What is a black hole?

A black hole is a region of space packed with so much matter that its own gravity prevents anything from escaping – even a ray of light. Black holes can form when massive stars run out of fuel and collapse under their own weight, creating such strong gravity that they disappear from view. Although completely invisible, a black hole exerts a gravitational pull on surrounding matter.

Find out more from HubbleSite:

How is a black hole created?

A common type of black hole is produced by certain dying stars. A star with a mass greater than about 20 times the mass of our Sun may produce a black hole at the end of its life.

In the normal life of a star there is a constant tug of war between gravity pulling in and pressure pushing out. Nuclear reactions in the core of the star produce enough energy and pressure to push outward. For most of a star’s life, gravity and pressure balance each other exactly, and so the star is stable. However, when a star runs out of nuclear fuel, gravity gets the upper hand and the material in the core is compressed even further. The more massive the core of the star, the greater the force of gravity that compresses the material, collapsing it under its own weight.

For small stars, when the nuclear fuel is exhausted and there are no more nuclear reactions to fight gravity, the repulsive forces among electrons within the star eventually create enough pressure to halt further gravitational collapse. The star then cools and dies peacefully. This type of star is called a "white dwarf."

When a very massive star exhausts its nuclear fuel it explodes as a supernova. The outer parts of the star are expelled violently into space, while the core completely collapses under its own weight.

If the core remaining after the supernova is very massive (more than 2.5 times the mass of the Sun), no known repulsive force inside a star can push back hard enough to prevent gravity from completely collapsing the core into a black hole.

From the perspective of the collapsing star, the core compacts into a mathematical point with virtually zero volume, where it is said to have infinite density. This is called a singularity.

Where this happens, it would require a velocity greater than the speed of light to escape the object's gravity. Since no object can reach a speed faster than light, no matter or radiation can escape. Anything, including light, that passes within the boundary of the black hole -- called the "event horizon" -- is trapped forever.

Do all stars become black holes?

Only stars with very large masses can become black holes. Our Sun, for example, is not massive enough to become a black hole. Four billion years from now when the Sun runs out of the available nuclear fuel in its core, our Sun will die a quiet death. Stars of this type end their history as white dwarf stars. More massive stars, such as those with masses of over 20 times our Sun’s mass, may explode as supernovae and eventually create a black hole.

Since light has no mass, how can it be trapped by the gravitational pull of a black hole?

Newton thought that only objects with mass could produce a gravitational force on each other. According to Newton’s theory, the force of gravity should not affect light. Einstein discovered that the situation is a bit more complicated than that.

First he discovered that gravity is produced by a curved space-time. Then Einstein theorized that the mass of an object actually curves space-time. Mass is linked to space in a way that physicists today still do not completely understand. However, we know that the stronger the gravitational field of an object, the more the space around the object is warped. In other words, straight lines are no longer straight if exposed to a strong gravitational field; instead, they are curved.

Since light ordinarily travels on a straight-line path, light follows a curved path if it passes through a strong gravitational field. This is what is meant by "curved space," and this is why light becomes trapped in a black hole. In 1919, a team led by Sir Arthur Eddington proved Einstein’s theory when they observed the bending of starlight when it traveled close to the Sun. This was the first successful prediction of Einstein’s General Theory of Relativity.

One way to picture this effect of gravity is to imagine a sheet of rubber stretched out. Imagine that you put a heavy ball in the center of the sheet. The weight of the ball will bend the surface of the sheet close to it. This is a two-dimensional picture of what gravity does to spacetime in four dimensions. Now take a little marble and send it rolling from one side of the rubber sheet to the other. Instead of the marble taking a straight path to the other side of the sheet, it will follow the contour of the sheet that is curved by the weight of the ball in the center. This is similar to how the gravitation field created by an object (the ball) affects light (the marble).

What does a black hole look like?

A black hole itself is invisible because no light can escape from it. We can't see black holes, but we can find them by examining their effects on objects around them.

We identify suspected supermassive black holes in the center of galaxies by studying the orbits of stars and clouds of gas in that vicinity and the speed with which they move. If those motions indicate the presence of more mass than can be accounted for by counting the stars in that area, the best explanation for the extra mass is a black hole.

When a smaller black hole and a star orbit each other, the black hole can be identified if it pulls matter from its companion star. As the matter swirls into the black hole it heats up, emitting x-rays that can be detected by astronomers.

What is escape velocity?

Escape velocity is the speed a moving object must have in order to escape from the gravitational pull exerted by another object. For example, the escape velocity from Earth’s surface is equal to about 6.8 miles per second (11 km/s). Anything that wants to escape Earth’s gravitational pull must go at least 6.8 miles per second (11 km/s), no matter what the thing is – a rocket ship or a baseball. The escape velocity of an object depends on how compact it is; that is, on the ratio of its mass to radius. A black hole is an object so compact that, within a certain distance of it, even the speed of light is not fast enough to escape.

Is a black hole a giant cosmic vacuum cleaner?

The answer to this question is "not really."

The gravity around a black hole remains normal unless you get extremely close. If the Sun suddenly became a black hole (which isn't actually possible), the Earth and all the other planets would continue to orbit it just as though nothing had changed.

The behavior of gravity doesn't change until an object approaches to the point where it's within a few times the radius of the event horizon, the boundary marking the region around a black hole from which not even light can escape. At that point, objects begin to lose the ability to maintain stable orbits, and inevitably spiral into the black hole.

So to return to our theoretical example, if the Sun became a black hole, objects would have to be as close as about 6.2 miles (10 km) to the black hole's center before they began spiraling in.

How many types of black holes are there?

According to theory, there might be three types of black holes: stellar, supermassive, and miniature black holes – depending on their mass. These black holes would have formed in different ways.

Stellar black holes form when a massive star collapses. Supermassive black holes, which can have a mass equivalent to billions of suns, likely exist in the centers of most galaxies, including our own galaxy, the Milky Way. We don't know exactly how supermassive black holes form, but it's likely that they're a byproduct of galaxy formation. Because of their location in the centers of galaxies, close to many tightly packed stars and gas clouds, supermassive black holes continue to grow on a steady diet of matter.

No one has ever discovered a miniature black hole, which would have a mass much smaller than that of our Sun. But it's possible that miniature black holes could have formed shortly after the "Big Bang," which is thought to have started the universe 13.7 billion years ago. Very early in the life of the universe the rapid expansion of some matter might have compressed slower-moving matter enough to contract into black holes.

Another division separates black holes that spin (possess angular momentum) from those that don't spin.

When were black holes first theorized?

Using Newton’s Laws in the late 1790s, John Michell of England and Pierre-Simon Laplace of France independently suggested the existence of an "invisible star." Michell and Laplace calculated the mass and size – which is now called the "event horizon" – that an object needs in order to have an escape velocity greater than the speed of light. In 1915, Einstein's theory of general relativity predicted the existence of black holes. In 1967 John Wheeler, an American theoretical physicist, applied the term "black hole" to these collapsed objects.

What evidence do we have for the existence of black holes?

Astronomers have found convincing evidence for a supermassive black hole in the center of our own Milky Way galaxy, the galaxy NGC 4258, the giant elliptical galaxy M87, and several others. Scientists verified the existence of the black holes by studying the speed of the clouds of gas orbiting those regions. In 1994, Hubble Space Telescope data measured the mass of an unseen object at the center of M87. Based on the motion of the material whirling about the center, the object is estimated to be about 3 billion times the mass of our Sun and appears to be concentrated into a space smaller than our solar system.

For many years, X-ray emissions from the double-star system Cygnus X-1 convinced many astronomers that the system contains a black hole. With more precise measurements available recently, the evidence for a black hole in Cygnus X-1 -- and about a dozen other systems -- is very strong.

Find out more from HubbleSite:

How does the Hubble Space Telescope search for black holes?

The spectra detected by STIS in galaxy M84. Read more about it.

A black hole cannot be viewed directly because light cannot escape it. It can, however, be identified by its effect on the matter around it. Matter swirling around a black hole heats up and emits radiation that can be detected. Around a stellar black hole, this matter is composed of gas. Around a supermassive black hole in the center of a galaxy, the swirling disk is made of not only gas but also stars. An instrument aboard the Hubble Space Telescope, called the Space Telescope Imaging Spectrograph (STIS), was installed in February 1997. STIS is the space telescope’s main "black hole hunter." A spectrograph uses prisms or diffraction gratings to split the incoming light into its rainbow pattern. The position and strength of the line in a spectrum gives scientists valuable information. STIS spans ultraviolet, visible, and near-infrared wavelengths. The instrument can take a spectrum of many places at once across the center of a galaxy. Each spectrum tells scientists how fast the stars and gas are swirling at that location. With that information, the central mass that the stars are orbiting can be calculated. The faster the stars go, the more massive the central object must be.

STIS found, for instance, the signature of a supermassive black hole in the center of the galaxy M84. The spectra showed a rotation velocity of 400 km/s (248 miles per second), equivalent to 1.4 million km (.86 million miles) every hour. Earth orbits our Sun at 30 km/s (18.6 miles per second). If Earth moved as fast as 400 km/s (248 miles per second), our year would be only 27 days long.

Find out more from HubbleSite:

What are gravitational lenses and how do they work?

Any huge amount of matter, like a cluster of galaxies, creates a gravitational field that distorts and magnifies the light from distant galaxies that are behind it but in the same line of sight. The effect is like looking through a giant magnifying glass. It allows researchers to study the details of early galaxies too far away to be seen with current technology and telescopes.

Smaller objects, like individual stars, can also act as gravitational lenses when more distant stars pass directly behind them. For a few days, light from the more distant star temporarily appears brighter because it is magnified by the gravity of the closer object. This effect is known as gravitational microlensing.

HubbleSite and STScI are not responsible for content found outside of hubblesite.org and stsci.edu

Return to question list for Exotic

Return to FAQ home