on and that Cygnus (the Swan) is overhead, with the bright star Deneb very close to being at Zenith.


In this image, where would the antumbra be? In this image where would the antumbra be?:

There is no antumbra in this image.

An antumbra only exists when the moon is further from the Earth than in that image, so that the moon appears smaller in the sky than the sun.

In that case, when the moon is perfectly aligned with sun, a ring of sun is visible behind the moon. The region in which the ring is visible is called the antumbra.

In the image, the moon is closer to the Earth, so when the moon and sun are aligned, the moon entirely blocks out the sun, and so there is a region of umbra.

That image shows a total eclipse. An antumbra is present only in an annular eclipse.


Does photon capture in a black hole (or any other "very compact" object) reduce the luminosity/photon flux from its self isotropic emission? I don't know if this is even a feasible question or if I have a point in asking so.
Let's say we can model the radiation coming from a very compact object, such as a BH with an accretion disc or any star with proper dimensions (for example, with its disc/gravitational radius within its photon sphere $R_\gamma = 1.5 R_s=3M$), as a black body radiating isotropically. I guess we can do so, since we cannot imagine any good reason for discarding this approximation in them!
What I try to ask is basically if we should modify the formula for blackbody emission or whatsoever bolometric magnitude/luminosity in such an object, given that we already know a radius and an effective temperature (we are using $L = 4\pi \sigma R^2T^4$ here). And also considering that due to the strong curvature some photons emitted in the surface of the disc/atmosphere will be returning inside and will never reach us...
Due to bending and lensing, supposedly we have only a sector from the compact object that will be sending photons outside, cf. MTW pp. 675 or Shapiro & Teukolsky pp. 352:
when measuring the subtended emission angles from a local static observer.
What I'm sure to know and explain with my own words is that the emitted photon flux reduces when we measure it as receivers on Earth, due to this capture.

Let's answer the question from the point of view of a stable neutron star.
The luminosity is
$$L_0 = 4\pi R_0^2 \sigma T_{0}^4$$
in the inertial frame of the neutron star surface (assume a non-spinning object).
For a distant observer, the radius of the neutron star is (e.g. see Haensel 2001)
$$R_{\infty}= R_0 \left(1 - R_s/R_0\right)^{-1/2}\, ,$$
the temperature is
$$T_{\infty} = T_0 \left(1 - R_s/R_0\right)^{1/2}\ ,$$
and the luminosity
$$L_{\infty} = L_0\left(1 - R_s/R_0\right)\ .$$
$R_s$ is the usual Schwarzschild radius.
I think the fact that $L_{\infty} < L_0$ is the effect you are talking about.


What is the real position of the tidal bulge? The wikipage on "tidal accelaeration" has this picture of the tidal bulge:

but says that:

The average tidal bulge is synchronized with the Moon's orbit, and
Earth rotates under this tidal bulge in just over a day. However,
Earth's rotation drags the position of the tidal bulge ahead of the
position directly under the Moon. As a consequence, there exists a
substantial amount of mass in the bulge that is offset from the line

On the other hand, this academical article
states the opposite:

as we move with respect to both tidal bulge and moon, the moon
crosses our meridian before we experience the highest tide.] How
early? Some books show misleading diagrams with the symmetry axis of
the tidal bulges making an angle of 30° or more with the moon. In
fact, the angle is only 3°, so the tides are late by about 24(3/360)60
= 12 minutes

through the centers of Earth and the Moon

Can you say what is the real position of the bulge? it is surely a hard fact not open to interpretations. Do you have access to sites that say where is now the moon and where is the tidal bulge?
According to the basic laws of conservation of momentum and energy, a mass moving to a greater radius should be slowed downm and if we add friction the lag of 12 minutes or more makes sense. If the highest point of the bulge is forward wrt to the vertical of the moon, can you explain what stronger force pushes the water in the direction of the spin?
I found this site that gives the position of sun and moon, but doesn't show tides

This animation, on the other hand, shows that there is no bulge
The red areas experience a rise over 1 m, the blue one a depression of over 1 m:

There is no great contradiction. The offset of the tidal bulge is about 3 degrees. It is exaggerated in diagrams for clarity. The diagram is correct, but not to scale.

This causes the tides to be slightly late. Imagine a person standing on the Earth of the diagram, with the moon directly overhead. The tidal bulge is on their left. The rotation of the Earth will take them towards the left (the moon is also orbiting but its motion is much slower), so a little later (12 min later) they will reach the maximum of the tide. The maximum is delayed by about 12 min.

Actual flows of water around the coast are driven by this tidal bulge, but are complex effects of local topography. The actual flows of water are highly non-linear, including multiple locations at which there is no tide.


Are black holes expected to contain the same ratio of dark matter to regular matter as the rest of the universe? Are black holes expected to contain the same ratio of dark matter to regular matter as the rest of the universe? I've heard that dark matter is distributed in halos around galaxies. Does that make it less likely to be ingested into a black hole?

(Short answer: No, scroll to the last point.)

It is irrelevant to an external observer whether the matter that fell into the black hole was dark matter or baryonic, by the no hair theorem. The only properties of a black hole from our point of view are mass, electric charge and angular momentum. (But of course we don't understand quantum gravity.)
From the point of view of matter which has fallen into the black hole, nothing special happens upon crossing the event horizon. This means that dark matter stays dark and baryonic matter stays baryonic when viewed from inside the black hole.
There is some controversy about how much dark matter exists in the universe. This recent article, for example, indicates in the abstract that more accurate modelling of galactic rotation curves could eliminate a large percentage of the expected non-baryonic dark matter. (Note as @pela indicated in the comments, that this author's papers have not been peer reviewed and could be suspect.) Obviously, the amount of dark matter in the universe would greatly affect the question's answer. I should note that the controversy is mostly composed of a small number of vocal scientists who appear disproportionately in the media. Following the mainstream news science sections, I get the impression that the death of dark matter seems to be announced once a month or so.
The formation of supermassive black holes is poorly understood. One hypothesis is that they may form by successive merger of stellar mass black holes. As there have recently been gravitational wave observations of such mergers, and as candidates for intermediate mass black holes have also been observed recently, I will assume here that this is how they form and that supermassive black holes are therefore made of roughly the same stuff as stellar mass black holes.
Black holes lose most of their mass during the formation process. It is important to always keep in mind whether we are talking about the mass of the stellar core which collapsed to form the black hole (this is often the "mass" of a black hole that is referred to when speaking about e.g. the minimum size black hole that can form from core collapse) or the mass of the black hole as seen by a distant observer after the supernova.
Dark matter particles can not lose much orbital energy by interacting with other matter nor by radiation, therefore will remain in orbit around a black hole rather than falling in, unless they happen by unlikely chance to hit it near the event horizon. This paper indicates that simulated supermassive black holes derive no more than about 10% of their mass from dark matter.

However, it must be said that some scientists suspect that dark matter is made of primordial black holes in the first place. There is also the theory of MACHOs (Massive Compact Halo Objects), that dark matter is composed of large compact bodies such as black holes, but it is believed by most that this theory can not account for the dark matter in the universe.


What is the projection of Earth's axis on the sphere with the Sun in center (see explanation) Seems like it is the most convenient SE community for this question..

Imagine the sphere, witch center is the Sun and radius is, say, ~1.5 distance from Sun to earth (in arbitrary time, precise number doesn't matter much).
EDIT: the center of sphere is 'attached' to the Sun.

Then, imagine, the Earth's axis projection on that sphere in time. It leaves some trace.

What will be the picture? Will it repeat itself through some period? What will be the "scansion" for different time periods?

EDIT: as reply to comments I'll add, that my interest is purely out of curiosity, and degree of required accuracy is low. I don't want high precision here, just rough pattern. As I understand this could be done, using mentioned Milankovitch cycles, but I'm not much into mathematics. Though, if I'll not find answer here, I will be forced (by myself) to figure this out, when i'll get some spare time heh.

It will be two images of the nearly-circular ellipse of the Earth's orbit (one for the north pole, one for the south), distorted by being projected onto the interior of a sphere, which means that the side of each orbit that has the axis tilted "towards" it will appear enlarged and closer to the ecliptic, and the side of each orbit that has the axis tilted "away" from it will be smaller and closer to the pole.

To a first approximation the pattern will retrace itself every year. The Earth's orbit is stable with respect to the Sun and the fixed stars. Over a period of tens of thousands to hundreds of thousands of years, the Earth's orbit will precess (causing the figures to rotate around the sphere), and nutation will also cause small wiggles at a much higher frequency.


Is Dark Matter moving? It sounds like dark matter makes up one quarter of the universe's mass-energy.  And it's density is fairly uniform.  But could it be simultaneously both uniform and moving - like currents in a lake.  Does it have a standing wave?  A propagating wave?  A rotation around some universal central point?  Local rotations around massive objects like low pressure systems on a weather map?

Yes and no. In general, Dark Matter tends to exist in the same places as galaxies. It isn't really distributed throughout all of space uniformly. It has voids and filaments just like normal matter does. In fact, on the whole, normal matter is a pretty good tracer of where Dark Matter is. In some cases Dark Matter can separate from normal matter, and it's believed by some that Dark Matter can clump in areas where normal matter doesn't, but more or less, on a cosmological scale, it is distributed and acts like regular matter. On short timescales, Dark Matter most just sits there. There isn't mass motion or propagating waves. On long timescales and in very large views, you can see actual motion and interesting structures (e.g., Baryonic Acoustic Oscillations - technically we observe these in normal matter, but Dark Matter would be affected in the same way).

To give you an idea of this, check out the Millennium Simulation (that's a YouTube video which moves around and views the results of the simulation). This was a set of simulations of the universe as a whole, in an attempt to test out current theories about our universe and see if those theories could produce a universe that looks like ours. Suffice to say, they were pretty successful. An image of the results of the simulation is shown below. This represents the universe at a very very large scale and you can see all the dark matter and how it is distributed.

You might also want to check out the Aquarius Simulation which was very similar in nature to the Millennium Simulation. However, unlike the video for the Millennium Simulation, this video shows the evolution of the universe. This really gives you an idea of the flow and motion of dark matter over the entire history of the Universe.


Are our 'observations' of or near a black hole affected by gravity induced time dilation? This answer made me wonder if matter/energy/events near a black hole are viewed (by us) in a very different time frame, how might that affect our measurements?

The people on the smaller planet would observe the people on the extremely large planet
moving in "slow motion". (note: due to gravitational time dilation)

On a related note, does light being 'bent' around a black hole experience time dilation or red/blue shift from our viewing perspective? (as possibly suggested by this answer).

The Schwarzschild black hole is the simplest, being just an isolated, nonrotated, uncharged black hole. If $\tau$ is the proper time measured by a clock moving along an arbitrary path in this spacetime, then in Schwarzschild coordinates
$$-\mathrm{d}\tau^2 = -\left(1-\frac{R}{r}\right)\mathrm{d}t^2 + \left(1-\frac{R}{r}\right)^{-1}\mathrm{d}r^2 + r^2\left(\mathrm{d}\theta^2+\sin^2\theta\,\mathrm{d}\varphi^2\right)\text{,}$$
where $R = 2GM/c^2$ is the Schwarzschild radius. If the observer is stationary, then neither the radial nor angular coordinates change ($\mathrm{d}r = \mathrm{d}\theta = \mathrm{d}\varphi = 0$). So, for example, relative the rate of time of stationary observers at some radial coordinates $r_1$ and $r_2$ is just
$$\frac{\mathrm{d}\tau_1}{\mathrm{d}\tau_2} = \frac{\mathrm{d}\tau_1/\mathrm{d}t}{\mathrm{d}\tau_2/\mathrm{d}t} = \sqrt{\frac{1-R/r_1}{1-R/r_2}}\text{.}$$

  how might that affect our measurements?

In the sense of the measurement process, it doesn't, since a measurement apparatus is not time-dilated relative to itself. In the sense of what results we get, then of course it does.

  So it would logically follow that events happening near a black hole (a very large gravitational field) would seem to us to happen much more slowly than if they happened outside that field.

Yes. If the first stationary observer is shifted closer to the horizon ($r_1\to R$), then $(1-R/r_1)\to 0$, so the above time dilation goes to $0$ as well. So, for example, if that observer shines a laser beam toward the other, it will be redshifted when it arrives to the other.

This is really the same thing as time dilation. The laser beam has a certain frequency, so it takes a certain amount of time for one oscillation. Thus a frequency shift is just another way of talking about time dilation, except that time dilation means that this happens to every process, not just light: e.g, if the first observer taps his foot at a certain frequency (say, a tap per second), the second observer will see him tapping at a lower frequency (say, a tap per two seconds). Slowed. We can can still call this "redshift" in the sense that the frequency was shifted in the same way, although the term is usually used specifically for electromagnetic radiation.

  On a related note, does light being 'bent' around a black hole experience time dilation or red/blue shift from our viewing perspective?

Light itself experiences only trivial time dilation in the sense that the proper time along any lightlike trajectory is exactly $0$. Above, we were talking about stationary observers, and light is not stationary. Rather, $\mathrm{d}\tau = 0$ for light beams, always. So strictly speaking, light doesn't experience anything at all.

However, if you mean what the stationary observers would get when measuring light bent around the black hole, then it is blueshifted when coming closer to the black hole and redshifted when moving away--in the sense that this would be what stationary observers closer or farther away would measure. The details of the path it took between the observers do not matter; the ratio of measured frequencies will just be the inverse of the time dilation factor between the observers.


What are the metrics of observation conditions? How do professional astronomers measure and describe (ambient) observation conditions?

I know smog, light contamination from the city lights, fog, clouds, rain/snow, humidity etc all contribute negatively to quality of the image, obscuring the celestial objects more or less, but I'm fairly sure there is a more precise measurements of viewing capability for any given location or time than "good weather and far from cities" or "Kinda can see M42 in Orion if you squint."

I'm not an observer, but there are a couple of quantities I know:

seeing: is a measure of atmospheric turbulence and airmass. Its units are arcseconds and is the measured size of a point source, typically a star, as it appears in the image. In short: atmospheric turbulences shift slightly the position of the star, so if you observed it for long enought, the star will appear blurred. Good places for telescopes have seeing of few arcsecond; Excellent ones reach about 0.6 arcsec. This link can give some more info
fog, cloud, rain, snow: you don't even bother opening the telescope domes as you won't be able to observe much
humidity: I think that part enter in the seeing. If you do just imaging what you get is some extra blurring of the image. If you do spectroscopy you will see a lot of atmospheric lines (due atoms and molecules, e.g. water, in the atmosphere) that you want to remove from the measured spectrum.


Is it possible to have a star orbiting around a brown dwarf? I know a single star is almost surely more massive than a single brown dwarf, but consider a case: a very small star is orbiting a very massive brown dwarf which has a very massive moon, such that the total mass of that brown dwarf and its moon is larger than that of the star. Not counting capturing, is this type of system possible (formed naturally)?

Yes

If a brown dwarf count as a star in this case, the solution is as easy as a smaller brown dwarf orbiting a larger one.

If not, it is still possible if you have two brown dwarfs orbiting close to each other, both just too small to be red dwarfs, together out-massing an orbiting red dwarf, just large enough to be counted as a star.

While I have no example of such a system, the Alpha Centauri system has to closely orbiting stars, with a more distant star orbiting, and Luhman 16 is a system with two brown dwarfs orbiting each other.


Has the velocity of photons coming from black holes already been measured? Has the velocity of photons, which are coming from black holes, already been measured? I am not aware of such measurements.

The reason for the question is the following: If space-time near black holes has some extra dimensions, than photons created in this higher dimensional space-time might have non-zero components of the wave-vector in these extra dimensions. On the earth however we see just our 3 or 4 components of the wave-vector, so we might see just the projection of the original wave-vector onto our (3,1)-dimensional space-time. In this case the velocity of the photons might be less than the standard velocity of light.

I have in mind a special model of our universe. I believe, that our universe is the horizon of a (4,2)-dimensional space-time. On this horizon (which is our universe) we do not see the extra dimensions, however, they should be visible near black holes. So, in this model, black holes are the windows into the extra dimensions.

No.

Photons can't come from the singularity of a black hole, or from beyond the event horizon.
While some very good candidates for black holes exist, none have been certainly observed.
We have never observed Hawking radiation: it remains theoretical.
We have observed radiation from the accretion disk of likely black holes, however the matter in this disc is orbiting in 'normal' spacetime.
Direct measurement of the speed of individual photons is pretty much out of the question. We can measure the speed of light by creating photons at one point and then timing when those photons arrive at another. You can't measure a photon's passing, as to measure it you would have to interact with it. If you interact with it, it is no longer the same photon. This leads to...
When we speak of a photon travelling from one point to another, remember that it is a quantum process: the photon interacts with matter and with itself. Photons are not classical particles. Everything about them needs to be considered through quantum electrodynamics.
The flashes from pulsars are coming from a region of intense gravity. They do propagate at the speed of light: pulsars measured from different locations flash at slightly different times due to light travel time. There is nothing special about the space-time outside of a black hole as compared to that around a neutron-star, only a difference of degree. Gravity is still pretty intense around a neutron star, $g\approx10^{12}\mathrm{ms^-2}$
Gravitational redshift from white dwarfs is well known. Gravitational redshift is predicted by relativity, which assumes a constant speed of light.
Constant speed of light is pretty fundamental to relativity. If a theory breaks this, a lot of other physics is broken (exceptional claims require exceptional evidence)


How many galaxies can be seen in a given patch of sky Similar to the question asked here, but my question is not about what can be seen with the naked eye but rather what might be seen if pointing my Newtonian reflector (full specs outlined below) at an average point in the sky. (This is why my questions is different from the potential duplicate...)

I was doing this last night because it was cloudless and ok seeing, and whenever I point the scope at a given patch of sky I always see a much larger amount of objects (obviously) but how many of these additional dots are galaxies and how many are just nearby red dwarfs or average but more distant stars in the milky way?

Do galaxies ever appear as points of light similar to faint stars, or are they always more diffuse like Andromeda is through binoculars or my telescope? To be clear here, I'm not talking about the galaxies in our local group that are close enough to be discernable as actually fuzzy objects, but rather more distant galaxies that might be very bright (i.e. large elliptical galaxies). I feel as though I'm answering my own question here and that galaxies are only seen as diffuse objects and not points of light, but I can't think of a good way to test this hypothesis with my equipment.

Assume ideal conditions (low light pollution, perfect seeing, etc.) although I am also curious to know how much this changes if the light pollution conditions worsen, particularly in my backyard. For reference, I can somewhat make out the milky way on a cloudless, moonless night. Andromeda is not visible without small binoculars on any night here (or it is and my eyes just aren't trained enough to find it without them).

A better way to phrase my question might be this. Imagine if I were to remove all the stars in the milky way so that the only objects in the night sky were other galaxies. How densely populated with the sky be to the naked eye, and how densely populated would it be through my telescope?

Thanks for taking to the time to read my question(s)! I hope to be able to give my own answers one day.
Clear skies everyone.

Telescope specs:
NexStar 130mm
Aperature: 130mm
Focal Length: 650mm
Eyepiece: 9mm, 1.25" Plossl

Do galaxies ever appear as points of light similar to faint stars

sure, just google for photos

  Telescope specs

it depends on your exposure time

How many galaxies does Hubble see?

A hellova lot!!

https://en.wikipedia.org/wiki/Hubble_Ultra-Deep_Field


What does less than one count from an x-ray detector mean? (Swift BAT detector) I'm reading a paper about a recent x-ray burst from a suspected magnetar (A. Dai et al 2016) where they show a light curve of a burst that lasted about 10ms. (Their figure 1). The value of count rate for the peak of the burst from the graph is $6\text{ counts/s}$ over a timespan of less than $0.01s$. This corresponds to about $6*0.01=0.06 \text{ counts}$ for that time period. What does this mean? Is it not counts of photons? What is 0.06 photons? Is there a step in the data analysis I'm missing?

(Fig. 1 Swift/BAT mask-weighted light curves in different energy bands)

I think there is a missing piece of information. The BAT is a coded mask telescope. The imaging is done by photons passing through a mask and falling onto an array of 32768 detectors. http://swift.gsfc.nasa.gov/about_swift/bat_desc.html

The "mask-weighted" light curve is produced after a complex ray tracing exercise using an estimate of the position of the source. Looking at some of the software specifications (eg https://heasarc.gsfc.nasa.gov/ftools/caldb/help/batmasktaglc.html ) it seems that the count rate is divided by the number of active detectors.

So in this case it looks like the detection rate was more like 200,000 counts/s in the peak 2ms of the event. There would be 400 detected counts, assuming there are no other corrections for sensitivity or vignetting. However,  I think there probably are - the mask must take out about 50% of these counts and there are gaps between the detectors. A background rate has also been subtracted. So overall I guess that the first point in the burst is due to around 100 detected photons, and so on.


If a Jupiter-sized sphere of a translucent liquid were to suddenly appear, what would the inner core consist of? Would the planet refract light? Assuming Jupiter is suddenly replaced, or if a giant sphere of fruit juice or other translucent mixtures/liquids were to appear bearing a similar radius to that of Jupiter, hypothetically , what would be the contents of the inner core? Would the pressure there be so high as to rip the liquids into their constituent atoms? Would light from a Sun shining on this planet be refracted through the planet's translucent body?

A Jupiter made of fruit juice would probably allow sunlight to enter the first few metres below its surface, before being absorbed.  Just think how far light penetrates the water of the ocean. So you wouldn't be able to see the sun through it. It wouldn't act like a lens.
If you magically created it with uniform density, it would be very very far out of equilibium. The central parts would compress and release huge amounts of gravitational energy in the process.  The effect would probably be dramatic.
If you created it with a density and temperature gradient to keep it stable, you'd probably get a mantle of ice and water (including exotic high pressure ices) Depending on your exact temperature profile the water would be supercritical (it is indistinguishable from very high pressure steam, and there is no boiling point). The centre might be supported, at least in part, by electron degeneracy.
And (again depending on the exact parameter you choose) there could be a water vapour atmosphere.
As Rob says in a comment, "I doubt whether anybody has spent their time working out what happens to a Jupiter-mass of fruit juice." But we can look at a slightly smaller planet made out of ammonia and water: Neptune and Uranus are mostly water and ammonia ices, under a thick a hydrogen/helium atmosphere.


Term for the period between planet alignments? Is there an established astronomical term for the period of time between alignments of pairs of planets?  For example, Mars, Earth, and the Sun roughly align ever 780 Earth days.  In my writing, I've variously call this period the 'alignment cycle' or 'conjunction cycle' or 'opposition cycle', but I'd like to know if there's a correct term for this.

The time between oppositions of Mars (or another planet) is called its synodic period. That is the length of time it takes for Mars to be in the same position in Earth's sky relative to the sun, and is about 780 days.

It contrasts with the sidereal period, which is the length of time the planet takes to orbit the sun.

There isn't a fixed number of days between conjunctions, so I don't think this has a name.


Oldest in the universe When the big bang happened and all that exist today came into existence from singularity, how do you determine which part of the universe is the oldest?  Its natural to think that looking back at the center of the universe, we would find the oldest galaxies(not sure if this is true); but isn't there a certain logic that the farthest from the center should have the oldest galaxies?  Is there a proven theory behind where we would find the oldest galaxies? or is this one of those space-time enigmas?

So, I agree with the other answers, that as we look further away from our Galaxy, we see galaxies as they were in the past. Therefore, by and large, these galaxies are younger than ours. That is, we see them as they were when they had an age that could be at most 13.7 billion (the age of the universe) minus their distance in light years.

Therefore to find the oldest objects in the universe, we must look locally. Probably the most ancient galaxies that we can study, are those that are thought to be the building blocks of larger galaxies. These are the "dwarf spheroidal galaxies", several of which have been identified as companions to the Milky Way and M31, and are probably remnants of a larger population of such objects. Their stellar populations appear to consist of mostly intermediate age stars through to extremely ancient stars.

Other ancients objects include the extremely metal-poor stars of the halo population of our Galaxy and a number of very metal poor globular clusters.

Estimates for the ages of such objects are uncertain, but are likely in the range 12-13 billion years old and so only a short time after the big bang.


Nature and mechanism of the short-term variability in radio strength of Sgr A*? The Phys.org article Cosmic filament probes our galaxy's giant black hole links to A Nonthermal Radio Filament Connected to the Galactic Black Hole? which discusses (among other things) the importance of achieving high dynamic range to study these filaments in the proximity of the strong emission associated with Sgr A*. It mentions the fluctuation of radio intensity from Sgr A* on time scales as short as five minutes (as well as to instrumental time-varying effects that also require attention and calibration).

The paper refers to Zhao et al. 2016, which in turn links to Zhao et al. 1999. But so far I can't track down either the nature of the short term variations seen in the radio brightness of Sgr A* nor the mechanism.

What is the nature of this variation (amplitude, power spectrum, any periodicity?) and what are the proposed mechanisms?

Nature of the variation in signal strength
From the paper Unprecedented variability of Sgr A* in NIR mentioned in Bit Chaser's comment, Do et al. record enormous variability in the strength of emissions from SGR A* in the NIR (Near InfraRed) band typically described at about 215 THz to 400 THz.  Here is a figure from their paper:

Here we see enormous variability on the order of tens of minutes.  In the radio spectrum, from Time Variations in the Flux Density of Sgr A* at 230 GHz Detected with ALMA written by Iwata et al., we see much less dramatic variability, without clear periodicity, with events again on the order of tens of minutes.  Here is a similar figure from their paper:

Iwata et al. also perform some Fourier analysis of their signal flux variability which reveals some "not-so-obvious" periodicities on the order of a minute or so.

Brinkerink et al. note that there are also periodic variabilities on longer timescales:

The emission from Sgr A* between frequencies of 20 GHz and 230 GHz
shows flux density variability of a few tens of percent on hour-long
timescales, up to 100% on month-long timescales, as well as occasional
flaring behavior (Dexter et al., 2013). In radio, Sgr A* has an
inverted spectrum (i.e., rising flux density with increasing
frequency) that peaks at the ’submm bump’, around 350 GHz, beyond
which the spectrum steeply drops in the infrared regime.

Proposed Mechanisms for the variation in signal strength
There are several proposed mechanisms for the signal strength variations.  Brinkerink et al. and Yusef-Zadah et al. propose a relativistic colliminated outflow from SGR A* that interacts with gas and dust nearby.  Yusef-Zadah et al. have this nice map from their paper: ALMA and VLA observations of emission from the environment of Sgr A*:

The authors also argue that

millimeter emission is produced by synchrotron emission from
relativistic electrons in equipartition with a ∼1.5mG magnetic field.
The origin of these is unclear but its coexistence with hot gas
supports scenarios in which the gas is produced by the interaction of
winds either from the fast moving S-stars, the photo-evaporation of
low-mass YSO disks or by a jet-driven outflow from Sgr A*.

Brinkerink et al. and Iwata et al. also talk about the synchrotron radiation, which occurs when charged particles are subject to an acceleration perpendicular to their velocity.  We would expect this for any "windy" star with a close periapsis path to the black hole, passing through the massive magnetic field associated with SGR A*.  An example of this cited in some of the literature is the S0-2 star, which has an orbital period of 16 years and the fastest known ballistic orbit, exceeding speeds of 1/60 the speed of light.


Why don't we see the gas behind the black hole? In computer simulations of what a black hole might look like and many artist renderings of black holes, they all seem to show the brightly glowing gas BEHIND the black hole due to gravitational lensing.  In all such images (with a "horizonal" accretion disk) this light appears as a thin band on (the "top" or "bottom" of) the event horizon, that appears more and more intense as it gets closer to the event horizon.

But in the recently released image of the black hole in M87, no such lensing effect is visible.  Rather, it looks more like a hazy version of plain ol'Saturnian rings.  Why is that?

This link (http://rantonels.github.io/starless/) contains several images, and some math on what I was expecting to see.  This animated gif of a black hole from various angles, I think, best shows what I was expecting:

The reason is that your animated gif shows a geometrically thin, optically thick accretion disk, whereas the disk around the M87 black hole is a geometrically thick, but optically thin disk.

The main difference is that you do not see any evidence of the structure of the disk in the picture - it is nearly transparent. You do however see plenty of gravitational lensing - that is what the bright ring is. Many of our sightlines to hot plasma run through a small annulus at the photon sphere (at 1.5 times the Schwarzschild radius) making it bright. Because of lensing, the apparent size of this ring, as viewed from infinity, is enlarged to a radius of 2.6 times the Schwarzschild radius. Contrary to your question, we see photons emitted from all around the black hole in that bright ring.

There are many GRMHD (general relativistic magneto-hydrodynamic) simulations that do agree with what is observed.


Are digital adaptive optics possible? Adaptive optics generally apply slight changes to mirrors to account for atmospheric turbulence. These generally require either a mirror to be broken up into smaller mirrors, each with their own actuator, or for a single large mirror to be slightly deformed by many different actuators.

Is it possible to do much of this digitally?

Couldn't software slightly attenuate or amplify each pixel at each time-slice collected from a passive mirror? These changes would depend on the distortions measured by a guide star or laser (this digital system would still require a guiding light)? I suspect this would probably be less accurate than true adaptive optics, but it would also be far less expensive (as you wouldn't need so many actuators).

Has this approach been studied and dropped? Is it ridiculous and not even theoretically possible? I'm curious if anything has researched this idea.

The problem with your approach is that the deformable mirror changes the phase of the light across the mirror, where the light is not focussed.  The light at the sensor array is focussed, and what you get is the intensity which is, roughly, the Fourier Transform of the phase front at the mirror.   By the time you have intensity, the phase information has been lost.

edit for clarity:

The sensor array measures the intensity, at which point the phase info is irretrievable.  If you remove the sensor array and re-measure past the focal plane, yes, you can get phase info -- see "Plenoptic cameras."

Now, there are other techniques -- you might be interested in searching for papers on "Lucky Imaging" , which basically takes as many images as possible and throws away the distorted ones.


Why is the number of Long Gamma Ray Burst (LGRB) event occurrences greater than the number of Short Gamma Ray Burst (SGRB) event occurrences? I collected  from NASA this histogram plotting the number of GRBs vs the duration of GRBs.

Now my question is, why is the number of Long Gamma Ray Bursts (LGRBs) events occurrences greater than the number of Short Gamma Ray Bursts (SGRBs) event occurences, since on that plot the area covered by the long duration GRBs is greater than short duration GRBs?

Long and short GRBs are thought to arise from different types of event, involving different types of star. Therefore the question you should be asking is why are their event rates so similar?!
Long duration bursts are thought to be produced in the death throes of rapidly, rotating massive stars - the hypernova model. Short GRBs are thought to be produced during kilonova explosions, after the merger of two orbiting binary neutron stars. Short GRBs are less luminous than long GRBs, but they are also systematically closer, and so their observed gamma ray fluxes and hence detectability are similar.
However, these are quite different things, so there is no reason that they should share similar event rates.


Floating Masses in Gas Giants This may be a very stupid question, as I know little about astrophysics, but if solids like Aerogel can float in gasses like sodium hexafluoride, could there be pressures high enough on gas giants to compress a gas to the point where solid particles such as dust collide and create floating landmasses?

Buoyancy, following Archimedes' law, depends on the volume the body and the density of displaced material. The force is
$$ F_{buoy} = \rho_{gas} \cdot V_{body}\cdot g$$
which has to act against the bodies' own weight $F_{weight} = -m_{body} \cdot g$.
Here $\rho$ is density, $V$ is Volume, $g$ the local gravity.

If you're referring to this video, then the aerogel that was created in the 1980's is essentially a high-tech balloon.

There are a few points that need discussion here.

1. Balloons the size of continents don't occur naturally anywhere.
I think we can take this as given.

2. The filling gas
acts to decrease $F_{weight}$ and its pressure must be enough to hold up the body volume. Else the body will collapse and the buoyancy decreases to zero.

A floating continent would need some appropriate filling gas in large quantities, that has lower density than the gas giant atmosphere.
Impossible is a strong word, but this seems very impossible to me. Also for the continent then be able to uphold its structure under its own weight would be very difficult.

3. Particles either float or experience drag.
If they float, they're coupled to the gas flows. Once they would grow larger, they start experiencing gas drag. When this happens, they loose their momentum and are dragged down into the depths of the planet.
They then will contribute to the structure of the deep planet, but are lost to any continent-building process.

So even if your floating continents seem impossible, I think is not a stupid question at all. I might just throw this on our students and test their understanding of buoyancy!


Taking Advantage of Time Dilation According to Einstein's theory of Special Relativity, one's speed in relation to a relatively stationary object would 'slow down' time on the moving object.

With this in mind, would it be possible to travel in a straight line away from Earth (stationary object) in a space craft able to travel at +90% the speed of light for 2 years and return with significant time dilation?

What technologies have we generated that could reach speeds for significant time dilation (+10 years)?

For the fist question it is theoretically possible, and it is explained in the popular Twin Paradox example.

For the second one I'm not sure if there are some advances but as far as I know there is an important problem about relativistic speeds: the mass of the object traveling at such speed increases dramatically to the point it would weight so much it would collapse(It wouldn't collapse actually).


How will "modern equipment" allow the Royal Observatory to now avoid some of the effects of light pollution at Greenwich? The article First Light: a new era for the Royal Observatory says (in part):

25 June 2018
The first modern, research-grade telescopes have just been installed at the Royal Observatory Greenwich, making the institution a working observatory once again after 60 years. Marek Kukula, Brendan Owens and Tom Kerss explain what this will mean for them, researchers, and the public.
"Modern equipment has allowed us to bypass a lot of the disadvantages that drove astronomy away from Greenwich in the middle of the twentieth century."

How exactly will "modern equipment" do this? A blocking filter for the bright yellow sodium line of some streetlights might have some effect, but with high pressure mercury lights and now broadband LED illumination, what other tricks do Greenwich astronomers have up their sleeves?

AMAT’s dual role is aptly matched to its name-sake. Annie Maunder explored the night sky and its phenomena as an astronomer, astrophotographer and science communicator in Greenwich during the 1890s and early 20th Century. She was hired as a human computer at the Royal Observatory in 1891, and though, as a government employee, she was required to give up her position when she married fellow Greenwich astronomer Edward Walter Maunder, she continued to participate in science, publishing books to inform and inspire others and returning to work at the Observatory during the First World War. She was also one of the first women to be elected as a Fellow of the Royal Astronomical Society in 1916. By the end of her life she had gained an international reputation, particularly for her work examining the Sun during eclipses for which she designed her own specialised cameras.

It's a little hard to guess what research can be done there -- Today, Greenwich is close to the worst place on Earth to have an observatory: At sea level, in the light dome of one of the largest cities in the world, in a country famous for its clouds and fogs.
The Wikipedia article on the observatory mentions the new telescope and says:

In 2018 the Annie Maunder Astrographic Telescope (AMAT) was installed at the ROG in Greenwich. AMAT is a cluster of four separate instruments, to be used for astronomical research; it had achieved first light by June 2018:

A 14-inch reflector can take high resolution images of the sun, moon and planets.
An instrument dedicated to observing the sun.
An instrument with interchangeable filters to view distant nebulae at different optical wavelengths.
A general-purpose telescope.

At a guess, they hope to do some sort of star position measurements which is one sort of astronomy that is less affected by haze and light pollution (though it needs steady air -- I have no idea what the air steadiness is like there). But even that is still better done with larger instruments in better locations, and by satellites such as Gaia.
It really sounds like a public outreach facility or an undergraduate training site more than a research facility.  It's also worth noting that the pictures show equipment that is at the serious amateur level -- I certainly know amateurs who have better equipment.


How are J1407b's rings possible? If planetary rings are maintained by staying within the Roche limit, how is it that  J1407b's rings extend so far? Surely the gravity and tidal forces should be low enough to allow a moon to form.

There are a few ways that a gigantic ring system outside the Roche limit might be possible.   Ring systems can extend well past the Roche limit if they are very faint or very young.   A faint/light ring system doesn't have enough mass to coalesce and a young system might not have enough time.   Young small moons with lots internal heat of formation can be volcanically active too, feeding a ring system.

Saturn's E ring is well outside it's Roche limit.

https://upload.wikimedia.org/wikipedia/commons/f/f7/Saturn%27s_Rings_PIA03550.jpg

The most likely explanation for it's enormous and very dense rings, however is that J1407b is very young and a lot of it's ring system will likely to coalesce into moons in time.  The breaks in it's ring system suggests that some moons are already clearing paths in it's rings.

It's mass is also very significant.   While difficult to estimate, it's thought to be about 10-40 Jupiters in mass.  Source.  That would give it a mass in the range of 33 to 130 Saturns and a Roche limit some 3 to 5 times (cube root of the mass), distance to Saturn's center, perhaps 6-10 times the distance from the surface surface.   Because it's ring system is about 200 times the size of Saturn's rings, the mass alone wouldn't explains part of it.    Mostly likely it's a combination of very massive planet/perhaps a brown dwarf star and young plant/star with a satellite system still in formation.

There might also be a very powerful magnetic field at play that helps prevent the ring debris from forming into satellites, but that's just speculation on my part.


What mechanisms limit the top speed of high altitude winds on Venus? According to http://www.universetoday.com/36816/winds-on-venus/, the high altitude winds on Venus travel at about 100 m/s.

I don't think frictional resistance is really what's limiting the speed of the winds. Here's a possible cause of resistance to the winds moving even faster.

Venus has a single convection cell per hemisphere.
The atmosphere cools by adiabatic expansion as it rises at the equator,

then because it's colder than the low altitude air, it emits radiation at a lower rate than the low altitude air
so it heat's by absorbing radiation from the low altitude air faster than it cools by emitting radiation
so the temperature drops less rapidly with altitude than that from adiabatic expansion.

As the air at the equator rises, it cools

but then it's colder than the rest of the air at that altitude
so it resists being accelerated upward by the high pressure of the air under it.

Also because of that resistance, it rises slowly enough that it heats up from absorption fast enough to not drop further in temperature as it goes higher preventing it from exerting a stronger force of resistance to the flow of the convection cells than the driving force for the convection.

I found some recent papers which, if nothing else, discuss apparent mechanisms.
Quoting from Schubert, 2016,

  The large-scale circulation of the upper atmosphere from ~90 to ~200
  km altitude (upper mesosphere and thermosphere1 ) is a combination of
  two distinct flow patterns: (1) a relatively stable
  subsolar-to-antisolar (SS-AS) circulation cell driven by solar (EUV-
  UV) and IR heating, and (2) a highly variable retrograde superrotating
  zonal (RSZ) flow, in part a continuation of the lower-atmosphere RSZ
  flow discussed above

Then, ESA says, in part,

  In 2006, average cloud-top wind speeds between latitudes 50° on either
  side of the equator were clocked at roughly 300 km/h. However,
  detailed cloud tracking studies revealed that these already remarkably
  rapid winds are becoming even faster, increasing to 400 km/h over the
  course of the mission. The reason for this dramatic increase is
  unknown.
  [emphasis mine]

Alternatively,  these guys think they have a working model:

  The atmospheric circulation in Venus is well known to exhibit strong
  super-rotation. However, the atmospheric mechanisms responsible for
  the formation of this super-rotation are still not fully understood.
  In this work, we developed a new Venus general circulation model to
  study the most likely mechanisms driving the atmosphere to the current
  observed circulation. Our model includes a new radiative transfer,
  convection and suitably adapted boundary layer schemes and a dynamical
  core that takes into account the dependence of the heat capacity at
  constant pressure with temperature. The new Venus model is able to
  simulate a super-rotation phenomenon in the cloud region
  quantitatively similar to the one observed. The mechanisms maintaining
  the strong winds in the cloud region were found in the model results
  to be a combination of zonal mean circulation, thermal tides and
  transient waves. In this process, the semi-diurnal tide excited in the
  upper clouds has a key contribution in transporting axial angular
  momentum mainly from the upper atmosphere towards the cloud region.
  The magnitude of the super-rotation in the cloud region is sensitive
  to various radiative parameters such as the amount of solar radiative
  energy absorbed by the surface, which controls the static stability
  near the surface. In this work, we also discuss the main difficulties
  in representing the flow below the cloud base in Venus atmospheric
  models

.


Can we see the stars at the edge of the Milkyway? This question proved much more difficult to formulate than I initially imagined. It concerns how far out in the Milky Way that we actually see with different telescopes. I know the Milky Way disk is about 100k light years wide, its halo a couple of times that wide, and most of the naked eye stars are within 1000 light years. I also know that when many stars are close together they are more visible, which means we can easily see for example the double cluster in Perseus at about 7000 light years with our naked eyes. But if you take a large telescope and peer into the distance, how easily do you see through the star fields until the point where you have no more foreground stars in the field of view, and all that is there are the numerous faint galaxies lurking in the back. For example, in the Hubble Deep field you see a few stars. Are we looking at the last few stars at the edge of the Milky Way, or are there more hidden in the field that we can't see because they are too far away/too faint?

I realize that there are dust clouds in the Milky Way obscuring the view, so if you look towards the galactic center you would definitely not see all the way to the other side. But lets assume we're looking perpendicular to the galactic plane. Lets also say that you're using a 16" telescope, allowing you to see maybe down to the 15th magnitude. Do you see the stars at the edge of the galaxy?

Another way of approaching the question... From what I've read it seems we estimate the Milky Way to contain about 250 billion stars. I also read that the Gaia satellite will map about 1 billion stars. Does that mean our best telescopes are only able to discern 1/250 of the stars in the Milky Way? Of course all stars on the other side the galaxy disk are hidden from us, so they surely account for a large portion of the invisible stars. But looking straight up from the galactic plane, can we see the last galactic stars before the empty intergalactic "void"?

Take a look at the HR-diagram. You have forgotten to take into account that stars come in vastly different luminosities.

For example, we can barely see all red dwarves out to a distance of 10pc, while we know that they vastly outnumber the more luminous stars.

The luminous red giants however we'll be able to see on side of the edge of the disc, but even Gaia will not be able to look through to the other edge. It's limit magnitude is 20, while extinction through dust in the direction of the galactic center is often cited to be up to 100th magnitude.
Hubble deep field galaxies for comparison are around 30th mag.

However, there is Baade's window, a region just above the central galactic plane that has low dust content on our line-of-sight there. We'll be able to spy quite some stars there.

If you're interested in the Gaia mission, you can find information here.


What is the abbreviation for solar time, hour angle of the sun? I understand that the hour angle and declination can be abbreviated as (HA Decl), and rectascension and declination as (RA Decl). The hour angle of a star is of course zero hours at the meridian.

The Sun however does have solar time 12 hours at the meridian, and the "solar" hour angle would better be 12 hours. Therefore I look for an abbreviation for use in computer programming for the pair (solar time, declination) or (sun hour angle, declination), and I can't make up them. I didn't anywhere find them naturally, say in Wikipedia or through Google.

I considered (SOL Decl), (SHA Decl), (ST Decl) or the german (SZ Decl). I don't want to use (HA Decl) for the sun, because HA is zero at the meridian, but as already said for the sun its hour angle should be 12 at the meridian and therefore for clarity and for programming purposes, another designation is needed.

My angles are local, there is no need for a compulsary prefix L- (local) or G- (Greenwich).

I know that in english usage is "apparent" sun hour angle, but in german it would be "true" sun hour angle, therefore the word element "apparent" should be absent.

The question is not about the formulas proper for hour angle or solar time.

Thank you.

Use SHA, solarHourAngle or some variant thereof, as James K said, if your programming language allows it. Longer variable names can be more cumbersome to type and have more possibility of mis-typing the name, but can save a lot of time by providing clarity of purpose when reviewing code at a later date.


Likelihood of a stable system with a dwarf planet's orbit inside that of a gas giant I keep thinking of various planetary system configurations, and would like to know:

What are the fundamental references based on Monte Carlo long-term simulation of planetary systems' evolution that put testable restrictions on the distribution of stable configurations around Sol-like stars (books, papers, software packages)?
How rare is the configuration wherein a dwarf planet (Ceres, I'm looking at you) is located in the inner system?

The main question is the first one, in the vein of "teach a man to fish...".

Related Q&A's here that do not answer my question:

 Is there a upper limit to the number of planets orbiting a star?
 Can you explain the pattern of Hill sphere sizes of the objects of the Solar system?
Could a habitable satellite of a gas giant have a stable subsatellite?

This is just a repository of useful links to software and papers along the way.

NEMO
On Toolboxes and Telescopes , by Hut and Sussman, (1986) in: The Use of Supercomputers in Stellar Dynamics, Springer Verlag, p 193-198.


Is the galaxy made of a nebula or the solar system? My book says on the topic 'Formation of stars' that a galaxy starts forming by the accumulation of hydrogen gas in the form of very large clouds called nebulae. Then local lumps of gas are formed. These lumps start getting even denser and eventually form gaseous bodies called stars.

That seems a fair explanation but in the topic 'Our solar system' my book says "The nebula from which our Solar system is supposed to have been formed, started its collapse and core formation some time 5-5.6 billion years ago and the planets were formed about 4.6 billion years ago."

So, is the nebula being talked about in the topic 'Formation of stars' same as the nebula being talked about in the topic 'Our solar system'?

This question came to my mind because the nebula being talked about in 'formation of stars' topic might be very large, but when talking about our solar system it is eventually formed into the Sun, so how can they talk about the nebula while describing about our solar system, as the solar system is something within our galaxy.

Any picture helping to visualize will be very nice.

No, those are two different nebulae.  There are stars in our galaxy much older than 5.6 billion years.  The problem is that "nebula" is a very generic term, so it can be a whole galaxy or even the remains of a single star.  It's not even clear what is meant by the nebula from which our Sun formed, it could be the giant molecular cloud if our Sun formed in a huge cluster, or it could be something called a "proplyd" (which is much smaller) if the Sun formed in relative isolation.  There are even proplyds that break off from giant molecular clouds in a kind of combined scenario where the GMC forms the proplyd and then the proplyd forms the star.  In any event, it is important to realize that the galaxy formed from a huge nebula some 10 billion years ago, and the Sun formed from a much smaller nebula much later.


Size and Mass of a typical "small" asteroid that impacts the Earth? I'm looking for some realistic data about the size (in meters) and mass (in kg) of a typical small asteroids that impact the Earth, without disintegrating in the atmosphere.

Any suggestion ?

For the asteroid not to break up it needs to either be big, or tough.
You can experiment with the impact effects calculator but you should notice that a rocky asteroid with a diameter of less than about 1km will partially break up on impact with the atmosphere, and there may be multiple craters formed.

If the asteroid is less than about 100m in diameter than the breakup will be complete enough that the resulting meteorites will fall at terminal velocity, and won't form craters in rock.

Iron meteorites are tougher, but even these are likely to break up if they are less than 500m in diameter.

Needless to say, these are not small events, and we have never actually witnessed any object hitting the Earth that is large enough to make it through our atmosphere in one piece.

The direct answer to the question is that there are no typical small impacts of bodies that don't break up. The Barringer "meteor crator" impactor was an iron asteroid, with a diameter of about 40 or 50m, that broke up on impact with the atmosphere, and fell in a circle of about 150m in diameter, forming the 1km diameter crater we see now. The mass of the impactor was about 1 to 2 million tonnes. It hit the atmosphere at about 15 to 17 km/s but by the time it reached Earth, it had slowed to less than 12 km/s. It exploded with the energy of about 10 Megatonnes of TNT. The dispersal of the fragments was not enough to from several craters. See a barringer type impact Or read about the impact


How much of the Universe is invisible to us, and how does it affect our theories? I watched an interview with Neil deGrasse Tyson on The Late Show with Stephen Colbert where he briefly mentioned a disturbing thought. It is something that I have never thought of before, and the idea borders on the eerie.

The idea is this (and here comes a mouthful): If all our knowledge of the Universe is based on observations of distant galaxies (and Milkyway stars), and we accept that the Universe is expanding at such an accelerating rate that at some point in the future only our nearest stars will be visible to whoever inhabits Earth, and that then those future inhabitants will need to explain the Universe based only on what they can observe, then it is possible that we are today trying to explain the Universe based on largely incomplete data.

Like, imagine that there is much more stuff out there, stuff that could help us explain some of the scientific "mysteries" such as dark energy/matter, but it is beyond our horizon. We don't know what is missing from our view.

And so I guess the question is this: How does this affect the validity of our current models of the Universe?

It well might be the case that universe beyond our observable universe is filled with pink flying elephants, but this just seems highly unlikely.

Our current models are usually based on the fact that universe is isotropic, ie there’s nothing special about our observable universe vs everything else there is. Obviously there are smaller and larger deviations, but  on larger scales these are thought to cancel out. This means the simplest position here is to assume all of universe looks more or less the same.

While we can’t currently / no longer observe what is going on on the other side of horizon, there is some evidence supporting above view. Firstly, cosmic microwave background measurements seems to support isotropic universe. Secondly, inflation models seem to provide explanation for isotropic
universe via early causal connection beyond our current observable universe, and as our early universe model is able to explain many other things to great detail, it gives some confidence to this claim as well.

Since early universe models do support the view universe outside our observable universe is pretty similar to what we can see now, this helps us to rely on observable universe for overall model building.

Also, there does not seem to be any large mass concentrations outside the observable universe, since these would be seen as bias for movement of galaxies and galaxy groups, which also supports that the rest of universe is at least very close to what we can observe directly. This gives further confidence that our current models are probably not affected much by things beyond our currently observable universe.

However, since everything outside our observable universe is now causally not connected to us, anything that might be going on over there does and will not affect us - thus it is not necessary nor valid to include any of that in our models either.

To summarize my answer: no, there’s probably no effect on universe beyond the observable horizon that would need to affect our models.


Why do constellation names change (eg from Cancer to Cancri) when used as part of a star's name? Cancri is what to Cancer?  or Leonis Minoris to Leo Minor?   Arietis to Aries ?  or Lyncis to Lynx.

Are they the Flamesteed names, the Latin names, Greek names?
What is their name?

These are the Latin word endings.

Latin nouns change their endings according to how the word is being used. English doesn't have this grammar. But note how the word "I" changes to "my". In Latin you would call "my" the genitive form of "I" The good people at https://latin.stackexchange.com/ can give more explanation.

For example, "Leo" is a third declension noun and its genitive "Of the lion" is Leonis. Third declension nouns have a genitive that ends in -is.  On the other hand "Cancer" is a second declension noun with Genitive "Cancri". A Latin student spends much of their first year learning all these word endings.

When you write "Alpha Cancri" you mean "the alpha star of the Crab", because "of the crab" is cancri.


Is Barnard 68 the only cloud so close to us that there are no stars between us and it? Regarding molecular clouds / absorption nebulae / Bok globules / obscuring clouds,

(I'm not sure if there's one generic term encompassing all those and any similar obscuring clouds - I just used "cloud" in the question title),

is Barnard 68 the only one so close to us that there are no stars between us and it?

If not, what is the list of such?  (Are we observationally confident about this, at this stage?)

I guess a related question would be: "about what is the density of bok globules near us?" ... in a cube of 1000pc on a side, about how many bok globules are we talking? One or two or thousands??

http://sci.esa.int/gaia/60207-barnard-68/

Barnard 68 has a surface area of around 10 square arcminutes on the sky and is at an estimated distance of around 160 pc. The volume of space encompassed by a sightline to Barnard 68 is therefore about 1 pc$^{3}$.

The the density of stars in the solar neighbourhood (hydrogen burning stars with mass above about 0.1$M_{\odot}$) is about 0.1 pc$^{-3}$. So it is not surprising that no stars are seen in front of Barnard 68.

At this distance, the brightness of the faintest stars would be around $V \sim 20$ so ought to have been picked up in various observations that have been performed on this cloud. Such stars would be more obvious in the near-infrared, but then you pick up background stars through the cloud and you would need an estimate of the distance.

I'm not actually aware of any clear statement that there are no faint stars in the foreground, though obviously you can't see any in the widely available visible pictures.

There are lots more Bok globules known which appear to have very little in the way of foreground stars. As I outlined in the calculation above - if they are small and nearby, then that is to be expected. You could also look at the list of "dark nebulae".

Other catalogues include the Lynds catalogue of dark nebulae or the catalogue of southern dark clouds by Hartley et al. (1986).


Can the asteroid belt be seen from Jupiter? Suppose that someone lives in the neighborhood of Jupiter. He may live in a space station around Jupiter, or in any ot it's moons (I believe that it doesn't matter for this question). If this person look to the Sun he can see the asteroid belt, since it is between them? If generally not, then he would be able to see larger bodies such as Ceres?

No. The asteroid Belt is on average about 2.6 AU from the Sun.

Earth is 1 AU from Sun, 1.6 AU from the asteroid belt.

Jupiter is 5.2 from Sun, 2.6 AU from the asteroid belt. I.e. much further away.

Also, it should be easier to see asteroids from inside their orbit since they reflect more sunlight towards us. From Jupiter you would see their shadowed sides. Ceres is to Jupiter what Venus is to Earth. Venus is inside of us just like Ceres is inside of Jupiter. When it is at its nearest, we can only see a thin rim of sunlight on one half of its edge. But when an outer object like Mars is at its closest to us, we see its entire hemisphere lit up by the Sun. Add to that that when Venus is at its closest to us, as when Ceres is at its closets to Jupiter, it is very near to the Sun on the sky and therefor lost in the bright Solar light.

Ceres wasn't discovered until year 1801. That's after the discovery of Uranus 20 AU away. You'd need an even better telescope to find Ceres from Jupiter than from Earth.