University Physics with Modern Physics (14th Edition)

Published by Pearson
ISBN 10: 0321973615
ISBN 13: 978-0-32197-361-0

Chapter 39 - Particles Behaving as Waves - Problems - Discussion Questions - Page 1313: Q39.5

Answer

The intergalactic medium is cold compared to the light-emitting galactic material.

Work Step by Step

In the intergalactic medium, most of the hydrogen atoms must be in the ground state, n=1, since they strongly absorb Lyman-$\alpha$ photons and move to the n=2 state. The Maxwell-Boltzmann distribution (section 18.5) tells us that the atoms must be at a relatively low temperature for so many of them to be in the ground state.
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