Now let's consider the galaxy. What is its size? And where is its center? These questions are tougher than they sound. When we look at the sky at optical wavelengths, we find that most of the stars lie in a flat plane; in other words, on a great circle on the celestial sphere. Similarly, when we look in the far infrared, where objects glow if they are only 60 to 100 degrees above absolute zero (or below -100 degrees Celcius), we again find that most of the emission is in a plane. (This emission comes from the cold dust in the interstellar medium.) We see the same flat plane in H I (atomic hydrogen) and molecular carbon-monoxide. But this doesn't tell us much about how the galaxy is laid out, or how big it is.

The key to measuring the size of our Galaxy lies in the globular clusters. When we search the sky for star clusters, we find that virtually all the open clusters are located very close to the plane of the Galaxy. This makes them difficult to study, due to the obscuring dust. The globular clusters, however, are not so clustered. They are located all around the sky, but not equally -- there are more in some directions than in others. Harlow Shapley noticed this, and found a way to use the globular clusters to locate the galactic center. To understand the method, however, we have to digress and revisit the topic of measuring astronomical distances.

As we have seen before, measuring astronomical distances can be tough; stellar parallaxes are extremely small, and, in fact, most stars are too distant to have any discernable parallax. We therefore need some other way to tell distance.

Actually, in astronomy, there are many ways to estimate distance, but virtually all are varients of the standard candle method. Consider the inverse square law of light, which says that the absolute luminosity of an object L, is related to how bright the object appears l by the distance ( l = L / r^2 ). Therefore, if somehow you know ahead of time the intrinsic brightness of an object, then, to measure its distance, all you need to do is measure how bright it appears. If you know ahead of time the absolute luminosity (or absolute magnitude) of an object, then the object is a standard candle.

Consider some star that is too distant for any parallax measurement. Now take a spectrum of that star. Suppose it's an A-star. We know that most stars are main sequence stars, and that the absolute magnitude of an A main sequence star is zero. We therefore know the absolute brightness of the star ahead of time. It becomes a standard candle, and all we need to do to estimate its distance is measure its apparent brightness.

The method of taking a spectrum of a star and using our knowledge of the main sequence to estimate absolute magnitude is called the method of spectroscopic parallax. This method can be applied to any star that is bright enough to take a spectrum of. But it does require the assumption that the star is on the main sequence.

If the star is in a cluster, then we can do even better. Because star clusters contain many stars at the same distance, you can plot apparent magnitude vs. spectral type and see which stars are main-sequence stars. You can then derive distance with the knowledge that your target stars are definitely main sequence stars. This method of obtaining a distance is called main-sequence fitting.

This is all well and good, except that the globular clusters are all extremely old, so their main sequence stars are all extremely faint. (All the bright O,B,A,F, and even G main sequence stars have run out of fuel.) But astronomers do get lucky, with a type of star called an RR Lyrae star.

After a star runs out of hydrgoen fuel on the main sequence, it goes up the giant branch, becomes a shell burner, and eventually ignites helium in its core. During this time, the star retreats from the giant branch and moves back towards the main sequence. It doesn't quite get there -- it's still much brighter than a normal main sequence star, but its not quite a giant star.

Some of these post-giant branch stars have a very interesting property. There is a narrow strip in the HR diagram called the instability strip and stars within them pulsate. They get bigger and smaller, brighter and dimmer, over a period of hours, days, or months. Some Pop II post giant-branch stars enter the strip after igniting helium in their core. These are called RR Lyrae stars. These stars are important, because when we observe them inside globular clusters, we find they all have the same average apparent magnitude. Since all the stars of a globular cluster are at the same distance, this means they all have the same absolute magnitude. And, when we compare RR Lyrae stars in different globular clusters (with known distances through main sequence fitting), we find that all RR Lyrae stars have the same average absolute magnitude. RR Lyrae stars are therefore standard candles.

RR Lyrae stars are fairly bright -- they have an absolute magnitude of about zero. (In other words, they are about 100 times brighter than the Sun.) Moreover, because all RR Lyrae stars have the same absolute magnitude, all we have to do to estimate their distance is to measure how bright they appear. We can therefore a couple of pictures of a distant globular cluster (one that is too distance to see main sequence stars), and identify the stars that have changed their brightness. These are (most likely) RR Lyrae stars. Once we find them, we measure how bright they appear (their apparent magnitude), and compare them to the absolute magnitude of an RR Lyrae star (which is known from measurements in nearby globular clusters with known distances). Since both the apparent and absolute magnitude is then known, the distance then follows.

When we do this and measure the distance (and direction) of all the globular clusters, we find that the system of globular clusters is not centered on us. Instead, globular clusters are scattered in all directions around a point 8 thousand parcsecs from us. That is the center of the Milky Way galaxy.

Thanks to globular clusters, astronomers have known the approximate position of the Galactic center for over half a century. As far as the discovery of the structure of the rest of the Galaxy, this proceeded at a slower pace. Because of all the dust of the Galaxy, it is difficult to see very far in the plane. Spectroscopic parallax of nearby stars, plus observations of gas, dust, and cool stars in the radio and infrared regions of the spectrum (where light passes through dust more easily), has given us a pretty good idea of what our galaxy looks like. But even now, some details are controversial.

We believe that, when viewed from the side, our Galaxy looks like a thin disk, with perhaps a slight bulge in the middle. The Sun is in this disk, about 2/3 of the way out from the center. When viewed from above, we believe one would see a spiral pattern of bright stars (with plenty of other stars between the spiral arms). Moreover, it is likely that our Galaxy has a bar crossing the center, more-or-less pointed in the direction of Earth.

In terms of stellar populations, the Galaxy can be thought of as being made up of two components. The first component is a roughly spherical distribution of Population II stars, which is densest at the center, but that goes out in all directions for quite some distance (say, 50 kiloparsecs). All these Population II stars orbit the center of the galaxy, but in all sorts of orbits, some going one way, some going the other. Occasionally, a Population II star will be seen in the Galactic plane (as it is passing through) but not very often. Near the center of the Galaxy, this spheroidal component is often called the bulge. Further out, as the stars become rarer and rarer, the component is sometimes called the halo.

Superposed on this spheroidal component is a disk comprised of Population I objects. The youngest, most extremely Population I stars (and gas) are confined to an extremely thin disk, and are located in spiral arms. Slightly older (but still Population I) objects are not confined to the arms, but are distributed more uniformly throughout a slightly thicker disk. The disk component rotates about the center of a Galaxy in the plane of the disk, with all the stars and gas moving in the same direction. The Sun is located about 2/3 of the way out in the disk, about 8 kpc from the Galactic center, and rotates about the galaxy at about 250 km/s. At that rate, the Sun goes around once every 200 million years, or so.

How does our Galaxy compare to other galaxies? It turns out that galaxies come in all sizes and in a variety of shapes. There are many ways of classifying these shapes, but the most famous is the tuning-fork diagram that comes from Edwin Hubble. It is called the Hubble sequence.

At one end of the tuning fork is a pure elliptical component, comprised entirely of Population II objects. Elliptical galaxies can appear spherical (called E0), or highly flattened (E7), or anywhere in between. Since there is no Population I component in these galaxies, elliptical galaxies contain no gas, dust, or young stars.

At the other end of the diagram are the spiral galaxies. Like the Milky Way, these galaxies contain both a Population I and Population II component. The relative sizes of these components, however, can change. An Sa galaxy has a large bulge (ie, spheroidal component) and a very smooth disk with relatively little dust and star formation. In Sb galaxies, the bulge is smaller, and the star formation greater. Consequently, the spiral component appears stronger, and H II regions in the disk are common. In Sc galaxies, there is almost no bulge, but the spiral pattern, which includes areas of dust, H II regions, and regions of bright, blue stars are prominent. Paralleling the sequence of spirals is the sequence of Barred spirals, in which the central bulge is augmented by a bar of Population II stars. For these SB galaxies, the spiral patterns emerge from the end of the bar. At the extreme end of the sequence are irregular galaxies, which are dominated by Population I objects, but which have no discernable center or underlying pattern. In this sequence, the Milky Way galaxy would probably be classified as an SBc.