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Answer to Question #28059 in Astronomy | Astrophysics for Maryam

Question #28059
5) Assuming that half of the mass of the Milky Way within the Sun's orbit is in the bulge, estimate the average mass density of the Milky Way's bulge. How does it compare with the density of giant molecular clouds?
Expert's answer
The bulge of the Milky Way is more spherical than the disk and is composed of stars which are more reddish than those in the disk (===>they are more evolved or low mass, but are still considered Pop I stars). The bulge is roughly 30,000 to 40,000 light years in diameter.

The central region of the Galactic Bulge (the nucleus) is quite interesting because it shows activity in type similar to that shown by Active Galactic Nuclei, AGNs (although at a considerably lower level).

* The core harbors
~ 1,600 stars per cubic light year. This is several 100,000 times as dense as the average stellar density of our Galaxy! Further, when one approaches the center of the Galaxy, one finds a dense cluster containing roughly 1 million stars with a stellar density around 10 million times as high as in the Solar neighborhood.

* Near the center
of the Milky Way lies the bright radio source Sagittarius A (Figure 23.25d). There is structure on scales ranging from 600 ly to what appears to be a ring of material on scales of 10-15 ly to structure on scales of 10 Astronomical Units (~80 light minutes ~ 1.5 billion km)--an Astronomical Unit is the average distance of the Earth from the Sun .

* Further
observations shed light on the nature of the central object. Just as one can determine how galaxies rotate and thus can gain a sense of the mass and mass distribution within galaxies, similar ideas can be applied to any gravitating system.

* Interestingly,
for the galactic center, one can infer the mass of the central object--according to the text, around 3.7 10 6 Solar masses or more--and must be fairly compact, < 45 astronomical units given the motion of the stars near the center of the Galaxy.

* Massive objects
with small sizes remind one of Black Holes . A black hole is an object which is so dense that not even light can escape from its surface.

Density of disk = 5*10^10 solar masses / 241 cubic kpc
Density of Milky Way + Halo: 10^12 solar masses / 113000 cubic kpc Density of Milky Way out to Andromeda: 10^12 solar masses / 2.1 billion cubic kpc Density of Local Group: 2*10^12 solar masses / 14 billion cubic kpc Density of region containing nearest 6 local groups: 10^13 solar masses / 268 billion cubic kpc. These give densities in solar masses per cubic parsec of about 0.2, 0.01, 0.000001, 0.0000001, and 0.00000003, respectively. This is to be compared with the typical average density near the Sun, which is usually given at 0.1 solar masses per cubic parsec; since the Sun is within the Milky Way disk, it is unsurprising that these densities are similar. Okay, now to the densities in g/cc. More specific numbers are given in the spreadsheet, but to emphasize our uncertainties in these estimations, here we only list the order of magnitude: Milky Way Disk 10-23 g/cc Milky Way Halo 10-26 g/cc Milky Way to Andromeda 10-29 g/cc Local Group 10-29 g/cc Nearest Local Groups 10-30 g/ccThese can also be interestingly compared to the average density of the universe which is also 10-29 g/cc.

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