You may have heard someone say, "My desk has become a black
hole!" You may have seen an astronomy program on television or
read a magazine article on black holes. These exotic objects
have captured our imagination ever since they were predicted
by Einstein's Theory of General Relativity in 1915.
Photo courtesy NASA/Space Telescope Science
Institute (J. Gitlin, artist)
Artist concept of the near vicinity of the
black hole at the core of galaxy NGC
4261
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What are black holes? Do they really exist? How can we find
them? In this edition of HowStuffWorks,
we will examine black holes and answer all of these questions!
What is a Black Hole?
A black hole is
what remains when a massive star dies.
HistoryThe concept of an object from which light could
not escape (e.g., black hole) was originally proposed by
Pierre Simon Laplace in 1795. Using Newton's Theory of
Gravity,
Laplace calculated that if an object were compressed
into a small enough radius, then the escape velocity of
that object would be faster than the speed of
light.
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If you have
read How Stars
Work, then you know that a star is a huge, amazing
fusion reactor. Because stars are so massive and made
out of gas, there is an intense gravitational field that is
always trying to collapse the star. The fusion reactions
happening in the core are like a giant fusion
bomb that is trying to explode the star. The
balance between the gravitational forces and the
explosive forces is what defines the size of the star.
As the star dies, the nuclear
fusion reactions stop because the fuel for these reactions
gets burned up. At the same time, the star's gravity pulls
material inward and compresses the core. As the core
compresses, it heats up and eventually creates a supernova
explosion in which the material and radiation blasts out into
space. What remains is the highly compressed, and extremely
massive,
core.
The core's gravity is so strong that even light cannot
escape.
This object is now a black hole and literally disappears
from view. Because the core's gravity is so strong, the core
sinks through the fabric of space-time,
creating a hole in space-time -- this is why the object is
called a black hole.
Photo courtesy NASA
Artist concept of a black hole: The arrows
show the paths of objects in and around the opening of
the black
hole.
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The core becomes the central part of the black hole called
the singularity. The opening of the hole is called the
event horizon.
You can think of the event horizon as the mouth of the
black hole. Once something passes the event horizon, it is
gone for good. Once inside the event horizon, all "events"
(points in space-time) stop, and nothing (even light) can
escape. The radius of the event horizon is called the
Schwarzschild radius, named after astronomer Karl
Schwarzschild, whose work led to the theory of black holes.
Types of Black Holes
There are two types of
black holes:
- Schwarzschild - Non-rotating black hole
- Kerr - Rotating black hole
The
Schwarzschild black hole is the simplest black hole, in
which the core does not rotate. This type of black hole only
has a singularity and an event horizon.
The Kerr black hole, which is probably the most
common form in nature, rotates because the star from which it
was formed was rotating. When the rotating star collapses, the
core continues to rotate, and this carried over to the black
hole (conservation of angular momentum). The Kerr black
hole has the following parts:
- Singularity - The collapsed core
- Event horizon - The opening of the hole
- Ergosphere - An egg-shaped region of distorted
space around the event horizon (The distortion is caused by
the spinning of the black hole, which "drags" the space
around it.)
- Static limit - The boundary between the
ergosphere and normal space
Photo courtesy NASA
Artist concept of a black hole and its
surroundings: The blackened circle is the event horizon
and the egg-shaped region is the
ergosphere.
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If
an object passes into the ergosphere it can still be
ejected from the black hole by gaining energy from the hole's
rotation.
However, if an object crosses the event horizon, it
will be sucked into the black hole and never escape. What
happens inside the black hole is unknown; even our current
theories of physics do not apply in the vicinity of a
singularity.
Even though we cannot see a black hole, it does have three
properties that can or could be measured:
- Mass
- Electric charge
- Rate of rotation (angular momentum)
As of
now, we can only measure the mass of the black hole reliably
by the movement of other objects around it. If a black hole
has a companion (another star or disk of material), it is
possible to measure the radius of rotation or speed of orbit
of the material around the unseen black hole. The mass of the
black hole can be calculated using Kepler's
Modified Third Law of Planetary Motion or rotational
motion.
How We Detect Black Holes
Although we cannot
see black holes, we can detect or guess the presence of one by
measuring its effects on objects around it. The following
effects may be used:
- Mass estimates from objects orbiting a black hole or
spiraling into the core
- Gravitational lens effects
- Emitted radiation
Mass
Many black holes have objects around
them, and by looking at the behavior of the objects you can
detect the presence of a black hole. You then use measurements
of the movement of objects around a suspected black hole to
calculate the black hole's mass.
What you look for is a star or a disk of gas that is
behaving as though there were a large mass nearby. For
example, if a visible star or disk of gas has a "wobbling"
motion or spinning AND there is not a visible reason for this
motion AND the invisible reason has an effect that appears to
be caused by an object with a mass greater than three solar
masses (too big to be a neutron star), then it is possible
that a black hole is causing the motion. You then estimate the
mass of the black hole by looking at the effect it has on the
visible object.
For example, in the core of galaxy NGC 4261, there is a
brown, spiral-shaped disk that is rotating. The disk is about
the size of our solar system, but weighs 1.2 billion times as
much as the sun. Such a huge mass for a disk might indicate
that a black hole is present within the disk.
Photo courtesy NASA/Space Telescope Science
Institute
Credit: L. Ferrarese (Johns Hopkins
University) and NASA
Hubble
Space Telescope image of the core of galaxy NGC
4261
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Gravity Lens
Einstein's
General Theory of Relativity predicted that gravity could
bend space. This was later confirmed during a solar
eclipse when a star's position was measured before, during
and after the eclipse. The star's position shifted because the
light from the star was bent by the sun's gravity. Therefore,
an object with immense gravity (like a galaxy or black hole)
between the Earth and a distant object could bend the light
from the distant object into a focus, much like a lens can.
This effect can be seen in the image below.
Photo courtesy NASA/Space Telescope Science
Institute
Credit: NASA and Dave Bennett (University
of Notre Dame)
These images
show the brightening of MACHO-96-BL5 from ground-based
telescopes (left) and the Hubble Space Telescope
(right).
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In the above image, the brightening of MACHO-96-BL5
happened when a gravitational lens passed between it
and the Earth. When the Hubble Space
Telescope looked at the object, it saw two images of the
object close together, which indicated a gravitational lens
effect. The intervening object was unseen. Therefore, it was
concluded that a black hole had passed between Earth and the
object.
Emitted Radiation
When
material falls into a black hole from a companion star, it
gets heated to millions of degrees Kelvin and accelerated. The
superheated materials emit X-rays, which can be detected by
X-ray telescopes
such as the orbiting Chandra
X-ray Observatory.
Photo courtesy CXC/S.Lee
Schematic of a black hole in a binary system,
showing the accretion disk around the black hole and
emission of
X-rays
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The star Cygnus X-1 is a strong X-ray source and is
considered to be a good candidate for a black hole. As
pictured above, stellar winds
from the companion star, HDE 226868, blow material onto the
accretion disk surrounding the black hole. As this material
falls into the black hole, it emits X-rays, as seen in this
image:
Photo courtesy NASA/CXC
X-ray image of Cygnus X-1 taken from orbiting
Chandra X-ray
Observatory
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In addition to X-rays, black holes can also eject materials
at high speeds to form jets. Many galaxies have been
observed with such jets. Currently, it is thought that these
galaxies have supermassive black holes (billions of solar
masses) at their centers that produce the jets as well as
strong radio
emissions. One such example is the galaxy M87 as shown below:
Photo courtesy NASA
Schematic diagram of active galactic nucleus
with a supermassive black hole at its
center
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Photo courtesy NASA/Space Telescope Science
Institute
Credit: NRAO, NSF, Associate Universities,
Inc., NASA, and John Biretta (STScI/Johns Hopkins
University)
The images on the
left and bottom are ground-based radiotelescope images
of the heart of galaxy M87. The image on the right is a
visible image from the Hubble Space Telescope. Note the
jet of material coming from
M87.
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It is important to remember that black holes are not cosmic
vacuum
cleaners -- they will not consume everything. So although
we cannot see black holes, there is indirect evidence that
they exist. They have been associated with time
travel and worm holes and remain fascinating objects in
the universe.
For more information on black holes and other space-related
phenomena, check out the links on the next page!
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