Milky way is the galaxy that includes the sun, the earth and the rest of our solar system. It also includes huge clouds of dust particles and gases that lie throughout it and depths of interstellar space. The term Milky Way also refers to the portion of the Milky Way galaxy that can be seen from the earth. This is the outer edge of the galaxy. On clear, dark nights, it appears to the naked eye as a broad, milky looking band of starlight stretching across the sky.
Milky Way
The Milky Way galaxy is shaped like a thin disk with a bulge in the center. Stars, dust and gases fan out from the central bulge in long, curving arms that form a spiral pattern. For this reason, astronomers classify the Milky Way galaxy as a spiral galaxy.
The diameter of the Milky Way galaxy is about 105 light years. A light yea
r is the distance that light travels in one year - about 9.4618km. The Milky Way is about 105 light years thick at the central bulge and much flatter towards the edges of the disk. The thick center represents a relatively high concentration of the stars at the center. Our solar system is located in the outskirts of the galaxy, about 25 x 103 light years away from the center.
All stars and star clusters in the Milky Way orbit the center of the galaxy, much as the planets in our solar system orbit the sun.
Differences between a Galaxy and a Constellation
- A galaxy is a collection of billions of stars whereas a constellation is a collection of only a very few stars.
- A galaxy does not form a definite pattern, which resembles the shape of an animal or a human being, but a constellation appears to resemble an animal or human being in its shape.
- There are billions of galaxies in this universe but only about 88 constellations are known at present.
The Structure of the Milky Way Galaxy
The three main parts of the structure of the Milky Way galaxy are the bulge, the disk, and the halo. Above is an edge-on diagram of the Milky Way with each of these noted.
The Disk
The disk is the flat rotating Frisbee shaped part of the galaxy. Almost all of the current star formation in the Milky Way takes place in the spiral arms of the disk within a few hundred light-years of the plane of the galaxy. The disk has a more sparsely populated component known as the thick disk, which extends farther from the plane of the galaxy (about 1000 light-years) than the rest of the disk.
The Bulge
The bulge is the dense roughly spherical concentration of stars at the center of disk. The center of the galaxy lies in the bulge, roughly 24,000 light-years from the sun.
The Halo
The halo is the sparsely populated extended region that encompasses the disk and bulge of the galaxy. The brightest members of the halo are large star clusters, known as globular clusters. Individual stars, known as halo field stars, also occupy this region of space.
Sample Volume
This is a rough representation of the region that our sample of RR Lyraes is contained in. This shows that our sample of RR Lyraes should contain members of the disk, thick disk, and halo populations.
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