Correct!

Recap: Your astronomy professor has tasked the class with determining the velocity of Andromeda with respect to the Milky Way. You thought of three possible ways to do this, one of which will give you the right answer. You've decided to try using Hubble's Law to find the velocity from M31's distance.

You checked your math and are confident that you have the right answer for the velocity of M31 using Hubble's Law. However, when you tell your professor your answer, she shakes her head.

She explains that although your general approach would work for distant galaxies, M31 is not actually distant enough. It is only very distant objects that participate in the "Hubble flow." For closer galaxies, such as M31, the effects of the expansion of the Universe that give rise to Hubble's Law are swamped by the local effects of gravity. In fact, M31 is part of the Milky Way's "local group" – a cluster of galaxies containing more than 50 galaxies. This means that M31 is gravitationally bound to our own galaxy and the local cluster. That gravity has a huge effect on M31's motion.

She invites you to keep trying.

Return to the beginning and try another approach