Galactic Structure

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Determining the structure of our galaxy is not an easy task because the solar system is stuck inside the Galaxy and we can only look in all different directions. Our situation is like you having to determine the layout of your hometown from just looking out on your front porch (or back porch) and not being able to move even across the street. The fact that you see a narrow band of stars tells you that our galaxy is shaped like a thin disk. If we lived in a more spherical galaxy, the stars would be distributed more uniformily around the sky. There is a hint of a bulge in the direction of the Sagittarius constellation. Careful star counts and determining their distances shows hints of a spiral pattern in the disk. The interstellar dust limits our view a small section of the Galaxy. However, clear evidence of the spiral structure in the disk comes from the 21-cm line radiation discussed in the previous section.

Our galaxy, the Milky Way, is disk-shaped with spiral arms in the disk. It has an elliptical bulge in the center and a spherical halo that is denser closer to the Galaxy center. It is about 100,000 light years across and our solar system is about two-thirds of the way out from the center. For comparison, our solar system with the Oort Cloud is about 1 light year across. A view of the Galaxy as seen from above is shown at the top of this page. From the side the Galaxy would look very flat because most of the stars are in the disk (see the figure below).

You can make a rough guess of the number of stars in our galaxy by dividing the Galaxy's total mass by the mass of a typical star (e.g., 1 solar mass). The result is about 200 billion stars! The actual number of stars could be several tens of billions less or more than this approximate value. Recently, astronomers have discovered that most of the mass of the Galaxy (and other galaxies) is not in the form of stars, gas, or dust. It is made of some other material, as yet unknown, and is given the descriptive name ``dark matter''. Note that this affects your guess of the number of stars in the galaxy! (Does it increase the number or decrease it?)

the flat disk of the Milky Way. The disk arrow points to the approximate location of the Sun

Our galaxy probably closely resembles the galaxy NGC 891 as seen edge-on. Note the prominent dust lanes going through the disk mid-plane and how flat the galaxy is.

AAT image of NGC 891

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last updated: 25 May 2001

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel