Interstellar Gas and Dust - the Interstellar Medium - Rejuvenate Galaxies with Young Stars

Discovery of the Interstellar Medium

Globules of dust and gas appear as shadows against a nebula Early observations with telescopes showed "Holes in the Heavens". Astronomical photography at the turn of the century revealed many more dark regions.
  • Eventually astronomers realized that these are dark clouds in space blocking our view of the stars behind them.
  • Dust distributed throughout the galaxy was discovered when the colors of some stars did not match the colors expected from their temperatures as deduced from their spectral types. For example, a star with spectral type B has a temperature of about 15,000°, which should look blue-white. However, it may appear to be quite red if it is behind a lot of dust, similar to a star with a temperature of only 4,000° or less that is free of dust.

===> Dust distributed throughout the galaxy preferentially transmits red light and absorbs or scatters blue light. (This is the same physical process that makes the setting sun look red and the sky look blue). Thus, stars appear both redder and dimmer if we view them through intervening clouds of dust.

 

Interstellar Material: Fuel for New Stars

 

 

 

 

The space between stars is not truly empty (if all the material lying between stars in the Milky Way could be collected, it would be equal to about 10% the mass of all the stars !)

Where new stars are forming, they light up the surrounding clouds of material, and also excite emission lines in the gas in the clouds. An example is the Orion Nebula*, shown to the right. To explore it in more detail, try this fly-through movie (produced for the Hayden Planetarium by the San Diego Supercomputer Center) en00500_1.jpg (18578 bytes)

*nebula is Latin for cloud, plural is nebulae

 

 

 

 

 orionmos.gif (152020 bytes)

Most interstellar gas is hydrogen and helium. Interstellar dust accounts for about 1% of the interstellar material and is a combination of carbon (graphite) and silicates (similar to sand in composition). In the table below, we compare the gas densities in different environments: the interstellar gas is very dilute compared with air at the surface of the earth, or even the best vacuum we can achieve in a laboratory.

Locale Density
g/cm3		 Distance between particles (cm)
Air in room 1.2x10-3 1x10-7
Nebula around forming star 1x10-8 5x10-6
Vacuum in lab 1x10-12 1x10-4
Orion Nebula 1x10-21 0.1
Typical Interstellar Space 1x10-24 1

We describe interstellar clouds in terms of their appearance and the state of the gas:

Reflection nebula: cloud contains dust particles that can scatter star light. These nebulae look blue in color because blue light scatters better than red light.

Emission nebula: cloud of gas and dust lying close to a hot star that ionizes the hydrogen in the cloud. Also called "HII Regions" -- HII means hydrogen atoms with their electrons gone (HI means regular hydrogen). These nebulae look red in color because hydrogen has a strong emission line in the red part of the spectrum. O or B stars are the only stellar types hot enough to create HII regions.

HI cloud: Cool gas, also called HI regions because the hydrogen gas is neutral, not ionized.

Molecular cloud: cloud of gas and dust that is dense and cool. May contain molecules like carbon monoxide, water, methane, alcohol and even more complicated ones. May have temperatures as low as 10°K. Are regions where stars form.

The horsehead nebula The horsehead nebula is a cold molecular cloud seen as a shadow against a background diffuse HII region. A compact HII region lies to the lower left.
darkmatter.jpg (4103 bytes)

 

 

 

"Dark Matter" by G. Rieke

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 childrensun.jpg (20028 bytes)

 

 

 

 

"Children of the Sun," by Roger Francois, The Electric Gallery, http://www.egallery.com/francois.html

 

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hypertext © G. H. Rieke

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