Post on 16-Dec-2015
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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