34
votes
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Where was the solar system born? Alternatively, how far back in time can we retrodict the location of the solar system?
Basically no, and not very far back at all.
Star forming regions generally last for at most 10 million years. The "nebula" in which the Sun was born is long gone so cannot be identified.
The ...
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25
votes
Is Dark Matter possible if there is dynamical friction?
Dynamical friction is considered in the study of dark matter.
The timescale for dynamical friction to cause a significant loss of kinetic energy is roughly
$$\tau \sim \frac{10^{5}}{\sqrt3 \ln \Lambda}...
- 158k
21
votes
Accepted
When will the number of stars be a maximum?
TL; DR Somewhere between now and a few hundred billion years time. (For a co-moving volume) Now read on.
If stellar remnants are included, then the answer is very far in the future indeed, if and ...
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18
votes
Where was the solar system born? Alternatively, how far back in time can we retrodict the location of the solar system?
..and to Cuivienen there is no returning
Nobody knows.
All the stars in the Milky Way Galaxy orbit around the center of mass of the galaxy.
The orbit of the Sun takes about 250 million years. The ...
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17
votes
How do we know that supermassive black holes can gain mass by means other than merging with other supermassive black holes?
The idea behind the paper (Shannon et al. 2013) that article is based on is to measure the gravitational wave background (GWB) produced by mergers of supermassive black holes, and determine which ...
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15
votes
Are there some (simple) mathematical models which can simulate the cellular/web structure of the universe?
I suppose it depends on your definition of "simple", but I would say "No".
Kepler simulations
A solar system is simple because you have one dominating mass with orbiting masses ...
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14
votes
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How can the (in my eyes quite ridiculous) conjecture of Sheldrake be disproven?
Firstly, thank you for your leveled and clear explanation of Sheldrake's essay. I agree with you that it is quite ridiculous to make such a bold claim when there is such little support for it even for ...
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14
votes
Accepted
Why wouldn't an elliptical galaxy collapse back into a spiral galaxy?
If an elliptical galaxy is free of gas then it can't really "collapse" effectively, since there is no mechanism to dissipate the kinetic energy of the stars.
In fact, once stars have formed ...
- 158k
13
votes
How can the (in my eyes quite ridiculous) conjecture of Sheldrake be disproven?
To be fair, Sheldrake credits Greg Matloff (2015) for this "dark matter is really the motions of 'volitional stars'" idea.
It's easy enough to show this won't work (I mean, aside from all ...
- 18.1k
12
votes
Accepted
What (actually) is the " deprojected half-light radius" of this almost-all-dark-matter Galaxy?
The half light radius is the radius from within which half the luminosity emerges.
"Deprojected" means that the authors must have fitted some model to the 2D distribution of light, which can then be ...
- 158k
11
votes
How do we know that supermassive black holes can gain mass by means other than merging with other supermassive black holes?
We know that black holes can gain mass other than merging with other black holes because we see high redshift quasars.
The luminosity of quasars is caused by the accretion of mass into their central ...
- 158k
10
votes
Accepted
Are there some (simple) mathematical models which can simulate the cellular/web structure of the universe?
In order to make a cosmic web pattern one can run a physics-based simulation approximating the flow of mass, as in Pela's answer. However, it is also possible to make "phenomenological models&...
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9
votes
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What does "unremarkable transverse peculiar velocity" mean exactly, and how is it calculated here?
"Peculiar velocity" is a fixed term and describes the velocity of an object relative to a defined rest frame.
Astronomy has the problem that you need different methods to measure the 3D ...
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9
votes
What happens to planets as they cross the Galactic equator?
Nothing whatsoever.
To be honest I find it a little hard to imagine why you think anything could happen.
The galactic equator is a plane that is roughly in the middle of the disc of stars that compose ...
- 127k
8
votes
Accepted
More recent data and simulations of "Milkomeda", the collision of the Milky Way and Andromeda galaxies?
Since the second data release (DR2) of the European Space Agency's Gaia mission there has been a revolution in astrometry, including measuring the motion of the Andromeda Galaxy.
On February this ...
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8
votes
Accepted
Have we discovered a Galaxy lacking in Dark matter?
I don't work in this field, but it looks like the answer is "yes". The paper in question is here. See also 2018 preliminary results (linked to in your OP) and 2022 follow-up.
My impression ...
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7
votes
In which direction does the Milky Way rotate?
The Milky Way has arms that form due to density waves. Like the majority of spiral galaxies, the arms are trailing. Individual stars orbit in circles (roughly), neither towards or away from the centre....
- 127k
7
votes
Accepted
What is the characteristic time of the evaporation of the galaxies?
The standard treatment can be found in (Binney & Tremaine 2008), but see also (Adams & Laughlin 1997) for a good treatment.
The overall timescale for galactic evaporation is $$\tau_{evap}= ...
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7
votes
Accepted
Does the Final-Parsec problem apply to stellar collisions?
The "final parsec problem" describes the difficulty in getting two supermassive black holes close enough together that their merger timescale due to gravitational wave emission becomes ...
- 158k
5
votes
Supergalactic and Equatorial coordinates
Equatorial coordinates have their equator and poles at the equator of the earth and the poles at the earth each projected onto the sky.
Supergalactic coordinates on the other hand have their equator ...
- 3,441
5
votes
How do galaxies move in space?
Galaxies move though space independently of the orientation of their axis of rotation. That this is true can be appreciated from the fact that their direction through space is relative; that is, in ...
- 39.1k
5
votes
Galaxy interactions
Galaxies are not so far from each other compared to their sizes as you might think.
The typical distance between galaxies is a few Mpc (1 Mpc, or megaparsec, is roughly 3 million light-years). While ...
- 39.1k
5
votes
Accepted
Why does the neutral hydrogen velocity have this characteristic behavior in the galactic plane?
The reason is detailed in depth in this pdf, which contains the following diagram:
Some key quantities:
$R_0$: Distance from the observer to the center of the Milky Way
$R$: Distance from target gas ...
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5
votes
Is the expected time for a star in an elliptical galaxy to collide with another star less than the average age of elliptical galaxies?
The collision timescale for a star in the solar neighborhood is1
$$t_c\simeq5\times10^{10}\text{ Gyr}\left(\frac{R}{R_{\odot}}\right)^{-2}\left(\frac{v}{30\text{ km s}^{-1}}\right)^{-1}\left(\frac{n}{...
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5
votes
Accepted
How do bars form in barred spiral galaxies?
It seems strange to me that a galaxy, which I imagine as a structure with some level of cylindrical symmetry, goes on to develop an elongated bar structure in its central region.
The general idea is ...
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5
votes
Accepted
Why do stars orbit much faster than gas in galaxies?
The velocity dispersion of giant molecular clouds in the vicinity of the Sun might be 30-40 km/s (this sounds too large to me actually), but their orbital speeds are of order 220 km/s, like the Sun.
...
- 158k
5
votes
What happens to planets as they cross the Galactic equator?
The Sun and planets are both pulled together by the gravitational field of the Galaxy. But the differential tidal effects could be felt by planets (or other objects) on orbits of size $\sim 10^5$ au (...
- 158k
4
votes
Direction of Earth’s poles relative to the Sun’s movement around the Galaxy
There is no difference in the number of cosmic particles.
You seem to imagine that there are static clouds of gas in the universe that the sun is moving through. This is not the case. The gas in the ...
- 127k
4
votes
Accepted
Examples of astrophysical objects NOT in virial equilibrium?
Not clear what the initial part of your question means. Objects can be in virial equilibrium without being in thermal equilibrium.
A clear exception to the virial theorem would be any system that is ...
- 158k
4
votes
Accepted
Orphaned star systems and intergalactic travel (in fictional context)
The Canis Major dwarf galaxy is about 8 kpc from the Sun, but is only 8 degrees below the Galactic plane (and further out than the Sun). So it is about 42,000 light years from the Galactic centre and ...
- 158k
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