Additional reading from www.astronomynotes.com
Defining the Milky Way is a bit difficult, because it is not one single coherent, solid object. Instead, The Milky Way is considered to be the sum of all the individual objects (stars, planets, nebulae, dust particles, etc.) that are gravitationally bound to each other. That is, if an object like a star or a star cluster feels a strong enough gravitational pull from the rest of the objects in the Milky Way that it cannot escape, it is considered to be part of the Milky Way. If we draw borders that enclose all of these objects, we can roughly define the shape of the Milky Way. In general, the word galaxy refers to a collection of gravitationally bound stars and associated material that is above some minimum size (to differentiate galaxies from massive star clusters).
Using this working definition, we can show that the Milky Way contains many billions of individual stars. Astronomers have found that these stars are not part of one single, homogeneous structure, but instead different populations of stars form somewhat distinct structures with different properties. For this reason, the study of an object like the Milky Way often is described as studying stellar populations. The figure below is a model for the different populations in the Milky Way.
The artist's conception above shows an image of the Milky Way disk with its spiral arm structure represented (labeled in blue). Also shown are the central bulge region (labeled in red) and the globular clusters (labeled in yellow). The wireframe in the background represents the halo.
We can describe these structures and their properties in more detail:
- The disk: The band of light that we see in the sky is part of the flattened, disk-shaped part of the galaxy. There are different ways to measure the size of the disk, but it is approximately 30,000 parsecs in diameter. The Sun is found in the disk, but, as Shapley found, we are about 8,000 parsecs from the center. The dimensions of the disk are similar in ratio to an old vinyl record. That is, the thickness is much smaller than the radius. Besides stars, the disk contains the majority of the gas and dust in the Milky Way. The stars that are found in the disk are primarily young stars with properties similar to the Sun (which should not be too surprising, since the Sun is a disk star). The young stars in the disk population are usually referred to as Population I stars.
- The bulge: The disk is not uniformly thick. In the very center, there is a thicker, roughly spherical region that has different properties from the rest of the disk. The stars in the bulge are in general older than the stars in the disk.
- The halo: Although the word halo implies a ring shape, in this case, we define the halo as the spherical region that completely encompasses the disk. This is the part of the Milky Way that contains most of the globular clusters, and so these objects are used to trace out the total extent of the halo. It is much larger than the disk and may even extend as far as 250,000 parsecs or more in radius. The density of stars in the halo (the number of stars per cubic parsec) is much lower than it is in the disk, so it is very difficult to identify enough of these objects to study this stellar population in detail. However, we have been able to show that the stars in the halo are primarily old and contain fewer heavy elements than the Sun. If you recall our lesson on star clusters, the properties of the stars in the halo seem to match very closely the properties of the stars in globular clusters. The old stars in the bulge and the halo are referred to as Population II stars.
Because we are embedded in the Milky Way's disk, it is quite difficult for us to discern the substructures in the disk. So our understanding of the Milky Way's structure continues to evolve as we study it in more depth. Below is a labeled image of the structure of the Milky Way's disk and bulge, which includes the latest updates for the location and densities of the Galaxy's spiral arms determined by observations with the Spitzer Space Telescope.