The Interstellar Medium
Gas and Dust Among the Stars

Outline

The Interstellar Medium

The interstellar medium is clearly evident as it blocks the light
from many stars in the Milky Way

An enlarged view in the direction of the center of our Galaxy, illustrating the blockage of light

Nebulae visible in the above enlarged view

Gas and Dust

Gas
- about 90% hydrogen (by no. of atoms)
- about 9% helium
- remaining 1% heavier elements
Dust
- like cigarette smoke
- C, N, silicates
- typically elongated 1/10,000 millimeter particles
- elongated shape inferred from its polarizing effect on light

90% of interstellar medium is hydrogen

(about 1 atom per cubic centimeter)

Hydrogen atom = proton + electron (HI)

ionized hydrogen (no electron, just protron - HII)

21 cm line from spin-flip in HI

Temperature
- few kelvins to few hundred kelvins
  - depends on nearness of stars to heat medium
  - 100 K is typical temperature (COLD)
Density
- gas typically 1 atom per cubic centimeter
  - compare this to gas on Earth
  - typical atmosphere = 2.7 x 10¹⁹/cm³
  - best "vacuum" achieved in lab on Earth = 10⁴/cm³
- dust typically 1 per trillion (10¹²) atoms
  - this is 1000 per cubic kilometer

while the density is low, the total amount of mass in the interstellar region is nearly as much as the mass contained in the stars themselves

Extinction
- Dimming of starlight due to interstellar medium
  - Star's are completely blocked by dust in some directions
Interstellar Reddening
- Why is the sky blue?
- Why is the sunset red?
- Answer: dust preferentially scatters blue light

Any fuzzy bight or dark patch on the sky

Absorption nebulae
- caused by absorbing gas
- eg. Horsehead Nebula
Reflection nebulae
- caused by starlight reflected from surrounding gas
- eg. Pleiades
Emission nebulae
- results when a cloud is heated by stars until it starts to glow
- eg. Orion Nebula

clouds of gas surrounding at least one newly formed O or B type star
typical temperature of 8000 K
typically composed of ionized hydrogen, and therefore called HII regions
Recombination of electron and protons into atom results in emission of radiation => emission lines
- region of Sagittarius with several fuzzy patches which are emission nebulae M8, M16, M17, and M20
- eg. Butterfly-shaped Emission Nebula, NGC 2346, and emission spectrum
some very rare lines are seen (referred to as "forbidden lines")
- example of "forbidden lines" is green light from the Orion Nebula (M42)
  - doubly ionized Oxygen
  - emitted very slowly, only in very "low-density" situation, not realized on Earth, even in laboratory

(aka Absorption Nebula)

Obscures visible light
Creates absorption spectra
Typically very cool regions (less than 100 K, down to few 10 K)
- emit infrared radiation
  - eg. right-most image is Rho Ophiuchi from the Infrared Astronomy Satellite
    - Note - wavelength icon for this image is incorrect

Ultraviolet radiation has helped in the study of the interstellar medium by allowing a map of our local cosmic neighborhood
Ultraviolet radiation can only be studied by instruments placed in orbit above the Earth's atmosphere
The International Ultraviolet Explorer (IUE) found a number of regions of interstellar space that are much thinner (5000 atoms/m³) and hotter (500,000 K) than expected
The Sun seems to reside in such a region
- a huge cavity, called the "Local Bubble"
- about 100 pc across, containing about 200,000 stars
- thought to have been carved out by multiple supernova explosions several hunderd thousand years ago
- Such supernovae would have been spectacular events in the sky

A probe of the interstellar medium that does not rely on the heat of the stars
"radio emission" from hydrogen when the electron flips its spin
This is a critical technique for "seeing" the interstellar medium which is dominated by hydrogen
particularly when it is not very warm

When the interstellar medium is cold (say 20 K) molecules can form, and the density often grows (to 10⁶ per cubic centimeter)

Molecules in space emit radio signals.
The wavelength of the signals indicate from which molecule it comes.
- Hydroxyl OH
- Water H₂O
- Ammonia NH₃
- Formaldehyde H₂CO
  - spectra indicate formaldehyde molecules exist around M20
  - contour map of amount of formaldehyde around M20
- Carbon monoxide CO - useful for spiral arm study
- Molecular hydrogen H₂
- Hydrogen isoCyanide HCN - see cloud around center of Milky Way
Hydrogen is the overwhelmingly dominant component, but it is harder to observe, and the other molecules are the observed tracers of the accumulation
These radio signals come from limited places in the sky