Black Holes - Black and Red: the colors of
gravity...
Black
holes are amongst the universe's family
of anomalies that we've just recently begun to understand in the recent
decades. The term black hole was laid to claim by John Wheeler in 1969,
yet the theory dates back over 200 years. In 1783, John Michell wrote a
paper declaring that a star that was massive enough would have a
gravitational influence so strong that light would not be able to
escape its surface. He also believed there were a number stars like
this in the universe, but because light could not escape their gravity
they would just be black voids in space. Michell also conjectured that
even though we could not see the stars' light we could feel their
gravitational influence. It took 200 years before Michells theories
could be put to the test, but it came. Of course
Re won the battle each and every
day, to shine his rays onto
the fertile lands surrounding the river Nile, bringing food and
prosperity to the realm. It's not surprising that the most important
god of Egypt was the sun, source of all wealth. The Pharaohs didn't
take on the name and depiction of Re for no reason. The sun was the
embodiment of life AND eternal life. But how eternal is the life of the
sun really?
A relatively small star
The
expected life span of our sun is
about 14
billion years. The sun is about one-third through that time, and can be
compared to a human being in her late twenties, still full of strength
and vigor.
In order to
understand black holes, one must understand the life
process of a star. Stars form when a large quantity of interstellar gas
- mainly hydrogen atoms - begins to contract due to self-gravity. The
colliding atoms begin heating up as they collide at greater rates and
at high velocities. Eventually, the collapse gets so hot that the atoms
no longer repel off of each other, but fuse together into helium atoms.
This is called thermonuclear fusion. Eventually, the heat produced from
these collisions counters the contraction of gravity and a star is
formed. Stephen Hawking's analogy works great: "It is a bit like a
balloon - there is a balance between the pressure of the air inside,
which is trying to make the balloon expand, and the tension in the
rubber, which is trying to make the balloon smaller." Inevitably, the
star will run out of nuclear fuel and will no longer be able to melee
with gravity. Thus, gravity wins the war and the star is doomed to
collapse; but it isn't necessarily doomed to a collapse so severe it
creates a black hole.
How will the sun die eventually?
During
the next billion years or so,
the sun
will become brighter by 10%. This will heat up our planet as a result
of a severe greenhouse effect. All of the oceans on earth will vaporize
and all life will be destroyed. After another 5.5 billion years
the sun will burn up all of its hydrogen fuel located in the core, and
then it will start using up the hydrogen from the layers surrounding
the core.
This will cause
the sun to swell like a big balloon. 2.5 billion years
later the sun will have become about 100 times bigger than its present
size. By this point it has swallowed Mercury, Venus and very probably
the Earth in the process of expansion. At that moment we call the sun a
Red Giant.
The sun's exhaust
gas, helium - generated through nuclear fusion - will
serve as the sun's new fuel, when it has devoured all of its hydrogen.
The standard hydrogen core can reach temperatures as hot as 100 million
degrees, while a helium core can reach up to 600 million degrees. The
temperatures increase and the fuel runs out quicker. The transition
from a G2 star (our sun) to a Red Giant is roughly 160 million years.
On the cosmic scale that's quite fast. The lifespan of a Red Giant is
only 1 billion years, compared to our sun's 10 billion years.
Once all the
sun's helium is consumed it will then eject enormous
amounts of matter into space. After it ejects its surface layers, the
sun will then cool down and contract to be an object with a very high
density, but only a few thousand miles in radius. We call this object a
White Dwarf. A teaspoon of white dwarf material would weigh
five-and-a-half tons or more on Earth. Yet a white dwarf can contract
no further; its electrons resist further compression by exerting an
outward pressure that counteracts gravity. This balance between
gravity and outward pressure, called electron degeneracy pressure, is
the reason why stars do not explode very soon after birth. Effectively
the sun is now around its dying years.
Shrinking Star
White
dwarfs are very common objects in
the
universe.Most of them are very dim and invisible to our eye and
telescopes. A very famous one is Sirius B. Astronomer W.Bessel was the
first to suspect that Sirius had an invisible companion when he
observed that the path of the star wobbled. In the 1920's it was
determined that Sirius B, the companion of Sirius, was a "white dwarf"
star. The pull of its gravity caused Sirius's wavy movement.
Here is an
X-ray image of the Sirius star
system located 8.6 light
years from Earth. This image shows two sources and a spike-like pattern
due to the support structure for the transmission grating. The bright
source is Sirius B, a white dwarf star that has a surface temperature
of about 25,000 degrees Celsius which produces very low energy X-rays.
The dim source at
the position of Sirius A - a normal star more than
twice as massive as the sun - may be due to ultraviolet radiation from
Sirius A leaking through the filter on the detector. The picture was
taken with the Chandra X-ray Observatory. Since its launch on July 23,
1999, the Chandra X-ray Observatory has been NASA's flagship mission
for X-ray astronomy, taking its place in the fleet of "Great
Observatories."
The picture to
the bottom right shows the same star system, now through
a 'normal' visible light telescope, to show exactly how small Sirius B
is compared to Sirius A, which is about 1.6 times the size of our own
sun, but 22 times the luminosity of our sun. Sirius B has a luminosity
of 1/400 of our sun, making it very dim.
