Gas, Dust, Nebulae
- 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,
...], 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.
- 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).
Where is all of this? Everywhere in and around the galaxy,
but mostly concentrated in the disk (along what is seen from
earth as the Milky Way), in warm or cool atomic H clouds. It accounts for
10–15% of the mattter in the disk.
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.
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
- 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.
page by luca bombelli <bombelli at olemiss.edu>,
modified 29 sep 2012