Galaxies Like the Milky Way

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If the source is moving away from the observer, the spectral line will be shifted to a longer wavelength.

The 21-cm Line

Slide 12

Wavelength and velocity

The relative change in wavelength is related to the source’s velocity.

Stationary

Moving slow

Moving fast

The 21-cm Line

Slide 13

Uses of the 21-cm line

The Milky Way has over 3 billion solar masses of hydrogen gas.

Enough hydrogen atoms so that the 21-cm line is easily detected by radio telescopes.

Therefore, it is the best way to map the structure of the galaxy.

The 21-cm Line

Slide 14

Example: Rotation of the Milky Way

The 21-cm Line

Image from Richard Powell www.atlasoftheuniverse.com