34 votes
Accepted

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 ...
ProfRob's user avatar
  • 152k
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}...
ProfRob's user avatar
  • 152k
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 ...
ProfRob's user avatar
  • 152k
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 ...
M. A. Golding's user avatar
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 ...
HDE 226868's user avatar
  • 36.6k
14 votes
Accepted

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 ...
Daddy Kropotkin's user avatar
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 ...
ProfRob's user avatar
  • 152k
14 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 ...
pela's user avatar
  • 38.3k
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 ...
Peter Erwin's user avatar
  • 16.7k
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 ...
ProfRob's user avatar
  • 152k
10 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 ...
ProfRob's user avatar
  • 152k
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&...
Anders Sandberg's user avatar
9 votes
Accepted

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 ...
planetmaker's user avatar
  • 19.4k
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 ...
Swike's user avatar
  • 3,916
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 ...
Allure's user avatar
  • 4,544
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....
James K's user avatar
  • 122k
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}= ...
Anders Sandberg's user avatar
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 ...
ProfRob's user avatar
  • 152k
6 votes

Why Milky Way and Andromeda are being drawn together if there was 'Big Bang'?

The evidence for expansion is that the redshift is proportional to distance. The redshift of a galaxy can be divided into two components: that due to the cosmological expansion, which stretches the ...
ProfRob's user avatar
  • 152k
6 votes
Accepted

Why don't globular clusters flatten with a galactic disc?

Globular clusters formed whilst the gas of the proto Milky Way was still approximately spherically distributed. The gas forms a dissipative system that loses energy and collapses (within the first ...
ProfRob's user avatar
  • 152k
5 votes
Accepted

Is our central black hole actually at the CG of the galaxy?

In simple terms: yes, the Milky Way's supermassive black hole (SMBH) is at the center of the galaxy, we know approximately where the center is (but not terribly precisely), and we should expect the ...
Peter Erwin's user avatar
  • 16.7k
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 ...
HDE 226868's user avatar
  • 36.6k
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}{...
HDE 226868's user avatar
  • 36.6k
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 ...
SpaceCore's user avatar
  • 3,003
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 ...
pela's user avatar
  • 38.3k
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 ...
pela's user avatar
  • 38.3k
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 ...
Peter Erwin's user avatar
  • 16.7k
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. ...
ProfRob's user avatar
  • 152k
4 votes

Why Milky Way and Andromeda are being drawn together if there was 'Big Bang'?

Scientists used to use following reasoning: most galaxies are red-shifted $\implies$ there was a 'Big Bang' That is an oversimplification, and a rather large one at that. There are multiple lines ...
David Hammen's user avatar
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 ...
James K's user avatar
  • 122k

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