![]() ![]() It is much larger than the disk and may even extend as far as 250,000 parsecs or more in radius. 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. 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.The stars in the bulge are in general older than the stars in the disk. In the very center, there is a thicker, roughly spherical region that has different properties from the rest of the disk. The bulge: The disk is not uniformly thick.The young stars in the disk population are usually referred to as Population I stars. 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). Besides stars, the disk contains the majority of the gas and dust in the Milky Way. That is, the thickness is much smaller than the radius. The dimensions of the disk are similar in ratio to an old vinyl record. The Sun is found in the disk, but, as Shapley found, we are about 8,000 parsecs from the center. There are different ways to measure the size of the disk, but it is approximately 30,000 parsecs in diameter. The disk: The band of light that we see in the sky is part of the flattened, disk-shaped part of the galaxy.We can describe these structures and their properties in more detail: The wireframe in the background represents the halo. Also shown are the central bulge region (labeled in red) and the globular clusters (labeled in yellow). The artist's conception above shows an image of the Milky Way disk with its spiral arm structure represented (labeled in blue). Source: Captured using the Partiview software and the Digital Universe Atlas The figure below is a model for the different populations in the Milky Way. For this reason, the study of an object like the Milky Way often is described as studying stellar populations. 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. Using this working definition, we can show that the Milky Way contains many billions of individual stars. 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). If we draw borders that enclose all of these objects, we can roughly define the shape of the Milky Way. 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. 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. Defining the Milky Way is a bit difficult, because it is not one single coherent, solid object. ![]()
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