This page was copied from Nick Strobel's Astronomy Notes. Go to his site at for the updated and corrected version.

Interstellar Medium and the Milky Way

All of the stars you can see at night and several hundred billion more are all bound together gravitationally into a huge cluster called a galaxy. Most of the stars in our galaxy are far enough away that they blend together in a thin band across the sky. If you are fortunate enough to view a dark sky outside of the glare of the city lights, you will see this milky band running through the constellations Cassiopeia, Perseus, Taurus, Monoceros, Vela, Crux, Norma, Sagittarius, Scutum, Aguila, Cygnus, and Lacerta. To people of long ago, this band looked like milk had been spilled along a pathway, so it was called the Milky Way. This chapter covers the radical discoveries made in the 20th century of the nature of the Milky Way and our place in it. The vocabulary terms are in boldface.

Interstellar Medium (ISM)

Though the space between the stars is emptier than the best vacuums created on the Earth (those are enclosed spaces devoid of matter, not the household cleaning appliances), there is some material between the stars composed of gas and dust. This material is called the interstellar medium. The interstellar medium makes up between 10 to 15% of the visible mass of the Milky Way. About 99% of the material is gas and the rest is "dust". The interstellar medium affects starlight and stars (and planets) form from clouds in the interstellar medium, so it is worthy of study. Also, the structure of the Galaxy is mapped from measurements of the gas.


The dust is made of thin, highly flattened flakes or needles of graphite (carbon) and silicates (rock-like minerals) coated with water ice. Each dust flake is roughly the size of the wavelength of blue light or smaller. The dust is probably formed in the cool outer layers of red giant stars and dispersed in the red giant winds and planetary nebulae.

a typical tiny piece of dust


Starlight passing through a dust cloud can be affected in a couple of ways. The light can be totally blocked if the dust is thick enough or it can be partially scattered by an amount that depends on the color of the light and the thickness of the dust cloud. All wavelengths of light passing through a dust cloud will be dimmed somewhat. This effect is called extinction.

Bok globules in IC 2948
Dark dusk clouds like these "Bok globules" in IC 2948 were once thought to be holes in the sky. The dark clouds block the light from the emission nebula behind.
Barnard 86 silhouetted against the stars
A dark cloud, Barnard 86, is silhouetted against a starry background. Stars form in the dark clouds. A young cluster, NGC 6520, probably associated with Barnard 86 is seen just to the left of it.

The discovery of the dust is relatively recent. In 1930 R.J. Trumpler (lived 1886--1956) plotted the angular diameter of star clusters vs. the distance to the clusters. He derived the distances from inverse square law of brightness: clusters farther away should appear dimmer. IF clusters all have roughly the same linear diameter L, then the angular diameter q should equal a (constant L) / distance. But he found a systematic increase of the linear size of the clusters with distance.

This seemed unreasonable! It would mean that nature had put the Sun at a special place where the size of the clusters was the smallest. A more reasonable explanation uses the Copernican principle: the Sun is in a typical spot in the Galaxy. It is simply that more distant clusters have more stuff between us and cluster so that they appear fainter (farther away) than they really are. Trumpler had shown that there is dust material between the stars! The extinction of starlight is caused by the scattering of the light out of the line of sight, so less light reaches us.

extinction and reddening of  starlight


star with and without dust in frontNot all wavelengths are scattered equally. Just as our air scatters the bluer colors in sunlight more efficiently than the redder colors, the amount of extinction by the interstellar dust depends on the wavelength. The amount of extinction is proportional to 1/(wavelength of the light). Bluer wavelengths are scattered more than redder wavelengths.

The 1/l behavior of the scattering indicates that the dust size must be about the wavelength of light (on the order of 10-5 centimeters). Less blue light reaches us, so the object appears redder than it should. This effect is called reddening, though perhaps it should be called "de-blueing". If the dust particles were much larger (say, the size of grains of sand), reddening would not be observed. If the dust particles were much smaller (say, the size of molecules), the scattering would behave as 1/l4. Trumpler showed that a given spectral type of star becomes increasingly redder with distance. This discovery was further evidence for dust material between the stars. If the Sun is in a typical spot in the Galaxy, then Trumpler's observation means that more distant stars have more dust between us and them.

You see the same effect when you observe the orange-red Sun close to the horizon. Objects close to the horizon are seen through more atmosphere than when they are close to zenith. At sunset the blues, greens, and some yellow are scattered out of your line of sight to the Sun and only the long waves of the orange and red light are able to move around the air and dust particles to reach your eyes.

preferential scattering of short wavelengths

Review Questions

  1. What is the interstellar medium composed of?
  2. How do astronomers know that the non-gaseous part of the ISM cannot be made of rocks but, rather, of small "dust" particles?
  3. How does dust make stars appear redder than they actually are?
  4. How does dust cause the extinction of starlight?
  5. Where is the dust thought to come from?


About 99% of the interstellar medium is gas with about 90% of it in the form of hydrogen (atomic or molecular form), 10% helium, and traces of other elements. At visible wavelengths, however, dust has a greater effect on the light than the gas. The presence of interstellar gas can be seen when you look at the spectral lines of a binary star system. Among the broad lines that shift as the two stars orbit each other, you see narrow lines that do not move. The narrow lines are from much colder gas in the interstellar medium between us and the binary system.

The hydrogen gas is observed in a variety of states: in ionized, neutral atomic, and molecular forms. The ionized hydrogen emits light in the visible band as the electrons recombine with the protons and the neutral atomic and molecular hydrogen emits light in the radio band of the electromagnetic spectrum.

H II Regions

H II regions are regions of hot (several thousand K), thin hydrogen emission nebulae that glow from the fluorescence of hydrogen atoms. The roman numeral "II" of H II means that hydrogen is missing one electron. A He III nebula is made of helium gas with two missing electrons. A H I nebula is made of neutral atomic hydrogen. Ultraviolet light from hot O and B stars ionizes the surrounding hydrogen gas. When the electrons recombine with the protons, they emit light mostly at visible wavelengths, and primarily at a wavelength of 656.3 nanometers (giving the hydrogen emission nebulae their characteristic red color). In this conversion of the ultraviolet energy, each ultraviolet photon produces a visible photon. The temperature of the stars causing the nebula to fluoresce can be estimated from this even though the O and B stars are hidden inside the nebula. Fluorescent light bulbs operate on the same basic principle except they use mercury vapor to produce ultraviolet light. The ultraviolet light is then converted to visible light by the phosphor layer on the inside of the glass bulb.

Below is a famous H II region called the Orion Nebula. It is the fuzzy patch you can see in the sword part of the Orion constellation. It is the closest large star formation factory to us and is explored in more detail in the stellar evolution chapter. Selecting the image will bring up a close-up of the heart of the nebula showing the four hot "Trapezium"stars (four O and B stars making a trapezoid figure) at the center of each of the images by AAO and HST.

the Orion Nebula
AAO image

Another large H II region is the Lagoon Nebula in the constellation Sagittarius. It is about 5000 light years away and spans 90 by 40 arc minutes in our sky. Converting the angular size to a linear size, the Lagoon Nebula is about 130 by 60 light years in extent (the Orion Nebula is only 29 by 26 light years in size). The complex interaction of the intense radiation from the hot stars lighting up the nebula, varying densities of the gas and dust, and temperature differences creates twisted, turbulent features in the nebula that have been explored by the Hubble Space Telescope. Selecting the image will bring up the high-resolution image of the heart of the Lagoon Nebula from the Hubble Space Telescope.

the Lagoon  Nebula
AAO image

Next to the Lagoon Nebula on our sky (but closer to us in space) is the Trifid Nebula, so-called because of the dust lanes that trisect the H II region behind them. The image below is nice one to illustrate the three types of nebulae: the red H II region behind a dark dust nebula (showing the effect of the extinction of light) and next to them a blue reflection nebula (showing the preferential scattering of shorter wavelengths).

the Trifid Nebula

O and B-type stars are only found in regions of star formation because they are young stars. These hot, very luminous stars do not live long enough to move away from where they were formed. Since stars form in clusters, where O and B stars are found, there are sure to be smaller, lower-mass stars still forming. The spectra of H II regions are much simpler than star spectra so they are easier to decipher. The composition and conditions inside the H II regions are easier to determine and understand than for stars, so H II regions provide a valuable tool for understanding the history of star formation in a galaxy.

H II regions also provide a convenient way to map the structure of a galaxy because they are so large and luminous. In our galaxy the H II regions are distributed in a spiral pattern. The best wavelengths to use to map the distribution of hydrogen, however, are in the radio band. Most of the hydrogen gas is not ionized because O and B stars are rare. Also, energy in the form of radio passes easily through dust.

21-cm Line Radiation

Most of the hydrogen gas in the interstellar medium is in cold atomic form or molecular form. In 1944 Hendrik van de Hulst predicted that the cold atomic hydrogen (H I) gas should emit a particular wavelength of radio energy from a slight energy change in the hydrogen atoms. The wavelength is 21.1 centimeters (frequency = 1420.4 MHz) so this radiation is called 21-cm line radiation. The atomic hydrogen gas has temperatures between 100 K to about 3000 K.

Most of the hydrogen in space (far from hot O and B-type stars) is in the ground state. The electron moving around the proton can have a spin in the same direction as the proton's spin (i.e., parallel) or spin in the direct opposite direction as the proton's spin (i.e., anti-parallel). The energy state of an electron spinning anti-parallel is slightly lower than the energy state of a parallel-spin electron. Remember that the atom always wants to be in the lowest energy state possible, so the electron will eventually flip to the anti-parallel spin direction if it was somehow knocked to the parallel spin direction. The energy difference is very small, so a hydrogen atom can wait on average a few million years before it undergoes this transition.

how hydrogen produces 21-cm line radiation

Even though this is a RARE transition, the large amount of hydrogen gas means that enough hydrogen atoms are emitting the 21-cm line radiation at any one given time to be easily detected with radio telescopes. Our galaxy, the Milky Way, has about 3 billion solar masses of H I gas with about 70% of it further out in the Galaxy than the Sun. Most of the H I gas is in disk component of our galaxy and is located within 720 light years from the midplane of the disk. What's very nice is that 21-cm line radiation is not blocked by dust! The 21-cm line radiation provides the best way to map the structure of the Galaxy.

Using 21-cm line radiation to Map the Galaxy

The intensity of the 21-cm emission line depends on the density of the neutral atomic hydrogen along the line of sight. Atomic hydrogen all along the line of sight will contribute to the energy received. You need a way to determine the distance to each clump of hydrogen gas detected. Then when you observe the Galaxy in different directions, you can get a three-dimensional picture of the Galaxy. Using the rotation curve, the doppler-shifted radio emission can be converted into distances to the hydrogen clouds.

The rotation curve is a plot of the orbital velocity of the clouds around the galactic center vs. their distance from the Galaxy center. The term "rotation" in this context refers to the motion of the galactic disk as a whole---the disk made of stars and gas clouds appears to spin. The gas clouds are assumed to move in the plane of the disk on nearly circular orbits. Jan Oort (lived 1900--1992) found in 1927 that stars closer to the galactic center complete a greater fraction of their orbit in a given time than stars farther out from the center. This difference in the angular speeds of different parts of the galactic disk is called differential rotation.

differential rotation

The rotation curve can be determined by looking at the doppler velocities of hydrogen gas along different lines of sight. The 21-cm emission will include contributions of hydrogen at different distances from the galactic center and different doppler shifts with respect to us. Some of the emission will be from gas clouds just inside the orbit the orbit of the Sun moving at slightly faster angular speeds than the Sun. They will have a small redshift. The part of the total emission coming from gas closest to the galactic center will have the greatest redshift because that gas is moving at the greatest angular speed. In the figure below the line from the galactic center to the fast moving gas (called "Rmin") makes a 90 angle with respect to our line of sight. Using basic trigonometry, the distance of the fast moving gas (at "A") from the galactic center = (the Sun's distance) sin[(galactic longitude)], where the galactic longitude is the angular separation between the cloud and the galactic center and sin() is the "trigonometric sine" function (it is the "sin" key on a scientific calculator). Angle by angle, strip by strip, the rotation curve is constructed from the maximum doppler velocity along different lines of sight.

Once the rotation curve is determined, the Galaxy's structure can be mapped. The 21-cm line profile has several doppler shifted peaks that are narrow and well-defined (see the figure below). Using the known rotation curve, you can convert the doppler speeds of the peaks to get the distance to the hydrogen producing each peak. The intensity of each peak depends on the density of the hydrogen gas cloud. The mapping surveys show that the hydrogen gas is distributed in a spiral pattern in a thin disk for almost all of the Galaxy.

geometry for mapping the Galaxy


Different types of atoms can combine in the coldest regions of space (around 10 K) to make molecules. The cold molecules are detected in the radio band. Most of the molecules are hydrogen molecules (H2) and carbon monoxide (CO). Actually, molecular hydrogen does not emit radio energy but it is found with carbon monoxide, so the radio emission of CO is used to trace the H2. Other molecules include such familiar ones as H2O (water), OH (hydroxide), NH3 (ammonia), SiO (silicon monoxide), CO2 (carbon dioxide) and over a hundred other molecules. Many of the molecules have carbon in them and are called organic molecules. The organic molecules and the water and ammonia molecules are used in biochemical reactions to create the building blocks of life: amino acids and nucleotides. The presence of these molecules in the interstellar medium shows that some of the ingredients for life exist throughout the Galaxy. Some carbonaceous meteorites reaching the Earth have amino acids in them---apparently, amino acids can be created in conditions too harsh for normal biological processes.

Molecular clouds

Based primarily on observations of the CO emission, astronomers find that the molecules in the interstellar medium are clumped together into clouds with masses anywhere from just a few solar masses to over a million solar masses with diameters ranging from several light years to over 600 light years. Molecules need to have some sort of shielding from the high-energy light from stars. Otherwise, the energetic photons would dissociate the molecules. Molecular clouds have dust in them. The dust grains may provide the shelter for molecules to form. Compared to the size of atoms, the dust grains are enormous and have many pits and recesses for atoms to congregate and combine.

Stars form in the molecular clouds. If the molecular cloud is cold and dense enough, it can collapse under its own gravity. Smaller fragments can form and produce stars (see the stellar evolution chapter for further details). The Milky Way has about 2.5 billion solar masses of molecular gas with about 70% of it in a ring extending from 13,000 to 26,000 light years from the center. Not much molecular gas is located at 4,900 to 9,800 light years from center but about 15% of the total molecular gas mass is located close to galactic center within 4,900 light years from the center. Most of the molecular clouds are clumped in the spiral arms of the disk and stay within 390 light years of the disk midplane.

Molecular Hydrogen and Carbon Monoxide Connection

Molecular hydrogen H2 does not produce radio emission. It produces absorption lines in the ultraviolet. However, the gas and dust become so thick in a molecular cloud that the ultraviolet extinction is too large to accurately measure all of the H2 in the interior of the cloud. Fortunately, there is evidence of a correlation between the amount of CO and H2, so the easily detected CO radio emission lines (at 2.6 and 1.3 mm) are used to infer the amount of H2. The CO emission is caused by H2 molecules colliding with the CO molecules. An increase in the density of the H2 gas results in more collisions with the CO molecules and an increase in the CO emission.

Another nice feature of the CO radio emission is that its wavelength is small enough (about 100 times smaller than 21-cm line radiation) that even medium-sized radio telescopes have sufficient resolution to map the distribution of the molecular clouds. The higher resolution of large radio telescopes can be used to probe the structure of individual molecular clouds. There is some controversy about how the molecules are clumped together in the clouds. Is one gas cloud actually made of many smaller gas clouds? There is some evidence that indicates that 90% of the H2 is locked up in 5000 giant molecular clouds with masses greater than 105 solar masses and diameters greater than 65 light years. The largest ones, with diameters greater than 160 light years, have more than a million solar masses and make up 50% of the total molecular mass. Other studies indicate that the giants are actually made of smaller clouds grouped together into larger complexes.

Review Questions

  1. What are the characteristics of the gaseous part of the ISM? Is the gas all at the same temperature and density? How do you know?
  2. What are H II regions and how are they produced? What is going on at the atomic level?
  3. Why would the presence of a H II region indicate the site of star formation?
  4. How does the gas far from any star make its presence seen in the optical wavelengths and radio wavelengths?
  5. How is the 21-cm line radiation produced?
  6. Why is the 21-cm line radiation so important for determining galactic structure and mass?
  7. How is the 21-cm line radiation used to determine galactic structure and mass?
  8. Which part of the Galaxy has greater angular speed? How do you determine the rotation curve of the Galaxy from the doppler shifts of the gas?
  9. Where are most of the interstellar molecules found and how are they detected?
  10. What is the importance of the discovery of organic molecules in the interstellar medium?
  11. If the hydrogen molecules produce no radio emission, how do you map its distribution over the entire galaxy?

nextGalactic Structure - Cepheids

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Is this page a copy of Strobel's Astronomy Notes?