Re: anatomy of a galaxy

"Matt Menge" <mspmenge@xxxxxxxxxxx> wrote in message news:df442072-5c51-48c6-97c0-d19855d215c1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I recently read an article that said our Milky Way Galaxy can be
subdivided into four sections: halo, bulge, thick disk, and thin
disk. However the article wasn't clear where the sections are
located. I was particularly confused about where the 'halo' was
located. The article sort of implies it is at the center of the
galaxy but the word (i.e. the meanings associated with 'halos') is
more suggestive of being on the outskirts.

Regards,

Matt

The halo is the largest part of the galaxy in terms of both mass and volume, but not in terms of light output. It is centered on the galactic core, but it extends as far as, if not further than,
the outermost parts of the disk. The globular clusters are part of the halo stellar population.

The stars of the halo are generally the oldest stellar population (c. 10-12 billion yr) and have highly inclined, often highly eccentric orbits. The orbits are not closed like planetary orbits because the mass of the galaxy is not concentrated at a central point. Halo stars pass through the galactic plane and when we see them relatively close to us they appear to have very high velocities compared to stars in the galactic disk.

Look at this image of The Sombrero galaxy M104 (where the halo is "supressed").

http://antwrp.gsfc.nasa.gov/apod/ap080308.html

and the same galaxy

http://antwrp.gsfc.nasa.gov/apod/ap990815.html

where it isn't.

In a relatively undisturbed spiral like ours, or M104, the halo is symmetric and approximately spherical, or a very thick oblate spheroid. Haloes contain a lot of mass but emit relatively little light because the stars are very old, so those still on the main sequence are mostly either highly evolved (faint white dwarves, horizontal branch, etc) or class K or later. A few, of course, are red giants but these are short-lived phases.

The nuclear bulge is a mix of halo stars and old disk stars.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

.

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