Besides Stars, What are Galaxies Made of?

Discovery of the Interstellar Medium, Another Key Part of Galaxies

Key points: What makes up the interstellar medium; nebulae of different kinds; HII and HI regions, molecular clouds

Globules of dust and gas appear as shadows against a nebula Interstellar dust had fooled astronomers about distances and colors of objects for more than a century. 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 15,000o, 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 4,000o or less that is free of dust.

Dust distributed throughout the galaxy preferentially transmits red light and absorbs or scatters blue lightbuttonex.jpg (1228 bytes). (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 - it is filled with gas and traces of dust (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





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Here is another view designed to enhance appearance of the gas in the Orion Nebula (from M. Robberto, Astronomy Picture of the Day, NASA, ESA)

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
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. Although mostly made of molecular hydrogen, they also contain molecules like carbon monoxide, water, methane, alcohol and even more complicated ones. They may have temperatures as low as 10 degrees K and are the regions where stars form.

The horsehead nebula is a cold molecular cloud seen as a shadow against a red background of a diffuse HII region. A compact HII region lies to the lower left, with its color dominated by the scattered star light. This picture therefore contains a reflection nebula, an emission nebula, and a molecular cloud. (from CFHT, J.-C. Cuillandre, Coelum

Test your understanding before going onbuttongrad.jpg (11232 bytes)

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John Flamsteed's constellation Andromeda, from Atlas Coelestis,

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darkmatter.jpg (4103 bytes)"Dark Matter" by G. Rieke




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