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
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
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 3.7 106
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.
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 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!
At the Event Horizon, we find that
space and time change
character, and 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!!
Spinning black holes, known as Kerr Black Holes
, have interesting properties. At the Static Limit,
spacetime is dragged forward at the speed of light, inside or which objects
cannot be at rest. Furthermore, in the region enclosed by the static limit
spacetime is dragged
forward by the rotation of the black
hole at speeds exceeding the speed of light with respect to a
The inner boundary of this region is the Outer
Event Horizon of the Black Hole. The region in between the Static
Limit and the outer Event Horizon is called the
Ergosphere. As an object falls through the
ergosphere, the black hole drags it forward,
speeding up its motion (gaining energy at the expense of the hole). Because
is outside the Outer Event Horizon,
the energized particle can escape to our Universe, stealing some of the
energy of the Black Hole! This is known as the Penrose
Process. The Penrose Process
is exceedingly efficient; an object with
mass M can enter with zero energy and then escape with
0.29 Mc2. This is a very efficient process.
In the nuclear fusion processes
which power the Sun, an amount of energy is released which corresponds to
Another interesting effect can occur inside a
Kerr Black Hole. The inner boundary of the ergosphere is the outer
Event Horizon of the hole, which implies there is an inner Event Horizon
for the hole. This leads to a strange occurrence. At the outer Event Horizon,
space and time change character, and an object inexorably moves in space
toward the inner horizon. At the inner horizon, space and time change
character again and the object no longer inexorably marches toward the
Singularity (the ring-like feature in the figure).
Instead, an observer could avoid the singularity (and perhaps
even pass through it) and emerge in another apace-time; also it is
possible that Closed Time-like Curves exist
Cosmic Censorship asserts that
naked singularities cannot exist (singularities must always be cloaked by
Event Horizons). Roger Penrose propsed Cosmic Censorship
in the 1960s becuase
of the fact that causality may break down singularities.
Cosmic Censorship has
not been proven for all circumstances; for example, in Kerr black holes naked
singularities are possible but it is not known under what physical scenario
they could arise. Interestingly, the possibility of
traversable wormholes is intertwined with the notion of
- The Schwarzschild radius of a
black hole whose mass is the same as that of the Sun is
3 [M/M(Sun)] kilometers.
A 3.7 million Solar mass black hole would then have a radius
of 11.2 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.
A Journey into a Black Hole (Youtube)
Infrared observations have revealed
the presence of several stars at the galactic center and have tracked their
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
- 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,
tidal forces arise.
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
nearly be 100 times as efficent as the typical nuclear reactions which occur
in the cores of stars (recall that the Penrose process was around
40 times as efficient).
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. Tidal forces arise because of the dependence of
the gravitational force on the separation of the masses. The near side of
the object is pulled from its center which, in turn, is pulled from the far
side of the object. Because the tidal force depends on the difference of
forces on an object, the strength of the tidal force depends strongly on
the separation of the objects. As the separation decreases, the tidal force
strongly increases in strength.
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.
Return to Lecture-2