MadSci Network: Astronomy

Re: Why do the centre of galaxies appear bright like the sun?

Date: Mon Mar 19 20:11:03 2001
Posted By: Angelle Tanner, Grad student, Astronomy, UCLA
Area of science: Astronomy
ID: 981895554.As

Your question is a good one and, in fact, I study the very types
of black holes you are referring to. You are right that light
cannot escape from a black hole, but what you don't realize is
that black holes, even the supermassive ones lurking at the
centers of galaxies, are extremely small. The supermassive
ones are not much bigger than one Astronomical Unit (AU) in
radius which is the distance from the Sun to the Earth. This 
leaves lots of space for other objects, like stars, to shine
brightly and give the galactic center the glow we see. 
The centers of all spiral galaxies like the Milky Way have a
puffed-up cloud of stars called a bulge. However, don't be 
fooled by our own Galactic Center. The bright patch of stars
you see toward the center of our galaxy, which is near
the constellation Sagittarius, is not the true center.
The bright patch is a region called Baade's window. The true
center lies to the side of the bright patch but is much fainter due to
all the gas and dust along the plane of our galaxy which
absorbs the light from the Galactic Center. 

So in conclusion, the centers of galaxies are bright because
there are a lot of stars in the galactic bulge. 
Black holes are not big enough to gobble up all the light from the many
stars in the bulge. 

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