The disk of the galaxy

The Galaxy's disk contains stars, gas, and dust. You became acquainted with these components in Chapters 17 to 19; this is a good place to review that material. Here we'll deal just briefly with the overall distribution of stars and the interstellar medium.

The Common Stars Recall from Section 18.4 the H-R diagrams for the brightest stars in the sky and for the stars within 6 pc of the sun (Fig. 18.16). These groups complemented each other. The brightest stars contained many very luminous ones and the nearest stars many very low-luminosity ones. One goal of studying the Galaxy is to find out, on the average, the properties of stars in a typical volume of space. For example, if we looked at a cube 100 pc on each side, how many stars would we find with the sun's luminosity? How many 100 times more luminous? How many 100 times less luminous? In other words, what would a star census of our Galaxy look like? To take such a census requires a combination of methods to get the distances and luminosities of many stars. For nearby stars, we use heliocentric parallax. For the distant stars (which turn out to be the most luminous ones), we employ both spectroscopic distances and statistical parallaxes. The final census, for stars of all spectral types, is called the luminosity function. It is simply (but hard to get!) a compilation of how many stars of a given luminosity can be found in a typical volume of space in the disk of the Galaxy.

What does this census reveal? First, that most stars have very low luminosities. Note that even the sun has a larger luminosity than most of the stars in the disk. Second, that although the very luminous stars are rare, they contribute most to the total starlight in the disk.

With the aid of the mass-luminosity relation, we can use the luminosity function to determine the amount of mass contained in stars. In a typical region of the Galaxy near the sun, there is about 0.04 solar mass of stars in a cubic parsec emitting 0.11 solar luminosity of visible light. The stars are spaced about 2 pc apart. That should give you an idea of how empty the Galaxy's disk is in terms of stars.

How thick is the disk? Since there is no sharp boundary to the Galaxy, the answer depends on what you mean by thickness. It also depends on which objects you chose to measure it. Here we're discussing stars, so let's use them, and we'll define the thickness as the distance from the plane where the star density falls to half its mid-plane value. Remarkably, if we use stars of different spectral types, we get different values for the thickness. For instance, О stars spread out about 50 pc above and below the plane, G main-sequence stars about 350 pc. Using G, K, and M stars to define the thickness gives a result of about 700 pc. Whichever of these values for the thickness you care to use, the Galaxy's disk is razor thin compared with its diameter of about 40 kpc. Where's the sun in the disk? It lies in the galactic plane, 8.5 kpc from the center.

The Interstellar Medium Revisited The gas and dust of interstellar space have a distribution very different from that of the stars (Color Plate 20). Let's look first at how neutral hydrogen. (H I) is arranged and then at molecular hydrogen (H2). The H I gas forms a thin layer that has a thickness of about 250 pc at the sun's location. But at greater distances from the center, the thickness increases considerably, reaching 3000 pc at the outer edges of the Galaxy. In addition to the gas widening out at the outer parts, it's warped; it bends "up" on one side and "down" on the other. It is thought that the warp is caused by tidal forces of the Magellanic Clouds. The average density in most of the disk is about 3 x 105 atoms per cubic meter, though somewhat less in the region beyond the sun. The total mass of H I is about 3 x 109 solar masses.

The inner part of the Galaxy, within 2000 pc of the center, is also peculiar. Much of the gas here lies in a thin disk tilted some 35° with respect to the galactic plane. The disk has a thickness of 100 pc and a maximum rotational velocity of 360 km/s. One model has the gas streaming on elliptical orbits, which gives the appearance of radial motions at some points, but there is no net expansion or contraction. If the stars also have this elliptical distribution, then the center of our Galaxy may have a bar through it. This disk contains some 107 solar masses of H I.

What about the distribution of H2? Because we cannot observe molecular hydrogen directly with radio telescopes, we look at carbon monoxide instead. There is considerable evidence that wherever we find CO, H2 must also exist. Why? Because if the interstellar environment is suitable for CO, it must also shelter H2.

The CO distribution does not in general follow that for H I. Observations indicate that the CO is most dense at a distance of 6 kpc from the Galaxy's center. Outward from 8 kpc, the density of CO takes a slide, whereas the HI density stays roughly the same. Inside 4 kpc, the CO density also decreases but not as rapidly as in the outer direction. The CO layer has a thickness of 125 pc.

Remember, the CO indicates the presence of H2, molecular hydrogen. Within the sun's distance from the Galaxy's center, about 93 percent of the hydrogen exists as H2. In contrast, outside the sun the hydrogen mostly takes the form of H I.

In one respect, the molecular distribution is like that of the H I: it shows the same tilted disk (or bar) structure in the innermost part of the Galaxy. This disk contains some 109 to 1010 solar masses of H2. The total mass in this disk, determined from the rotation of matter outside it, is only a few tens of billions of solar masses. This inner part of the Galaxy may be exceptionally rich in gas, rather than being predominantly stars.

Q The interstellar dust is mixed in with the gas. We presume that it more or less follows along with the gas distribution because we find dust in molecular clouds. Surveys of the reddening of stars show that the amount of reddening is generally the greatest in and around the plane of the Galaxy, where the gas is located.