Milky Way Description

There are a number of people who are unclear about what our galaxy, the Milky Way, is like so let me try to give a brief description. In the following description, I'll abbreviate parsecs with ``pc''. Remember that 1 pc = 3.26 light years and that the nearest star outside the solar system is about 1.3 parsecs away. I'll abbreviate kiloparsecs (1000 pc) with ``kpc''. I also use the term ``metal-poor'' and ``metal-rich''. A metal-rich star has a larger amount of atoms that are heavier than Helium. Since these heavier elements only come stellar nucleosynthesis, a metal-rich star has material that came from a previous generation of stars. A metal-poor star is made of material that was hardly (if at all) processed before. Generally, metal-poor stars tend to be older than metal-rich stars, though, alas, the correlation between metal-richness and age is poor! With this preface, here's the answer to the question:

What are the ``bulge'', ``stellar halo'', ``disk'', & ``dark matter halo'' of the Galaxy? Include a drawing in your written answer. Where are old stars likely to be found? Where are young metal-rich stars likely to be found? How do we know the dark matter halo exists? Explain how we know it is there! Using low-power binoculars, how could we determine that we live in a disk galaxy?

Answer: The components merge smoothly into each other with the stellar halo among the disk and the inner part of dark matter halo and the dark matter halo among the disk, stellar halo, and bulge, etc. The bulge is the elliptical-shaped center part of the Galaxy about 1-2 kpc in radius. It had lots of star formation early on, so now it is made of tens of billions of old, metal-rich! stars. The disk is the thin pancake-shaped part about 400 pc thick and 15-20 kpc in radius with the Sun 8-8.5 kpc from the center. The disk contains over 98% of the dust and gas in the Galaxy and has a few hundred billion stars. Some of these stars continue to form so the disk has some young metal-rich stars. The gas and dust are found in a layer that is thinner than the star layer (the gas/dust layer is the thin dark line at the midplane of the disk in the picture below and the star layer is the thicker light band). The stellar halo is a roughly spherical distribution of hundreds of millions of old, metal-poor stars that has increasing concentration of stars toward the center of the galaxy. It is about 20-30 kpc in radius and it may contain small amount of hot gas, but the disk contains the vast majority. Most of the globular clusters are found in the halo and like the halo stars, the number of them increases toward the galactic center. If we were at the center, we would see approximately the same number of globular clusters in any direction we looked in the sky. Since the globulars are found bunched up in one part of the sky, i.e., they are swarming around some other point in the galaxy, we know that we are not at the center.

The dark matter halo's presence is indicated from the rotation curve-the balance of the outward acceleration (from the orbital speed) and the inward pull of gravity so that the mass inside the orbit's distance from the galactic center, Enclosed Mass = Rotation speed^2 * Distance from center / G. The rotation curve is flat even though the light-producing matter's distribution says it should be falling. The dark matter halo may go out as far as 60-80 (or more!) kpc! We don't know what it is made of (brown dwarfs, black holes, neutrinos with mass?).

Using binoculars (or using the Voyager program on the Macintosh), we see a narrow band of stars (the ``Milky Way'') and a large number of gas/dust nebulae that are only found in disk galaxies or irregular galaxies. An irregular galaxy would have a much patchier distribution of material is various parts of the sky and would not produce the narrow band of stars. Only a disk galaxy would make the narrow band with a slight bulging in the direction of the galactic center. (For the Milky Way, the center is in the direction of the constellation Sagittarius.)

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last updated 29 Aug 95