Bulge and Nucleus of the Milky Way

The above picture shows the center of the Milky Way galaxy with the constellation Sagittarius superimposed on the image. The center of the Milky Way sits just above the spout of the teapot.

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 (the filamaents in Figure 23.25b) 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 around 4.3x10⁶ 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.

    Schwarzschild Radius, Event Horizon, and Singularity The escape speed from the Schwarzschild Radius is equal to the speed of light, c. Inside the Schwarzschild radius, the escape is even higher; it is larger than c. Because the speed of light is the fastest any massive object can move, this means nothing can escape from inside the Schwarzschild Radius not even light, hence the name. The Schwarzschild Radius then defines the Event Horizon; events which take place within the Event Horizon cannot leak out and affect the rest of the Universe! Matter once inside the Event Horizon inexorably flows to the center of the black hole getting compressed to infinite density in the Singularity of the black hole, the central point of the black hole!!

    The Schwarzschild radius of a black hole whose mass is the same as that of the Sun is 2.95 [M/M(Sun)] kilometers. A 4.3 million Solar mass black hole would then have a radius of 13 million kilometers, or less than a millionth of a light year. This is much smaller than the 10-15 light year sizes mentioned above for the gas ring seen around Sgr A. This evidence thus does not require the presence of a black hole at the center of our Galaxy. A more stringent and starting to become interesting constraint comes from the fact that because the density of stars near the nucleus of our Galaxy is huge, stars are expected to pass close to the very center of our Galaxy. Indeed, infrared observations have revealed the presence of several stars at the galactic center and have tracked their orbital motions over roughly the last 10-15 year period. The object S2 moves in an orbit whose size is only 950 Astronomical Units ~ 1 light-week which led to the mass estimate of 3.7 million Solar masses, and the motion of object S16 which passed within 45 Astronomical Units (a little larger than the size of Pluto's orbit about the Sun), less than 1 light-day of the center of the Galaxy. Although these are tiny, tiny orbits, they are still not small enough to resolve the black hole question. To nail the question of whether there is or is not a black hole at the center of our Galaxy requires that we peer into the nucleus of our Galaxy on scales of 3 million km or ~ 1/50-th of an Astronomical Unit (or an object whose size is much smaller than the orbit of Mercury about the Sun). This is a difficult proposition.

  • So, although it is suggestive that the core of our Galaxy might be a low-powered version of an AGN, the case has not yet been made conclusively.
  • Moving forward, however, let's imagine that there is a black hole at the center of our Galaxy to see why this is such an attractive ides. Because stars such as S2 and S16 pass close to the nucleus of our Galaxy, near the purported black hole of our Galaxy, strong tidal forces from the black hole arise and may rip the stars apart (much as comets are ripped apart when they pass close to the Sun) allowing their gases to be devoured by the black hole. In the process of devouring the stars, large amounts of energy are released as the material spirals into the potential well of the black hole. This energy release process may be up 100 times as efficent as typical nuclear reactions which occur in the cores of stars.

    How often must the black hole eat in order to power the activity of the nucleus of the Milky Way?

    The nucleus of our Galaxy produces energy at the the prodigious rate of 10³³ Watts; the Sun only produces energy at the rate of 4 x 10²⁶ Watts! Despite this huge power output, the black hole at the center of our Galaxy need only eat roughly 1 star every few million years to fuel its engine.

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