# At what rate are stars accelerating?

How fast stars accelerating away from a central point? Are they increasing in acceleration or decreasing? If possible could you please provide a plot of the acceleration of stars away from the central fixed point.

• Are you talking about the expansion of the universe? Because everything in the universe is moving away from everything else, not from a central point. – Featherball Dec 15 '15 at 18:01
• There is no central fixed point. – ProfRob Dec 15 '15 at 19:34
• Partly answered by astronomy.stackexchange.com/questions/11584/… – ProfRob Dec 15 '15 at 19:37

As has already been mentioned, stars within a galaxy do not generally expand. The stars in a galaxy are gravitationally bound together.

But as you observe more and more distant galaxies, you see a general trend that distant galaxies are moving away from us at a speed that's proportional to their distance. (There are some small variations due to the random motion of individual galaxies; for example, the Andromeda galaxy is approaching us, and will collide with the Milky Way in about 4 billion years.)

This does not indicate that everything is expanding from some central point. An observer in a galaxy a billion light-years away would see the same thing, all other galaxies receding at a speed proportional to the distance from the observer. This can be interpreted as space itself expanding. Going into more detail would require General Relativity, which I'm not competent to explain.

Your question is about the rate of expansion. The answer to that is what's called the Hubble Constant, which describes the ratio between a galaxy's speed of recession and its distance from us. The current best estimate of this ratio is 67 kilometers per second per megaparsec. A parsec is about 3.26 light-years; a galaxy one million parsecs away will be moving away from us at about 67 kilometers per second.

• "At what rate are stars accelerating"? "Are they increasing in acceleration or decreasing"? The question is not asking about the present rate of expansion, but about $\dot{H}$ or even $\ddot{H}$. – ProfRob Dec 16 '15 at 19:36

No part of the universe, at least on a large scale, is believed to be accelerating away from a central point. And, in any case, the movement of stars in galaxies is dominated by the effect of galactic gravitation rather than the expansion of spacetime.

The universe as a whole is believed to be expanding (the observational evidence for this is generally not questioned these days), but this is also believed to be happening everywhere at once. The big bang - the moment at which this expansion started also occurred everywhere at once and so there is no central point of the expansion.

My favourite analogy - which I have used on this site a few times now - is of the surface of a balloon as it is blown up - all the points on the surface move away from each other but there is no central point on the surface.

• The question was about the rate of expansion. You haven't addressed that. – Keith Thompson Dec 16 '15 at 17:26
• The question was how fast are they accelerating away from a central point. The answer is they aren't - as stated in the first sentence of my answer. – adrianmcmenamin Dec 16 '15 at 21:04
• no central point on the surface there is one on the inside – tox123 Mar 11 '16 at 1:39