Prof. Dale E. Gary
A. Interstellar Medium (ISM)
Solar wind and IPM (Interplanetary Medium)
- carries particles and magnetic fields outward
- defines the Heliosphere (region of space dominated by the Sun)
- each star has a similar region of domination around it
- region between the stars
- consists of particles and magnetic fields, from stars and "primordial"
- "particles" can be either gas or dust
Gas vs. Dust:
There are three aspects of the Interstellar Medium in this photograph. The blue nebula at upper right is a reflection nebula from dust, which scatters blue light from the stars. The red nebulae seen in several places (e.g., Lagoon nebula below center) are emission nebulae from gas, shining in the red light of hydrogen-alpha. In several places, especially middle left, the background stars appear to be less numerous due to interstellar absorption from dust, which obscures the more distant stars. Photo by Steve Mazlin and Jim Misti.
Dark Nebulae and Extinction
Extinction (= Absorption + Scattering) means reduction of overall brightness of objects seen through dust. Distance modulus equation is modified when we take dust into account:
m - M = 5 log d - 5 + A
where A is amount of absorption in magnitudes. For A = 1, star is reduced in brightness (flux) by a factor of 2.5. If A were uniform throughout galaxy, then A = kd (proportional to distance) but A is not uniform.
When you see dust from the side, it appears BLUE and polarized. This polarization does not necessarily depend on grains being elongated.
1. Pleiades 2. More Pleiades 3. Wider yet
Here is another beautiful example of the three aspects of the Interstellar Medium. Reflection nebulae from dust are seen in the vicinity of hot, luminous stars on both left and right. The red nebulae at left, including the cone nebula, are emission nebulae from gas, shining in the red light of hydrogen-alpha. The absorption by dust is obvious throughout the region. The gas and dust is obviously sculpted by light and energy from the luminous stars on the left, forming bow-shaped features. An open cluster of stars appears near the center of the photo. Photo by Adam Block and Tim Pucket.
Nature of Interstellar Grains
Again, 99% of ISM by mass is gas, and there are 1012 gas "particles" for every dust grain.
Interstellar Absorption Lines
Video (shock wave slams into ring)
Interstellar Radio Lines
- Lowest energy state of hydrogen
- Electron in one of two spin states "up" or "down"
- Define "up" to be parallel with proton spin--then this is higher energy state--would rather be antiparallel
- Energy difference between "up" and "down" corresponds to radio photon of 21 cm wavelength
- Spontaneous flip occurs only once every few million years!
- Collisions in ISM occur once every 400 years, so most flips are due to collisions (no radiation)
- Rare photon emission, but there are enormous numbers of H atoms along each line of sight, so line is strong!
- Electrons recombining with hydrogen in H II regions can attach in very high excitation levels, then transition to slightly lower ones, i.e. n = 105 to n = 104. These low-energy transitions give off low-energy photons in the radio region of the spectrum, giving another way to study H II regions.
- CO rotational line, with J = 0 to 1 transition at 115 GHz, and the 1 to 2 transition at 230 GHz. These correspond to mm wavelengths--radio again.
- Molecules that have been detected: CO, CN, OH, H2CO, CH3OH, H2O, NH3, HCN, HC3N, HNCO, more
- Most molecules, however, are good old H2 (molecular hydrogen).
- Chance of these forming in ISM seem very small (one collision every 400 years) but
- dust helps
- provides place to collect and hold atoms while molecules form over millions (perhaps billions) of years
- shadows fragile molecules from dissociating radiation
- Widespread existence of molecules has implications for extraterrestrial life.
Inside a molecular cloud -- OMC-1: Image
Star formation in M16: link