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Black Holes
An artist's drawing a black hole named Cygnus X-1. It
formed when a large star caved in. This black hole pulls
matter from blue star beside it. Image
Credit: NASA/CXC/M.Weiss
One of the most perplexing mysteries of the universe is the nature
of a black hole. Less than a century ago, black holes were dismissed
as implausible and not supported by mathematics. Improved
technology and more advanced mathematical models have given
scientists much more information about what black holes are.
Now scientists know that there are black holes at the center of all
galaxies, including our own, the Milky Way. They theorize that it’s
the pull of these black holes that help keep galaxies together.
The next chance for scientists to examine a black hole growing in our
galaxy will be in March and April of this year. The black hole in the
center of the Milky Way is called Sagittarius A*, and it is pulling a gas
cloud towards it.
Illustration of Sagittarius A*, the black hole
at the center of the Milky Way. Source: New
York Times
How does a Black Hole Form?
When it dies, an average-sized star, like our Sun, will lose its outer layers and compress to
about the size of the Earth, becoming a white dwarf.
For a star 20 times the size of our sun, this compression is much more extreme, creating a
gravity force so intense, it would crush the Earth to the size of a marble.
What makes a Black Hole Black?
Escape velocity is how fast something must be traveling to escape the gravitational pull of an
object. The escape velocity of Earth is 11.2 km/second.
The speed of light is 299,792 km/second. The escape velocity at the edge of a black hole (the
event horizon) is greater than the speed of light. That means that nothing, not even light, can
get out.
Anatomy of a Black Hole
Because no light escapes them, black holes could not be seen until the 1960s. Then, new
telescopes using x-ray and radio waves were developed. Scientists must study how space
around a black hole is acting to even know to point the telescope in a black hole’s direction.
The main parts of a black hole are the event horizon, accretion disk, and singularity.
Misconceptions about Black Holes
Black Holes suck in everything around them.
o Black holes aren’t vacuums and don’t suck in matter. They affect objects around
them with their gravitational force.
Stars increase their gravitational pull as they become black holes
o Actually, the gravitational force of a black hole is equal to whatever its star’s
gravitation force was. For example, if our Sun ever became a black hole (which is
highly unlikely because of its size), it would have the same gravitational pull it
does now; it would just be MUCH smaller—about four miles across.
Black holes facilitate time travel
o Albert Einstein proved that gravity affects time. Gravity slows time down.
Because there is so much gravity in a black hole, time appears to slow almost to
the point of standing still. One minute in a black hole’s event horizon would equal
over a thousand years on Earth.
Black Holes throughout History
1783: Geologist John Michell posits that there could be a body in space so massive, not
even light could escape it.
1796: Mathematician Pierre-Simon Laplace proposed a similar idea.
1915: Albert Einstein developed the theory of general relativity. He showed that light
acts as both a wave and particle; therefore, gravity influences lights’ motion.
1915: Karl Schwarzschild solved the Einstein’s equations describing point mass—the size
an object would have to be compressed to before light could not escape due to the
gravity of its mass. The size of this object is called the Schwarzchild radius. If an object
is smaller than this radius, light cannot leave it—it is a black hole.
1939: Robert Oppenheimer theorized that neutron stars three times bigger than our
sun’s mass would collapse into black holes.
1958: David Finkelstein showed that the Schwarzchild radius is equal to a black hole’s
event horizon. This is the start of the golden age of black hole research.
1964: First use of the term Black Hole. It was adopted more readily in 1967.
Sources:
It’s Snack Time in the Cosmos. By Ron Cown. New York Times. http://www.nytimes.com/2014/02/18/science/its-snacktime-in-the-cosmos.html?_r=0
Black Holes: Star Eater. By Michael Finkel. National Geographic Magazine.
http://ngm.nationalgeographic.com/2014/03/black-holes/finkel-text
Recommended Books:
The Perfect Theory, by Pedro G. Ferreira
The Universe in the Rearview Mirror, by
Dave Goldberg
Gravity’s Fatal Attraction: Black Holes in
the Universe, by Mitchell Begelman
Gravity: How the Weakest Force in the
Universe Shaped our Lives, by Brian Clegg