Most globular clusters like M13 contain a few RR Lyrae type variable stars. If one assumes that all such stars have absolute magnitude about 0, then the spatial distribution of globular clusters about our galaxy, the Milky Way, may be ascertained by noting the apparent magnitudes of these stars. In 1917 the American astronomer Harlow Shapley found that the globular clusters form a spheroidal system centered upon a point in the Milky Way located in the direction of Sagittarius at a distance of 25,000 to 30,000 light years. He supposed that this point represents the center of our galaxy, about which the globular clusters are pursuing elliptical orbits of great size.
Individual RR Lyrae stars have been found as far away as 50,000 light years either side of the galactic plane, indicating that the galaxy possesses a corona, or halo, with an overall diameter of about 100,000 light years. However, most of the stars of our galaxy are, like the Sun, pretty close to the galactic plane.
The Sun and the other stars comprising the galactic disk are known to be moving about the galactic center much like the rings of Saturn revolve about that planet. The distance of the Sun from the galactic center is known to be about 30,000 light years. If we could determine the period of this orbital motion, we could estimate the mass of the galaxy, or at least that part located within the orbit of the Sun. This would entail treating the galaxy as if its mass were all at the center. We would simply use Kepler's 3rd law in the form
To estimate the period P we may proceed as follows: The velocities of the globular clusters relative to the earth can be determined spectroscopically. If one accepts the notion that the spheroidal system of globular clusters defines a "rest frame," then the motion of the Sun may be inferred. In this way one discovers that the Sun's orbital speed is 200 to 300 km/sec. This corresponds to a period about 2 x 108 yeaers. Using Kepler's law we then find that the mass of the galaxy is about 2 x 1011 times the mass of the Sun. Of course, not all this mass need be in the form of stars.
The presence of obscuring dust in the galaxy led to a great deal of difficulty in establishing the true nature of our galaxy. These difficulties were only overcome during the 20th century as a result of technological advances. The early observers overestimated the distance of stars which are within our galaxy. As a result they arrived at estimates of the galactic size which were too low by a factor ten, for they considered many of the stars to lie outside the confines of the galaxy.
As the Sun's light is reddened as it sets, because of scattering of light within the thicker layer of atmosphere, so the light of distant stars is reddened by passing through much dust between the star and us.Hence a star which is in reality blue can appear red to us. Fortunately the tell-tale spectrum of the star will still identify the star as a blue giant, so if one is careful one should not fall prey too often to error.
Just as the sky appears blue to us, light reflected by dust in the galaxy appears bluer than the star supplying the light. Modern color photography has yielded nothing less than spectacular portraits of objects which appear rather pale visually.
Actually in interstellar space there is gas as well as dust, although the optical effects of the gas are not very obvious. Even though its density may be greater than that of the dust, the larger dust particles absorb visible light much more readily. It doesn't take much cigarette smoke in a room to absorb a significant amount of light.
How can interstellar gas be detected? Most of this gas is probably hydrogen, the main constituent of stars, although a recent article in the Scientific American listed many complex molecules which have been detected in interstellar gas. Except near stars the hydrogen gas would consist of neutral atoms, while near stars the absorption of ultraviolet light would cause the ionization of the hydrogen, so one would have free protons and electrons instead of neutral hydrogen.
When a free electron passes close to a free proton, there is a certain probability that the electron will be captured and, according to contemporary atomic theory, energy will be radiated in the visible portion of the spectrum. The great Orion nebula M42 shows evidence of such emission. Of course, the electrons are not captured indefinitely, for the absorption of ultraviolet radiation from the nearby stars can ionize the neutral hydrogen atom again. The rates at which these competing processes take place can be calculated on the basis of modern quantum theory.
While fluorescence provides an important way to detect interstellar hydrogen ionized by ultraviolet light emitted by a nearby star, neutral hydrogen emits very little visible radiation. Nevertheless, one may often infer the existence of interstellar neutral hydrogen because it can absorb visible light of certain wavelengths and give rise to a dark line spectrum superposed upon the spectrum of starlight. As even more important clue to the existence of neutral interstellar hydrogen is provided by the radio emission to which it gives rise.
The picture of the galaxy which emerges involves a thin dirty disk containing most of the visible stars, together with an extensive spherical halo populated by globular clusters in which there is little or no dust. It is observed that the brightest stars in the globular clusters are red, not blue. These stars, however, are brighter than the RR Lyrae variables in the cluster, so we must conclude that the red stars are red giants. We shall see that the globular clusters are extremely old, when next week we review stellar evolution.
Far smaller than the globular clusters are the open clusters found near the galactic plane. The Hyades constitute a famous open cluster in Taurus, containing more than 200 stars. RR Lyrae stars are never found in such open clusters. The brightest stars in open clusters are much bluer than those in the globular clusters. We shall see that these are younger stars.
The brightest main sequence stars tend to be grouped in very small associations, containing 5 to 50 stars. The Soviet astronomer Ambartsumian emphasized that such associations cannot have terribly long lifespans, for as the stars comprising the association revolve about the galactic center those closer to the galactic center will pull ahead of the others. One can estimate that a typical associattion will only exist for a period of a few million years. From this and other evidence one may conclude that the brightest main sequence stars must be very young stars, presumably formed recently out of the interstellar dust and gas prevalent in the galactic plane.