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The Milky Way Galaxy |
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The Milky Way Galaxy |
Galaxies are clusters of stars, gas, dust, and ???. Galaxies serve as the primary indicators for our study of the structure of the Universe. We will spend the next several lectures talking about the properties of normal galaxies (with particular emphasis on our own galaxy, the Milky Way galaxy) and Active Galactic Nuclei (AGNs). We will concentrate on the aspects of galaxies which we will use in our discussions of the evolution of the Universe.
The Milky Way galaxy is classified as a spiral galaxy (typical examples: M83, and M104). There are also spiral galaxies with central bars known as Barred Spirals (NGC 1365). There are also other kinds of galaxies among which are included those known as elliptical galaxies (for example, see M87, NGC 4697), Irregular galaxies (for example, see N4449), and S0 galaxies. Overall, typical large galaxies have the approximate properties,
Mass | 109 to 1012 M(Sun) |
Diameter | 105 to 106 ly |
Number of Stars | 108 to 1012 stars |
Luminosity | 108 to 1011 L(Sun) |
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The Milky Way is a spiral galaxy similar to M51 and M104 (Sombrero galaxy). It is composed of a thin disk, a large spherical halo, and central bulge. The disk contains youngish stars (Population I stars), gas, and dust which orbit about the center of the Galaxy (and so is the site of ongoing star formation--O & B Stars and HII regions). The halo is composed of older stars (Population II stars [e.g., in globular clusters]), which orbit the center of our galaxy in randomly inclined orbits. The halo contains the majority of the mass of our Galaxy but negligible amounts of gas and dust. The Bulge contains gas, dust, and stars similar to the halo of our Galaxy. There is also activity near the center of the Bulge.The Sun is in the disk of the Milky Way galaxy in a spiral arm, roughly 2/3 of the way out from the Bulge of the Galaxy to the edge of the visible disk. The stars, gas,and dust, in general, orbit around the center of the disk of our Galaxy. |
Mass | ~ 2x1011 M(Sun) (visible disk) |
~ 6x1011 M(Sun) (total?) | |
Disk | ~ 100,000 light years (visible) |
~ 300,000 light years (total?) | |
~ 1,000 light years (thickness at the Sun's orbit) | |
Luminosity | ~ 2 x 1010 L(Sun) |
Number of stars | ~ 4 x 1011 stars |
Stellar density | ~ 1 star per 125 cubic light years |
Gas density | ~ 1 atom per cubic centimeter (a sugar cube) |
Note--the above stellar density implies that stars are separated by roughly 5 light years (1 light year = distance light travels in 1 year ~ 6 trillion miles ~ 9.6 trillion kilometers). The Sun's nearest neighbor is Proxima Centauri at a distance of ~ 4.3 light years.
Note--There are 4x1011 stars in our Galaxy and the mass of the Galaxy is at least 6x1011 times the mass of the Sun. The average mass of a star in our Galaxy is ~0.3 times the mass of the Sun. This suggests that a lot of the material of our Galaxy must be in some nonluminous form, the so-called dark matter .
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