THE INTERSTELLAR MEDIUM

What stars are made from.

COMPOSITION

• gas: mostly hydrogen, 9% helium; deficient in heavier elements (from spectroscopy)
• dust: probably includes silicates (rock), carbon and iron; dirty ice (methane, ammonia, water ice, etc.) like comet material

DENSITY

Gas: 1 atom per cubic cm -- about 10^-24 times less dense than water

Dust:

• even less dense than gas -- typical distance between grains, 100 m
• size of particles, 10^-7 m: comparable to wavelength of visible light (determined from the way they scatter light)

NEBULAE

A nebula is a fuzzy patch (from Latin word for "cloud"); most nebulae are irregularly shaped clouds of dust and gas

1. EMISSION (BRIGHT) NEBULAE

regions of hot glowing gas, typically made to glow by new O- or B- (hot) stars

Color: predominantly red; this is one of the hydrogen emission lines (H)

Composition: hydrogen (90%) and helium (9%) (from spectroscopy)

Size: so large that we can measure their angular width, and from their distance, determine their size (a few parsecs)

109 bright nebulae were catalogued by Charles Messier in the 18th C while looking for comets. (Some of the Messier objects turn out to be galaxies or globular clusters.)

2. DARK NEBULAE

observed primarily from the fact that their dust blocks light from stars behind them. The whole plane of the Milky Way is filled with dust.

The dust scatters light, blue (short wavelength) more strongly than red (long wavelength)

• This means the particles are comparable in size to the wavelength of visible light.
• If brightly lit, we observe the scattered (blue) light (3. REFLECTION NEBULAE)
• Light from bright objects passing through the nebula will be reddened, if not completely obscured

Composition: Mostly gas, but it's the dust that blocks the light.

Temperature: The dust is observed to give off black-body radiation in the IR, indicating temperatures of 10-100 K (e.g. Rho Ophiuchi)

21-CM RADIATION -- very long wavelength (21-cm) electromagnetic radiation (radio waves) emitted by neutral hydrogen. ("hyperfine" splitting.)

because the wavelength is large compared to the size of dust particles, it is not appreciably blocked by the dust. This enables us to "see" into the dust clouds, and to detect the presence of dark nebulae in regions where there aren't stars behind them to reveal their existence

INTERSTELLAR MOLECULES

In some dense dark nebulae, radio-wavelength emissions, first discovered in the 1960's, indicate that atoms are combined into molecules.

In clouds, molecules are protected by the dust from harsh radiation that would break them apart. Also, the dust may provide sites for atoms to stick and combine (catalysis)

Fairly complex molecules have been found in space, including some simple organic molecules

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