21 votes

Questions about spiral galaxy arms

Actually, the stars and nebulae that make up the spiral arm are only temporarily part of that spiral arm. Spiral arms are more like sound waves where individual particles move around a more or less ...
user avatar
  • 3,311
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 ...
user avatar
  • 114k
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 ...
user avatar
  • 33.7k
15 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 ...
user avatar
14 votes
Accepted

Has the great Andromeda Galaxy ever collided with any galaxies?

Galaxies grow through cosmic time by accretion of the surrounding matter. Some of its mass increase happens through smooth accretion of gas, but much also happens through merging with small clumps of ...
user avatar
  • 31.9k
14 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 ...
user avatar
  • 14.7k
13 votes
Accepted

Leaving the Milky Way

Not really, for the same reason that you cannot travel west by jumping up in the air and let Earth rotate underneath you, such that you land a little farther to the west. The reason is that standing ...
user avatar
  • 31.9k
12 votes

Questions about spiral galaxy arms

To add to Dieudonné's excellent answer, I'd like to say that spiral arms are only really prominent in the blue part of the spectrum (massive stars tend to be blue and short-lived), while in infrared ...
user avatar
  • 5,276
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 ...
user avatar
  • 114k
11 votes
Accepted

Could dark energy be negative gravity?

Could dark energy (the mysterious accelerating expansion of the universe) be explained by "negative gravity"? But it already is "negative gravity". In general relativity, the stress-energy tensor $...
user avatar
  • 7,674
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 ...
user avatar
  • 114k
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 ...
user avatar
  • 10.7k
8 votes
Accepted

What happens to galaxies when they die?

Well, it would be useful to define what a 'dead' galaxy is. Probably the most simple method would be a galaxy that is no longer producing new stars. We might also consider a galaxy that no longer ...
user avatar
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 ...
user avatar
  • 3,536
7 votes
Accepted

What mechanism causes oscillations of the solar system's orbit about the galactic plane?

The cause for the oscillations perpendicular to the galactic plane is the gravity of the non-spherical mass distribution (needed for a plane Kepler ellipse) in the Milky Way. Simplified, there is a ...
user avatar
  • 11.2k
7 votes
Accepted

What are the arguments against the Feng and Gallo thin disk explanation of galactic rotation curves?

Feng & Gallo have published a series of extremely similar papers, all of which essentially claim that they have "discovered" a major flaw in the way (some) astrophysicists think about rotation ...
user avatar
  • 114k
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}= ...
user avatar
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 ...
user avatar
  • 114k
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 ...
user avatar
  • 114k
6 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....
user avatar
  • 87.6k
5 votes
Accepted

Distribution of Stars in Milky Way and globular cluster analogy

Particles in a gas approximate to point-like objects that interact roughly elastically through short-range forces when they collide, but are otherwise non-interacting. Stars interact gravitationally ...
user avatar
  • 114k
5 votes
Accepted

Could discoid galaxies be expanding?

No, for several reasons. there are no radial velocities in the required range observed, for example in the Milky Way galaxy. the reservoir of stars moving out from the centre would quickly be drained,...
user avatar
  • 5,276
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 ...
user avatar
  • 14.7k
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 ...
user avatar
  • 33.7k
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}{...
user avatar
  • 33.7k
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 ...
user avatar
  • 2,400
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 ...
user avatar
  • 31.9k
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 ...
user avatar
  • 31.9k
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 ...
user avatar
  • 14.7k
4 votes

Are there a lot of collisions between stars in the core of the galaxy?

The answer is yes. Stellar collisions are events that occur mainly in Globular clusters due to their high stellar density. The likelihood of these events is highly dependent on the stellar density. As ...
user avatar

Only top scored, non community-wiki answers of a minimum length are eligible