# Tag Info

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 ...
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### 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 ...
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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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### 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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### 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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### 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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### 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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### 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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### 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. ...
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### 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 (...
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### 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 ...
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### 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