As you go closer to the center of the Milky Way, the
distribution
of stars becomes vertically thicker. This is not an actual thickening
of
the disk, but rather a sign of a different component of the galaxy -
the
galactic bulge.
Disk galaxies often have central bulges -- look at
the nearby Andromeda galaxy, for instance.
Unlike the disk, the distribution of stars in the galactic bulge is not exponential. It was once thought to be spheroidal, with an effective radius - the radius which contains half the light - of a few kpc.
It's hard to study the galactic bulge due to dust -- as much as 28 magnitudes of extinction exist within a few degrees of the center. But the dust is patchy, and there are holes which we can peek through, such as Baade's window. The line of sight through Baade's window passes within 500 pc of the galactic center, so we can study bulge stars.
Bulge stars are mostly old (ages > 9 Gyr) and have a range of metallicities (-1 < [Fe/H] < 0.5) with an average metallicity a little under solar ([Fe/H] = -0.2). There is evidence for a complex formation history.
The mass of the bulge is about 1010 Msun
, or about 1/6 that of the disk.
Recent results indicate that
the bulge is not spheroidal,
but is more likely bar shaped.
First, the COBE infrared data showed that the bulge is bigger on one side.
Secondly, if you look at the distances to stars on either side of the bulge, on average stars on one side of the bulge are closer than stars on the other. This can't happen if the bulge is axisymmetric!
The current thinking?
