Planets, Stars, Galaxies, and the Universe

The Interstellar Medium (ISM)


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When you observe the night sky, you see the stars as pinpoints of light against a black background. You have also probably been told that outer space is a “vacuum”—that is, that, other than stars and planets, it is very empty. It is true that space is so empty that it is a more perfect vacuum than we can create in the laboratory on Earth; however, space is not completely empty. Here are some images that show some obvious examples of regions in space that contain some material between the stars:

In all of these images, you see different regions of space that are either filled with what appears to be glowing gas (similar to a neon light), or dark regions that are obscuring the stars behind them. All of these objects are called nebulae, a word which is Latin for "cloud." Taken as a whole, the gas and dust that fills the space between stars is called the interstellar medium.

If you look at the images at the links above in some depth, you'll see some or all of the following:

  • Clouds that glow an almost uniform, bright red;
  • Dark clouds that appear to block all or most of the light of stars or bright clouds behind them;
  • Clouds that glow a blue color very similar to the blue of our sky.

At this point, let's address the physical processes responsible for the appearance of these various nebulae, and give them more descriptive names.

Emission Nebulae

As the name implies, emission nebulae emit emission spectra, not continuous or absorption spectra. If you refer to the lesson on the production of emission lines, what is required for a cloud of gas to emit an emission spectrum is for electrons in the gas to be in energy levels above level 1. As they move down to the ground state, they will emit photons of specific energies, producing emission lines at those specific energies. One point we glossed over in that lesson is: How do the electrons get into the higher energy levels in the first place? In the case of emission nebulae, there is a clue usually quite apparent in the image. For example, in the image below of massive stars in NGC 6357, you see a large region of glowing red emission nebula surrounding a number of massive stars (although there is no way to tell just from looking at the picture that they are massive stars).

Massive stars in NGC 6357
Figure 5.1: Telescopic image of massive stars in the star-forming region NGC 6357 illuminating a red emission nebula in their local environment.
Credit: APOD

The massive stars give off a lot of ultraviolet light, which the hydrogen gas absorbs. The electrons in the H atoms absorb so much energy that they are actually stripped completely from the H atoms, creating a sea of hydrogen nuclei and free electrons. This is a process called ionization. The free electrons can recombine with the hydrogen nuclei, and then, as they cascade down to the ground state, they emit photons. Because the clouds are primarily made of hydrogen, the spectrum of a typical emission nebula shows strong hydrogen emission lines. The strongest hydrogen line in the visible part of the spectrum is H-alpha at 656.3 nm, which is in the red part of the spectrum.

Astronomers refer to neutral hydrogen atoms as HI (pronounced "H-one") and they refer to ionized hydrogen nuclei as HII ("H-two"). For this reason, the glowing red emission nebulae found surrounding massive stars are often referred to as "HII Regions."

We will talk about these later, but there are objects known as planetary nebulae that also give off emission spectra. Here is an example emission spectrum from a planetary nebula taken by Prof. Robin Ciardullo of Penn State:

Two dimensional plot of an example of an emission spectrum from a planetary nebula
Figure 5.2: Emission Spectrum
Credit:Robin Ciardullo, Penn State Astronomy & Astrophysics

On the x-axis is wavelength in Angstroms (1 Angstrom is 1/10 of a nanometer, so 656.3 nm = 6563 Angstroms), and on the y-axis is the relative flux (or apparent brightness). You can see that there are several very prominent emission lines—the H-α line at 6563 Angstroms, the H-β line at 4861 Angstroms, a helium line, and several oxygen lines, for example. In this nebula, the oxygen line at 5007 Angstroms is stronger than H-α, so this nebula will appear more green than red. This is seen in the central regions shown in the photo below.

Image of the Ring Nebula (M57)
Figure 5.3: Telescopic image of the Ring Nebula (M57) showing color variations in the inner region vs. the outer region.
Credit: APOD

Dark Nebulae

In regions where there are a lot of stars (such as in the APOD of a Giant Molecular Cloud), or regions with bright emission nebulae (such as in the Hubblesite image of a Bok Globule), we can see dark nebulae that appear to be blocking the background light sources behind them. These dark nebulae are also interstellar clouds, but unlike the emission nebulae, they are very cold (10 K, as opposed to about 10,000 K for emission nebulae) and very dense. These clouds are called molecular clouds because the conditions in them are just right for the creation of molecular hydrogen. Be aware that two hydrogen atoms that have linked together form the molecule H2 (which is also pronounced "H-two", but should not be confused with HII). Another constituent of these dark clouds is interstellar dust. The dust particles are very tiny grains of different solids, and if you compare their chemical makeup to common materials on Earth, they are similar in many ways to soot and sand. As photons of light encounter a dark cloud of gas and dust, we observe two effects: first, the light is extinguished—that is, the background light sources appear dimmer than if there was no intervening cloud. This effect is referred to as interstellar extinction. The second effect is referred to as interstellar reddening. As the name suggests, the light from a background source will appear redder than if it had not passed through the cloud. As photons of light pass through the cloud, they get scattered from their original paths. Blue light scatters more than red light, so less blue light and more red light from the background object makes it through the cloud. If you look closely at the APOD image of Barnard 68, you will see that all the stars visible near the edge of the cloud look red or orange, while those just outside the boundaries of the cloud look blue or white, which is caused by this reddening effect. Again because of the wavelength dependence of scattering by dust, most of the infrared light will actually pass through the cloud. See the image below of Barnard 68 taken through infrared filters. You can see the reddening, but you can also see right through the cloud!

Telescopic image of Barnard 68 taken through infrared filters showing the reddening of starlight by this foreground cloud.
Figure 5.4: Barnard 68 taken through infrared filters.

Reflection Nebulae

The bright blue regions seen in some of the images above are reflection nebulae, and they are also caused by the scattering of light by dust particles. The light from a star encounters dust particles and reflects. In most cases, the particles are the correct size to scatter blue light more efficiently than red light, so the reflection nebula appears to us in the reflected blue light from the nearby star. Since the light we see is reflected starlight, the spectrum of the reflection nebula is similar to the spectrum of the star's light.

More complex nebulae

Finally, let's consider some of the images of the regions that include overlapping nebulae of different types. First, look again at the very famous picture of a region of sky called the Eagle Nebula.

Image of the Gaseous Pillars from the Eagle Nebula
Figure 5.5: Hubble Space Telescope image of gaseous pillars visible in the Eagle Nebula
Credit: Hubblesite

The dark pillars are part of the edge of a molecular cloud. Some nearby, very bright stars (off the edge of the picture) are illuminating these pillars of gas, and their intense radiation is causing the outer layers of the molecular cloud to evaporate (this process is called photoevaporation). What astronomers have found is that this process of photoevaporation is revealing that inside the densest knots in the pillars are newly formed, baby stars. If you study carefully some of the other images, for example the panorama of Carina, you can find similar structures to the ones seen in the Eagle Nebula.

In fact, we see that all of the youngest stars in the sky live in regions with a lot of interstellar matter. It seems clear that there must be a connection between the stars and the interstellar gas. By observing these regions in many wavelengths of light, astronomers have found that new stars are formed out of the gas and dust in these clouds. The process is not completely understood, but in the following section, we will discuss how stars form.

Want to learn more?

The Eagle Nebula image is one of the most famous Hubble images ever taken, and so, PBS has made a video on the making of the Eagle Nebula image.