It is thought that a black hole forms when a large star burns out. While a star is still active it creates an outward force that counteracts the inward force of gravity. When a star burns out, and this outward force is no longer there, the gravitational force causes the star to collapse inward on itself. If this mass is equal to or greater than three solar masses (3 of our suns) a black hole forms.
The gravitational force within a black hole is so great that nothing can escape it. Anything that enters or gets near a black hole is gone for good because the speed needed to escape the gravitational force within a black hole is too great. Even light doesn't travel fast enough to escape from black holes.
The reason the gravitational pull is so great is because a black hole's mass is very dense, but its size is very small. If you keep an object's mass the same, but the object becomes smaller and more compact, the gravitational force increases, which is exactly what happens in a black hole. It is equivalent to smashing an entire planet down to the size of a small marble, without losing any of its mass.
Parts of a Black Hole
The event horizon is what we think of as the surface of a black hole. It is spherical and is the boundary in which nothing can leave the gravitational force. Once something passes through the event horizon it is never seen again. The larger the mass of the black hole, the larger the event horizon.
The singularity is the center of the black hole. It is thought that the mass continues to shrink within itself until this center becomes infinitely dense. Anything pulled into the black hole goes toward the singularity and becomes part of its mass. Unfortunately, we don't currently have any mathematical or physical theories that can fully explain a singularity.
In the popular media you'll often hear references to worm holes at the singularity. Worm holes are tunnels or passages where you can supposedly travel to other universes or galaxies. However, there is no conclusive evidence that these exist. In fact, no one really knows what lies at the point of the singularity.
The accretion disk isn't actually a part of the black hole. Instead it is a flattened cloud of gases swirling around the event horizon.
Black holes are invisible and we can't see them. However, we can see the accretion disk and this is one way that we "see" black holes. The reason we can see the accretion disk is because it emits X-ray radiation. Scientist use X-ray telescopes to find black holes by looking for X-ray radiation from the accretion disk. The source of the radiation is from objects that have become too close to the black hole. Objects that get close to the event horizon get torn apart, they heat up, and begin to emit radiation.
NASA has several satellites in the Earth's orbit. One of these is the Chandra X-ray Observatory and it has X-ray telescopes that help us detect black holes. The photo on this page was taken by a telescope on board this satellite. The photo shows the accretion disk of a supermassive black hole in Galaxy NGC 1068. Its mass is 5 million solar masses. The light area of the photo is a cloud of cool gas and dust that surrounds the black hole, called the torus.
In the vaccuum of space light travels in a straight line. However, if light gets close to a black hole the gravitational pull causes it to become curved.
Objects near a black hole are affected by its gravity and begin to move faster than they normally would. In addition to looking for radiation, scientists can find the location of black holes by looking for space objects, such as stars and gases, that are moving faster than normal.
There are three different kinds of black holes:
- Stellar Mass - has the mass of about one star
- Intermediate Mass - has the mass of up to 1000 stars and may eventually turn into supermassive black holes
- Supermassive - has the mass of millions of stars
Scientists believe that there is a supermassive black hole in the center of the Milky Way galaxy and possibly in the center of all galaxies. This suggests that perhaps these black holes may be involved in galaxy formation.
Photo of black hole: Courtesy of NASA Marshall Space Flight Center (NASA-MSFC).