Black Holes

On 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."  The     escape 
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)