Chapter 14: The Milky Way

The components of the interstellar medium — atoms, molecules, and dust grains — directly affect the light emitted by stars. When light from a distant star passes through the material between stars, the interaction changes the light's properties, such as its intensity in different colors. The physical laws that describe these changes are complex but can be reduced to several basic principles.

The spectrum of the Sun, with dark aborption lines labeled as the element or molecule they represent.

When electromagnetic radiation of any kind (ultraviolet light, visible light, infrared, radio waves, and so on) interacts with individual atoms or molecules, radiation can only be absorbed or emitted according to the difference between fixed energy levels in the atom or molecule. The result is a set of sharp spectral features – this is also how we study the spectra of stars. When electromagnetic radiation interacts with much larger particles such as dust grains, the type of interaction depends on the chemical composition of the particles and their sizes relative to the wavelength of the light. Also, the appearance of the light from a star, and the appearance of the particles it illuminates, may depend on the direction from which the observer looks.

The interstellar medium is composed of particles that span a wide range of sizes. A hydrogen atom has a radius of 5 × 10^-11 m; typical interstellar molecules have sizes 10 to 20 times larger, ranging up to 10^-9 m or 1 nm. Interstellar grains are tiny, but they are made of very large numbers of atoms or molecules. Grains range in size from 5 × 10^-9 m to 10^-6 m, or a wide range of 5 to 1000 nm, similar to the range of sizes of the particles found in cigarette smoke. For comparison, the average wavelength of visible light is 500 nm.

Radiation interacts differently with each of the three types of interstellar particles — atoms, molecules, and dust grains. In reality, the interstellar material is always a mixture of gas and dust, but it is easier to understand the effects if we imagine separate interactions of light with atoms, with molecules, and with dust grains. Typically, the interstellar material is concentrated in a cloud. The Stefan-Boltzmann law tells us that hot stars emit far more energetic (short wavelength) photons than cool stars. Luminous, hot stars are also young, and they are therefore less likely to have drifted away from the gaseous and dust-filled region of their birth. As a result, a nebula is much more likely to be lit up by a hot star than a cool star.

As light with a broad range of wavelengths enters the nebula, photons with certain wavelengths will have just the right energy to excite the gas, or knock electrons from lower to higher energy levels. Photons with a short wavelength may even have enough energy to ionize the gas, or knock electrons clear out of the atoms. Each time a photon excites or ionizes an atom, that photon is absorbed. When the atom re-emits the photon, it may do so in any direction. Thus photons with wavelengths corresponding to the electron energy transitions in atoms are redirected, while the much greater number of photons that do not correspond to an electron transition passes through the gas undisturbed. The effect is to subtract energy from the light beam at specific wavelengths. Looking at the star through the nebula, we would see absorption lines created by the interstellar material.

 

An example of how a spectrum is affected by whatever material is in front of the object of interest.

As the electrons cascade back down through the energy levels of the atoms, they create emission lines. The photons in these emission lines leave the nebula in all directions so that an observer off to one side would see the nebula glowing in the various colors corresponding to the strongest spectral lines. This is an important astronomical example of Kirchoff's laws of radiation. The colors of a nebula are hard to see with the eye, even with large telescopes, because the light's intensity is low and the eye's color sensitivity is poor at low light levels (the reason why a moonlit scene looks less colorful than in daylight). The eye also cannot sum up all the photons it receives over many seconds into one image. However, sensitive films and electronic detectors can record the colors accurately. Some spectral lines are much more effective at removing the energy from a nebula than others. Since hydrogen is the most abundant gas, and since the red Hα emission line is one of its strongest transitions, many clouds of excited gas glow with a beautiful deep red color. Another important transition of oxygen can give a nebula a beautiful blue-green tinge.

Starlight also causes a nebula to glow with thermal radiation. When a gas absorbs photons, it is gaining energy. As electrons are freed from atoms, they collide with other electrons and atoms. The effect of these collisions is to increase the speed of the average particle. This means that the temperature of the gas increases. Starlight can therefore heat up a remote gas cloud. The balance between heating by absorption and cooling by re-emission governs the temperatures of the gas and the dust. A cloud very close to a star is heated to a temperature of several thousand degrees Kelvin, with thermal emission that peaks at visible wavelengths or in the near-infrared. A cloud far from a star is much colder — a temperature of tens or hundreds of degrees Kelvin — and the thermal emission peaks at far infrared or radio wavelengths. Thermal radiation travels in all directions and is seen by observers in any direction from the cloud. Remember that thermal radiation is seen as the smooth part of a spectrum, not as individual emission lines.

Hydrogen energy diagram

Starlight can also interact with molecules in a nebula. Molecules are two or more atoms bound together by weak electrical forces. Like atoms, molecules have characteristic spectra that are related to the structure of their internal energy levels. Molecular spectra are typically more complicated than those of single atoms because the rotations and vibrations of molecules add together many closely spaced energy states. When observed with a low spectral resolution, this can give the appearance of an absorption or emission band rather than a single narrow line. Tiny amounts of energy can cause vigorous rotation in molecules, which explains why many molecular absorption and emission features appear at low-energy infrared or sub-millimeter wavelengths. Molecular spectral features are observable even in very cold, low-density interstellar environments.

 

Finally, starlight can interact with interstellar dust grains. Dust grains are much larger atoms and molecules and their interactions with photons are quite different. The larger particles affect colors over a much broader range of wavelengths than individual spectral lines or bands. There are two important observational effects. Grains absorb incoming optical and ultraviolet radiation and re-emit it in the far infrared, with a thermal spectrum that reflects the cold temperature of the grains. The result is a general dimming of starlight at all wavelengths, called interstellar extinction. Since the dust grains are constantly colliding with the individual atoms and molecules, all these different-sized particles are in equilibrium and they all have the same temperature.

Many dust grains, thinly distributed throughout the interstellar gas, produce a general haze or "interstellar smog." By contrast, when dust grains are concentrated in distinct clouds, their effects can be much more dramatic. Sometimes dust can almost extinguish the background light. An observer who looks at the dust cloud from the side will see the blue light scattered out of the beam, however, so that a nebula illuminated in this way will have a bluish color. The light from a reflecting nebula arises from scattering by dust rather than atoms or molecules. The blue color of the nebulosity is caused partly by dust scattering and partly by the fact that the light being reflected comes from a hot, blue star.

The blue color of the sky is caused by Rayleigh scattering of sunlight by the gases in the Earth's atmosphere. The above image shows the degree to which Rayleigh scattering scatters blue light more intensely than red light.

You don't have to visualize remote regions of space to understand the phenomena of extinction and reddening. Just step outside! Why is the sky blue? Earth’s atmosphere is full of gas molecules and tiny dust particles. The molecules and many of the dust particles are much smaller than the wavelength of light, so scattering is more efficient for short wavelengths of light. Light from the Sun must pass through these particles before it can reach our eyes. Blue light is therefore removed from the Sun’s beam of light. So where does the blue light go? Air molecules and dust particles scatter the blue light many times until it reaches our eyes with nearly equal intensity from every direction. Therefore, we see the sky as blue. We know that the blue color is not a property of the air itself, since the air in a jar or a room is transparent and colorless.

 

Dramatic Rayleigh scattering in the atmosphere after sunset

Why is the setting Sun red? When the Sun is high in the sky, we look through the minimum amount of gas and dust. Thus the reddening is minimal, and the Sun is perceived as yellow. At sunset, the sunlight passes through much more gas and dust, shifting the light to the red end of the spectrum.