A black hole is the velocity necessary to take one away from one’s own gravitational force. For example, the escape velocity of earth is equal to 11 km/s. anything that wants to escape earth’s gravitational force or pull must go at least 11 km/s, no matter what the thing is . The escape velocity of an object depends on how compact it is; that is, the ratio of its mass to radius. A black hole is an object so compact that, close to it, even the speed of light is not fast enough to escape. A common type of black hole is the type produced by some dying stars.
A star with a mass greater than 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 to push out. 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 the “white dwarf. ” When a very massive star exhausts its nuclear fuel it explodes as a supernova. The outer parts of the star are sent into space and the core falls under its own weight.
To create a massive core a progenitor (ancestral) star would need to be at least 20 times more massive than our Sun. If the core is very massive (approximately 2. 5 times more massive than 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. Then the core compacts into a mathematical point with zero volume, where it is has infinite density. This is referred to as a singularity. When this happens, escape would require a velocity greater than the speed of light. No object can reach the speed of light.
The distance from the black hole at which the escape velocity is just equal to the speed of light is called the event horizon. Anything, including light, that passes across the event horizon toward the black hole is forever trapped. Newton thought that only objects with mass could produce a gravitational force on each other. Applying Newton’s theory of gravity, means that since light has no mass, the force of gravity couldn’t affect it. 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 and radius of an object (its compactness) 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 curved. 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 the 1920’s Sir Arthur Eddington proved Einstein’s theory when he observed starlight curve 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 piece of rubber sheet 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 space in three 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). A black hole is invisible because no light can escape from it. In fact, when black holes were first hypothesized they were called “invisible stars. ” If black holes are invisible, how do we know they exist? This is exactly why it is so difficult to find a black hole in space!
However, a black hole can be found indirectly by observing its effect on the stars and gas close to it. For example, consider a double-star system in which the stars are very close. If one of the stars explodes as a supernova and creates a black hole, gas and dust from the companion star might be pulled toward the black hole if the companion wanders too close. In that case, the gas and dust are pulled toward the black hole and begin to orbit around the event horizon and then orbit the black hole. The gas becomes compressed and the friction that develops among the atoms converts the kinetic energy of the gas and dust into heat.
X-rays are created. Using the radiation coming from the orbiting material, scientists can measure its heat and speed. From the motion and heat of the circulating matter, we can tell the presence of a black hole. The hot matter near the event horizon of a black hole is called an accretion disk. John Wheeler, a famous theorist, compared these double-star systems to watching women in white dresses dancing with men in black tuxedos in a poorly lit room. You see only the women, but you could tell the men were there because of all the moves the women did.
Looking for stars with invisible partners is one way in which astronomers search for possible black holes. The gravity of a black hole is not special. It does not attract matter at large distances differently than any other object does. At a long distance from the black hole the force of gravity falls off as the inverse square of the distance, just as it does for normal objects. Mathematically, the gravity of any spherical object is as if all the mass were concentrated at one central point. Since most things have surfaces, you feel the strongest gravity of an object when you are on its surface.
This is as close to its total mass as you can get. If you penetrated a spherical object with a constant mass density, getting closer to its core, you would feel the force of gravity get weaker, not stronger. The force of gravity you feel depends on the mass that is interior to you, because the gravity from the mass behind you is exactly canceled by the mass in the opposite direction. Therefore, you will feel the strongest force of gravity from an object, for example a planet, when you are standing on the planet’s surface, because it is on the surface that you are closest to its total mass.
Penetrating the surface of the planet does not expose you to more of the planet’s total mass, but actually exposes you to less. Now remember the size of a black hole is quite small. Gravity near a black hole is very strong because objects can get extremely close to it and still be exposed to its total mass. There is nothing special about the mass of a black hole. A black hole is different from our ordinary experience not because of its mass, but because its radius is gone. Far away from the black hole, you would feel the same strength of gravity as if the black hole were a normal star.
But the force of gravity close to a black hole is strong because you can get so close to the black hole’s total mass! For example, where we are standing, on the surface of the Earth, it is 6378 km from the center of the Earth. The surface is as close as you can get to the total mass of the Earth. Therefore, it is where you will feel the strongest gravity. If the Earth was to become a black hole (which is impossible) and you stayed at 6378 km from the new Earth-black hole, you would feel the same pull of gravity as you do now. For example, if you’re normal weight is 110 lbs, you would still weigh 110 lbs.
Since the mass of the Earth hasn’t changed, and your distance from it hasn’t changed, you would feel the same gravitational force that you would on the surface of normal Earth. But if the Earth was a black hole, it would be possible for you to get closer to the total mass of the Earth. Let’s say that you weigh 110 lbs standing on the surface of normal Earth. As you get closer to the Earth-black hole you would feel a stronger and stronger force. If you got past 3189 km, (which is half the radius of normal Earth) of the Earth as a black hole you would weigh 440 lbs. That’s very heavy!
If you were to dig to 3189 km of the center of the earth you would weigh about 60Ibs, since the inside of the Earth’s mass is smaller, it’s more comparable to you. Think about the Sun. If the Sun was to also become a black hole (which is impossible) the Earth would keep its normal orbit and would feel the same force of gravity from the Sun as usual, since the distance is still the same. In order to be “sucked up” by a black hole, you have to get very close, if not you will experience the same force of gravity as if the black hole was still the normal star it used to be.
As you get close to a black hole, the effects of relativity will become important. For example, the escape velocity gets bigger, which is defined as “the minimum velocity needed to escape a gravitational field” and in time it reaches the speed of light and then things like the “event horizon” effect will start to happen. Once you’re past the event horizon you cannot turn back because the black hole’s force is too strong for even the speed of light. Stars can only become black holes if they have large masses. For example, our Sun isn’t massive enough to become a black hole.
When the Sun runs out of the nuclear fuel in its core, four billion years from now it will die, but it will not become a black hole. Stars like this type die as white dwarf stars. Stars that are more massive stars, like those with masses that are over 20 times our Sun’s mass, will probably create a black hole sooner or later. When a massive star runs out of nuclear fuel it won’t be able to carry its own weight causing it to die. When this happens the star becomes hot and some fraction of its outer layer, which usually contains some nuclear fuel, will activate the nuclear reaction again and cause an explosion which is known as a supernova.
The rest of the inside of the star, the core, continues to fall down. The star may become a neutron star and stop falling, or if it keeps falling apart it may become a black hole. This all depends on the mass of the star. The dividing mass of the core, which determines what will happen to it, is about 2. 5 solar masses. It is thought that to produce a core of 2. 5 solar masses the ancestral star should begin with over 20 solar masses. A stellar black holes is a black holes which was formed from a star. There might be three types of black holes, they are stellar, supermassive, and miniature black holes. It all depends on their size.
These black holes have also formed in different ways. I already described how stellar black holes were formed. Supermassive black holes probably exist in the centers of most galaxies, including our own galaxy, the Milky Way. They can have a mass that is euqual to billions of suns. In the outer parts of galaxies there are huge distances between stars. Although, in the area that is the center of galaxies, stars are packed very close together. Because everything in the central region is packed, a black hole in the center of a galaxy can get bigger and bigger as stars that are near it get sucked into the black hole.
By measuring the velocity of stars that are orbiting close to the center of a galaxy, we can assume that there is a supermassive black hole and estimate the mass. In the supermassive black hole, there are sometimes two jets of hot gas. These jets can be up to millions of light years in length. They are probably caused by the contact of gas particles with strong, turning magnetic fields that surround the black hole. Some observations made with the Hubble Space Telescope have given us the best evidence to date that supermassive black holes exist. It is not known how miniature black holes develop, but there are some ideas.
First, is that they might have been formed right after the “Big Bang,” which is thought to have started the Universe about 15 billion years ago. At that point the rapid expansion of some matter might have compressed slower matter creating black holes. Some scientists believe that black holes can evaporate and explode. The time that is needed for evaporation would depend on the mass of the black hole. Massive black holes would need a longer time than the age of the universe. Miniature black holes are thought to be able to evaporate at the current age of our universe.
For a black hole formed at the time of the “Big Bang” to evaporate today its mass must be about 1015g (i. e. , about 2 trillion pounds), a little more than twice the mass of the current Homo sapien population on planet Earth. During the final part of the “evaporation,” a black hole would explode with a force several trillion times greater than the most powerful nuclear weapon. So far, there is no proof that miniature black holes exist. With the use of Newton’s Laws in the late 1790s, John Michell of England and Pierre LaPlace of France suggested an invisible star could exist.
Michell and LaPlace estimated the mass and size,( which is now known as the event horizon) that an object needs for it to have an escape velocity that is bigger than the speed of light. In 1967 John Wheeler, an American theoretical physicist, described these collapsed objects as black holes, and the name just stuck like that. Astronomers have found evidence for supermassive black holes. They based this on measurement of gas around the black hole. In 1994, a measurement from the Hubble Space Telescope showed a huge mass at the center of a galaxy (M87).
The mass was about 3 billion times the mass of our Sun and appears to be concentrated into a space smaller than our solar system. For a lot of years x-rays from the double-star system Cygnus X-1 convinced many astronomers that the system had a black hole. With better measurements tools, the evidence for a black hole in Cygnus X-1 is very strong. A black hole is not seen since light can’t escape it. In order for a black hole to be found the matter surrounding it has to be seen. Matter that surrounds a black hole heats up and gives off radiation so that it can be found.
Around a stellar black hole this matter is made up of gas and dust. Around a supermassive black hole in the center of a galaxy the swirling disk is made of gas and stars. In February of 1997 an instrument called the Space Telescope Imaging Spectrograph (STIS) was installed in the Hubble Space Telescope. STIS is the space telescope’s black hole hunter. A spectrograph uses prisms to create rainbow patterns. The STIS can take a spectrum of many places at the same time. Each spectrum tells scientists information about the gas around stars. With that information, the mass that the stars are orbiting can be estimated.
The more massive the central object is, the faster the stars go. STIS found a supermassive black hole in the center of the galaxy M84. There was a rotation of 400 km/s, which is equal to 1. 4 million km every hour. The Earth orbits our Sun at 30 km/s. If Earth moved as fast as 400 km/s our year would be only 27 days long. The Advanced Camera for Surveys, which was installed in March 2002, represents the third generation of science instruments flown aboard the Hubble Space Telescope. It has a wider field of view, better image quality and better sensitivity.
The new camera has double the field of view and has much more capabilities than the Hubble. The telescope with ACS’s technology will make it ten times more useful and it will also last longer. ACS is probably going to work better than all the other instruments flown on the Hubble Space Telescope, because of ACS’s increased wavelength range. Since the ACS was created to study some of the earlier activities in the universe, ACS will see from ultraviolet to infrared light. On the inside, the new instrument has three different cameras each doing different things.
The wide field camera, the high-resolution camera, and the solar blind camera. For example, with a greater field view than the Hubble’s, ACS’s wide camera can do surveys of the universe. Astronomers will use it to study the galaxies in more detail, which will show more clues about how our universe has grown. The camera will take excellent pictures of the inside regions of galaxies. It will search for other stars or planets and will take close-up images of the planets in our own solar system. The solar blind camera, will focus on hot stars producing radiating in ultraviolet wavelengths.
