Galaxies Like the Milky Way

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Slide 1

Hydrogen in the Milky Way

Hydrogen is the most abundant element in our galaxy.

The 21-cm Line

Slide 2

Electronic Energy Levels

Atoms emit a photon of light at a specific

wavelength when an electron falls to a lower

energy level.

The 21-cm Line

Slide 3

The resulting dark line spectrum identifies hydrogen.

Hydrogen is detected using a technique called spectroscopy.

The 21-cm Line

Slide 4

An Element’s Signature

An element’s spectrum acts as is its signature.

Each line will always appear in the same location.

The 21-cm Line

Slide 5

Hydrogen only has only one electron spinning around its nucleus.

How can just a spinning electron have energy levels?

The 21-cm Line

Slide 6

The electron spin energy of a hydrogen atom is slightly different depending on whether the spins of the proton and electron are in the same direction or opposite directions.

YES!

The 21-cm Line

Slide 7

The 21-cm Line

If the spin of the electron changes from the

higher energy level to the lower energy one,

a photon with a wavelength of 21 cm is emitted.

The 21-cm Line

Animation from Nick Strobel’s website www.astronomynotes.com

Slide 8

A 21-cm Hydrogen Spectrum

The 21-cm Line

Slide 9

Spectral Lines and Motion

Spectral lines can tell us whether the source is stationary or moving.

If the source is moving, the spectral line will be shifted (called a Doppler shift).

Source at rest

Source moving away

Source moving closer

The 21-cm Line

Slide 10

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The spectrum line shift tells us the motion of a source (stars, galaxies, gases, etc.).

The 21-cm Line

Slide 11

Shift in wavelength

Stationary

Moving away observer (red shift)

Moving toward observer

(blue shift)

If the source is moving toward the observer, the spectral line will be shifted to a shorter wavelength.