The Interstellar
Medium

As we have noted above, the region between the stars in a galaxy like the Milky Way is far from empty. These regions have very low densities (they constitute a vacuum far better than can be produced artificially on the surface of the Earth), but are filled with gas, dust, magnetic fields, and charged particles. This is commonly termed the interstellar medium.

Approximately 99% of the mass of the interstellar medium is in the form of gas with the remainder primarily in dust. The total mass of the gas and dust in the interstellar medium is about 15% of the total mass of visible matter in the Milky Way.

Gas in the Interstellar Medium

Of the gas in the Milky Way, 90% by mass is hydrogen, with the remainder mostly helium. The gas appears primarily in two forms
  1. Cold clouds of atomic or molecular hydrogen
  2. Hot ionized hydrogen near hot young stars

The clouds of cold molecular and atomic hydrogen represent the raw material from which stars can be formed in the disk of the galaxy if they become gravitationally unstable and collapse. Although such clouds do not emit visible radiation, they can be detected by their radio frequency emission.

HI and HII Regions

Ionized hydrogen is produced when the ultraviolet radiation emitted copiously by hot newly-formed stars ionizes surrounding clouds of gas. The characteristic beautiful red colors of emission nebulae like the Orion Nebula (M42) or the Trifid Nebula (Ref) shown in the adjacent figures are produced by visible light emitted when electrons recombine with the ionized hydrogen in these regions. Such regions of ionized hydrogen are called HII regions, while cold un-ionized hydrogen clouds are termed HI regions (with the I and II referring to the ionization state of the hydrogen). The disk and its spiral arms are heavily populated by both HI and HII regions.

The Orion Nebula is relatively nearby, about 1500 light years away in the same spiral arm of the galaxy as our own Sun. The image shown above (Ref) is a mosaic of Hubble Space Telescope images showing the inner 2.5 light years of this large nebula (which is visible as the middle "star" in Orion's sword). As we have already seen, M42 is the location of many stellar nurseries where stars are being born.

Dust in the Interstellar Medium

The Horsehead Nebula
Interstellar dust grains are typically a fraction of a micron across (approximately the wavelength of blue light), irregularly shaped, and composed of carbon and/or silicates. Absorption of light by dust causes large dark regions in our galaxy and in other galaxies, as this picture of the Milky Way looking toward the center indicates. These dust clouds are visible if they absorb the light coming through them. We then refer to these clouds as dark nebulae such as the adjacent Horsehead Nebula. On the other hand, light can reflect from clouds of dust and gas, giving rise to sometimes beautiful reflection nebulae.

Dust has two major effects on light passing through it:

  1. The light is dimmed by the dust; this is called interstellar extinction.

  2. The light that does pass through the dust is depleted in blue wavelengths because the size of the dust grains favors scattering blue light. This is called interstellar reddening, because the resultant transmitted light is more red than it would have been otherwise. This implies that transmitted light will be more red, but reflected light will be more blue. (On Earth, the blueness of the sky is due to similar effects in scattering of light from molecules in the atmosphere.)
Quantitative observations in astronomy must correct for both extinction and reddening of light by the interstellar medium.

The image adjacent left shows an example of a reflection nebula called the Witch Head Nebula, because of its shape, but you can call it IC 2118 if you are less poetic or an astronomer (Ref). It is associated with the bright star Rigel in the constellation Orion (Rigel is offstage to the right of the image shown), and is about 900 light years away. The reflection nebula is blue for two reasons: the hot star Rigel supplying the light being reflected is blue, and as noted above the fine dust in the nebula is more efficient at scattering blue light than red light because of the size of the dust particles.

Another example of a reflection nebula is shown below. The Pleiades Cluster is a young cluster of predominantly blue stars that is visible to the naked eye. There is still some dust left from the nebula in which they formed, and light reflecting from that dust causes the blue haze around each star of the cluster (Ref).

The red part of the Trifid Nebula is an emission nebula powered by the hot star in its center. The blue part of the Trifid Nebula is a reflection nebula, reflecting light from a hot star centered in that part of the nebula. Thus, the Trifid Nebula is both an emission and reflection nebula. Likewise, the Orion Nebula shown above contains both emission and reflection nebulae within it (as well as absorption nebulae).

The image below left illustrates a portion of the sky near the star Antares (which is the brightest star in the image). It contains many examples of all three kinds of nebulae that we have discussed: red emission nebulae, blue reflection nebulae, and dark absorption nebulae (Source). The image below right is another example of a beautiful nebula. It is about 9000 light years away in the constellation Carina and is called NGC 3372 or The Great Nebula in Carina (Source). This nebula contains the massive active star Eta Carina.


The Source of Interstellar Dust

The exact nature and origin of interstellar dust grains is unknown, but they are presumably ejected from stars. One likely source is from red giant stars late in their lives. In particular, stars on the asymptotic giant branch of the HR diagram (AGB stars) are known to eject much of their envelope into space and this could be a significant source of interstellar dust grains.

The Distribution of Aluminum-26

One important probe of the interstellar medium is the distribution of the radioactive isotope aluminum-26. It can be detected by gamma ray detectors on satellites (gamma rays are completely absorbed in the atmosphere) because it emits gamma rays of a particular energy that are fingerprints for Al-26 nuclei, just as emission lines in the optical spectrum at particular frequencies are fingerprints for atoms. The distribution of Al-26 is important because (1) it can be produced in various processes in stellar evolution, and (2) it decays radioactively to magnesium-26 with a half-life that is short on astronomical scales (about a million years), so if it is seen it must have been produced very recently. The following figure shows an all-sky map in galactic coordinates of the observed distribution of Al-26, obtained using the COMPTEL instrument aboard the orbiting Compton Gamma Ray Observatory (Ref).

We don't know the distance to these Al-26 sources. But since the intensity of the gamma rays from a source decreases as the square of the distance to the source and we do not expect enormous concentrations of Al-26 anywhere, this map presumably represents Al-26 concentrations within our own galaxy. This distribution of Al-26 is not understood in any detail. It is concentrated in the plane of the galaxy, and since it can't be far from where it was formed and the most likely process that produce it involve massive stars (for example, supernova explosions and nucleosynthesis in aging massive stars), it probably is correlated with star-forming regions in the galaxy.


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