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Black Holes in Astronomy

by Ron Kurtus (revised 12 August 2005)

A Black Hole is a star or sun that has so much gravity that nothing can escape it, not even light. Because of this, it appears like a black blob or hole in space. Black Holes have some unusual properties and are useful in finding out more about the nature of space and the Universe.

Questions you may have include:

• How can gravity stop light from escaping?
• How big is a Black Hole?
• What are some special properties of a Black Hole?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

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Time = 6 min. 13 sec.

Stopping light from escaping

The force of gravity determines how high an object can go up when propelled at a given velocity. If the speed of the object is fast enough, it can escape the gravitational field and go off into space. A Black Hole has such a strong force of gravity that even light cannot escape its grasp.

Throwing a ball

When you throw a ball up in the air, the height it will go depends on how hard you throw it (or its initial velocity) and the force of gravity.

Looking at the Gravity Equations lesson, you can calculate how high the ball will go when thrown at a given velocity with the equation x = v² / 2g, where x is the height, v² is the velocity times itself or squared, and g is the gravitational acceleration of 32 ft/s² or 9.8 m/s².

Thus, if you throw a ball up at v = 9.8 m/s, it would go to a height of x = 4.9 m.

Since the acceleration of gravity on the Moon 1/6 that of on Earth, a ball will go 6 times higher on the Moon than on the Earth, with the same initial velocity. If you could throw a ball from the surface of the Sun, it would take a much greater velocity to go the same height, since the gravity on the Sun is so much greater.

Escape velocity for a rocket

If the ball or a rocket went at a high enough velocity, it would escape the gravitational field and go off into space. This is called the escape velocity.

Escape Earth

An object must go about 26,000 miles per hour to escape the Earth's gravitational field. That is approximately 7 miles per second or 11 kilometers per second.

Although Newton's gravitational equations are only approximates for distances close to Earth, the equation x = v² / 2g can be used to get a rough approximate of the distance where the object would escape the Earth's gravity. x = 6200 km or 4300 mi.

Escape Sun

The Sun has a mass 300,000 times that of Earth, so the escape velocity from the Sun is about 600 km/sec. That means that a rocket would have to travel at about 1,500,000 miles per hour to escape from the surface of the Sun.

(Obviously, a rocket or ball that was on the surface of the Sun would burn up, because of the intense heat. We are just using it as an example to illustrate the different escape velocities.)

Escape velocity far too low for light

Since light travels at 186,000 miles/second (300,000 km/sec), you can easily see that its speed exceeds the escape velocity of the Earth and even the Sun.

Size of Black Hole

For a sun or star to be a Black Hole, it would have to have so much mass and gravity that its escape velocity would be greater than 300,000 km/sec, such that even light would not escape.

Heavier than Sun

It is estimated that such a star would have to have a mass of 1,000,000 (1 million) times that of our Sun in order for it to be a Black Hole. If you could weigh the Sun and compare it to a Black Hole, it would be like comparing a grain of sand to a bowling ball.

Actually smaller

Surprisingly, the Black Hole might actually be smaller in diameter than our Sun. A Black Hole has such a strong force of gravity that it actually compresses its mass into a smaller object. Don't forget that the Sun is like a hot gas or liquid, so that the material could be compressed into a smaller space.

Properties

A Black Hole has some interesting properties.

Gains energy

Although it is a sun and very hot, none of its light escapes. That means a Black Hole does not lose mass or energy like our Sun does. In fact, it gains energy and mass by sucking in nearby matter.

Difficult to see

Since no light leaves the Black Hole, it does not shine like other objects in space. This makes it very difficult to find in the black background of space. Astronomers think they have found Black Holes by noticing background stars temporarily disappearing at different viewing angles. Still, they are not 100% sure that what they saw was a Black Hole or some obstruction in space.

Horizon

Since gravity decreases as the square of the distance from an object, there is a distance where the escape velocity of a Black Hole becomes less than the speed of light. This is called the Black Hole's horizon. Outside the horizon light is allowed to escape, but inside that horizon, nothing could escape. Scientists have visualized what would happen at and near that horizon.

Summary

A Black Hole has so much gravity that even light cannot escape it. The escape velocity of a planet or star depends on its gravity. The gravity of a Black Hole is a million times that of our Sun and thus it has a million time the mass of the Sun. Astronomer only think they have seen Black Holes in observations through their telescopes.

Answers to Readers' Questions

Celebrate your successes

Resources

The following resources can be used for further study on the subject.

Web sites

Black Holes Frequently-Asked-Questions by Ted Bunn - thorough explanation of Black Holes from the University of California-Berkeley

Virtual Trips to Black Holes and Neutron Stars by Robert Nemiroff - explanations and MPEG movies showing how things look from NASA (most computers can show these short animations)

Black Holes and Beyond - a more complex explanation of Black Holes from the University of Illinois

Astronomy Resources

Books

You can purchase these books in your local bookstore or through Amazon.com.

Exploring Black Holes: Introduction to General Relativity by Edwin F. Taylor, John Archibald Wheeler, Benjamin/Cummings Publishing (2000) $37.33 - top ranked book on the subject

Black Holes and Time Warps: Einstein's Outrageous Legacy by Kip S. Thorne, Frederick Seitz and Stephen Hawking, W.W. Norton & Company (1995) $18.95 - popular book, including views of Hawking

Top-rated books on Black Holes

Miscellaneous

Mini-quiz to check your understanding

1. What determines the velocity required to escape a planet or star?

2. If a Black Hole has more mass than our Sun, how could it be smaller in diameter?

3. What do they call the distance from a Black Hole where light can escape?

If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.

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