Ch 09-01 Interstellar Medium (ISM)
- Nebulae
- Emission Nebulae
* Abundant Hα,
Hβ... lines
* T ~ 104 K
* n ~ 102 atoms/cm3
(Note that n ~ 2.5 x 1019 molecules/cm3 in the air)
* Ionization and recombination
* HII regions
* Forbidden lines, e.g. [OIII 4959Å][OIII 5007Å]
- Reflection nebulae
* Reflected stellar spectra, by dust
- Dark nebulae
* Appearing dark in visible light; Barnard objects, Bok globules
- Interstellar extinction, interstellar reddening
Interstellar absorption lines
- Dust emissions at mid- and far-infrared
An optical image towards the Galactic Center
(130 degree across)
The Spitzer 8μm image toward the Galactic Center
(1.6 degree across, about 230pc)
- Radio observation
- 21-cm line (due to the coupling of electron spin and the proton magnetic moment)
=> HI cloud
- 2.6-mm line (due to the rotation of CO) => molecular clouds
(CO as tracers of H2,
NCO /
NH2 ~ 10 -4)
More than one hundred of different molecule species,
both organic and inorganic, have beem found
in the interstellar medium.
- Components in the ISM
- Different dust abundance and nearby stellar environments result in the appearance
of emission nebulae (HII regions), reflection nebulae, and dark nebulae.
- Corona gas is believed to come from both hot young stars and the death of stars -- supernova explosion.
Hot corona gases (X-ray emitting, represented in blue) can be seen in this star forming region.
They come from the newly formed OB stars (in the right part of this figure)
and also from some supernova explosion events (in the left part).