Astronomy/ Black Holes term paper 12808

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BLACK HOLES

A black hole is the most powerful, most mysterious phenomenon in the universe. The gravity within a black hole is so intense that not even light, can get away from its force. A black hole is so dense that 100 million suns would be compressed to a globe 6 million kilometers in diameter. Our sun is only 1,390,000 kilometers in diameter. It is even theorized that there is a massive black hole at the center of every galaxy. These black holes have a single point called a singularity that supposedly has infinite mass. Therefore a black hole could conceivably swallow an entire solar system in seconds if the event horizon could expand that quickly . These giant, destructive, mysterious beasts are the most complex puzzles in the universe.

WHAT IS A BLACK HOLE?

A black hole is one form of a "dead" star. A "dead" star may: shrink until it is a white dwarf, a few thousand miles across, about the size of the earth . It could shrink smaller then a white dwarf until it is a neutron star, about 20 miles across. Neutron stars have a huge density, about a million tons per square inch. A dead star may also keep on shrinking until it is just a point in space with an infinite density- a black hole.

WHY DOES A STAR SHRINK?

A star shrinks because gravity tries to pull the particles in the star together and make it collapse . If the star is less than two or three solar masses, it has enough exclusion principle repulsion to support it against gravity. If it is more than two or three solar mass units, it will probably collapse into a black hole. Common sense would say that a star with a smaller mass would be in the smallest form- a black hole . The reason it is the other way around has to do with more particles of matter attracting more gravity.

ARE BLACK HOLES LIKE VACUUMS AND SUCK EVERYTHING IN SIGHT?

Black holes only suck in everything within a certain distance from its center. That distance is called the event horizon. If anything were to get inside the event horizon, there would be no return . If planets and stars are outside the event horizon, they would be safe. They would orbit the black hole just as planets in our galaxy orbit the sun.

WHAT DOES A BLACK HOLE LOOK LIKE?

Nothing, it is just black. There are two parts to a black hole, the singularity and the event horizon. The singularity of the black hole is the center. Within a certain distance of the singularity, (the distance called the event horizon), nothing would be able to escape the hole. Since black holes are black, they are very hard to detect. If a black hole wasn't black, this is what it would look like: (next Page)

THE EVENT HORIZON

When people talk about the black hole, they're usually referring to its event horizon. If you wanted to get theoretical, it is the first sphere of light (which is, of course, constantly expanding at the speed of light) that does not expand (due to the extreme curvature of spacetime) . Rs, the Schwarzschild radius. Once something passes beyond the event horizon, it can never leave the black hole and is doomed to a painful stretchy demise. Your rocket ship can come close to the event horizon and can move away from it. This all has to do with something called escape velocity . Like when rockets are launched on the earth, they have to go a certain speed or else they'll just plummet back to the earth. Escape velocity: the speed needed to leave a gravitational field. Black holes have escape velocities just like anything else; it's just that the numbers are orders of magnitude bigger than the escape velocities of planets . The closer to the black hole's event horizon, the greater the escape velocity. At the event horizon the escape velocity is the speed of light. Things with mass can't go the speed of light, so, if they get to or go closer than Rs, they cannot escape the black hole . Beyond the event horizon the escape velocity is greater than the speed of light. Light rays that go beyond Rs can't even get out.

WHAT HAPPENS WHEN YOU CROSS THE EVENT HORIZON?

At first, nothing noticeable unless you consider the closure of the light cone to be significant. Had you been looking out the back window, you would have seen the view of your universe shrink to a dot and disappear; but there is no Star Trek force field effect to tell you when you cross. What you see and feel next all depends on your reaction to the fact that you are now going to face a painful stretchy death. You could fire your rockets full blast in the hopes of surviving a little longer by braking, or you could shut off the engines to make your last few moments as comfortable as possible.

IF BLACK HOLES CAN'T BE SEEN, HOW COME WE KNOW THEY EXIST?

The black hole concept was made by a German astronomer named Karl Schwarzschild in 1916. He based his concept on Albert Einstein's theory of relativity. We had no proof that black holes existed until 1994, when astronomers used the Hubble Space Telescope to uncover the first evidence that black holes exist. They measured the gasses around the center of the galaxy M87 and found a solar mass of about 3 billion. Second and third black holes were discovered in 1995 in the galaxies NGC 4258 and NGC 4261.

CAN BLACK HOLES EVENTUALLY FORM IN OUR GALAXY?

No, astronomers say that our sun will end in a white dwarf. Even if the sun did collapse and form a black hole, the earth and other planets would continue to orbit just as they do now, because no planet would be inside the event horizon.

WHAT HAPPENS WHEN SOMETHING GETS SUCKED UP IN A BLACK HOLE?

Astronomers don't have enough knowledge about black holes to answer that.

Some astronomers think one possible answer to this question might be when something is swallowed by a black hole, it stays in the singularity as long as the black hole exists, just like something sucked up by a tornado. Astronomers think that a black hole may end with a large explosion, as large as millions of nuclear bombs exploding. Anything that had been swallowed by the black hole would be just a particle in space after the explosion.

IF WE CAN T SEE THEM, THEN HOW DO WE FIND STUFF OUT ABOUT THEM?

It may be black, but it's not completely hidden. We can tell the size of the event horizon (the black hole's "mass"), whether it's charged or not, and whether it's spinning and at what rate. Remember that a star can collapse to form a black hole? All information about what the star is made of is lost. The star could have been made of antimatter or matter, and it wouldn't make a difference in the resulting black hole. We can't peer past the event horizon to see the singularity...even if we could, the singularity is a damn strange place. If the collapsing star were not quite spherical, those deviations were burned off during the collapse so all black holes have the same general shape. What differs from hole to hole is the mass, charge, and angular velocity.

TYPES OF THEM

There are two kinds of black holes. A static, which is a Schwarzschile black hole, and a charged, which is a Reissner-Nordstrom black hole.

Crossection of a static black hole:

WHY IS IT CALLED THE STATIC BLACK HOLE?

It has no charge and it is not rotating. This is your standard, idealistic, simple black hole. It is also called a Schwarzschild (Swar - shild, not Swarts - child) black hole. The interesting places for this one are the photon sphere, the event horizon, and the singularity. What happens to you near the black hole, all depends on your distance in Schwarzschild radii. Let us say that you're flying towards a static black hole in a brand new, spaceship. You're approaching the black hole slowly, since only a fool would charge full-speed into one.

The black hole itself is very plain and quite difficult to see. It's the space around it that is interesting. You see multiple images of many of the stars. That galaxy that you know is behind the black hole appears as a ring around the black hole, commonly called an Einstein ring. Why you see the galaxy at all, when the black hole is between you and it, and why it appears as a ring is from the bending of light due to the strong force of gravity of the black hole. Say you have an iron marble and a bar magnet. If you roll the ball near enough the magnet, it veers towards the magnet. The marble ends up tracing a slightly bent path versus the straight path it would have traced had it not encountered the magnet. Now replace the magnet with the black hole, and the marble with a light ray, and you've got it. The light from the 'hidden' galaxy peeks around the black hole and looks like a ring.

CROSSECTION OF A CHARGED BLACK HOLE:

The singularity of a charged black hole is the same as that of a static black hole with the exception that it is possible for the singularity to exist without any protective horizons.

Truly, there is one other important difference: you can avoid the singularity of a charged black hole, whereas you must eventually encounter that of a static black hole. Encountering a singularity is not something you want to do, I can assume. In a static black hole, once inside the event horizon, that's it end of your life. However, should you survive the trip between the outer event horizon and the inner one of a charged black hole, you could in theory turn around and leave the black hole back and return to your own universe, or you could go into another universe.

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