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Hydrogen in the Milky Way

Hydrogen is the most abundant element in our galaxy.

The 21-cm Line

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Electronic Energy Levels

Atoms emit a photon of light at a specific wavelength when an electron falls to a

lowerenergy level. The 21-cm Line

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The resulting dark line spectrum identifies hydrogen.

Hydrogen is detected using a technique called spectroscopy.

The 21-cm Line

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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

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• Hydrogen only has only one electron spinning around its nucleus.

• How can just a spinning electron have energy levels?

The 21-cm Line

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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

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

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A 21-cm Hydrogen Spectrum

21 cm Galactic Center Hydrogen Line

0

10

20

30

40

50

60

21.11 21.115 21.12 21.125 21.13

Wavelength (cm)

Flu

x

The 21-cm Line

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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

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The spectrum line shift tells us the motion of a source (stars, galaxies,

gases, etc.).

The 21-cm Line

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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.

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

The 21-cm Line

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Wavelength and velocity

The relative change in

wavelength is related to the source’s

velocity.

Stationary

Moving

slow

Moving fast

The 21-cm Line

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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

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Example: Rotation of the Milky Way

The 21-cm Line

Image from Richard Powell Image from Richard Powell www.atlasoftheuniverse.cowww.atlasoftheuniverse.comm