Bulge and Nucleus of the Milky Way

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 the stars are still considered Population I stars). The bulge is roughly 6 kpc x 4 kpc (20,000 x 13,000 light years) in size.

The central region of the Galactic Bulge (the nucleus) is quite interesting because it shows activity similar to that seen in active galactic nuclei, AGNs (although at a lower level). It is studied primarily in the IR and radio because it is a very dusty region.

• The core harbors ~ 1,600 stars per cubic light year. This is several hundred thousand times as dense as the average stellar density of our Galaxy!^*. In the Solar neighborhood, stars are found roughly every 5 light years. In the galactic center, stars are roughly 0.1 light years apart.

• Near the center of the Milky Way lies the bright radio source Sagittarius A. There is structure on scales ranging from 600 ly (the filamaents in the figure below, Figure 23.23 from text) to what appears to be a ring of material on scales of 10-15 ly (Figure 23.23) to structure on scales of 10 Astronomical Units (~80 light minutes ~ 1.5 billion km).
  

  The center of the Galaxy is the lower bright region to the left of the bright spot. In the figure below, the light blue region is Sagittarius A East, a Supernova remnant. The red spiral pattern is Sagitarius A West with Sagittarius A*, the galactic center at the center of the spiral.

  

• 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, one infers the presence of an object with mass ~2.6x10⁶ M(Sun) and that the object is compact, much smaller than 10 A.U. (80 light minutes).

  • Recall from the properties of black holes that the radius of a black hole is 3 [M/M(Sun)] km. A 2.6 million M(Sun) black hole has a radius of 8 million km ~ 0.05 Astronomical Units, much smaller than the inner edge of the disk, 10-15 light years or the size inferred from variability, 80 light-minutes ~ 10 Astronomical Units. The case has not yet been conclusively made that there is a black hole at the center of our Galaxy, however, it is likely that a black hole does lurk at the center of our Galaxy.

  • The large number of stars in the nucleus of our Galaxy combined with the black hole (Sgr A*) suggests a way the strong radio emission can be powered (stellar orbits). The stars in the nucleus of the Galaxy orbit about the large black hole in the center of the Galaxy. The black hole disrupts stars that pass by too closely (through tidal forces). The disrupted stars then flow (accrete) onto the black hole. As the material flows onto the black hole, it speeds up and gains energy as it is pulled inward by the gravity of the black hole. The material from the disrupted star forms a disk around the black hole from where it slowly spirals down the drain that is the gravitational potential well of the black hole driven by viscosity (which heats the disk). The spiraling gas gives up its energy of motion as it is eaten by the black hole, producing the high energy particles that lead to the strong emission emanating from the Galactic center.

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^*The extremely high stellar density (and the active center) help to define the habitable zone of the Milky Way Galaxy (see Lineweaver, Fenner, & Gibson 2004, Science ). Lineweaver et al. argue that, today, the habitable zone is from 7 kpc to 9 kpc (or 23,000 light years to 30,000 light years). For a simulation which summarizes their results, see here.