Astronomy 124

The Interstellar Medium

Before we begin our extensive discussion regarding the evolution of stars, we must start with a discussion of the stuff out of which stars are made, namely the gas and dust that is present throughout the galaxy. This material is collectively known as the interstellar medium: the material between the stars.

One tends to think of outer space as consisting solely of stars separated by great distances with a complete vacuum between them. While it is true that space is mostly empty, there are regions that contain considerable material. This is relative of course. By earthly standards such regions still constitute a pretty good vacuum. For example, the "typical" gas density in space is one atom per cubic centimeter. The best man-made vacuum is about 10¹² atoms per cubic centimeter. A cc of the air in the room has about 10¹⁹ atoms in it. The interstellar medium is not uniform in density. Although on average its density is one atom per cc, it can have densities up to a thousand or a million atoms per cc.

The interstellar medium is composed of two classes of material: gas and dust. Gas is composed of molecules or atoms of types of gasses, hydrogen being the most abundant. Carbon monoxide, CO, is another common form of gas, as are oxygen and nitrogen. Dust is composed of little bits of solid matter. Dust is very small, on the order of microns (10^-6 meters). Dust is made of compounds of carbon and silicon in various forms (graphite grains, silicate grains) and out of ices (such as water, carbon dioxide, ammonia). Temperatures have to be pretty low to have the icy particles (100 K and below). The other kinds of dust and grains can exist at higher temperatures, although if the temperature gets too high all kinds of dust will be destroyed. The composition, nature, structure, etc. of dust particles remains an area of some uncertainty in astronomy.

The interstellar medium manifests itself to the astronomer in various phenomena. The most obvious perhaps are the emission nebula. (Nebula is the latin word for "cloud." Its plural is nebulae.) Emission, you will recall, occurs when a cloud of gas is warmed up by some source of continuum radiation, say from a nearby star. The various atomic energy levels are excited by this radiation, and as the electrons jump back down to their lower energy states they emit photons at distinct spectral wavelengths. Since hydrogen gas is the most common form of gas, hydrogen emission lines are most often observed. In particular, a specific transition in the hydrogen atom corresponds to red light and color pictures of these emission regions appear red. This is the so called H-alpha emission line of hydrogen. It is the same line that made the chromosphere of the sun appear red.

Figure: The Trifid Neubla (M20). This nebula exhibits many of the features we are describing here: the red light comes from hydrogen Balmer emission, excited by the ultraviolet radiation from a hot star embedded in the nebula. The dark regions criss crossing the nebula are obscuring dust. The blue region to the left of the main body is a "reflection nebula" so named because it consists of dust that is scattering starlight into our line of sight. Dust scatters short wavelength (blue) light preferentially, hence the blue appearance.

Emission nebula are also known as HII regions. The explanation of this nomenclature is this: The H means Hydrogen and the Roman numeral II means that the hydrogen is ionized (its electron has been stripped off by high energy photons). H I (H-one) is neutral hydrogen meaning that it still has its electron. He II would be singly ionized helium, and He III would be doubly (fully) ionized helium. These designations become more important for atoms with larger numbers of electrons as it is necessary to keep track of how many electrons an atom has if you are to properly characterize the emission spectrum from that atom.

If you would like to see an emission nebula, go out into the night and look at the constellation Orion. One of the "stars" in Orion's sword will appear to be fuzzy. This is not a star at all but an emission nebula. If you can get hold of a small telescope or a good pair of binoculars take a look at it. It will appear to be a glowing cloud rather than a brilliant point.

The power to keep an emission nebula glowing is provided by hot, high energy stars in the interior of the nebula, specifically only O and the hottest of the B type stars (B0 and B1). The ionization of hydrogen (and helium which is also seen in an ionized state in the nebulae) requires high energy ultraviolet photons such as will be emitted from stars of those spectral types. The size of the HII region is determined by how many ionizing UV photons there are. Typically a hot O star can ionize hydrogen out to a distance of a few tens of parsecs.

You can do an observing project to see dust. Go out on a dark, moonless night and look at the milky way. The milky way is the disk of the galaxy stretched out like a band across the sky. The stars are so distant and so dense that one sees a diffuse glow rather than individual stars. However, the band is not uniform. There are regions where there is no glow. These areas are dust lanes, places where the dust is sufficiently thick that it blocks the visible light coming from more distant stars. This is an extreme example of interstellar extinction which is the dimming of star light caused by the presence of interstellar material. In the dust lanes the extinction is complete. But in other regions it is only partial. Interstellar extinction will be important in computing absolute magnitudes since it will reduce the apparent magnitude of a star over and above the effect of distance. Extinction is quantified in terms of so many "magnitudes" of extinction, i.e., 5 magnitudes of extinction between us and some other star means that the apparent magnitude we observe will be 5 magnitudes larger than what it would be without the interstellar medium. One tries to correct for this by estimating the extinction and then adjusting the star's magnitude appropriately. The average extinction for interstellar haze is about 2 magnitudes per 10 kpc in distance. Notice that extinction is distance dependent; the more stuff you go through the worse it is. This further complicates matters.

Another way that dust manifests itself is in the reflection nebula. Unlike emission nebula which produce light directly from emission from atoms in the nebula, a reflection nebula merely reflects light from some other source, typically a nearby star. Dust particles can scatter photons of light (think of a photon of light hitting a dust particle and bouncing off). The ability to scatter light depends on the size of the particle and the wavelength of the light. Light photons with wavelengths much bigger than the sizes of the particles are much less affected by the particles than photons with wavelengths comparable in size to the particles. Now the dust in interstellar space has diameters ranging from 0.1--1 micron (10^-6 meters). This is 100 to 1000 nm which of course is right in the range of the wavelength of visible light. In practice this means that blue light gets scattered more by the dust than red light. This also explains the blue sky of the Earth by the way. You are seeing blue light preferentially scattered by air molecules (which are very small). Reflection nebula therefore have a bluish appearance. Reflection nebula are often seen close to emission nebula.

This preference for dust to scatter short wavelength light causes another effect known as interstellar reddening. What this means is that there is more light lost from the blue end of the spectrum than from the red end as the light pass through space from the star to you. In practice this means that the U and B apparent magnitudes increase in value faster than the V magnitude. This of course would mess up your calculation of color indices. So again to get accurate photometry one must include a color correction to take into account the effect of reddening. This is expressed in terms of magnitudes in the color indices. A terrestrial example is seen in the highly reddened sun at sunset. The atmosphere is scattering out the blue light leaving the red. If some volcano has sent up lots of fine dust into the air then the reddening becomes even more pronounced. Extinction is a fairly hard thing to measure directly but reddening isn't. By studying the spectrum of a star we get its spectral classification. This gives us a corresponding color index. Comparing this with the observed color index we can deduce the amount of reddening and hence the amount of dust between us and that star. From this we can estimate the total absorption and compute our corrected apparent magnitude.

To summarize the effect of dust: It can absorb photons of light and it can redden a continuum spectrum of light by preferentially scattering out the blue end of the spectrum. The effect of extinction is to increase the apparent magnitude number, that is to decrease the apparent brightness. Reddening increases the color index number, making the star appear redder than it otherwise would.

Example : A star has is a B8 spectral type Luminosity Class V (main sequence). However, its observed peak in its spectrum is in the blue, corresponding to a temperature of around 8000K. What's right or wrong here? A B8 spectral type is a very hot star. From data in the book we know that its temperature should be between 20,000K, and this corresponds to a black body that peaks in the ultraviolet. Hence this star is severely reddened by dust. It therefore follows that it must also be experiencing considerable extinction of its light.

We have talked about gas and dust, and about how we can detect them through emission and about scattering in HII regions and reflection nebula. There are more types of interstellar regions than just those two type of nebula however. One of these is the cold, dense giant molecular cloud. These clouds are not glowing in visible light, although they are associated with some nearby HII regions. Molecular clouds are detected in the radio frequencies. They mainly contain molecular hydrogen H₂, i.e., two hydrogen atoms bound together into an ordinary molecule of hydrogen (NOT to be confused with HII, or ionized hydrogen). What is generally detected however is radio emission from other molecules. The molecules (such as carbon monoxide, hydroxide OH, water, even formaldehyde, ammonia, and alcohol to name just a few) have complex emission spectra associated with their rotational and vibrational states. These are low energy states, hence the photons they emit are in the radio region of the spectrum. Astronomers using radio telescopes have mapped out these great clouds of molecules. Particularly good as a tracer is the carbon monoxide molecule CO. Densities in them range from several hundred to a thousand particles per cc. Temperatures tend to be low, down around 10K.

Hydrogen is the most abundant element in the universe so we should be able to detect lots of it. However, most of it will be at very low temperatures, too low to be ionized or to have many atoms with electrons up in the higher energy orbits. Hence they don't emit any of the visible photons we associate with HII regions. However, radio emission can be detected from neutral hydrogen, or HI regions. This radio emission comes from the difference in energy levels between a proton and its electron (the hydrogen atom) with their "spins" aligned and one where their spins are anti-aligned. (Another way to think of it is as the alignment or antialignment of the intrinsic magnetic N/S axes of the electron and proton.) The point is this: if the electron flips from being aligned to being antialigned it goes from a higher to a lower energy state and emits a photon. The wavelength of that photon is 21 cm. Twenty-one centimeter emission is a very important diagnostic for mapping out the presence of neutral hydrogen in the galaxy, and in other galaxies.

To summarize the states of interstellar gas:

1. HII regions: hot (10000K) ionized hydrogen surrounding hot O and B type stars. Detected by optical emission, particularly the red H alpha emission line. Also HII is found in the warm intercloud medium which has lower density of 0.01 atoms per cubic centimeter.
2. HI regions: cold (100K) regions of neutral hydrogen detected by 21 cm radio emission.
3. Molecular clouds: cold (10K) dense regions containing molecules including molecular hydrogen H₂. Detected in CO radio emission. Often associated with HII regions near or within the molecular cloud.
4. Coronal gas: very hot gas (up to a million degrees), very low density. Gas is probably produced by exploding supernovae.

Copyright © 2005 John F. Hawley. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 124 at the University of Virginia. Reproduction, distribution, and commercial uses are prohibited.