Black Holes (837 words)

Black HolesOn December 3, 1995, astronomers using the Hubble Space Telescope identified a black hole in the galaxy
NGC 4621, located 100 million light-years from Earth in the direction of the constellation Virgo.
This is the second super-massive black hole that astronomers using the Hubble Space Telescope
have found. Astronomers believe that the mass of this black hole is about 1.2 billion times the mass of our
sun, but it is concentrated in a space that is not any bigger than our solar system
There are two puzzling questions that astronomers are trying to answer. The black hole is fueled by the
galaxy and its 800 light-year-wide spiral disk of dust. Before the discovery of this black hole, astronomers
did not think that there was any dust in elliptical galaxies like NGC 4261. Currently they believe that the
disk of dust is the remnant of a smaller galaxy that fell into the core of NGC 4261. The black hole will
swallow up the gas from the smaller galaxy over the next 100 million years. Researchers believe that while
the gas is being swallowed by the black hole, the process will produce some amazing fireworks.

The second puzzling question that astronomers are trying to answer is why isn’t the black hole at the center
of the galaxy? According to images from the Hubble Space Telescope, the black hole is 20 light-years
from the center of the galaxy, but since the black hole is so massive it is hard to explain how it could have
been moved. One idea is that the black hole is moving itself. Some astronomers think that the disk of dust
serves as a “fuel tank.” The black hole sucks in the material which is absorbed by gravity, compressed, and
heated to tens of millions of degrees. This theory would explain why radio telescopes have observed radio
jets, or hot gas exhausts from the black hole’s area. The exhaust may be pushing the black hole across
space, as a jet engine gives thrust to a plane.
According to Ted Bunn, an astronomer at Berkley University, “A black hole is a region of space that has so
much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull, not
even light.”
So, what does that mean? The following example will explain the nature of a black hole. A ball is thrown
up into the air. It rises for a while, but it eventually falls. If the ball is thrown fast enough it is able to
escape the gravitational field and continue rising. This is called the “escape velocity.” Theescape
velocity depends on the mass of the planet, which affects the gravitational field. A object on Earth would
have to travel at 25,000 m.p.h. to escape the gravitational pull of Earth. Now imagine an object with a
gravitational pull so large and escape velocity so high that even visible light could not escape it. This is a
black hole.
How are black holes formed? To our best knowledge, black holes are created at the end of a very
large star’s life cycle. The star collapses into a white dwarf, which is the smallest, dimmest, and hottest of
all stars. Matter is packing continually tighter and tighter together and gravity is increasing. The white
dwarf will collapse into itself when it runs out of fuel, thus creating a black hole. Therefore only very
massive stars can form black holes because only large stars have enough mass and gravitational pull to
collapse into itself.

What are the parts of a black hole? There are two main parts to a black hole. The first one is the
event horizon and the second is the singularity. The event horizon is the spherical surface that marks the
boundary of the black hole. You can pass into the black hole through the horizon, but you cannot pass back
out of it. Once you have crossed the horizon, you are doomed to “hit” singularity.

The singularity is the center of the black hole and all that astronomers know about it is that the gravitational
pull is so large that anything reaching it would be torn to pieces.

Astronomers are continuing to research photos of the black hole in the galaxy NGC 4261 sent
back by the Hubble Space Telescope. These photos will hopefully help astronomers to better understand
the mysteries of black holes.

Bibliography
1. Anonymous. “Black Holes.”
http://ericir.syr.edu/Projects/Newton/11/blchole.html
(27 Aug. 1996)
2. Asimov, I. (1978) How Did We Find Out About Black Holes?
New York: Walker and Company
3. McIrvin, Matt. “Some Frequently Asked Questions About Black
Holes.”http://skyron.harvard.edu/bh_faq.html
(15 Sept. 1996)
4. The Students of the Astronomy Department of the University of
Cambridge.”Observational Evidence for Black Holes”,
“Introduction to Black Holes”, and “Black Holes and Critical
Phenomena.”http://damtp.cam.ac.uk/gr/public/
(27 Aug. 1996)