Interstellar Gas

• What is it? Mostly atoms (on average, 1 atom per cm³, of which 90% H, 9% He, enriched in heavier elements by supernovas) [or 70% H, 28% He by mass], and some molecules [CO, H₂O, NH₃, CH₃OH, ...], at temperatures from a few K to 10,000 K or more.
• How do we see it? If heated by stars, it emits light; Far from stars, it is mostly transparent, except for new absorption lines appearing in the spectra of stars behind it; But it emits radio waves, like the 21-cm hydrogen lines, which are not absorbed by dust and can be detected by radio telescopes, and if ionized, other radiation as well.
• Origin: Hot ionized gas comes from stars (stellar wind and explosions), cools down over millions of years and forms atoms and molecules, mostly H₂.

  Interstellar Dust

• What is it? Elongated grains made of millions of atoms (10^-7 m in size) [a silicate core surrounded by an organic layer, encased in ice]; on average, 1 every 100 m³.
• What does it do to light? It scatters and absorbs light at all wavelengths, making light dimmer (especially blue light, causing reddening), emits depending on its temperature [and possibly polarizes it].
• What else does it do? Dust grains catalyze chemical reactions between atoms that stick to their surfaces and form molecules; They also keep heavier atoms "hidden" in their cores (e.g., interstellar Mg, Si, Al, Ca, Ti, Fe are mostly in dust grains).
• How do we find it? If it does not glow, look for its effect on starlight going through it; Makes open clusters look too dim for their distance, stars too red for their spectral type.
• Origin: It forms in supernova explosions and stellar winds from old stars (like ashes).

Hot, Warm and Cool Clouds - Bright Nebulae

• Appearance: Some are lit up by newly formed stars inside or near them (like the Pleiades), others made up of material ejected by old stars, as with supernova remnants and planetary nebulae.
• Emission/ionization nebulae: Glowing clouds of hot gas, mostly red from H alpha, or green from O, from which we can get an emission spectrum.
• Reflection nebulae: They contain more dust and reflect light from bright stars inside, mostly blue [examples are Barnard's Merope nebula IC 349, and IC 2118].
• Other types and examples: Bubbles and fountains above the galactic disk; Many of the 109 "fuzzy" Messier objects, mostly in the galactic plane; The Herbig-Haro objects that glow around newly formed stars, and giant stellar nurseries like NGC 604 in the galaxy M33, or the Tarantula Nebula in the LMC.

  Cold, Molecular Clouds - Dark Nebulae

• Appearance: The darker regions of the Milky Way; Some are parts of larger nebulae with bright parts, near hot young stars that erode them; They dim starlight and add absorption lines, and emit infrared radiation and radio waves.
• Examples: Horsehead nebula, Gas Pillars in the Eagle nebula, and Coalsack.
• What are they? Cold, dense places (up to many thousands of particles/cm³) where new stars are more likely to form; 150-300 ly across, they may contain more than a million solar masses each and last for 10-100 Myr; there are a few thousands of them in our galaxy.

How Does the ISM Evolve? What Role Does it Play?

• Effect of stars: Stellar wind, planetary nebulae, and supernovae heat up the clouds, add heavy elements and dust grains, form bubbles (shock waves); Here on Earth, we get cosmic rays.
• Other effects: Intergalactic wind may penetrate and heat diffuse clouds.
• Molecular clouds: New star formation and star clusters.
• Dust: It is important as a seed for planet formation.
• Future of ISM: The galaxy will slowly lose its ISM (to brown and white dwarfs, neutron stars, black holes); Star formation will slow down in another 50 Gyr or so and eventually stop, unless something happens...

  Our Own Neighborhood

• What does it look like? We seem to be inside a "local bubble" in a network of cavities in the interstellar medium, probably carved by massive star explosions millions of years ago.
• Does the ISM affect us? The Solar System is moving [at 25 km/sec (900 mph), in the direction of Scorpius]; As a result, an interstellar wind with that speed is blowing through it.
• Can we detect the ISM? Some spacecraft have detected radio waves from the heliopause, others like ACE can analyze ions and atoms in space, and we find interstellar dust grains in meteorites; The walls of the Local Bubble seem to be causing a twinkling of radio waves from quasars, and NASA launched the CHIPS orbiting UV telescope to study it in 2003.