Black Holes

Most black holes are the result of a massive star explosion. As the star explodes, its core is crushed by gravity until it disappears from sight. Gravity becomes so extreme that it overwhelms all other forces in the universe. Not even light can escape.

Image at right: A growing black hole, called a quasar (courtesy of NASA/JPL-Caltech).

Not all stars will end their lives as black holes. A star must have a mass at least 25 times larger than our Sun to end in a supernova explosion powerful enough to create a black hole. Fewer than 1 out of 1000 stars in our Milky Way are massive enough to become a black hole.

Features

The densest objects in the universe, black holes are aptly named because they emit no light at all. A black hole is surrounded by a spherical boundary, called an event horizon, through which light and matter can enter but can't escape. The event horizon marks the "point of no return" around the black hole. The center of the black hole is called the singularity. The singularly is where all the mass of the black hole is concentrated. Some physicists think that singularities might be as small as a single atom, and others think that they may be infinitely small, but no one really knows for certain yet.

Even though scientists cannot directly see black holes, they are able detect them through their effects on surrounding material, such as gas and dust. As this material is pulled into a black hole, it is stretched and heated to millions of degrees. This hot material emits x-ray light that astronomers can detect with satellites.

It is now thought that almost all galaxies contain super-massive black holes, with masses equivalent to millions of Suns, in their centers.

Learn more about the formation of black holes from National Geographic.

Missions

: Chandra X-Ray Observatory (Courtesy of NASA)

NASA's Chandra X-ray Observatory, which was launched and deployed by the Space Shuttle Columbia in July 1999, is the most sophisticated x-ray observatory built to date. Information from Chandra in 2007 helped scientists to learn that black holes forming from dying stars can be much larger than they had realized.

Launched on June 11, 2008, the Fermi Gamma Ray Space Telescope (formerly named GLAST) is providing astronomers with a superior tool to study how black holes, known for pulling matter in, can accelerate jets of gas outward at amazing speeds.

Learn more about missions exploring black holes on NASA's website.

Myths, Stories, and More

: An artist's view of a star that wandered too close to a supermassive black hole (NASA/CXC/M.Weiss)

Many popular myths about black holes have been perpetuated by Hollywood. Movies and television often portray black holes as time-traveling tunnels to another dimension or cosmic vacuum cleaners sucking up everything in their vicinity. The truth is that the physics of black holes is so complex that scientists are still trying to understand their properties. But one thing that is certain is that from well outside the event horizon of a black hole, the gravity of a black hole is no different from the gravity of a normal star. In fact, if the Sun somehow became a black hole, the orbit of the Earth wouldn't change at all! (But it would get a lot colder and darker here on Earth!)

Discover more black holes stories at Amazing Space.

Earth Matters

: The Cygnus X-1 system consists of a black hole with a mass about 10 times that of the Sun in a close orbit with a blue supergiant star with a mass of about 20 Suns.

The nearest black hole, Cygnus X-1, is about 1,600 light-years away, so we don't have to worry about any black hole threats to the Earth. Our Milky Way galaxy contains some 100 billion stars, and roughly one out of every thousand stars that form is massive enough to become a black hole. If you could travel by spaceship to a black hole, what might you see? Beside complete darkness, you might see an accretion disk—super hot gas and dust caught in orbit around the black hole.

Explore more questions about black holes at the Ask an Astronomer website.