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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