Would the (orbitting) speed of the Earth change if suddenly the sun disappear and the Earth is thrown into space? Usually an orbit is considered as an acceleration. So if the Earth has a speed of about 100.000km/h around the sun.
Now I think there is some relativity involved, so from which point of view shall we observe those speeds? But lets imagine that we stand on mars which stays still compared to the Earth and the sun, only the Earth is moving.

Shall we measure a different speed when the Earth is thrown into space? Would it be more of less than 100.000km/h? Or is there something missing?

Velocity is a vector quantity with both a magnitude and direction. The magnitude of velocity is speed. During the Earth's nearly circular orbit, the speed changes little, but the direction of motion changes, and this change of direction is the acceleration. The Earth is always accelerating towards the Sun.

When we speak of velocity one should mention the frame of reference: we measure the velocity of the Earth wrt the the Sun. The sun is therefore by definition "at rest" (we should actually use the centre of mass of the Earth-Sun system, an also include the gravitational effect of the other planets, I pass over these details) This frame approximates an inertial frame of reference

Mars doesn't stand still relative to either the sun or the Earth. We can use a frame of reference in which Mars is at rest, but this is then a non-inertial frame, it rotates, so in this frame both the Earth and Sun are moving, and accelerating due to the non-inertal motion of the frame.

If we (magically) remove the sun (but not the frame of reference in which the sun is at rest) then there will be no force on the Earth, and it will move in a straight line at constant velocity, relative to the frame in which the sun was at rest. It would move at the same speed as its orbit, about 100,000 km/h (in that frame of reference)


Could the Sun's magnetosphere block extraterrestrial contact? I was reading how certain strong magnetic fields can block or alter radio waves so bearing this in mind would it be possible for the Sun's magnetosphere to block attempts to establish radio contact from other solar systems if the signal is weak enough?

The Sun's magnetosphere is too weak to have a significant effect on extra solar radio waves. If it did, we wouldn't have any radio telescopes as it would block the signals they detect.


In planetary and astronomical science, what exactly is, or is not, a tidal force? I'm wondering exactly in which situations forces between bodies are, and are not consider to be tidal forces in the context of planetary and astronomical science.

If two rigid, non-deformable spherical masses with shperically symmetric mass distributions orbit each other,  can we say there are no tidal forces or tidal effects there?

If one of them has a permanent, static deformation (e.g. quadrupole moments and higher) are there tidal forces then? If both do?

Or are dynamic, induced deformations required before we invoke the "T-word"?

Can tidal forces be radial, such that they do not tend to raise or lower an orbit over time, or must they have tangential components, like the one that is slowly raising the Moon's orbit?

Is it possible to draw a clear line here, at least within the scope of this SE site and This month's focus tag which is tidal-forces?

According to that tag's Tag Info:

  There is no usage guidance for this tag … yet!

Slightly related: Constraints on the mass distribution within each body such that their mutual orbits are Keplerian?

The "tidal force" is simply a manifestation of good old inverse-square-law gravity.  It doesn't need a rigid body to be present to manifest.

Consider a giant circular ring of dozens of space stations established in interstellar space. They're far away from any star and just sit there happily doing whatever it was they were set up to do.  Suddenly, a star sweeps in from stage left (perhaps a fast-moving star created by a supernova-disrupted close binary system).  What happens to the ring of stations?

They are attracted towards the rogue star, of course and start moving noticeably towards it when it gets close enough.  They accelerate towards it pulled by the force of the star's gravity, which follows the inverse square law. But they don't all accelerate at the same rate. The station on the edge of the circle closest to the star accelerates fastest, stations half-way around the circle accelerate, but not as much, and the station on the far side of the circle accelerates least.

Soon the circle becomes an oval with its long axis pointing towards the star.  This is the tidal effect.  Let's say that the rogue star was dark -- a neutron star, perhaps, which had been stripped of its accretion disk by the explosion.  What would the people on the stations observe? They wouldn't notice the acceleration per se because they're in free fall.  Unless they were doing doppler spectroscopy of distant stars, they wouldn't detect their velocity (at least not at first), but they would see their neat ring of stations turning into an oval. From their point of view, some force was tugging on them and dragging them out of alignment. That's the tidal force.

From the point of view of the omniscient observer (us!) there are no tides, there's just simple gravity.  But from the point of vie of the freely falling observers in the gravity field, most of the effects of gravity disappears and what's left looks like a force.

Wikipedia has an article on the tidal force which gives a mathematical treatment. (Personally, I don't think the article is terribly clear, though it appears to be correct.)


What is the shape of Universe? What we can assume by the shape of universe. As I know when we talk about universe we talk about multi dimension structure but that structure should have some shape. I wonder that if we look the Universe from a far distance, how it will look like?
I know people will tell that its infinite we can't go outside universe and see, but assume if its true that universe has some end and its too far and out of our reach due to our limitation but exists.
I feel that its a kind of myth or assumption that universe is infinite everything is finite in this universe.
Do you remember in early times around 1000 years before humans used to think that earth is flat and infinite?
But after many years we reach to conclusion that earth is round.
Don't you think we are misunderstanding universe the same way we did before for Earth?

When we talk about the "shape" of the universe. It is its intrinsic shape, not the embedding of the universe within a larger space.
It does make sense to talk about the intrinsic shape.  Properties like "curvature" can be measured from inside a shape, you don't need suppose that the universe is "in" something to talk about its shape.
Now the space of the universe is three dimensional, this creates visualisation problems.  So I'm going to first talk about an imaginary 2d universe:
A 2d universe could be flat, with a curved boundary (like a disc)  Or it could be flat with no boundary (an flat infinte plane).  Or a 2d universe could be intrinsically curved.  If it has positive curvature it could be like the surface of a ball (positive curvature, finite with no boundary) Or shaped like a bowl (positive curvature)  Alternatively it could have negative curvature: it could be shaped like a "Pringles crisp"  These have negative curvature (this can happen with or without a boundary). Finally there could be regions with different curvature: positive in some parts, negative in others.
The same possibilities exist for our universe:  It could be flat (with or without boundary)  It could be positively curved (This is intrinsic curvature and your brain can't visualise this for a 3d shape)  Or it could be negatively curved (again, don't try to visualise this for a 3d shape, you can't).  In the case of positive curvature, it could be finite and unbounded, but for negative curvature, it must either have an edge, or be infinite.
So mathematically these are the possibilities. What does the science say?
We can measure the curvature on large scales.  Our measurements are not perfect, there is some room for error.  We measure the large scale mass/energy density of the universe, since curvature is caused by gravity, and gravity is caused by mass and energy. If the mass/energy density is > 1 then space would be positively curved.  We actually find that the energy density is 1.00±0.02  That is the universe is either flat, or very nearly flat.
No there are profound problems with supposing that the universe has a boundary. But there are also problems with supposing it to be infinite.  Scientifically, no edge has ever been detected. So when we build models of the universe, we will generally suppose to have no edge.
Thus our "best guess" is that the universe is uncurved and infinite.


How many science instruments can be used in parallel with the Hubble Space Telescope? The Hubble Space Telescope offers a parallel obervation mode. Observing programmes such as CANDELS and CLASH use WFC3 and ACS in parallel fields. These are also the only instruments of which I have heard that they are being used in parallel. Here are my questions:

Is it possible to observe with more than two instruments in parallel? If so, what is the limit, and why?

Why aren't all observations done with as many instruments as possible turned on?  Granted, a lot of the data would be useless / difficult to analyze (like spectra of a random patch of sky), but on the other hand it might lead to serendipitous discoveries. HST won't last forever, so why not get the most use possible out of it?

Note that this is about the science instruments - I assume the guidance sensors will be working alongside the others anyway. (Or is that wrong?)

There are two types of parallel observations: Coordinated Parallel and Pure Parallel.

Coordinated Parallel means part of the same proposal, so it has to have the same scientific justification: you have to come up with a good case for why the different observations would be useful for the same project. (This is case with CANDELS.)

Pure Parallel means something independent of the main observation, which I think is more what you're asking about. The proposal guidelines currently note that these are allowed only when the primary observation is spectroscopy (with the COS or STIS instruments).

One factor is that you need some specification for how you use a given instrument: what filter, what exposure time, what spectroscopic grating, etc. This requires that someone consider a plausible science case for what are effectively random pointings. (And you need to make sure that you're not accidentally going to try taking a long exposure of Vega, which might actually damage an instrument.)

And, yes, I gather there are data-storage and overhead limits, as well as the idea that running the data transmitters at maximum may shorten their lives. From the WFC3 Instrument Handbook (e.g., here):

  The primary restriction on parallel observations, both coordinated and
  pure, is that they must not interfere with the primary observations:
  they may not cause the primary observations to be shortened; and they
  must not cause the stored-command capacity and data-volume limits to
  be exceeded.

and

  In order to prolong the life of the HST transmitters, the number of
  parallels acquired during each proposal cycle is limited. Proposers
  must provide clear and strong justification in order to be granted
  parallel observing time.

(I think the practical answer to your "how many instruments" is two, though I haven't seen an explicit statement to that effect. This page includes specific limitations for individual instruments -- e.g., which ones can be used in Pure Parallel mode and with what other instruments.)


How do they know the non-uniformities seen on the disk of π¹ Gru are real and not artifacts? @RobJeffries' answer to What is this web on the surface of the Sun? explains why convection cells on some other stars can be far larger than they are on our Sun and includes a near-infrared VLT interferometric image of π¹ Gru from Paladini et al. 2017 as an example.

The paper says:

  For this purpose, we use the image reconstruction software SQUEEZE14, based on a Markov chain Monte Carlo (MCMC) approach to the regularized maximum likelihood problem. To assess the reliability of the image we also use a different image reconstruction algorithm, MiRa15.

Of course it's likely that the reconstruction is correct, but I'm still wondering how the possibility that the non-uniformities have arisen through some combination of instrumental and algorithmic effects have been minimized.

Question: How do they know the non-uniformities seen on the disk of π¹ Gru are real and not artifacts? For example, are there similarly generated images of stellar disks of similar angular sizes not expected to have such large convection cells that show more uniform disks?

 click for full size

The paragraph you quote continues...

  The images from SQUEEZE and MiRa have very similar characteristics and are shown in Fig. 1 (see also the Image Reconstruction section in Methods)

In that section various tests are described including the reconstruction of simulated data, with similar interferometric coverage and signal to noise, from spotted and spotless (uniform) stars with limb darkening.


Does spacetime return to being flat? I wrote an experiment in HTML5 and Javascript to try to show what is happening to the curvature of spacetime as a body moves through a region.

As time progresses, I'm using the g=Gm/r2 equation to move the spacetime grid toward the object. When a grid point is within the body, I use g(1-h/R) to modify the bending according to the acceleration at the surface. This assumes uniform density.

The result is that it takes a very large mass and very long time for any sort of noticeable curvature to occur (as you'd expect). By decreasing the Earth's radius by a factor of 3, I was able to see the curvature very well as in the screenshot below.

What occurred to me then is that the the warping of space would remain long after the body of mass had moved on.

Does this warping in space ever return to truly "flat" or is the fact that we almost never orbit through the same absolute region of space mean we never experience this existing warping?

If light went through this "wake" would it bend according to the spacetime curvature created by the long distant planet? Or have I gone wrong in my model?

This demo uses real values except that the radius is shrunk by 3x to increase its gravitation effect.

earth.radius = 6.371e6 / 3;
earth.mass = 5.972e24;
spacetimeGrid.extent = 4.5e7;

You can run the demo here: https://dl.dropboxusercontent.com/u/2236585/spacetime/index.html

To answer the question

  Does spacetime return to being flat?

Yes it does. The picture you have calculated is not correct. It is not like that in gravity a solid body plow through spacetime and creates this valley like dragging a bowling ball throw snow would. The curvature of each point is affected by the mass and changes as the mass moves. So if the mass moves away far enough from a given point in space time the curvature becomes flat again.


Which elements are an indication of habitable exoplanets? Using spectroscopy the chemical composition of exoplanets atmosphere is determined. As a scientist, what spectral lines would you look for?

Which elements are relevant in pursue of a second earth?

While not strictly relevant to your question, I'm very much looking forward to what the James Webb Telescope might tell us about exo-planet atmospheres.   That's probably the thing I'm most looking forward to in astronomy discoveries over the next 5-10 years.

The one thing I've heard that they are looking for specifically is a combination of Oxygen (which implies Photosynthesis) and CH4 which, in combination with oxygen implies bacterial digestion of plant matter (aka cow farts).   Those 2 things together are of particular interest because CH4 and O2 tend to interact with each other, so, if not regularly produced they aren't likely to both be present, at least, in measurable amounts.   Other factors like plate tectonics, Methane escaping from the ground or under the oceans, comet impacts and perhaps even lightning might make such readings a little bit unreliable, but as I understand it, that combo, CH4 and O2 is the big one to look for.

Other interesting things might be a CO2/O2 ratio, also implying photosynthesis and perhaps telling us something about plate tectonics or O2 respiration.   Variation in H20 content which could tell us something about weather, if the planet has freezing/thawing periods like Mars or vast deserts and few oceans and little water or occasional hurricanes.   Dust particles could tell us something about the soil or volcanic activity.  I would think NH3 content in abundance might be an indicator of little or no life, but I'm not positive there.  Water with ammonia oceans might be possible and still have life, though we might find that not a good planet to live on.

Finally habitable is a term that needs a bit of exploration.  Lets use the Earth as an example some 2 billion years ago, there was Oxygen in the atmosphere, there was CO2 in the atmosphere, there were oceans, there was a magnetic field, there was even an ozone layer, so it had everything Knu8 listed, but, there were no trees, there wasn't even any soil.  There might have been a thin microbial layer living on the rocky surface or, if too hot, perhaps in caves.  There were no fish, there wasn't even any good plants or krill like things growing in the oceans for fish to eat.  It was mostly one celled life mostly just in the oceans and perhaps lakes.  Earth 2 billion years ago might not be our first choice for habitation.  It was also prone to photosynthesis driven snowball earth periods as well as having no soil and no trees.

There's also the question as to whether we'd want to inhabit an "earth-like" planet.  Bacteria can be pretty hostile and if there's life on the planet, it might not be safe to settle, and there would also be ethical questions.   If there's no life, then the Ethics isn't an issue, but there's likely a lot more terraforming.   It's all relatively moot given the difficulty of that kind of space travel, but "habitable" is a more difficult question than it first appears.   Our technology might be able to make a dead planet habitable over time, where as a planet with life on it might appear more habitable but could pose unforeseen problems.  I'm not sure which would be more ideal.

I'm not an expert in this, but what to look for in exoplanet atmospheres for habitability is a little complicated and I've only touched on it briefly.   Fun thing to think about though.


Quesions about extinction power law I am curious to know If dust extinction were to vary as a power-law so that $A_{\lambda}$ proportionate to $\lambda^{\beta}$ , what value of beta
would give rise to Rv =3.1 which is the average in diffuse ISM. I can see a plot but I wanted to see how to derive Rv=3.1 mathematically

The value is $\beta \simeq -1$ (e.g. see  Cardelli et al. 1988). The extinction would not be well fitted by a power law, except over a very narrow wavelength range.


Stars in constellation patterns by catalogue number? I need a resource that can tell me the stars that make up a constellation (pattern), preferably by Henry Draper or some catalogue number? All i can find is lists of every star in a constellation (area) or incomplete lists of stars by name.

If this doesn't exist can you suggest a way to work it out myself?

If you visit https://github.com/Stellarium/stellarium/tree/master/skycultures you see several sky cultures that include various constellation shapes (you also get this directory when you download stellarium).

As an example, lets look at https://github.com/Stellarium/stellarium/tree/master/skycultures/western_rey, in particular the file https://github.com/Stellarium/stellarium/blob/master/skycultures/western_rey/constellationship.fab and line number 127 which reads:

Aql 10 98036 97649 97649 97278 97278 96229 96229 95501 95501 97804 99473 97804 95501 93747 93747 93244 95501 93805 93805 93429

The "Aql" stands for the for the constellation Aquila, the Eagle (https://github.com/Stellarium/stellarium/blob/master/skycultures/western_rey/constellation_names.eng.fab verifies this)

The "10" means there are 10 lines that make up the markings for Aquila. Note that these lines are specific to the H.A. Rey skyculture. Different cultures will have different lines.

The next 20 number represent the lines themselves. For example "98036 97649" is a line from star 98036 to star 97649, and "97649 97278" is a line from star 97649 to star 97278. Since these two lines share star 97649, they are connected, but that's not always the case with adjacent lines. For example "95501 97804 99473 97804" represents a line from star 95501 to star 97804 and another unconnected line from star 99473 to star 97804.

The next question: what exactly IS star 98036?

If you look at https://github.com/Stellarium/stellarium/blob/master/stars/default/name.fab you'll see it's Beta Aquilae, but not all stars have names in this file, and the name isn't particularly helpful in finding the star itself.

Stellarium itself stores this data in the "catalog files" at https://github.com/Stellarium/stellarium/tree/master/stars/default with the format described at http://stellarium.sourceforge.net/wiki/index.php/Star_Catalogue_Format

However, this can be ugly. Since Stellarium uses the standard Hipparcos catalog, you can visit ftp://cdsarc.u-strasbg.fr/pub/cats/I/239/ and download "hip_main.dat.gz". The format is described at http://cdsarc.u-strasbg.fr/viz-bin/Cat?I/239

The line relating to star 98036 starts:

H|       98036| |19 55 18.77|+06 24 28.6| 3.71|...

which tells us star 98036 is at J2000 right ascension 19h55m18.77s, declination +06 degrees, 24'28.6" declination, and has a magnitude of 3.71. This agrees almost perfectly with https://en.wikipedia.org/wiki/Beta_Aquilae (the declination is slightly off, but this may be because Beta Aquilae is a double star, and Hipparcos references the brighter star and Wikipedia references the midpoint of the double star-- or something).

There are other files in the stellarium github (eg, culture-specific star names) that also use the Hipparcos numbering designation.


Can we create neutron star by pressing earth? Or other objects? Everywhere I can hear, that if we press anything to the right size, we create black hole.

Can we create neutron star by pressing, for example, our planet? And what if we press something bigger, like Jupiter?

It depends on what you mean by "can".

If you want to make the Earth into a black hole, you would have to compress it down to about the size of a marble.  That is impossible.  That is not just impossible with the technology of today, that is impossible with anything less than magic.

However, if we did make an Earth-mass black hole, it would be stable. (Except to Hawking radiation)

Now if you could somehow compress the Earth into a slightly smaller space, you would need to overcome electron-degeneracy and at some point before making your black hole you would form neutron matter. This is impossible. Nothing can do this, not now and not ever.

An Earth-mass piece of neutron matter, however, would not be stable. Its own gravity is not sufficient to keep it from expanding, so it would very very rapidly explode. The same with Jupiter. A body needs to be larger than about 15% of the mass of the sun to have enough gravity as a neutron star to be stable. Jupiter has about 0.1% of the sun's mass

We can't create a small neutron star by compressing the Earth, unless we use magic, and if we stop squeezing it would explode.


Gravity of a gaseous planet without a core Both Jupiter and Saturn have rocky cores.   Is there such of a thing as a gaseous planet without a core?   And would a planet without a core have gravity?

The gravitational force on a small mass on the outside of a planet is always the Newtonian $$F_{G}=-\frac{GM}{r^2},$$ so any planet, and particularly, any mass in the universe produces a gravitational field acting on everything else. So if, for example, the mass is $M=2\times 10^{27}\rm kg$ (i.e. one Jovian mass), then the gravity field outside the planet will always be the same (apart from Tides, higher order moments), no matter whether the mass is in Hydrogen or solids.
For the gas giants Jupiter and Saturn in our solar system, the mass in heavy refractories (i.e. everything heavier than Helium) is about $M_{\rm ref}\approx 15-20 \rm m_{\oplus}$, where $\rm m_{\oplus}$ is an Earth mass.
The rest of $M$ is hydrogen/helium. For Jupiter this is 300 $\rm m_{\oplus}$, Saturn about $75 \rm m_{\oplus}$.
This is a relatively large number of refractories in those gas giants, compared to solar composition, which is why we think that they have been formed via core-accretion, see Pollack (1996).
However there is another idea of how to form gas giants out there, which is that of gravitational disc instability, see Boss (2002). This idea posits that at very massive protostellar discs, which form planets, can become unstable and fragment into large clumps, which form gas giants directly. Those disc instability giant planets would have solar metallicity, i.e. a Jupiter-mass planet would have a refractory mass of only $M_{\rm ref} \approx 3 \rm m_{\oplus}$.
Those refractories would presumably sink to the planetary center and form a small core. Exoplanets that were found at large semi-major axis distances (hundreds of AU, compared to the Jovian 5 AU) from their stars, such as YSES 2b, are candidates for such disc instability models, and hence would host such small core. But that is as small of a core as it gets, you cannot have a core much much less massive than this.


Looking for help in understanding how black holes can move I'm looking for help in understanding how black holes can move through space. This is probably a trivial question for anyone who knows astronomy, but probably due to my lack of education on the topic, I can't come up with an answer or visualization on my own.

Specifically, my confusion is as follows:

I saw Dr. Kip Thorne in a documentary on black holes state that there is no longer any matter in a black hole once it forms. He said something like, the matter was there, but it has been crushed out of existence and basically has been transformed into the energy that produces such a great curvature of spacetime.
From that description, it seems to me that within the event horizon of a black hole it is simply this enormous "dent" in spacetime itself that is moving around.
I also saw Dr. Andrew Hamilton in another documentary state that within the event horizon of a black hole (again paraphrasing), space is falling toward the singularity so fast that it effectively drags everything with it (including light).
Based on 1 (no matter remains in the black hole) and 2 (space is being pulled towards the gravitational singularity from all directions), I try to picture the black hole moving, and it seems to me that if, say, the black hole moves in one direction, I can see how its event horizon is also moving, bringing that bit of space into it. But how can the other end of the black hole let space "escape" in order to complete the motion?

I am pretty sure I am just missing something here, because as I have learned, black holes have been found in galaxies and solar systems and many other places where they have to be moving. But I want to understand why it is that space itself can cross the event horizon when other things can't. And along those lines, it seems amazing to me that a black hole could be in essence a kind of scar in spacetime that can also move through it.

Thanks for any help and sorry for the wordy question.

UPDATE: Thanks for the answers so far. I hope someday I'll be able to comprehend them fully. :-)

I would like to add some further context that better illustrates my question.

If you go to the video here and watch the first minute or so (up to around 31:00)

http://youtu.be/Niurc_6xIYg?t=29m48s

you can see Andrew Hamilton explaining what I have a question about along with an animation to illustrate the description.

On the one hand, we have spacetime curved in such a way that the black hole structure remains intact - that is, there is a constant event horizon generated by that extreme curvature. But on the other hand, Dr. Hamilton describes a flow of spacetime between the event horizon and the inner horizon, and he says it is plausible that within that area, space itself (which has no substance, in his words) can be described as flowing in and being flung back out.

I don't understand how space can have these two opposing properties - (1) a curvature that is maintained in order for the black hole to exist, that is, a curvature generated because matter can interact with the space and (2) a flow of space that sounds like it is not subject to a restriction of crossing back out of the event horizon.

It's that apparent difference between (1) and (2) that I want to understand better.

Thanks!

I saw Dr. Kip Thorne in a documentary on black holes state that there is no longer any matter in a black hole once it forms. He said something like, the matter was there, but it has been crushed out of existence and basically has been transformed into the energy that produces such a great curvature of spacetime.

Isolated black holes are indeed vacuum solutions to general relativity. Thus in particular, the mass and energy density is identically zero everywhere in spacetime. Because of some technical issues, this does not imply that black holes have no energy or mass; rather, it means that we can't directly think of them as an integral of mass or energy density. See also this question.

  I also saw Dr. Andrew Hamilton in another documentary state that within the event horizon of a black hole (again paraphrasing), space is falling toward the singularity so fast that it effectively drags everything with it (including light).

For an isolated black hole in a particular frame field, this is a valid picture. For example, the Schwarzschild spacetime in Gullstrand–Painlevé coordinates can be interpreted as Euclidean space falling into the black hole. However, the spacetime is itself is stationary (even static) in the geometrical sense—there is a timelike field representing a direction in which spacetime geometry is left unchanged (this corresponds to Schwarzschild time, in fact). Similarly, rotating black hole spacetimes are stationary.

But generally speaking, you really can't think of black holes as some sort of suck-holes for space. It's just an analogy that applies to simple situations (isolated black holes, with nothing else in spacetime) and then only in a particular frame or coordinates. That's not to say it's useless--e.g., acoustic horizons in fluids are interesting analogies to event horizon--but don't take it too literally. See also this question.

  Based on 1 (no matter remains in the black hole) and 2 (space is being pulled towards the gravitational singularity from all directions), I try to picture the black hole moving, and it seems to me that if, say, the black hole moves in one direction, I can see how its event horizon is also moving, bringing that bit of space into it.

So what's the problem?

  But how can the other end of the black hole let space "escape" in order to complete the motion?

For realistic black holes, there is no "other end". If you really want to tie everything together, then interpret Dr. Thorne's comment about stuff falling into the singularity as crushed out of existence as also applying to space. It doesn't come out of any "other end". It stops existing.

But really, that's past the point where the fluid analogy make sense anyway. I think Dr. Hamilton might note that the full maximally-extended Schwarzschild spacetime is a black hole, so one may be able to think as it "coming out" of the corresponding white hole, but he would also tell you that this white hole is a mathematical artifact that doesn't have anything to do with actual astrophysical black holes.

  But I want to understand why it is that space itself can cross the event horizon when other things can't.

Of course they can. In the infalling-fluid analogy, they're carried along by the space that's being sucked in. Perhaps you're thinking of the fact that it takes an infinite Schwarzschild time for an object to reach the event horizon. But that doesn't imply that things can't cross the horizon; rather, it's just a symptom of Schwarzschild coordinate chart not covering the horizon.

Intuitively, think of a coordinate chart as a "grid" drawn on a patch of spacetime. That patch may be the entire spacetime, or it might be just a piece of it. In the case of the usual Schwarzschild coordinates, it's just a piece... one that simply doesn't cover the horizon.

  Regarding the last part, I meant space crossing the event horizon the other way - out of the sphere demarcating the event horizon - whereas material objects go one way - in but not out, spacetime itself does not seem so restricted, at least not in the sense of "pinning" a black hole to one part of itself.

OK, your edited question provides some more context. But the answer is completely the same: what Dr. Hamilton is talking about in the video is the maximal analytic extensions of the rotating black hole solution, which does contain a passage into a completely different region (sometimes called different 'universe'). This is very much analogous to the maximally extended Schwarzschild spacetime, which contains a wormhole that comes back out of a white hole in another spacetime region, except that the extension of the rotating Kerr black hole spacetime contains an infinite chain of such connected regions.

I haven't watched the entire video, but it's clear that's what they're talking about when they say "in a ship propelled by pure mathematics", because the causal structure of such spacetimes is well-known. Once again, I direct your attention to Dr. Hamilton's own page, linked above, that explains that this dual waterfall picture is an artifact of mathematical idealization and not something that actually happens in reality.

However, even if you take the maximal analytic extension of such black hole spacetimes overly seriously, it's important to emphasize that you don't come back out into the same region, but rather a different one connected by a wormhole inside the horizon. If you're up to it, I also recommend Dr. Hamilton's conformal diagrams of said black hole spacetimes, which make it quite clear that's what going on is a whole "chain" of black holes and white holes.


What part of the milky way do we see from earth? When looking up at the night sky, we can sometimes observe a bright band of stars stretching across it. I know this is our galaxy but exactly what are we looking at?

Could we look at the centre of the galaxy or is there another spiral arm blocking our view?
How does our view change as earth rotates around the sun? (E.g. What part of the milky way do we see in a winter night sky)

Fraser Cain:

  We’re seeing the galaxy edge on, from the inside, and so we see the
  galactic disk as a band that forms a complete circle around the sky.

  Which parts you can see depend on your location on Earth and the time
  of year, but you can always see some part of the disk.

  The galactic core of the Milky Way is located in the constellation
  Sagittarius, which is located to the South of me in Canada, and only
  really visible during the Summer. In really faint skies, the Milky Way
  is clearly thicker and brighter in that region.

If you want to know more, watch this video from Fraser Cain which explains it in details:

https://www.youtube.com/watch?v=pdFWbEwsOmA

You can find your answer in the first minute


Umbra, penumbra and antumbra I'm trying to understand following schema of the wikipedia page here.

Image subtitle is "Umbra, penumbra, and antumbra, for a lunar eclipse".

Distances and sizes are not to scale.

If distance Earth-Sun is about 150 Gm (gigameters) and Sun diameter is 1.4 Gm, $\arctan 1.4/150 = 0.57º$ (degrees). Thus, the Sun is seen from Earth with an angle of approx. 0.57º (0.01 radians). The Blue and red lines in schema should be near to parallel.

That means the point where umbra and antumbra touch is (being 12 Mm the approx diameter of Earth) at a distance of about $\frac{150}{1.4\times0.012}=1.3 \mathrm{Gm}$ from Earth.

As distance Earth-Moon is only 0.400 Gm, it seems impossible that Moon was never in the antumbra zone of the Earth.

I'm doing a basic error somewhere, but I do not know where.

The calculations seem reasonable. The conclusion is wrong.

The Earths antumbra starts 1.3 Gm from Earth. At less than 1.3 Gm there is umbra, penumbra and non-shadow. The moon is 0.4 Gm from Earth.

$$0.4 < 1.3$$

Hence it is impossible for the moon to be in the Earth's antumbra.

However, the Moon's antumbra forms at almost exactly one lunar distance. We notice that the moon and the sun are almost exactly the same size in the sky. When the moon is further from the Earth, an annular eclipse can form, as a small part of the Earth will then be in the moon's antumbra.

I've made a scale model of the sun-earth-moon shadows, at https://ggbm.at/CbbvmBFq


How can you find the distance to a galaxy using right ascension and declination? For example, Galaxy NGC 3689 has the right ascension of 11:28:11 and declination of +25:39:40.0. Using these values, how can we find the distance to the galaxy? If it helps, its radial velocity is 2727 km/s.

You can't use the RA and dec.  These tell you the direction that the galaxy is in, relative to the stars.  It is possible and normal for two galaxies to have the same (or very similar) RA and dec and be at completely different distance.
You can estimate the distance using the Hubble law $v= H_0 d$, if $v$ is measured in km/s and d is in mega-parsecs the value of H_0 is approximately 70 (measuring this value accurately is a major problem in cosmology)
So $D= 2727/70 \approx 39$ So we estimate the distance to the galaxy to be 39 million parsecs  (multiply by 3.26 to convert to millions of lightyears).  This can only be an estimate, as the actual value of $H_0$ is not know with certainty, and the galaxy can have particular motion relative to the Hubble flow (for example, Andromeda is actually blue shifted, but it isn't a negative distance from us, as this value would suggest!)


seasons elongating and shortening I have heard of years with longer summers and winters and shorter springs and falls and vice versa because of a change in the earths axis.

Is this from the procession that causes the season switch every 25,800 years or is it from some other change in the earth's axis like angle change and not revolution at the same angle(which I think the procession is)?

You seem to have two types of seasons confused.  Astronomical seasons are relatively stable in their durations, while seasons as defined by the weather, can vary by several weeks.  For example, snow 3 weeks after spring begins or freezing weather before winter begins.  This is what is meant when we say an early summer or a late winter.  It simply means that the weather usually associated with that season is later or earlier than usual.

Generally, the lengths of the seasons ranges from about 88 to 93 days every year.

For 2014 (Northern hemisphere season lengths)
Length of spring = 92d 17h 54m
Length of summer = 93d 15h 37m
Length of autumn = 89d 20h 34m
Length of winter = 88d 23h 42m

The axis of the earth has nothing to do with it unless we are considering a time span extending to many thousands of years.  The Earth's axis hasn't changed enough in recorded history to significantly effect the seasons.

The astronomical seasons are defined by the geocentric position of the sun and have little to do with the weather.

March Equinox Beginning of spring in the northern hemisphere = Beginning of autumn in the southern hemisphere, when the sun reaches declination 0 degrees while crossing the celestial equator northward.

June Solstice Beginning of summer in the northern hemisphere = Beginning of winter in the southern hemisphere, when the sun reaches right ascension 90 degrees (6h).

September Equinox Beginning of autumn in the northern hemisphere = Beginning of spring in the southern hemisphere, when the sun reaches declination 0 degrees while crossing the celestial equator southward.

December Solstice Beginning of winter in the northern hemisphere = Beginning of summer in the southern hemisphere, when the sun reaches right ascension 270 degrees (18h).

These astronomical season lengths vary by only a matter of minutes either way each year, while the seasonal weather can vary considerably and winter-type weather can persist even after astronomical spring has begun (a late or short spring), but it is not yearly changes in the axis of the Earth that causes these annual weather variations.

SEASONAL STATISTICS FOR THE YEAR 2014 @ TZone = UT-05:00 (New York City)
BEGINNING DATES, TIMES TT AND LENGTHS OF THE SEASONS AND SEASONAL YEARS

Equinox or Solstice      Season Length     Since Previous     Until Following
E 2014 Mar 20 11:57:58   92d 17h 54m 23s   365d 05h 54m 51s   365d 05h 48m 15s
S 2014 Jun 21 05:52:21   93d 15h 37m 26s   365d 05h 47m 17s   365d 05h 46m 41s
E 2014 Sep 22 21:29:47   89d 20h 34m 21s   365d 05h 44m 56s   365d 05h 52m 40s
S 2014 Dec 21 18:04:08   88d 23h 42m 05s   365d 05h 52m 01s   365d 05h 44m 56s

Perhaps these links will help:
http://neoprogrammics.com/equinoxes_and_solstices/
http://neoprogrammics.com/equinoxes_and_solstices/seasonal_statistics/


Dimensions of a black hole How big can a black hole become and  how small can a black hole become?(minimum and maximum dimensions of a black hole)

Primoridial black holes can be found (hypothetically; there is no experimental evidence yet) of any small size above the Planck mass. Stellar black holes, however, cannot have a mass below the TOV limit (1.5 to 3 solar masses)

There does seem to be an upper limit1 of 50 billion solar masses. However, I suspect2 that this takes into consideration formative constraints (i.e. the constraints posed on the formation of such a BH); and does not prohibit such a black hole from existing. After all, the Schwarzschild metric certainly does not impose limits on the size of a black hole.

Note that talking about the limiting dimensions of a black hole is slightly meaningless as the dimensions change in different reference frames. It is far easier to talk about the mass of a black hole; the radius can be calculated in various frames from that information.

1. Natarajan, P. and Treister, E. (2009), Is there an upper limit to black hole masses?. Monthly Notices of the Royal Astronomical Society, 393: 838–845. doi: 10.1111/j.1365-2966.2008.13864.x

2. but cannot confirm yet; I will have to read the paper more thoroughly


Explaining gravity assist to a 6-year old We've been talking about Cassini with my 6-year old daughter recently and I've mentioned that it picked up more speed by passing close to Venus (twice), Earth and Jupiter. But then, I've struggled to explain how did it pick up speed, why and how does it work.

How would you recommend to explain/demonstrate the concept of "gravity assist" to a 6-year old?

I like the trowing a ball against a speeding train analogy.

The important factor is that Jupiter is moving, quite fast.  About 13 km/s.   The other important factor is that Jupiter is very massive, so it has a lot of gravity and it can draw objects towards it, causing their direction to bend.

The tricky concept for a 6 year old is that gravity is a zero-sum game.   The object that falls towards Jupiter, in this case it's a hyperbolic orbit but regular orbits work too, so as it falls towards it, it adds velocity then flying away from it, it loses the same amount of velocity, so the net velocity relative to Jupiter is zero.  No change.

But because Jupiter is moving, Jupiter can tug the spacecraft along with it.  The velocity relative to Jupiter is unchanged but the velocity relative to the sun changes, Jupiter is like the train that the ball bounces off of.

The opposite can happen too.   Jupiter can be used to slow a spacecraft down. It depends on the angle of approach, like throwing a ball against a train while it moves away from you.

You can also do this experiment below (with 2 balls not 3 to represent the gravity assist, though it's a cooler experiment with 3).  It's the same principal.   Bouncing off a moving object (Jupiter) is very different than bounding off a flat surface that isn't moving.

https://www.youtube.com/watch?v=2UHS883_P60

All that said, I'd still point out that this is a tricky concept and not to worry if they don't understand it.


Star rising times in a different place given the latitude and time of one place I don't know astronomy that well, but I am preparing for the second stage of IJSO, and here in India they do add some questions like these ones.

  A star is seen rising from Kolkata (23.5N 92 E) at 7 pm IST. What time can it be seen rising from Mumbai (19N 72E)?

If someone could explain how I might go about solving this, and some surrounding theory so I can grasp the concept, I'll be really grateful.

"Time" in astronomy has many meanings. In essence, let us look at something called Sidereal Time.

On a fixed day, the difference between local and sidereal time is roughly the same. Sidereal time is the sum of the star's right ascension (constant) and its hour angle. The rising hour angle is given by the expression

$$\arccos(-\tan (p) \tan(d)),$$

where p is the latitude and d is the declination.

Since the latitude is "roughly" the same we only need to take longitude into account. This means that the time when an event happens, since it happens at the same sidereal time happens $S_2-S_1$ (S are local sidereal times) later in the second city (the difference is roughly the same since the difference between local and sidereal times is constant for this day).

Now, it happens 80 minutes later since 20 degrees to 360 is 80 min to 24h.


In what ways can force be applied to a black hole Besides gravity I have never heard of another force pushing or pulling a black hole. Does an object falling into a black hole push the black hole when it crosses the event horizon? What happens to the kinetic energy of an object that enters a black hole?

Aside from gravity, electromagnetic forces can change the momentum of a charged black hole.

When something falls into a BH momentum is conserved according to every observer, although they may have very different ideas of how that happens. A distant observer will see the new black hole as having some momentum and so moving in some direction relative to the trajectory of the old black hole. It's not really appropriate to describe it as pushing, though. From the object's perspective, nothing special happens as it passes the event horizon. From a distant observers perspective, the gravitational field of the moving object merges with that of the black hole, and the result is the field of a slightly bigger and slightly moving black hole.


how to define the universe itself is expanding greater than than the speed of light? since we observe galaxies and other steller objects in very tiny tiny scale, and how we can understand that the speed of expansion on universe is higher than the speed of light ?
even using the powerful telescope we just see the lights from a galaxy not the object itself in real time, also the lights are red shifted as well. So what's the basis of proving univers's expansion rate is higher than of light speed?

You don't see anything in real time.

When you check your phone, you look a nanosecond back in time. When you look at the Sun,$\!^\dagger$ you look 8 minutes back in time. How do you know your phone and the Sun are still there? Because you have a model of how the world works, and that model tells you that, to the best of your knowledge, they won't just disappear.

Likewise, we have a model of how the Universe works, based on theories and observational evidence for these theories. This model predicts that the Universe expands in such a way that two objects recede from one another at a rate proportional to their distance (the so-called Hubble-Lemaître law). Hence, for large enough distances, this rate exceed the speed of light. This is described by general relativity, and is not in conflict with the speed of light being a fundamental "speed limit", since that is a prediction of special relativity.

In physics, we can never be 100% sure of anything, but we can have more or less faith in our models. If you're asking more specifically how can we trust the models, then that would become a longer description of the assumptions entering the hypothesis, the theoretical framework, the equations, the observational evidence, etc.

Actually, the fact that we look farther back in time, the more distant an object is, has helped us developing our models, since this allows us to observe the Universe not only today, but throughout its entire history. This is one of the coolest differences between astronomy and other sciences, I think.

The redshifting of light is also sometimes helpful. The earliest galaxies (i.e. the most distant ones) are often only visible due to a certain type of light called Lyman $\alpha$. This light is ultraviolet, and hence doesn't easily penetrate our atmosphere. But when observing distant galaxies, the light has been redshifted into the optical or even infrared, and so makes it possible to observe from ground, rather than form space-based telescopes (which is much more expensive).

$^\dagger$Don't.


Stellarium 0.14 or later has a checkbox under Configuration, Tools, Planetarium options:
Include nutation.
If I uncheck it, the RA and Dec of date for those stars on 2016/1/1
match yours within 0.2 arcsecond.

Some libraries distinguish between astrometric and apparent positions,
aberration
making up to 20 arcseconds difference.


Does the axial tilt of Mercury and Venus vary? The axial tilt of Mars varies greatly over time, with a tilt varying between 15 and 25° over the last million years, and varying even more over the last 5 million years. The Earth's tilt on the other hand is stabilized by the presence of a large Moon, so its tilt doesn't vary so much.
Venus and Mercury don't have any moons at all, so does their axial tilt also vary a lot over time? If not, why not? Do we have enough data to know how much their axial tilt has varied over time?

There is almost no data available on the internet as such.
Though, the theory that they (Mercury and Venus) are stabilised by the Sun's Tidal Force seems very likely as they are much closer to the sun than any other planet. The Sun very easily dwarfs these planets in both size and gravitational and tidal attraction.
I found this Wikipedia page that I think might be useful : https://en.wikipedia.org/wiki/Axial_tilt
It says that it 'might' be that they are stabilised due to the Sun.
But this article I found changed my mind : http://hosting.astro.cornell.edu/~jlm/publications/2005I09374.pdf
It clearly says that Mercury's obliquity is not consistent with time. It also contains other info that makes it must-read if you are interested in this topic.
And although I couldn't find anything convincing on the subject of the obliquity of Venus,  I found this one article that seemed quite interesting :
http://articles.adsabs.harvard.edu/cgi-bin/t2png?bg=%23FFFFFF&/seri/AJ.../0075/600/0000273.000&db_key=AST&bits=4&res=100&filetype=.gif
Venus' obliquity is 177.4 degrees, to be exact, according to this : http://solarviews.com/cap/misc/obliquity.htm
It also displays the approximated obliquity of other planets in the Solar System.


What (actually) defines an Aten-class near-earth asteroid? I just read in the EarthSky.org article Hours after discovery, asteroid swept by that the near earth asteroid 2016 QA2 recently passed the earth at about 85,000 km. The article says that it is a member of the Aten group of asteroids. That article links to this very cool GIF http://www.virtualtelescope.net/2016qa2_28aug2016.gif, however it's 40MB so it takes a while to load.

Wikipedia says:

  Aten asteroids are defined by having a semi-major axis of less than one astronomical unit (AU), the distance from the Earth to the Sun. They also have an aphelion (furthest distance from the Sun) greater than 0.983 AU${}^2$.

${}^2$http://neo.jpl.nasa.gov/neo/groups.html

That link shows the following table and drawings. The text says:

  NEAs are divided into groups (Aten, Apollo, Amor) according to their perihelion distance (q), aphelion distance (Q) and their semi-major axes (a).

Are $q$ and $Q$ really perihelion and aphelion respectively? The table says Aten asteroids have $Q>0.983$ but in the drawing for Aten asteroids it says $Aphelion<1.0167$. I noticed these values could be reciprocals, so there may be something more going on here. But at least on the surface there seems to be some contradiction.

Could someone confirm these definitions with an external source - and help me understand the basis for these limits that deviate from 1 AU by only  +/- 1.7%?

The image below is from this explanation: Near Earth Asteroid / NEO Classifications Based on Locations. It might be helpful for discussion.

An Earth-crossing asteroid has either
an aphelion larger than Earth's perihelion ($Q \gt q_\oplus$, 0.983 AU),
or a perihelion smaller than Earth's aphelion ($q < Q_\oplus$, 1.017 AU).
For those which have both, the semimajor axis $a$ determines whether it is an Aten or an Apollo - subject to change by a close encounter with Earth.

This Minor Planet Center
blog post
is consistent with the JPL table.
I think the diagram used the wrong inequality for Aten aphelia.


Does a White hole lies behind a Black hole? While reading about a White holes, I stumbled upon a question "Where does the matter emitted by a White hole come from?" There seems to be 2 possible candidates:

It was the matter engulfed by a Black hole.
Matter was created inside the White hole.

My search on wiki stated that:

According to Penrose Diagram of Schwarzschild radius, objects first leave the White hole and then may enter into a Black hole. But, due to Hawking radiation, a Black hole can achieve thermal equilibrium with gas of radiation. This equilibrium state is reversal invariant, implying that time reversal of a Black hole is again a Black hole. This means that a Black hole and a White hole are same objects.

My 2nd candidate suggests that the big-bang was a white hole. I thought of ruling this out because it requires the existence of a white hole before the big-bang.

This forced me to draw the conclusion that a white hole must either be inside a black hole or behind it.

Is this correct (provided that we haven't yet proven the existence of a White hole)? Where am I wrong?

No, there is probably no such thing as a white hole in real life. A white hole is basically a time-reversed version of a black hole. Just as a black hole cannot be destroyed in classical General Relativity, a white hole can never be formed in GR. So while a white hole is technically a thing that could exist, there is no way to form one unless it was there at the beginning of the universe. We don't have any reason to think white holes existed at the Big Bang, so we think they don't exist (and never will).


If universe is expanding why is the distance between planets not increasing? General relativity tells us space is expanding.Then why does the distance between planets and heavenly bodies increase?

Distances between planets and stars do not expand as long as they are bound together by gravity sufficiently. This is true even at the scale of an entire Galaxy.


Why can't stars be multicolored like gas giants? Gas giants like Jupiter and Saturn have bands of different colors in their atmosphere. These are due to the rotation of the planets. Stars rotate too, so why do most stars have patches/blotches of color rather than having latitudinal stripes?

Just rotation is the wrong tree to bark up on.
You see color variations on gas giants due to differences in composition, i.e. ammonia vs. sulfuric acid clouds on Jupiter, which are transported differently on the rotating planet in the up/downwelling bands.
Spots on stars originate due to very different physics. At the temperatures which are prevalent on stellar surfaces, molecules are mostly dissociated and ionized, we call this state a plasma, so no more colour effects due to ammonia or others. Star spots are concentrations of the local magnetic field, which is coupled to the plasma dynamics. The magnetic field pushes the gas aside and lets the surface cool locally. This makes dark spots that you see on stars like our sun.
There is a transition region in terms of surface temperature of 2000-3000K, at the masses of brown dwarves. Those failed stars seem to exhibit dark bands on their surfaces, which are thought to be due to absorption from exotic, high-temperature molecules like TiO and VO (Titanium and Vanadium oxide).


Could a Trappist-like planetary system exist around a sun like ours? So, a couple of years ago, the TRAPPIST sun was discovered to have 7 different earth-like planets orbiting it – three of them in the habitable zone. Nothing like this has ever been seen before, or found again since. And the main reason it's possible at all is because of the orbital resonance of the planets – they orbit in a pattern which causes the system to be stable long-term. And this whole system of planets take place closer to the TRAPPIST star than Mercury orbits our sun.
This is interesting for a lot of reasons, but one that appealed to me was that so many planets could fit – and be stable – in such a small space. I was under the impression that you couldn't fit that many planets so close together.
(Another interesting fact: The distance between TRAPPIST-1B and TRAPPIST-1H is less than the distance between the two edges of our sun's Goldilocks zone.)
So my question is this: could such a system be stable around a larger sun – a sun like ours? And could such a system work with the planets further out – in the habitable zone? Would it be stable or am I missing something?

There are a couple of systems around solar-mass stars that follow the pattern of tightly-packed planets orbiting close to the star. These include:

Kepler-11 (0.961 solar masses), a system of six low-density sub-Neptune planets orbiting within 0.5 au of the star
HD 10180 (1.06 solar masses), a system of six Neptune-size planets of which four are within 0.5 au of the star.
Kepler-90 (KOI-351) (1.2 solar masses), a system with eight planets within 1 au of the star ranging in size from Earth-sized to Jupiter-sized.

In the case of HD 10180, planet g is located in the liquid water zone, though its mass indicates that it is an ice or gas giant planet and thus not habitable. Kepler-90's outermost planet is located at 1 au, but the star is more luminous than our Sun so it might be too hot to support liquid water and is in any case a Jupiter-sized planet. So systems like TRAPPIST-1 with packed inner planetary systems clearly exist around stars like the Sun, although the difficulty of finding small planets at sufficient distance from the star means that so far we haven't found such systems with terrestrial planets in the liquid water zone.
The question of how many planets you could pack into the habitable zone has been investigated by Kane et al. (2020), "Dynamical Packing in the Habitable Zone: The Case of Beta CVn", where they investigate the maximum number of planets that can dynamically fit in the habitable zone by spectral type, and also consider in detail the specific case of the G-dwarf Beta Canum Venaticorum. From the paper:

Our analysis
presented here demonstrates that in fact the dynamical limitations to the packing of HZ terrestrial planets
is ∼5 planets for most spectral types, and ∼6 planets
for stellar masses $\gtrsim 0.7 M_\odot$. Packing 7 planets in the
HZ is possible within certain specific stellar mass and
architecture regimes, but becomes vulnerable to MMR
perturbations that compromise the dynamical stability
of such configurations.

(MMR = mean motion resonance)
Whether real systems actually reach such a dense packing in the habitable zone is unknown, and they note some caveats to their analysis:

There are some caveats to the analysis presented here
that are worth repeating. First, Section 2.1 describes
the use of 3 Gyr isochrones, which are adopted in order
to include stars at the higher mass end of the stellar
mass range. However, the star will evolve with time,
as will the HZ (Gallet et al. 2017), resulting in an increase in both the width and orbital distance of the
HZ. Subsequently, planets located at the inner edge of
the HZ at 3 Gyr will transition into the Venus Zone
(Kane et al. 2014) where runaway greenhouse scenarios may become dominant. Second, our simulations account for Earth-mass planets and circular orbits. Higher
masses and non-zero eccentricities will reduce orbital
stability within the HZ, but lower mass planets may allow for additional planets, with the exception of strong
resonance regions.

(HZ = habitable zone)


Where is the North of the Universe Does it exist North of the Universe? (Which is probably used for determining coordinates of far galaxies.)
If so, then where it is?
On what is it aligned? (Is it orientated on Earth, Sun, Milky Way, Local group or on something more bigger?)

Sort of... There is a system called the International Celestial Reference System (ICRS) which has center at the Solar System Barycenter (normally inside the Sun but not the same as the Sun's center) and which has the x and y axis in the plane of the Earth's equator and the z-axis ("North" if you will) pointing towards the Celestial Pole. The x-axis points towards the equinox where the plane of the Earth's orbit and the plane of the Solar system (the ecliptic) cross (on average; this gets complicated quickly as everything related to the Earth is wobbling about on different timescales).

Note although this reference system is close to the older mean equator and equinox J2000 or FK5 system it's not actually the same as it is now defined by a set of several hundred very distant extragalactic radio sources which are called the International Celestial Reference Frame (ICRF) and which we assume don't move on the sky. The latest version of this frame is called ICRF3 and was formally adopted at the IAU General Assembly last year in Vienna. More details on how the reference systems and frames work at the USNO page on the ICRS


How far out can the sun keep celestial objects revolving? Astronomers define the Solar System as the distance under the influence of gravity from the Sun.

I read that the Oort cloud is probably the farthest object that surrounds the sun. The sphere of this icy cloud is thought to surround the Sun at up to a distance of 50,000 AU, making its total diameter nearly two light years.

How far can the sun's gravity extend to hold celestial bodies around it? The Oort cloud may be the known farthest body but its edge doesn't mark the end of the sun's effective gravitational pull.

The suns gravitational pull extends unlimitedly far and, if the universe were otherwise empty, a body could orbit it (very slowly) at unlimited distance.

However, the universe is NOT otherwise empty. The nearest other stars are about 4 light years from the Sun at the moment, so a body more than two lightyears away will typically feel equally strong attraction from other stars as from the Sun, and so will not really orbit the sun.

That's a bit of an over-simplification, because (a) the sun is not surrounded by other stars 4 l.y. away in all directions (here is a chart of nearby stars) so a body might orbit in such a way that it didn't pass between the sun and any of its closest neighbours; (b) not all stars have the same mass, many are smaller than the Sun but some (eg Sirius) much bigger, so their gravitational influences vary and (c) the stars are all moving, so that from time to time one comes quite a bit closer, disrupting orbits.


Property of elliptical orbits I've studied that the major axis of a planets orbit = ( radius of perihelion + radius of aphelion) . But then  how can we calculate the value of the minor axis if we are provided with the values of radius of perihelion and radius of aphelion only ?

If $r_p$ is the perihelion distance (the closest distance of the planet in its elliptical orbit from the Sun), and if $r_a$ is the aphelion distance (the farthest distance from the Sun), the major axis is
$$
2a = r_a+r_p
$$
where $a$ is the semi-major axis (half of the major axis).
The eccentricity of the orbit can be calculated from the perihelion and aphelion distances using
$$
e = \frac{r_a-r_p}{r_a+r_p}
$$
Once the eccentricity is known, the semi-minor axis can be calculated from
$$
b = a\sqrt{1-e^2}
$$
The minor axis, of course, is twice the semi-minor axis, or $2b$.


What was the greatest spatial extent of the Asteroid Belt prior to the Late Heavy Bombardment? Reading the article On the asteroid belt’s orbital and size distribution (Gladman et al. 2009), there is an interest point made:

  asteroids a
  few kilometers and smaller in size are recent collisional shards of
  larger objects since their lifetime against collisional destruction is
  much less than the age of the Solar System. As such, these bodies preserve information about collisional breakup, the dominant
  physical process that has shaped the present asteroid belt.

This suggests that there would have been a lot more asteroids etc in the main belt at the time prior to the Late Heavy Bombardment (~4 billion years ago) - which is after the major planets had formed.

My question is what was the greatest spatial extent of the Asteroid Belt in the period just before the Late Heavy Bombardment?

(I am after peer reviewed resources).

There have been some studies based on observations on the best preserved impact craters of that time in the inner solar system, namely those on Mercury and the Moon.  In the paper, Impact-induced compositional variations on Mercury (Edgard et al. 2014), state that based on the nature of Mercury's craters, they have developed a

  New dynamical simulations of the early evolution of the asteroid belt hint at the presence of additional
  asteroids in a region interior to the present-day belt, known as the “E-belt”.

Through their modelling and simulations, they have determined that the 'E-belt'

  The recent reevaluation of the early history of the inner Solar System suggests the primordial asteroid belt could have originally extended well into the Mars-crossing zone

The 'Hungarias' are thought to be a remnant of this belt - as shown in the diagram below

Image attribution:
By Agmartin (Own work) [CC-BY-3.0], via Wikimedia Commons


Array of telescopes with single orbiting collector? Might such a thing give better images than we could get from other virtual telescopes with multiple collectors? Or is having multiple collectors not a limiting factor? Could a single collector compensate for atmospheric distortion in a way multiple collectors cannot?

We sometimes use arrays of radio telescopes spread over a wide distance to increase the effective aperture. I think that array is called a virtual telescope, but I might be wrong. A normal radio telescope has a collector in the same way that a satellite dish has a collector. The reflector of a radio telescope is parabolic so as to focus parallel waves to a single point. That works because dish and collector are in a fixed geometry. The reflectors of my imagined telescope would need to turn so that they reflected waves from whichever part of the sky was being observed onto the collector. That turning would change the required curvature for perfect parabolic-ness.

The telescope I'm imaging would have a (perhaps geostationary) satellite as the collector and an array of flat-ish reflectors spread over hundreds of kilometers working together to collect waves from the same part of the sky. Being non-parabolic, the reflectors wouldn't be very efficient, but they could compensate by being numerous.

The collector wouldn't have to be geostationary, because the reflectors already need to compensate for the earth turning; adding the satellite's movement into the equation wouldn't be terribly complicated.

If I understand your question, what you're talking about is one of the things that has been on the astronomical wish list for ever.  There are two ways to combine separated telescopes.  One of them (the easy way) is to just overlay the images from the two telescopes.  In this way you get to double the light you collect, but you have to take care to overlay the two images very well, such that they have the same magnification, rotation, etc.  The resolution you get doing it this way is only the resolution you get from either individual telescope, so the main reason for doing this is to increase you signal-to-noise (it doesn't make sense to do this if you're looking at a bright scene).

The other way to combine multiple telescopes, which is along the lines of what you are thinking, is to phase the two telescopes, meaning that not only are you laying the images on top of each other, you are also doing it so that the two images have the same phase, meaning they have traveled exactly the same distance from source to collector.  What you gain by doing this is that the resolution you get this way is what you would get if you had a telescope aperture that circumscribed the two telescopes.  For instance, if you had two 10 cm telescopes separated by 10 meters, you'd end up with an image that had a resolution of a 10 meter diameter telescope; however it would only have the light collection capability of the two individual telescopes, so though you have the resolution of a 10 meter telescope, you have the light collection capability that is only $\frac{2*{0.05}^2}{5^2} = 0.02\%$, meaning that you'd need to take an exposure of 5000 seconds to get the same signal you'd get with a 1-second exposure with a 10 meter telescope.

The reason you can easily make these virtual telescopes with radio telescopes is that the level you need to match path length goes as a fraction of the wavelength you're looking in.  Radio waves are many many meters long, so if you need to match the path length to, say, 1/50th of that, you're talking distances that are on the order of many centimeters or even meters.  What this means is that the radio telescopes can measure the incoming radio waves and time-tag them with atomic clocks; the ability to time-tag and the level that we can synchronize separate clocks results in phasing errors that easily fit within the phasing requirements.  Radio astronomers can record the data at each telescope then after the fact combine their signals in a computer.  However, for visible light, a fraction of a wavelength is now on the order of nanometers.  We currently don't have the ability to time-tag optical data to be able to go back later and do the phasing, which means the phasing has to be done real-time.

So the challenge to do what you are suggesting is that we need to phase the separate telescopes in real-time.  This means that we need to hold the optics in the separate telescopes so that they are not only matched to the nanometer level, but you need to hold them to that level as you are imaging.  We can easily do that on an optics table in the lab, which is basically what you are doing when you make a Michelson interferometer, but doing it on separated satellites that are moving around, vibrating, etc., it is a bear of a challenge.


Why would a planet form a cometlike tail under conditions of weaker rather than stronger solar wind? This question is prompted by one posted on the worldbuilding stack exchange:
https://worldbuilding.stackexchange.com/questions/189701/could-this-planets-tail-be-visible
I understand that the ionized gas tails of comets are formed by the effect of solar radiation on the comet followed by a push from the solar wind.
In the article on Venus I found and then an article on a gas giant exoplanet linked in the comments, both planets formed a tail under conditions of low solar wind.  Text and links to these sources below.
http://www.esa.int/Science_Exploration/Space_Science/When_a_planet_behaves_like_a_comet

The observations were made in August 2010 when NASA’s Stereo-B
spacecraft measured a drop in solar wind density to 0.1 particles per
cubic centimetre, around 50 times lower than normally observed; this
persisted for about 18 hours.  As this significantly reduced solar
wind hit Venus, Venus Express saw the planet’s ionosphere balloon
outwards on the planet’s ‘downwind’ nightside, much like the shape of
the ion tail seen streaming from a comet under similar conditions.
“The teardrop-shaped ionosphere began forming within 30–60 minutes
after the normal high pressure solar wind diminished. Over two Earth
days, it had stretched to at least two Venus radii into space,” says
Yong Wei of the Max Planck Institute for Solar System Research in
Germany, lead author of the new findings.

 A giant comet-like cloud of hydrogen escaping the warm Neptune mass exoplanet GJ 436

According to our simulation, the stellar radiation pressure
counterbalances ≲70% of the star’s gravity pull on escaping atoms,
which is much less than in hot Jupiter systems, in which radiation
pressure takes over stellar gravity by factors of 3 to 5 (ref. 20).
The low stellar radiation pressure at GJ 436b allows the formation of
a large coma and tail of escaping atoms, co-moving with the planet
although not gravitationally bounded to it (Fig. 4).

When I think of wind forming a tail I think of a smokestack on a windy day.  More wind = longer tail.  I do not understand why weaker solar wind would allow a planetary tail and stronger would prevent it.

I think the first link that you sent has an interesting explanation that we can extrapolate from - here's the relevant section:

The new observations settle a debate about how the strength of the
solar wind affects the way in which ionospheric plasma is transported
from the dayside to the nightside of Venus.
Usually, this material flows along a thin channel in the ionosphere,
but scientists were unsure what happens under low solar wind
conditions. Does the flow of plasma particles increase as the channel
widens due to the reduced confining pressure, or does it decrease
because less force is available to push plasma through the channel?
“We now finally know that the first effect outweighs the second, and
that the ionosphere expands significantly during low solar wind
density conditions,” says Markus Fraenz, also of the Max Planck
Institute and co-author on the paper.

What I interpret is this: while solar winds do cause plasma to form a tail, or stream, a very strong solar wind will limit how voluminous that tail can be. Thus, a slightly weaker solar wind would have less of a limiting effect on the volume of the flow of plasma.
This pressure-reducing effect outweighs the effect of there being less force to actually move the plasma - as said:

“We now finally know that the first effect outweighs the second, and that the ionosphere expands significantly during low solar wind density conditions"

So, this comes down to which effect is stronger - is it the expansion of the tail due to less confining pressure, or the decreased force pushing the plasma into a channel/tail? Observationally, it seems to be the first.
Though, from here, I would also assume that there is some sort of lower limit - or equilibrium - at which the size of the tail/channel reaches its maximum. If the solar wind were far too weak, there would likely not be a tail at all. So, rather than "lower solar wind causes a larger tail", I think this phenomenon would be better phrased as "excess solar wind can decrease the size of a comet-like tail, specifically around planets". It's not that lower solar wind causes a larger tail, it's that there is a sort of mid-point at which the solar wind creates optimal pressure and conditions for the tail to reach its maximum size.


Is a trefoil orbit around a trinary star valid? Recently, I have been experimenting with weirdly-shaped orbits with a program called "My Solar System 2.04." While looking for interesting orbits of a spacecraft in a trinary star, I found an type of quasi-orbit called of a trefoil, as shown in this clip. Is this orbit possible in real life, and are there any examples of them that we know of?
Edit: The objects in the video are color-coded.

Because you're asking for a spacecraft, there is some wiggle room because spacecrafts can make adjustments.   An orbit around the Moon isn't stable both because of proximity to a much more massive body (the Earth) and due to the Moon's unusual lumpiness, but spacecraft can and have orbited the Moon when the orbit of least change is selected around the Moon and the spacecraft is able to make adjustments.
In your scenario (which I'll get to in a second), and doing some bad math which I won't retype here, the trefoil orbit of the smaller body (green), if it's a spacecraft does work.  I'm not sure why anyone would need a spacecraft to have such an orbit, but in your 3 star system, a trefoil orbit like that with occasional adjustments does work.
The problem is, your 3 star system would never happen.
It appears to be 2 central stars (Yellow and Blue) in a circular orbit around each other with Pink, more distant, in a circular orbit around the Yellow-Blue barycenter.   That much is fine, though both orbits being near circular is unlikely, but for a 3 star system to be stable, it needs to resemble 2 separate Kepler orbits, because an actual 3 body orbit where the 3 bodies are all massive and close, is always unstable.   You did the central 2 stars orbit each other and the 3rd, star orbits the inner two, but your Pink star is too close.
Your Pink star is 289 distance from the barycenter of Yellow-Blue and Yellow and Blue are 84 distance apart for a ratio of 3.4 to 1.  That's too close to be realistic.  There should be some eccentricity as well.
As your system is set up, Pink would exert tidal forces on Blue-Yellow and the system would destabilize.  Granted, you could avoid tidal forces depending on your units and the distance in the system, if you make the distances that much greater than circular orbits become less likely.
A final point, while you've set up a system where Green passes neatly between the Yellow-Green barycenter and Pink over several orbits, over the long term that would inevitably destabilize.   Green is also in a retrograde orbit which is less common.   You've set up an interesting system, but you didn't run it enough times to see the long term outcome, so it's problematic for several reasons.  Even if you balanced it just right so that it runs for a number of orbits, your system is unlikely to happen the way you designed it.
That said, certain trefoil orbits might be possible at least for a reasonable amount of time.  What's required is a 3/4 resonance and two massive bodies.  If you want to view the trefoil from a fixed point or barycenter.   If you view from one of the orbiting bodies, then I can give you an example.   Titan and Hyperion kind of do it, though both orbit Saturn in simple Kepler orbits when viewed relative to Titan, Hyperion has a trefoil orbit.
Your example of a retrograde orbit that passes between the two central stars (Yellow-Blue) and the outer star (Pink) probably a lot more rare and less long term stable (I would think), but running long term simulations is the stuff of computers, not my rough calculations.
However, with so many asteroids and comets moving about, maybe something like what you've set up might be rare but possible, if you could find a 3 star system that somewhat resembles your system, and that might be the biggest problem.
I did some looking and most 3 star systems are defined as "binary" with "companion" that is, 2 stars that orbit each other and a 3rd more distant star that much more slowly orbits the 2 inner stars.   I only checked a few though.
You might try to recreate your system with the 3rd star - Pink, at least twice or three times as far away, to get a more realistic set of circumstances.  It doesn't look like you can enter resonance orbits in your input, but try to look for orbital periods that work out to 3 to 4.  Orbits often look like petals of a flower when viewed from the right vantage point, so an orbit kind of like your setup might happen in the right circumstance.


Does Einstein's theory of gravity say anything about the direction of space Why is every representation of Einstein's theory of gravity depicted like this:

and not like this:

And many other ways it can be depicted. They suggest that the fabric of space-time is a flat plane

There are numerous ways to try to explain space-time by analogy, and the flat plane is one of them. Another is the inflating balloon. The are all only analogies. Either of your pictures above is equally valid, however the one "the right way up" is simpler to understand.
For humans, who can easily conceive of 3 dimensional space, an example that works in 3D is essential. If you have ever tried to visualise more than 3 space dimensions, you'll find it very tricky.


Mercury's spin-orbit resonance When was it confirmed that Mercury has a 3:2 spin-orbit resonance and by whom (research group/radio observations...)? The first suggestion was made by Giuseppe Colombo in 1965.
Its proximity to the sun posed some problems in nailing the orbit and rotation of Mercury, but what made it eventually possible to measure it? Was there a breakthrough or development of a technology or was it just "steady scientific progress"?

The first accurate determination of the rotation period and the first hints that the rotation period was different from the orbital period (as originally expected) was in 1965 from 430 MHz radar echo measurements from the now-collapsed Arecibo Observatory (RIP) in Puerto Rico. These were reported in a very short paper (1/2 page) in Nature by Pettengill and Dyce 1965. This was followed by a slightly longer paper by Peale and Gold (Peale and Gold, 1965, Nature) giving the theoretical context which says (after discussing tidal friction expected from the Sun on Mercury):

For a planet on a circular orbit the final condition would then be one of synchronous rotation like the motion of the Moon with respect to the Earth. Mercury's motion around the Sun takes 88 days and for synchronous rotation the sidereal period would thus be 88 days also. The observed value of 59 ± 5 days differs markedly from this (see preceding communication).

(the preceding communication is the radar measurement of Pettengill and Dyce)
As the OP says, the later November 1965 Nature paper by Giuseppe Colombo seems to be the first to raise the possibility of the rotation period being 2/3rds of the orbital period and therefore it being in a 3:2 spin-orbit resonance:

IN a recent communication by S. J. Peale and T. Gold the rotational period of Mercury, determined from radar Doppler-spread measurements to be 59 +/- 5 days ....snip...
A very nearly uniform rotational motion of 58.65 sidereal-day period, that is 2/3 of the orbital period, may indeed be a stable periodic solution.

This is borne out by the 1967 paper by Dyce, Pettengill & Shapiro 1967AJ..72.351 (free to view) which discusses the determination of the rotation periods of both Venus and Mercury via radar and discusses what lead to the misleading earlier optical results for the rotation period of 88 days. It also states (page 358):

In fact Colombo (1965) pointed out the possibility that Mercury's rotation period might be 58.65 days i.e. exactly two-thirds of its rotation period. This value of the rotation period would apparently make Mercury unique in the solar system in having its axial motion locked to its orbital motion in such a manner.


Which came first: black holes or galaxies? In other terms, did galaxies grow around black holes at their center?

Note: I am not an astrophysicist. If someone with the relevant expertise finds my analysis of the source paper to be invalid, I would appreciate your help with correcting my answer here.
This is a somewhat complicated question to answer. Black holes may precede galaxies in the form of primordial black holes which according to some models arose in the early Universe from quantum fluctuations. Primordial black holes are currently hypothetical. Supermassive black holes (that you find at the center of galaxies) may not come from these, however. There are a few possible ways that supermassive black holes may have formed:

Molecular cloud / dwarf galaxies collapsing to form supermassive stars (SMS) which later further collapsed into supermassive black holes (SMBH)
Dwarf galaxies forming massive stars which collapse into massive black holes, which later coalesce into SMBHs
Primordial black holes accreting sufficient matter to become massive black holes, which later coalesce into SMBHs

Depending on how strictly you define "galaxy" it does seem most likely that the supermassive black holes formed inside early galaxies, but a type of black hole (primordial black holes) may have preceded galaxies.
Source:

Formation of the First Supermassive Black Holes - Bromm and Loeb

UPDATE 10/21/2013:
This just in, recent observations have ruled out the possibility that SMBHs gain mass only through merging with other black holes. For more, see this Astronomy.com article.


Do boxy bulges indicate barred spirals? Some face-on spiral galaxies show barred structure in their central bulges, whereas some edge-on spiral galaxies have boxy bulges. I wonder if there is any relation between these two. In particular, do boxy bulges indicate barred spirals?

Yes, they do.
Bars in massive galaxies undergo a vertical "buckling" phase about one or two billion years after they form; when this settles down (after about a billion years), the result is that the inner 1/3-2/3 of the bar is vertically thick, with the outer part of the bar still as flat as the rest of the galaxy disk.
When seen side-on, the thick part of the bar looks like a peanut (double-lobed), often with a hint of an X-shape; when seen at an intermediate angle, it looks more rectangular and "boxy". This is the situation for the Milky Way, in fact. (When seen with the bar end-on, the result looks very round, like a stereotypical spherical bulge.)
You can also see this when the galaxy isn't perfectly edge-on: some bars in galaxies inclined at 45 or 60 degrees (where 90 degrees = edge-on) have rectangular interiors with thinner, offset outer regions; this corresponds to the vertically thick/peanut-shaped inner part of the bar and the vertically thin outer part.
We've even seen the signature in a couple of galaxies of a bar in the middle of forming the peanut-shaped bulge (the "buckling phase").
(In principle, you could get a boxy bulge from some kinds of minor mergers, where a small galaxy has merged with a larger galaxy. But if these exist in real galaxies, they're much rarer than the boxy bulges that are part of bars.)


Could Mars have oil? What events could have to happen to Mars (which is thought to once look like Earth) that may have depleted the atmosphere and erase all proof of life ever existing at least on the surface?

I know an asteroid could melt the planet but could there have been an event that could wipe the surface while leaving deposits under the ground intact?

There is no proof that Mars melted but there is proof that there at least was oceans.

http://en.yibada.com/articles/18182/20150309/red-planet-once-held-large-shallow-ocean-mean-positive-signs.htm

Where did Mars water go?

Planet solidifying.

No molten core iron -> no magnetosphere -> solar wind strips atmosphere.
Without atmosphere to cycle in exposed water also leaves.

Why did Mars freeze solid & Earth has not (yet).

is further from sun
smaller means less volume to hold heat
Earth may have the iron cores from two planets (moon forming impact)
Radioactive decay?

On oil, possibly.
If there was life there and it colonized the land and it found it needed to create a scaffolding structure to compete for photons such as ligand as happened here, and there were several hundred millions years until other life there figured out how to digest those ligands then there would be a layer there like our Carboniferous era in which a large amount of carbon is sequestered underground.

But chances are vanishingly slim.
If there was life there it most likely

recycled carbon just as we have here for all time except that little blip* after trees showed up and before bacteria figured out how to digest them.
did not survive long enough to need to compete for photons by growing very tall

(*)  300,000,000 / 4,300,000,000  is about 1/14th of life on earth's history.


Celestron Nexstar 102GT Good day folks.
My Grandson would like to start using this Telescope Celestron Nexstar GT102, has been in the basement for years, and unfortunately, we appear to have misplaced the eyepieces.
Just wondering if anyone had any insight what would be a reasonable eyepiece for me to buy so he can see the moon at the very least decently enough.
Also, would Amazon be a good start for eyepieces that way I can get them fairly cheap and quick.
Thanks in advance

The manual says the original eyepieces had 1¼ inch barrels and focal lengths of 25 mm, 20 mm, 12.5 mm, 8 mm (all reasonable) and 4 mm (not so much).
If I had to pick two replacements for a beginner, I'd get a 25 mm and a 10 mm.
You can probably get an entry-level Plössl by Meade, Celestron, or Orion for US \$20-25 from the Cloudy Nights classifieds or \$30-40 new.


How can an accretion disk form around the central star of a planetary nebula? I was reading the 2012 paper A tidally destructed massive planet as the progenitor of the two light planets around the sdB star KIC 05807616 about the possible origin of two exoplanets that orbit a subdwarf B star. The researchers suggested that they could have been formed by the fragmentation of the metal core of a gas giant that was orbiting the star while it was still in the main sequence. Just at the moment when it became a red giant and absorbed the planet, which was fragmented by tidal forces when it reached the core.

Here is an excerpt from the summary:

  We suggest the following alternative scenario for the formation of the two planets (section 2). A single massive planet of mass $m_P \gtrsim 5M_J$ went through the CE evolution inside the RGB envelope. It spiralled all the way to the center. The released gravitational energy is behind the removal of the stellar envelope, as is commonly the case with CE evolution. The massive planet reached the tidal destruction radius (eq. 1). The gaseous mass of the planet was lost and part of it formed a temporary accretion disk around the core, that now is the EHB star. The metallic core of the massive planet was tidally destructed into several Earth-like bodies immediately after the gaseous envelope was removed. Different bodies had different energy per unit mass. Some of them spiral-in and were further destructed by the core, while other survived at orbital separations of $\gtrsim 1R_⊙$ within the gaseous disk. The bodies interacted with the disk and among themselves and migrated, as planets around young stars do. Two of the bodies survived and reached an almost resonance. These are the observed Earth-like planets.

I infer that at the time the gaseous accretion disk was formed around the core, the planetary nebula was still in formation, since the high temperatures of the core must have ionized the surrounding gas, at least for a few thousand years.

I understand that the gas that is expelled from the interior of a dying star in its red giant phase does so at several kilometers per second. My question is: why the gaseous accretion disk that formed around the core was not immediately ejected into space as was the case with the gas from the nascent planetary nebula? Why was it gravitationally tied to the core while the surrounding gas was escaping from gravity?

The surrounding gas has already escaped, mostly during the thermal pulsation phase of the AGB progenitor. A common envelope phase just accelerates that process.

The envelope is relatively easy to remove (at only a few km/s as you say) because it escapes from a position a long way (several au) from the centre of the star where gravity is weak and is driven by radiation pressure acting on dust that can form in these cool outer regions.

In contrast, the gas stripped by tidal disruption of the planet is close to the core of the star, which is only a few thousand km in radius. The escape velocity there will be thousands of km/s and the gas will be fully ionised and comparatively transparent to radiation and hence radiative acceleration.


Could evaporating hot Jupiters have metallic hydrogen on their surfaces? Jupiter is believed to have metallic hydrogen in its core. And gas giants that migrate to become hot Jupiters are believed to evaporate, have their atmospheres blown away by their nearby star. Can metallic hydrogen (or some other exotic materia) be detected on the surface of such a planet, or does it require high pressure to exist? What happens to the metallic hydrogen? Does it release much energy if and when it is transformed?

Metallic hydrogen is an odd substance. When you push hydrogen atoms very close together, their electrons can come free, and move around, instead of being tightly bound to the atomic nuclei. As this form of hydrogen would conduct electricity, it behaves like a metal. At least this is the theory. Nobody has been able to produce enough pressure to actually make any metallic hydrogen in the lab.

However, the pressure inside Jupiter should be high enough to form metallic hydrogen. In extrasolar planets there could be large amounts of metallic hydrogen too.

However as soon as you release the pressure from metallic hydrogen, it turns back to normal molecular hydrogen. So it could not exist on the surface of a "hot jupiter", even one from which the outer layers had been stripped away by the solar wind. The metallic hydrogen that had been in the interior would change back into molecular hydrogen as it approached the surface.


Why doesn't the vertical light beam get out of a black hole? I'm asking this question as a follow-up to if light has no mass why is it affected by gravity?

Imagine you’re standing on a gedanken planet, shining a laser beam straight up into space. The light goes straight up. It doesn’t curve, and it doesn’t fall back down. Now imagine it’s a denser more massive planet. The light still goes straight up. It still doesn’t curve, and it still doesn’t fall back down. Let’s make it a really massive planet. That light still goes straight up. It still doesn’t curve, and it still doesn’t fall back down:

But when we make our gedanken planet so massive that it’s a black hole, all of a sudden light can’t escape. Why? Why doesn’t the light get out? Why doesn't the vertical light beam get out of a black hole?

There is no "up" direction within the event horizon.

Most people get fixated on the speed of light, or energy or whatever. They're like, if light was faster, could it escape the black hole? If my rocket had bigger engines, could I escape? The problem is, all these questions make no sense. You can't get out because there is no way out.

A black hole is formed when gravity is so strong, it ties spacetime into a giant knot. Space is tied into itself. It's not just a little bent; it gets curved until it closes onto itself.

Inside the event horizon, all paths lead to the center. No matter how you're turning, which direction you're looking, you're actually facing the center. It's hard to visualize, but that's how it is. This is not normal spacetime, it's something unlike anything you've thought about.

Starting from point A inside the event horizon, there is no path you could draw that leads to point B outside. All paths lead to the center. Spacetime is really sick and broken.

This is the real reason why nothing gets out of a black hole.