Milky Way Summary

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Here is a summary description of our Galaxy: the Milky Way. For a sense of
scale, remember that 1 parsec = 3.26 light years and that the nearest star
outside the solar system is about 1.3 parsecs away. A ``metal-rich'' star has a
larger percentage of atoms that are heavier than helium. Since these heavier
elements only come from stellar nucleosynthesis, a metal-rich star is made of
material that came from a previous generation of stars. A metal-poor star is
made of material that has undergone little if any processing. 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? Where are old metal-poor stars likely to be
found? Where are young metal-rich stars likely to be found? How do you
know the dark matter halo exists? Explain how it is known to be there! Using
low-power binoculars, how could you determine that you 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 1000 to 2000 parsecs 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 parsecs thick and 15 to 20
thousand parsecs in radius with the Sun 8000 parsecs from the center. The disk
contains over 98% of the dust and gas in the Galaxy and has a few hundred
billion stars. Some 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 above 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 to 30 thousand parsecs 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 the solar system was at the center, you would see approximately the
same number of globular clusters in any direction you 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, and we are not at the center.
The dark matter halo is denser toward the center. It extends further out than
the stellar halo.

The dark matter halo's presence is indicated from the rotation curve---a way
to measure the amount of gravity inside a star's orbit, so that the mass inside
the star's distance from the galactic center = (orbital speed)2 ×
(star's distance from the galactic 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 to 80 (or more!) thousand
parsecs! It is not known what the dark matter is made of (brown dwarfs, black
holes, neutrinos with mass?).

Using binoculars you see a narrow band of stars (the ``Milky Way'') and a
large number of gas/dust nebulae that are found only in disk galaxies or
irregular galaxies. An irregular galaxy would have a much patchier distribution
of material in 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: 25 May 2001

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

Author of original content: Nick Strobel