Interstellar medium is the material between the stars, made up of dust and gas. In places the matter is very thin and tenuous, other places the dust is very thick and can obscure the light from faraway stars.
This is the view we have, looking toward the center of our galaxy. The dark areas are dense clouds of interstellar dust. The brightly colored clouds are emission nebulae (reddish) and reflection nebulae (blue).
This video shows what it would look like to travel through the dust lanes and on toward the center of the Milky Way. From our viewpoint at the current time, the galaxy center lies in the direction of the Sagittarius constellation. This won't change much in our lifetime, since it takes some 250 million years to orbit the galaxy.
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Dust grains in space are similar to dust on Earth, small particles made of clumps of atoms and molecules. Since they are so small, they tend to block short wavelengths of light more effectively than they block longer wavelengths. This takes away more blue light than red light, and so, reddens the light as it passes through.
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This reddening produces a very different effect on the observed spectral lines than redshift caused by the Doppler shift. Recall that the Doppler shift actually increases the wavelength of the light, having the effect of shifting spectral lines against the background of the continuous spectrum.
Interstellar reddening doesn't shift the lines, it just decreases the strength of the lines. Since it decreases the blue end of the spectrum more than the red, the light appears redder as a result. The overall dimming of light when it passes through dust is called extinction. If we know how the star should look to us, from its spectral class and luminosity class, and we can see how much the light has been reddened and dimmed, we can estimate how much dust and gas must lie between the star and us.
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We can also see that the starlight becomes polarized as it passes through the interstellar dust. This means that the grains must be elongated and at least somewhat aligned to each other. Light passing through a polarizing medium tends to make electrons oscillate along the grains much more than across the grains. This tends to preferentially absorb light that is aligned with the grains. The dust grains must have some magnetic properties, since they are most likely aligning along the magnetic fields present in interstellar space.
21 centimeter radiation
Protons and electrons have an intrinsic property we call spin. Now, we know that protons and electrons are not really like spinning little marbles. They have wavelike properties as well as particle-like properties.
A hydrogen atom consists of an electron bound to a proton. Electron spins are quantized properties. That means they are either spin-up (parallel to the proton) or spin-down (antiparallel to the proton). The spin-down state has lower energy.
Recall that when we studied the excited states of an electron, we saw that an electron could be raised to an excited state when a photon of the correct energy was absorbed, and then dropped back down to ground state by emitting a photon.
An electron in the interstellar medium can raise its spin state to being parallel to the proton by absorbing a photon that has the right energy. The electron then releases a photon and drops back down to its lower energy spin state. The wavelength of this photon is 21 centimeters. It creates a line in the spectrum that correlates to this wavelength. By observing how strong the 21-cm line is in an absorption spectrum, we can tell how much hydrogen gas is present in the interstellar medium.
The image above shows the hydrogen gas, mapped by using the detected 21-cm radiation. The map shows the abundance of hydrogen, as well as the Doppler shifted velocity. Here, Blue indicates the gas is moving toward us and green indicates it is moving away. This map shows the whole sky, in a Mercator-type projection. The bright streak through the middle is the plane of the Milky Way galaxy, and the bright spots are the Magellenic clouds.
Emission and reflection nebulae
In general, a nebula is a cloud in space made of dust and gas (mostly hydrogen and helium). If the nebula is dense enough to obscure starlight behind it, we see it as a dark patch. In the next chapter, we will see that the material in a nebula can gravitationally contract to form new stars. If there are stars shining inside a nebula, the energy in the light will heat up the cloud and make it glow. We call this kind of nebula an emission nebula because the hot gas produces an emission spectrum.
The above image shows the Bubble nebula, a huge bubble of dust and gas expanding from the stellar wind from a massive central star. In the upper left is another object, a giant molecular cloud. The radiation from the nebula is heating up the molecules in the molecular cloud, causing it to glow. The spectrum of the light from molecular matter contains spectral lines not seen in a simple gas. Molecules are more complex, and can have excitation states arising from rotation and vibration within the molecule.
This animation from hubblesite.org shows what it would be like to make a flyby of the Bubble nebula.
This image shows a collection of an emission nebula, reflection nebulae dust lanes. Can you pick them out?
The emission nebula (NGC 2170) is the red cloud in the upper left, with dark dust lanes to its right. The blue reflection nebulae can be seen nearby. By the way, those four bright rays extending from many of the stars are not real. They are an effect called "lens flare" that arises from the light interacting with the lens.
The interstellar medium consists of gas and dust lying between the stars. Exploring the properties of the interstellar medium is aided by the fact that hydrogen gas can be mapped by the 21 cm radiation emission caused by spin-flipping of the electrons. Interstellar reddening is caused by the preferential absorption of blue light over red light.