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The Structure of Our Universe

The Milky Way Galaxy

Stars are organized into large, gravitationally-bound groups called galaxies. Our galaxy is the Milky Way Galaxy. It is a type of galaxy known as a spiral galaxy, because the visible gas and stars in the galaxy form arms that twist around the center of the galaxy. With a mass that is a billion times the mass of the Sun, our Galaxy is as large as a spiral galaxy can be; most galaxies are much smaller than the Milky Way.

The galaxy has two visible parts and an invisible part. The visible parts are a disk of stars and gas and a smaller central bulge. The invisible part is a halo of low-luminosity stars, and perhaps other types of gravitating material. The material in the halo stabilizes the galactic disk.

The central bulge contains at its core a very massive compact object; under the theory of general relativity, this compact object is a black hole. The mass of the central object is estimated to be 3.6 million times the mass of the Sun. The orbits of stars close to this central object have no preferred direction, and the shapes of these orbits are set solely by the central object. These orbits are closed ellipses, similar to the orbits of the planets around the Sun. Farther out from the central object, the mass of the stars exceeds the mass of the central object, which causes the orbits of the stars to be open.

The disk defines the orbital plane of the youngest stars in the galaxy. The disk is much thinner than its diameter. The orbits are nearly circular around the center of the Galaxy. Regardless of how far a star in the galactic plane is from the center of the Galaxy, it travels around galaxy at a velocity of about 210 km s-1.

The galactic disk contains not only stars but also gas and dust. In fact, new stars in the disk are born from this gas and dust. The gas and dust are opaque to light, blocking are view of most of the galactic disk. Because of this, most of the stars we see in the sky are relatively nearby. The dust in the Galaxy totally obscures the center of the Galaxy from view at optical frequencies. What we can see of the disk appears on the sky as a band of haze that completely encircles us; this band is called the Milky Way.

The galactic disk contains spiral arms that are composed of gas and stars—these are visible to radio telescopes. These spiral arms resemble those seen in other galaxies, twisting tightly around the center of the Galaxy. The gas in the spiral arms is dense enough cause the birth of new stars. For this reason, the spiral arms are where one finds the brilliant, short-lived blue stars.

The final component of the galaxy is largely invisible. This is the galactic halo. Its existence is known through two effects: the observation of old, high-velocity stars that are passing through the galactic plane, and the existence of a stable galactic disk containing stars all moving at about the same speed. The high-velocity stars are high velocity relative to the Sun; these stars are moving perpendicular to galactic disk. The velocity of the stars within the disk, a velocity that is independent of distance from the galactic center, gives a measure of how mass is distributed within the galaxy, while the presence of the disk requires this mass to be somewhat spherically distributed. Because all of the gas in the galaxy is in the galactic plane, the galactic halo is a region incapable of creating new stars. Because only the smaller stars live long lives, the stars in the halo are invisible unless they are close to the Sun. There is some speculation that the mass in the halo is in a form unknown to physics.

While most of the mass moving within the galaxy is in individual stars, gas, and perhaps a more exotic form, a small amount of mass moves as tightly-bound systems of stars. These are the globular clusters. These spherical star systems contain hundreds of thousands of gravitationally-bound stars within a radius of tens or hundreds of parsecs. All of the stars within a globular cluster are of the same age, which provides a useful laboratory for stellar evolution. These systems orbit the center of the galaxy, and the first determination of the galactic center's located came from the positions of the globular clusters on the sky.

The Sun is in the galactic disk, about half-way from the galactic center to the outer edge of the disk. All of the stars we see in the sky are part of this disk. The stars that make that band up are close to us, because dust in the disk eventually blocks our view. At its distance of 7.6 kpc from the galactic center, traveling at 210 km s-1, the Sun orbits the Galaxy once every 230 million years, so during its 4.5 billion years of existence, the Sun has orbited the Galaxy only about 19 times.

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