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Black Holes and Neutron Stars
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- What is Hawking radiation?
- Does gas from a nearby star orbit a black hole? How big is the event horizon?
- Wouldn't a black hole have the same gravitational pull as a star with the same mass?
- Where does all the stuff black holes suck in go? Could black holes actually be portals through time and space?
- Is it possible for a Black Hole to split or change size under strong gravitational influence?
- Are there Black Neutron Stars? Is there a point where it is possible to have a neutron star with an escape velocity greater than light?
- If black holes continue becoming infinitely smaller, then wouldn't they at some point fail to affect anything?
- Why are time and light so intertwined?
- How hot is it around a black hole? Does the gravity pull in heat too?
- I heard that the event horizon is spreads out at the speed of light. Is that true?
- Can stars form outside of a galaxy?
- Why did the Soviets call black holes "frozen stars"?
- If time stops inside a black hole, wouldn't the black hole cease to exist -- being left behind in time at the moment of its creation?
- I've read that atoms do not survive the collapse to a black hole. Does this mean then, that black holes are not composed of atoms?
- Are there any black holes near our Solar System? If so, could they affect our solar system any time soon?
Mike Reynolds writes, "I read in an article last year that back in 74' I believe, Stephen Hawking said that radiation could escape from black holes. Although in small amounts the radiation stream was detectable and that was a way astronomers could find black holes. Is this an agreed upon theory? I was also wondering how far the closest black hole to us is .I had also read there might be one in the Milky Way."
Hawking radiation is the result of particle pairs that are separated by a black hole. In "empty" space, fluctuations of electromagnetic and gravitational fields produce a pair of "virtual" particles--particle pairs that exist only briefly. They appear, move apart and then back together, annihilating each other. One particle has positive energy, the other has negative energy; added together, their energies equal zero.
When virtual particle pairs appear near a black hole's event horizon, one of the particles may fall into the black hole, leaving the other particle free. The particles with negative energy that fall into the black hole contribute negative energy, thus reducing the total energy of the black hole. This decreases its mass (remember E=Mc2--energy and mass are equated), and the black hole eventually evaporates away. (source: Hawking, Stephen W., A Brief History of Time: From the Big Bang to Black Holes. New York: Bantam Books, 1988.)
Because our own Galaxy's center is largely obscured from view by interstellar dust, it's difficult for astronomers to see what is there. Radio and infrared observations show stars and gas moving very fast near the center. But as you get closer to the center, you would expect stars to slow down because there is less mass to exert gravitational force--most of the matter is located farther out. Also, the high rotational speeds observed suggest a huge mass to keep the gas molecules from flying away. Scientists figure an object with a mass of several million times that of the Sun resides at the center of our galaxy, and that object is most likely a black hole (see http://www.sciencenews.org/sn_arch/10_5_96/fob1.htm for more information). Observations of other galaxies suggest a similar mass distribution.
New black holes are constantly being discovered. I don't know which is currently considered to be closest to Earth.
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TCG.010 writes, "If a black hole was next to a star then gases from the star or planets from the star or the star itself would not be sucked in unless they were in the event horizion regardless of their mass. Is this correct? Also how big is the event horizion?"
Something far away enough from a black hole will orbit it as the earth orbits the Sun. But some things get sucked in. Gaseous material from a nearby star, for example, forms a disk (called an accretion disk) as it spirals in to cross the event horizon and join the singularity.
Why doesn't the gas just orbit the black hole like a star or a planet might? First, the material is pushed towards the black hole by the companion star's solar wind. And you have to consider the gravitational effect of the gas atoms on each other--it's not a simple 2-body model of gravitation.
The size of a black hole event horizon depends on the mass of the singularity--the size is directly proportional to its mass. There's a pretty simple formula:
R=3M
where R is the radius in km and M is units of solar mass. So, a black hole 6 times as massive as the Sun is 18 km in radius.
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Matt Reece writes, "I am a ninth grader that is interested in science. I have a question about black holes. I can understand why they have such a strong gravitational pull based on Einstein's model of gravity. However, when Newton's law is considered, the immense gravitational pull ceases to make sense. By Newton's law F=GMm/d^2 a black hole would seem to have the same gravitational pull as a star with the same mass, regardless of their volumes. So my question is this: is this simply a point where Newtonian mechanics is no longer applicable, or is there another explanation for the apparent discrepancy? Thanks."
You are right--a black hole would have the same gravitational pull as a star with the same mass. Both Newton and Einstein deal with the same thing--just in different ways. Newton's laws quantify the mechanics behind a force that one body seems to have on another (or more accurately, that bodies have on each other). Einstein disposes of the notion of a force and explains the effect as curvature of space-time. Newtonian thinking says that galaxies that are billions of light years apart affect each other because of gravity; Einstein says they affect each other because of how their masses curve space-time. Newton's laws are descriptive--they tell what happens, where Einstein's theories attempt to tell how. Except for rare cases, Newton's laws work very well, and can be used to describe the gravitational pull that a black hole has on objects around it. The laws of physics only break down at the singularity, where space and time do not exist as we know them.
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Adam Tobin writes, "Some scientists have a theory that whatever enters a black hole instantly disintegrates or "disappears". Do you think if we sent astronauts in the near future to go in and examine a black hole, they could actually discover that a black hole is amazingly a time or warp gate to another part or planet in the universe. Could this be possible?"
Shawn Rajan writes, "What is the outlet for the immense mass and energy that black holes suck in? Is it a quasar?"
Derk-Jan ter Horst writes, "When material gets sucked into a black hole where does it go?"
Any matter that gets sucked into the event horizon becomes part of the black hole singularity--a single point that has mass but no volume. As matter joins the singularity, it increases not only the mass of the black hole, but also the size of the event horizon.
Does that mean that a black could keep growing until it consumes everything? One of the things that could prevent a black hole from swallowing up the whole universe is distance--the fact that galaxies are so far apart. Even if a black hole was to swallow up a whole galaxy, it would contain all the locally available mass and could grow no more. The event horizon of the black hole would be very large, but it would not span the enormous distance that exists between galaxies. The gravitational effect of the black hole on neighboring galaxies would be the same as when it was a galaxy.
Another scenario that suggests black holes will not consume everything is proposed by Stephen Hawking. In "empty" space, fluctuations of electromagnetic and gravitational fields produce a pair of "virtual" particles--particle pairs that exist only briefly. They appear, move apart and then back together, annihilating each other. One particle has positive energy, the other has negative energy; added together, their energies equal zero.
When virtual particle pairs appear near a black hole's event horizon, one of the particles may fall into the black hole, leaving the other particle free. The particles with negative energy that fall into the black hole contribute negative energy, thus reducing the total energy of the black hole. This decreases its mass (remember e=mc2--energy and mass are equated), and the black hole eventually evaporates away.
Some people say black holes are gateways to other parts of the universe, possibly containing a wormhole. A wormhole is a theoretical shortcut from one place in space-time to another. This shortcut can be accomplished by traveling in a "straight" line to connect two points of curved space-time. Instead of traveling the distance from point A to point B on the curved plane of space-time, you can reach point B faster by cutting across directly to point B.
Astrophysicist Lawrence M. Krauss points out the unlikeliness of this scenario: what the wormhole goes through is not 3-dimensional space--that is what the wormhole connects. The curved space-time is all there is to the Universe; there is nothing for the wormhole to exist in. Krauss has more to say about wormholes in his book, The Physics of Star Treck.
Krauss, Lawrence M. The Physics of Star Treck. New York: HarperCollins, 1995. (ISBN 0-465-00559-4).
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David Reith writes, "Is it possible for a Black Hole to split or change size (ex. become eliptical) under strong gravitaional influence (another larger Black Hole)?"
A black hole's size is determined by its mass, and its shape by its spin. The more mass it has, the larger the event horizon. The faster it spins, the more it bulges out at its equator. According to Kip Thorne, two black holes can collide and join to form a single black hole. As they collide, their shapes are distorted. But during collision is the only time that scientists think black holes change shape due to gravitational influence of another black hole. (source: Thorne, Kip. Black Holes and Time Warps: Einstein's Outrageous Legacy. New York: W W Norton & Co., 1994.)
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Mike Whitenton writes, "Are there Black Neutron Stars? More exactly, What is the escape velocity of a 3 solar mass neutron star? Is there a point where it is possible to have a neutron star with an escape velocity greater than light? An after thought to the last question: Would the escape velocity of a single neutron be closely related to that of a neutron star at it's surface?"
Escape velocity is how fast something needs to break away from the gravity of something else. Escape velocity depends on two things: the mass of the thing you want to escape from and its radius. Note that your mass (or the mass of the thing that is escaping) doesn't matter. The formula for figuring out escape velocity is:
where G = 6.7 x 10-11(the gravitational constant), M = the mass, in kg, of the object you want to escape from, and R = its radius in meters.
The Earth's escape speed, for example, is 11 km/s. (If you want to figure it you, the mass of the earth is 6 x 1024 kg, and the radius is 6.4 x 106 meters.) (source: Zeilik, Michael., and John Gaustad. Astronomy: The Cosmic Perspective. New York: John Wiley & Sons, Inc, 1990.)
So what's the escape velocity from a 3-solar mass neutron star? Since I'm already throwing numbers around, we might as well do some math. We'll solve the escape velocity equation using 3-solar masses (5.967 x 1030 kg) and a radius of 15 km (neutron stars come between 10 and 20 km in radius).
So the escape velocity from a 3-solar mass neutron star with a radius of 15 km is about 2.3 x 108 m/s, which is about 77% of light speed. That's the minimum speed something--whether it's a single neutron or and astronaut--needs to break free from a neutron star's gravity. (You can figure out all the variations for neutron stars--mass between 1.4 and 3 solar units, and radius between 10 and 20 km--if you want. You'll always come up with an escape velocity below light speed.) Since light travels faster than a neutron star's escape speed, light cannot be trapped. Neutron stars aren't black.
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Xwing writes, "If black holes continue becoming infinitely smaller, then wouldn't they at some point be so small, and have such a small event horizon, that they would fail to affect anything?"
Gravity depends on how much mass there is. The more massive something is, the more gravitational pull it has, and its size and shape make no difference. If the Sun suddenly collapsed to form a black hole, it would still be 1.989 x 1030 kg, and the Earth would still orbit around it.
The size of a black hole's event horizon depends also on mass because the event horizon is a gravitational boundary--the distance from the black hole singularity that nothing can escape. So, although the black hole singularity is compressed into a point, it still has mass. And mass determines gravitational influence and the size of the event horizon.
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Jim Hunter writes, "Why are time and light so intertwined? I have heard that if you can beat light to its destination you have traveled into the future (or the past?) Is it only related to the fact that light allows observation and that by exceeding its velocity you change the observational timeframe - or is there something more to it?"
I don't think there is more to it than what you said. The speed of light is a constant in the universe and is the same for all observers. Light speed is the fastest you could get to a certain destination, and it also limits the time it takes to get there. By exceeding light speed (by warping through space-time of perhaps going through a wormhole), you can arrive at your destination before it's possible for you to. This is time travel.
But there's a problem here. I stated that light speed is the fastest you can go and then talked about going faster than light speed. I violated my own premise that nothing can go faster than light speed. (Actually, anything with mass has its own problems even reaching light speed.) I think faster-than-light travel through space-time will remain the stuff of science fiction.
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Joe McMahon writes, "At the "horizon," do scientists know how hot it is? I mean sure we may revolve around a black hole but how hot is it there? Does the gravity pull in heat too?"
A black hole does not burn and doesn't generate heat. Heat is the thermal energy that matter contains--it's the motion of its particles. Any matter that gets sucked into a black hole takes the thermal energy with it.
The temperature near a black hole horizon would depend on many things, depending on the amount of infalling matter. (As matter falls into a black hole, it heats up.)
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Joe McMahon writes, "I heard that the horizon is spreading out at the speed of light is that true?"
The size of a black hole's event horizon depends on how massive the singularity is and will only increase if the singularity becomes more massive. For the event horizon to expand at the speed of light, the black hole would have to suck in a lot of matter. How much? You can figure out the radius of the event horizon using the equation:
R = 3M
where R = the radius of the event horizon in km and M = the mass of the singularity in solar units (mass of the sun). To expand at speed of light, enough mass (M) would have to fall into the black hole each second to allow the radius of the event horizon to grow 3 x 105 km each second:
3 x 105 = 3M
1 x 105 = M
So a black hole would need to suck in 100,000 solar units--100,000 of our suns--each second to expand at the speed of light. There just isn't that much mass available every second.
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Scott Myers writes, "Can stars form outside of a galaxy? Can intergalatic dust clump together to reach the critical mass required to start a nuclear reaction? Have there been any stars detected oustide of a galatic system?"
The density of intergalactic dust is very low--not enough for a star to form. That all happened when the universe was young and matter clumped together to form galaxies. The most likely candidate for intergalactic dust is ionized hydrogen at a density of less than 4 x 10-30 kg/m3. That's spread out pretty thin. (Source: Zeilik, Michael., and John Gaustad. Astronomy: The Cosmic Perspective. New York: John Wiley & Sons, Inc., 1990.)
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E Seis writes, "Why did the Soviets call black holes "frozen stars"?"
Before John Wheeler coined the term "black hole" in 1967, scientists used different names for these strange collapsed stars. Through the 40s and 50s, scientists reasoned that as a star collapsed, its surface would reach light speed. The speed of the inward collapse would cancel out the light still being emitted by the star, and the star would appear to freeze at that point. For this reason, the Soviet physicists called black holes "frozen stars." (Source: Thorne, Kip. Black Holes & Time Warps: Einstein's Outrageous Legacy. New York: Norton, 1994)
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Robert Viagas writes, "I've never had trouble grasping the physical effects in a singularity. However, your page asserts that time itself stops inside a black hole. If this were true, wouldn't the black hole cease to exist -- being left behind in time at the moment of its creation? The fact that black holes continue to exist night after night and affect things around them is evidence that time affects black holes quite normally."
Black holes are strange things. From our point of view (our reference frame), we see that black holes continue to exist, and intuitively conclude that time must also exist at the singularity. But if our reference frame was that of the singularity, the whole history of the universe would flash by. That's because time passes in "normal" space-time but not in a black hole singularity.
How quickly or slowly time passes, in fact, depends on where you are. An experiment done in 1960 at Harvard University showed that time runs slower in the basement of the physics building that it does 74 feet higher on the roof. This experiment confirmed a prediction of Einstein's general relativity: the more curved space-time is (more gravity), the slower time goes. The infinitely dense black hole singularity produces infinite space-time curvature and, thus, infinite slowing of time. (source: Krauss, Lawrence M. The Physics of Star Trek. New York: BasicBooks, 1995.)
Physicist Kip Thorne explains it another way. A black hole singularity is where matter is compressed into zero volume. Matter without volume suggests infinite density, But Nature abhors infinities, and at the singularity, something called quantum gravity takes over, rips space and time apart, and leaves only a timeless remnant of space called quantum foam. (For more detailed information, see Thorne, Kip. Black Holes & Time Warps: Einstein's Outrageous Legacy. New York: Norton. 1994.)
Time standing still or not existing at all violates our intuitions. But the physicists say it's so. Most of us have to take them at their word, because we cannot read or speak the mathematical language they use to explain the unintuitive.
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CHREYES writes, "I've been doing a report on black holes and I've read that atoms do not survive the collapse to a black hole. Does this mean then, that black holes are not composed of atoms?"
Atoms don't even survive a star's collapse into a neutron star. When a star collapses to form a neutron star, the tremendous pressure from the gravity causes something called inverse beta decay. Inverse beta decay occurs when an electron and a proton combine to make a neutron and a neutrino. So the atoms are ionized (stripped of their electrons) and the protons in the nucleus are converted to neutrons. As the pressure continues to increase, the atomic nuclei fall apart to form a gas of mostly neutrons (hence, the name neutron star). Then, the increasing pressure causes the neutron gas to pack into a crystalline solid. That's the core of the neutron star--no atoms, mostly just neutrons.
When a star with mass greater than 3 times the mass of the sun burns out, it collapses into a black hole. All of its matter is compressed into zero volume, a point called the singularity. Matter without volume suggests infinite density. But Nature abhors infinites, and something called quantum gravity takes over at the singularity. Quantum gravity rips space and time apart and leaves only a timeless remnant of space and time called quantum foam. For more detailed information, see Thorne, Kip. Black Holes & Time Warps: Einstein's Outrageous Legacy. New York: Norton, 1994.
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Bill Brooks writes, "Are there any black holes near our Solar System? If so, how far away are they, and could such items affect our solar system any time soon?"
New black holes are constantly being discovered. I don't know which is currently considered to be closest to Earth. But scientists now think that galaxies, including the Milky Way Galaxy, have gigantic black holes at their centers because of the high concentration of mass measured there. Our own Galaxy's center is largely obscured from view by interstellar dust, so it's difficult for astronomers to see what is there. Radio and infrared observations show stars and gas moving very fast near the center. But as you get closer to the center, you would expect stars to slow down because there is less mass to exert gravitational force--most of the matter is farther out. Also, the high rotational speeds observed suggest a huge mass to keep the gas molecules from flying away. Scientists figure an object with a mass of several million times that of the Sun resides at the center of our galaxy, and that object is most likely a black hole (see http://www.sciencenews.org/sn_arch/10_5_96/fob1.htm for more information). Observations of other galaxies suggest a similar mass distribution.
I don't think there are any black holes close enough to pose a threat to the solar system. If a black hole did come our way it probably wouldn't affect us very much. The intense gravity of the black hole might just nudge the solar system out of the way. Imagine bug floating on the surface of the ocean. If a luxury liner were to head straight for it, the waves pushed forward by the ship would probably push the bug out of harm's way. In much the same way, the gravity from a black hole or neutron star (which is greater than the gravity of the sun) might push the solar system out of the way, the sun would go into orbit around the black hole, and we would notice nothing. When I talk about the sun orbiting a black hole, I'm talking about really big distances.
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Black Holes and Neutron Stars
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