Chapter 19: Between the Stars: Gas and Dust in Space

To understand how stars are born, we must first understand something about the raw material they are made from. That raw material is the gas and dust that lies in the vast regions between stars, called interstellar matter. The term interstellar medium is used to describe the entire collection of interstellar matter.

In many places, material has aggregated into "clouds" that we refer to as nebulae. Nebulea emit radiation in the infrared and radio frequencies. They are sometimes lit up by the scattered light of nearby stars producing beautiful mixtures of color.

19.1 The Interstellar Medium

Most of the material between stars is gas. The most abundant elements in this gas are hydrogen and helium, the same materials that stars are composed of. Only 1% of material is not gas, and it is tiny frozen particles commonly called interstellar dust.

The gas in the interstellar medium is not distributed uniformly, but tends to come together in large "clouds". These clouds are what are seen in the images in this chapter, particularly Figures 19.1 and 19.2. The density of the gas in the clouds is incredibly low, only about 1000 (103) atoms per cubic centimeter. For comparison, the air around us has about 1019 atoms per cubic centimeter, and the best vacuums achieved on Earth have about 107 atoms per cubic centimeter. While the density in the cloud is low, the clouds can be seen because they are large, typically tens of lightyears across.

19.2 Interstellar Gas

The colors produced in the clouds tell us about their temperature and composition. Red is a common color, and comes from the red emission line of the hydrogen atom. In order to produce emission from hydrogen, a significant amount of hydrogen (basically all) must be ionized by ultraviolet radiation. Then, periodically, an electron will recombine with a hydrogen ion, then it will emit radiation as it makes its way to the ground state.

Regions where the hydrogen is ionized are called H II regions. In this notation, H I is unionized hydrogen, H II is ionized hydrogen, and Fe III is doubly ionized iron. H II regions are generally near hot stars that emit lots of ultraviolet radiation. H II regions contain other ionized elements as well, but the red hydrogen line dominates, so they appear red.

Neutral Hydrogen Clouds

When the hydrogen is not near a hot star, then it will remain cold and unionized (neutral). Neutral hydrogen clouds are harder to see, they don't emit lots of visible light to give away their presence. They do produce absorption lines in spectra, but it is hard to tell if the absorption is due to a separate cloud of hydrogen, or hydrogen around a star, except in special circumstances. One special case is if a binary star lies behind the hydrogen cloud. Such a case was seen, and the absorption lines due to the binary stars moved due to the doppler shift, but the lines from the cloud didn't.

There are also other special cases where lines of other atoms or molecules can be seen in the cloud to help identify it.

The 21 cm Line

But by far the most powerful method for observing cold hydrogen clouds is to detect the 21 cm radio emission from hydrogen.

Recall that hydrogen is formed by a proton with an electron orbiting about it. Both the proton and electron possess a property called spin. The spin of a particle is like the spin of a top, hence the name. The proton and electron both have spin, and due to the strange world of quantum physics, the spin can be either "up" or "down", but not in between. (This actually makes life easier, we only have two possibilities to consider.) The spin by itself wouldn't have any effect, but along with the spin comes a magnetic field.

The proton and electron behave like two bar magnets, they attract each other when N of one is aligned to S of the other, and they repel each other when N is aligned with N. This difference in attraction/repulsion produces a small difference in energy between the configuration where the proton and electron spins are in the same direction and the configuration with the spins in opposite directions. The difference in energy means that what we normally treat as a single energy level is really two energy levels, slightly separated. If an electron in a hydrogen atom flips the direction of its spin, it can absorb or emit a photon with an energy equal to the difference between the two energy levels. This energy corresponds to a photon with wavelength of 21cm.

Detecting the 21cm radiation from hydrogen clouds was accomplished in 1951 using a special radio antenna (horn).

Ultra-Hot Interstellar Gas

It was a surprise to discover hot interstellar gas. Astronomers now understand that this gas is heated by supernovae explosions which will be discussed in ch. 22.

19.3 A Model of the Interstellar Gas

Table 19.1 summarizes some of the different types of interstellar clouds. Notice that temperatures range from about 10 K to 106 K, and densities from 105 per cubic centimeter down to 10-3 per cubic centimeter, with the highest densities going with the lowest temperatures.

19.4 Cosmic Dust

Dark Nebulae

Astronomers see great dark voids in the background of stars. For a long time they wondered if these were areas lacking in stars, or if there was something that blocked the light of stars from reaching us. Careful study has shown that these regions are filled with dust that attenuates (reduces) the light.

Dark nebulae are opaque in visible light but glow brightly in the infrared. The infrared light follows a blackbody spectrum, so we can relate the peak wavelength to the temperature of the cloud, typically 10K to 100K. The Milky Way has many regions of dark nebulae, especially along its axis.

Reflection Nebulae

Some clouds reflect light from nearby stars making them visible to us. The small dust grains tend to scatter blue light more efficiently than red, making them appear blue to us. Of course, gas and dust can be mixed in a nebulae, giving us a beautiful mix of colors as seen in the Trifid nebula shown in Figure 19.10 (19.12 in the secondedition).

Interstellar Reddening

Interstellar dust tends to scatter blue light and transmit red light. If the cloud is lighted from the side, we see the reflected (scattered) blue light. If the cloud is lighted from behind, we see the transmitted red light. Not only does the cloud transmit red light, it makes the stars behind it appear more red -- their blackbody spectrum appears more red -- leading one to assume that the stars are relatively cool. But the spectral lines can indicate that the stars are hot. The contradiction is due to the reddening of the starlight by the dust cloud.

19.5 Cosmic Rays

Along with gas and dust, interstellar space is filled with fast nuclei, protons, electrons, and positrons (anti-electrons). These are collectively known as cosmic rays. Cosmic rays are constantly striking the Earth. Many of them are absorbed in the Earth's atmosphere, but some make it to the surface. In an area of 1m2 on the surface there is about 1 cosmic ray striking every second. The total energy in cosmic rays striking the Earth is comparable to the total energy in starlight striking the Earth.

The source of cosmic rays is not readily apparent from their direction. Cosmic rays are charged particles, and charged particles follow a curved path when they pass through a magnetic field. Magnetic fields are found throughout interstellar space. Magnetic fields are associated with planets, stars, black holes, and entire galaxies. On their journey from their source to us, cosmic rays are constantly curving and changing direction making it impossible for us to determine their origin. The preferred candidates for a source of cosmic rays are supernova explosions (ch. 22). It is estimated that cosmic rays travel around the Milky Way about 30 times before striking something.

© Robert Harr 2002