1. 1 Hydrogen in the Milky Way Hydrogen is the most abundant element in our galaxy. The 21-cm Line

2. 2 Electronic Energy Levels Atoms emit a photon of light at a specificphoton wavelength when an electron falls to a lower energy level. The 21-cm Line

3. 3 The resulting dark line spectrum identifies hydrogen. Hydrogen is detected using a technique called spectroscopy. The 21-cm Line

4. 4 An Element’s Signature An element’s spectrum acts as is its signature.element’s spectrum Each line will always appear in the same location. The 21-cm Line

5. 5 Hydrogen only has only one electron spinning around its nucleus. How can just a spinning electron have energy levels? The 21-cm Line

6. 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

7. 7 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 www.astronomynotes.com

8. 8 A 21-cm Hydrogen Spectrum The 21-cm Line

9. 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

10. 10 The spectrum line shift tells us the motion of a source (stars, galaxies, gases, etc.). The 21-cm Line

11. 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. If the source is moving away from the observer, the spectral line will be shifted to a longer wavelength. The 21-cm Line

12. 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

13. 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

14. 14 Example: Rotation of the Milky Way The 21-cm Line Image from Richard Powell www.atlasoftheuniverse.com www.atlasoftheuniverse.com