1. Radio Waves Interaction With Interstellar Matter
A cosmic journey brought to you by McGourty-Rideout

2. Interstellar Medium Gaseous - almost all hydrogen
H I- neutral hydrogen one proton and one electron( most)
H II ionized hydrogen- - no electron just a proton - near hot stars
Dust – C, N, and silicates

3. 21-cm line Radiation
Radio emission from hydrogen when an electron “flips its spin”
H I gas emits a particular wavelength of radio energy from an energy change in the hydrogen atoms. The wavelength is 21.1cm (frequency = MHz)
called 21-cm line radiation
Temperatures between 100 K to about 3000 K

4. 21-cm line Radiation
An electron moving about a proton can have a spin in the same direction as the proton or in the opposite direction
The energy state of an electron spinning opposite is slightly lower than the energy state when both are in the same direction

5. 21-cm line Radiation
Atoms always want to be in the lowest energy state
If the spins are in the same direction, the electron will eventually flip to the opposite spin direction
The energy difference is very small, so a hydrogen atom may wait a few million years before it undergoes this transition

6. Uses of 21cm Line
The Milky Way alone has about 3 billion solar masses of H I gas so enough hydrogen atoms are emitting the 21-cm line radiation to be easily detected with radio telescopes
Not blocked by dust
Best way to map the structure of the galaxy
Radio emission leads to penetration

7. Waves Transformed in Space
Slowed down & refracted by interstellar gas and plasma
Spectral lines absorbed by interstellar molecules & atoms
Doppler shifted by moving sources

8. Waves Transformed in Space
Slowed down & refracted by strong gravitational fields
Red shifted by gravitational sources
Polarization rotated by magnetic fields