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nykkyo · Apr 28, 2011, 01:37 AM

Why is space separating only between galaxies and not between stars in a galaxy. If space was being created betqween stars, they would be separating, and the galaxy would be nomore.

jcaron2 · Apr 28, 2011, 05:19 AM

The stars in the galaxies are attracted to each other by gravity. Thus they tend to stay together.

ebaines · Apr 28, 2011, 07:52 AM

Space is indeed expanding between stars. But the expansion rate is dependent on the distance between objects, and is only about 73 Km/sec per MegaParsec. If you convert this to units that are more appropriate for distances between stars, it's equivalent to 0.027 meters/sec per light year. In other words: if the distance between two stars is 1 LY, then the expansion of the universe causes them to move apart at 0.027 m/s. That's such a small number that this motion due to expansion is completely swamped by the actual relative motions of the two stars. Even for stars that are hundreds or thousands of LY's apart it's not far enough to measure the effect. The effect of expansion can't be measured except over very large distances - on the order of multiple megaparsecs. One Megaparsec is 3.26 million light years, or just a bit farther than from here to the Andromeda Galaxy.

TUT317 · Apr 28, 2011, 02:49 PM

Originally Posted by ebaines

Space is indeed expanding between stars. But the expansion rate is dependent on the distance between objects, and is only about 73 Km/sec per MegaParsec. If you convert this to units that are more appropriate for distances between stars, it's equivalent to 0.027 meters/sec per lightyear. In other words: if the distance between two stars is 1 LY, then the expansion of the universe causes them to move apart at 0.027 m/s. That's such a small number that this motion due to expansion is completely swamped by the actual relative motions of the two stars. Even for stars that are hundreds or thousands of LY's apart it's not far enough to measure the effect. The effect of expansion can't be measured except over very large distances - on the order of multiple megaparsecs. One Megaparsec is 3.26 million light years, or just a bit farther than from here to the Andromeda Galaxy.

Hi ebaines,

Do you think the question also relates to why galaxies are able to maintain their shape despite angular momentum? In other words, one would normally expect a spiral galaxy to exhibit the phenomenon of its arms being tightly wound around its core? A bit like a ice skater two draws their arms in while rotating about an axis?

Galaxies tend to maintain their shape over time. This seems to be one argument for the existence of dark matter.

Tut

nykkyo · Apr 28, 2011, 10:01 PM

Does that mean 73 km/s every sec through-out the megapoarsec or 73 km/s for megaparsec secs?

nykkyo · Apr 28, 2011, 10:07 PM

Or does it mean 73/megaparsec km/s

ebaines · Apr 29, 2011, 06:04 AM

The farther away something is, the faster it is receding. Hence the units of Hunbble's constant is velocity per unit of distance. So if you look at an object with is 1 megaparsec away it will be receding from you at a speed of 73 Km/s due to the expansion of the universe. If you look at an object with is 2 Megapasecs away it will be receding at 2 x 73 = 146 Km/s. An object 10 Megaparsecs away receeds at 730 Km/s, etc.

Now for extra credit: if an object that is 1 Megaparsec from us is receding away at 73 Km/s, how long ago were both it and us in the same place? You'll have to convert Megaparsecs to Km, to get the units for Hubble's constant into 1/s. The inverse of that is your answer, and is equal to the age of the universe.