# Tag Info

### 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 ...
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### 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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### 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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### 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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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 \$...
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### 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 ...
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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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### 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 ...
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### 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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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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