What is a black hole?

A black hole is an object that is so compact (in other words, has enough mass in a small enough volume) that its gravitational force is strong enough to prevent light or anything else from escaping.

The existence of black holes was first proposed in the 18th century, based on the known laws of gravity. The more massive an object, or the smaller its size, the larger the gravitational force felt on its surface.
John Michell and Pierre-Simon Laplace both independently argued that if an object were either extremely massive or extremely small, it might not be possible at all to escape its gravity. Even light could be forever captured.

The name "black hole" was introduced by John Archibald Wheeler in 1967. It stuck, and has even become a common term for any type of mysterious bottomless pit. Physicists and mathematicians have found that space and time near black holes have many unusual properties. Because of this, black holes have become a favorite topic for science fiction writers. However, black holes are not fiction. They form whenever massive but otherwise normal stars die. We cannot see black holes, but we can detect material falling into black holes and being attracted by black holes. In this way, astronomers have identified and measured the mass of many black holes in the Universe through careful observations of the sky. We now know that our Universe is quite literally filled with billions of black holes.
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Do Black Holes Obey the Laws of Gravity?

Black holes obey all laws of physics, including the laws of gravity. Their remarkable properties are in fact a direct consequence of gravity.

In 1687, Isaac Newton showed that all objects in the Universe attract each other through gravity. Gravity is actually one of the weakest forces known to physics. In our daily life, other forces from electricity, magnetism, or pressure often exert a stronger influence. However, gravity shapes our Universe because it makes itself felt over large distances. For example, Newton showed that his laws of gravity can explain the observed motions of the moons and planets in the Solar System.

Albert Einstein refined our knowledge of gravity through his theory of general relativity. He first showed, based on the fact that light moves at a fixed speed (671 million miles per hour), that space and time must be connected. Then in 1915, he showed that massive objects distort the four-dimensional space-time continuum, and that it is this distortion that we perceive as gravity. Einstein's predictions have now been tested and verified through many different experiments. For relatively weak gravitational fields, such as those here on Earth, the predictions of Einstein's and Newton's theories are nearly identical. But for very strong gravitational fields, such as those encountered near black holes, Einstein's theory predicts many fascinating new phenomena.
How big is a black hole?

All matter in a black hole is squeezed into a region of infinitely small volume, called the central singularity. The event horizon is an imaginary sphere that measures how close to the singularity you can safely get. Once you have passed the event horizon, it becomes impossible to escape: you will be drawn in by the black hole's gravitational pull and squashed into the singularity.

The size of the event horizon (called the Schwarzschild radius, after the German physicist who discovered it while fighting in the first World War) is proportional to the mass of the black hole. Astronomers have found black holes with event horizons ranging from 6 miles to the size of our solar system. But in principle, black holes can exist with even smaller or larger horizons. By comparison, the Schwarzschild radius of the Earth is about the size of a marble. This is how much you would have to compress the Earth to turn it into a black hole. A black hole doesn't have to be very massive, but it does need to be very compact!

Some black holes spin around an axis, and their situation is more complicated. The surrounding space is then dragged around, creating a cosmic whirlpool. The singularity is an infinitely thin ring instead of a point. The event horizon is composed of two, instead of one, imaginary spheres. And there is a region called the ergosphere, bounded by the static limit, where you are forced to rotate in the same sense as the black hole although you can still escape.
What types of black holes are there?

Black holes often look very different from each other. But this is because of variety in what happens in their surroundings. The black holes themselves are all identical, except for three characteristic properties: the mass of the black hole (how much stuff it is made of), its spin (whether and how fast it rotates around an axis), and its electric charge. Amazingly, black holes completely erase all of the other complex properties of the objects that they swallow.

Astronomers can measure the mass of black holes by studying the material that orbits around them. So far, we have found two types of black holes: stellar-mass (just a few times heavier than our Sun) or supermassive (about as heavy as a small galaxy). But black holes might exist in other mass ranges as well. For example, recent observations suggest there may be black holes with masses between stellar-mass and supermassive black holes.

Black holes can spin around an axis, although the rotation speed cannot exceed some limit. Astronomers think that many black hole in the Universe probably do spin, because the objects from which black holes form (stars for example) generally rotate as well. Observations are starting to shed some light on this issue, but no consensus has so far emerged. Black holes could also be electrically charged. However, they would then rapidly neutralize that charge by attracting and swallowing material of opposite polarity. So astronomers believe that all black holes in the Universe are uncharged.
Can I safely orbit a black hole?

It is possible to be near a black hole without falling into it, provided you move rapidly. This is similar to what happens in the solar system: Earth does not fall into the Sun because we move around it at a speed of some 67 thousand miles per hour. But the orbits near a black hole can have various interesting shapes, whereas those in the solar system are always elliptical (and almost circular).

Suppose that you are near a black hole and launch a spaceship to study it up close. If you start too slow, you will spiral into the black hole. If you start too fast, you will fly into the far off distance. At intermediate speeds you will orbit the black hole in a complicated pattern. There is exactly one launch speed that will put you on a circular orbit. This provides a stable vantage point if you start far from the black hole, but it is like playing Russian roulette if you start too close. In that case, even the smallest movement on your ship will drastically change your orbit. You might drift away from the black hole, but if you are unlucky you will spiral into it.
What happens when I drop a clock into a black hole?

According to Einstein's theory of general relativity, massive objects create distortions in space and time. Near a black hole, these distortions become so strong that time behaves in unexpected ways.

Imagine that we are on a spaceship near a black hole. We drop a clock into the black hole and compare its time to that of our onboard clock. The falling clock runs progressively slower. It never crosses the event horizon, but stays frozen there in space and time. The falling clock also becomes continuously redder, since its light loses energy as it escapes from the black hole's vicinity.

By contrast, if we were falling with the clock, time would appear to behave perfectly normally. We would see no slowdown as we approached the event horizon. We would cross the horizon without any perceptible change, and our color would not appear to change. This is the principle of relativity: things can appear different depending on whether you are moving or standing still.

What happens when I fall into a black hole?

Let's assume that you start outside the event horizon of the black hole. As you look toward it, you see a circle of perfect darkness. Around the black hole, you see the familiar stars of the night sky. But their pattern is strangely distorted, as the light from distant stars gets bent by the black hole's gravity.

As you fall toward the black hole, you move faster and faster, accelerated by its gravity. Your feet feel a stronger gravitational pull than your head, because they are closer to the black hole. As a result, your body is stretched apart. For small black holes, this stretching is so strong that your body is completely torn apart before you reach the event horizon.

If you fall into a supermassive black hole, your body remains intact, even as you cross the event horizon. But soon thereafter you reach the central singularity, where you are squashed into a single point of infinite density. You have become one with the black hole. Unfortunately, you are unable to write home about the experience.
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How can a star become a black hole?

A star shines because its center is so hot and dense that hydrogen nuclei fuse together, creating tremendous energy. It lives for millions or billions of years while the inward pull from its own gravity is balanced by the outward pressure from nuclear fusion. Its life ends when the nuclear fuel has been used up. First the star swells, brightens and cools to become a red giant. Then it collapses into a compact stellar remnant, much smaller than our Sun but of similar mass.

Stars less than eight times heavier than the Sun die relatively peacefully. The outer layers are shed in a stellar wind, making the star temporarily visible as a planetary nebulae. The remnant is about the size of the Earth and is called a white dwarf. Heavier stars die in a spectacular supernova explosion. If the star was moderately heavy, the remnant is a neutron star: a dense ball of neutral elementary particles, squeezed into a space little more than 10 miles across. Extremely heavy stars (more than 25 times heavier than the Sun) have no means to withstand their own gravity as they die. They collapse completely to a black hole.

We can see examples of the life cycle of stars all around us in the sky. Our own Sun is a fairly typical medium-sized middle-aged star. The star Betelgeuse is a well-known red giant. Planetary nebulae and supernova remnants can both be spectacular sights, even through a small telescope. Good examples are NGC 7027 and the Crab nebula, respectively. Albireo is an example of a binary star system in which two stars orbit around each other. More than half of all stars live in such systems. If one of the stars in such a binary system evolves into a black hole, then the system can sometimes be observed as a bright X-ray source. In our own Milky Way galaxy this is the case for example in Cygnus X-1. More examples can seen in other nearby galaxies, such as in M33. The following pages describe the properties of these objects, and their connection to stellar evolution in general:

* Sun (Ordinary star)
* Betelgeuse (Red giant)
* NGC 7027 (Planetary nebula)
* Crab nebula (Supernova remnant)
* Albireo (Binary star)
* Cygnus X-1 (X-ray binary)
* M33 (extragalactic binaries)



How do astronomers find the mass of a black hole?

Black holes often have stars or gas orbiting around them. It is then possible to measure the mass of the black hole, just by measuring the speed of the orbiting material.

Consider the case in which a star and a black hole orbit around their mutual center of gravity. Although we can't see the black hole, we can see the star. With accurate observations, we can measure the speed of the star as well as the size of the orbit. Once these have been measured, the laws of gravity tell us exactly what the black hole mass is.

For example, let's assume that a star like our Sun orbits a black hole. Suppose that we measure the speed of the star to be 117 miles per second, and that we measure the diameter of its orbit to be similar to the distance of the planet Mercury from our Sun. This implies that the star orbits the black hole once every 12 days. The laws of gravity then tell us that the black hole must be 10 times more massive than our Sun.

The supermassive black holes in the centers of galaxies can often be measured using this method. For example, the mass of the black hole in the center of our Milky Way galaxy was calculated by measuring the speeds of individual stars that orbit around it. This showed that the black hole is three million times more massive than our Sun. And the mass of the black hole in the center of the nearby Andromeda galaxy has been calculated by measuring the average speeds of all the stars that orbit around it. This showed that Andromeda's black hole is 30 million times more massive than our Sun. The following pages provide more details about these galaxies and their supermassive black holes:

* Milky Way center
* Andromeda (Spiral galaxy)