University of California, San Diego
Center for Astrophysics & Space Sciences

Gene Smith's Astronomy Tutorial

The Interstellar Medium

Although space is very empty and the stars in the Milky Way are very far apart, the space between the stars contains a very diffuse medium of gas and dust astronomers call the interstellar medium (ISM). This medium consists of neutral hydrogen gas (HI), molecular gas (mostly H2), ionized gas (HII), and dust grains. Although the interstellar medium is, by several orders of magnitude, a better vacuum than any physicists can create in the laboratory there is still about of 5-10 billion M of gas and dust out there, comprising approximately 5% of the mass of visible stars in the Galaxy.

Neutral Hydrogen Gas

The Milky Way Galaxy is filled with a very diffuse distribution of neutral hydrogen gas which has a typical density of about 1 atom/cm3 (10-24g/cm3). The interstellar medium is far too cool to excite the UV or optical transitions of hydrogen, but there is a feature at 21 cm wavelength in the radio produced by the spins (magnetic fields) of the hydrogen atom's nuclear proton and orbiting electron. Because the proton and electron are spinning distributions of electric charge they create minute magnetic fields which interact, creating a small energy difference between the state in which the poles are aligned versus counter-aligned. This energy difference corresponds to the energy of radio waves at 21-centimeters. Every once in a while (about once per 500 years) hydrogen atoms will collide, exciting an atom into the higher energy spin-aligned configuration. It will take as long as 30 million years for the atom to jump back to the lower energy state via a spin-flip, emitting 21 cm radio emission.

(Diagram courtesy of Dr. Terry Herter, Cornell University)

The neutral hydrogen is distributed in clumpy fashion with cool, denser regions that astronomers call "clouds" but which are more like filaments. These regions have a typical temperature of about 100K and a density between 10--100 atoms/cm3. Surrounding the clouds is a warmer lower density medium with about 0.1 atom/cm3 and T ~ 1000K.

Molecular Clouds

Comparatively dense (nH2 > 1000 molecules/cm3), cold (T ~ 10K) clouds of molecular hydrogen and dust, known as molecular clouds or dark clouds are the birthplaces of stars. We do not detect molecular hydrogen directly, but infer its characteristics from other molecules, most often CO. Over 50 other molecules have been detected including NH3, CH, OH, CS and molecules as complex as ethyl alcohol (C2H5OH - the stuff in whisky) have been found in Milky Way molecular clouds. The Horsehead Nebula (Messier Nebulae, Web Nebulae) to the right is produced by the incursion of a plume of dust from a molecular cloud, covering the lower half of the image, into a region of ionized hydrogen. A Giant Molecular Cloud (GMC) may have a mass of 106M and a diameter of 150 l.y. Within the GMCs are warm dense corse of order 2-3 l.y. in diameter, with T~100K and densities as high as n~107-109 molecules/cm3. It is in these regions where the star-formation process begins. There are thousands of GMCs in the Milky Way, mostly on the Spiral Arms and concentrated toward the Galactic Center. The total mass of molecular gas is estimated to be about equal to, or perhaps somewhat less (~25%) than, the mass of HI gas.
These maps show the Molecular Gas in the plane of the Milky Way and in the Galactic Center region.

Ionized Hydrogen Regions

Material left over from the formation of young, hot stars represents the most spectacular component of the ISM, the ionized hydrogen or HII regions like the Orion Nebula ( Messier Database, Web Nebulae), shown below in this HST mosaic. Here is the Near Infrared view of the Orion Nebula.

Massive O and B stars, recently formed in molecular clouds (remember - massive stars live fast & die young!) ionize the gas left over from their formation heating it to a temperature, T ~ 10,000K and causing it to fluoresce producing an emission-line spectrum.

Ultraviolet photons from four massive stars called the Trapezium in the nebula have sufficient energy to strip the electrons completely away from - ionize - hydrogen atoms. This requires a photon of energy greater than 13.6 eV or wavelength less than 912Å in the ultraviolet. If a hydrogen atom absorbs a photon with wavelength less than 912Å the atom is ionized with the "extra" energy going into the kinetic energy of the electron. Collisions between electrons "thermalize" this energy heating the nebular gas to a temperature of about 10,000K. Collisions between electrons and ions in the gas excite the ions to higher energy levels producing emission features of O+,O++,N+, S+, etc. as shown in the above spectrum. Electrons recombining to upper level in hydrogen and helium cascade through many energy levels down to the ground state producing the emission features of H & He.

The Orion Nebula is a bubble on the side of a much larger Giant Molecular Cloud complex. The GMC contains a lage cluster of newly formed stars

Scroll down this page to see a selection of HST images of Orion showing circumstellar disks surrounding pre-main sequence stars of solar mass.

Principal Constituents of the ISM
  Total Mass
"Cloud" Mass
HI gas ~5 x 109   0.1-10 100-1000
H2 gas 1-5 x 109 105-106 103-105 ~10
Dust ~5 x 107     ~40
HII gas   100-1000 103-104 10,000

Interstellar Dust

About 1% of the mass of the ISM is in the form of tiny grains of dust about the size of particles of cigarette smoke. We have already described how this dust obscures the plane of the Milky Way from our view. We know something about the characteristics of this dust from the way that it scatters visible and ultraviolet photons. The effect of dust is to dim the light from distant objects (interstellar extinction) in the Galaxy and redden the colors (interstellar reddening) because red light is not scattered as efficiently as blue light.

The Constellation Orion:
Visible Light ( S. Kohle & T. Credner, Univ. Bonn) Infrared ( IRAS Image, IPAC/Caltech)

A graphical exhibition of the effects of dust at different wavelengths is shown by the visible and infrared images of the constellation Orion above. Dust scatters and obscures visible wavelengths where stars emit most of their light (note Betelgeuse the bright red giant at Orion's left shoulder). Dust is largely transparent in the infrared, but at temperatures of about 40K, emits strongly at wavelengths between 50-100 m.

We know that dust grains are elongated, perhaps needle shaped, with sizes of about 1000Å, about the wavelength of light that the grains scatter most efficiently. Dust characteristics vary somewhat from place to place in the Galaxy, but a typical grain is believed to be composed of carbon in a graphite-like crystal structure, mixed with silicates (eg MgSi03 like olivine). Nearly all of the elements like Carbon and Silicon in the ISM are tied up in dust. In molecular clouds the grains appear to be coated with a water-ice shell. Ned Wright at UCLA has developed a fractal model for interstellar dust grains shown in this APOD.

Ned Wright's Fractal Dust Model

Reflection Nebulae

Dust is responsible for the blue haze around the Pleiades star cluster (Messier Database, (Web Nebulae); this nebulosity is called a reflection nebula resulting from blue light from the hot B-stars being scattered toward us from dust surrounding the cluster stars.

The Pleiades Star Cluster
Credit & Copyright: D. Malin (AAO), AATB, ROE, UKS Telescope

Why is the sky blue?

Supernova Remnants

Supernova remnants like the Crab Nebula enrich the ISM with elements heavier than helium as the expand into the ISM with speeds of several thousand km/s.

  • Astronomy Pictures of the Day: Supernova Remnants
  • X-Ray images of SNR

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    Conducted by Gene Smith, CASS/UCSD.
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    Last updated: 26 April 1999