event - definition - What is ? What is ? definition : event Event may refer to: ==Gatherings of people== * Ceremony, an event of ritual significance, performed on a special occasion * Convention (meeting), a gathering of individuals engaged in some common interest * Event management, the organization of events * Festival, an event that celebrates some unique aspect of a community * Happening, a type of artistic performance * Media event, an event created for publicity * Party, a large social or recreational gathering * Sporting event, at which athletic competition takes place ==In science, technology, and mathematics== * Event (computing), a software message indicating that something has happened, such as a keystroke or mouse click * Event (philosophy), an object in time, or an instantiation of a property in an object * Event (probability theory), a set of outcomes to which a probability is assigned * Event (relativity), a point in space at an instant in time, i.e.

a location in spacetime * Event (synchronization primitive), a type of synchronization mechanism * Event (UML), in Unified Modeling Language, a notable occurrence at a particular point in time * Event (particle physics), refers to the results just after a fundamental interaction took place between subatomic particles * Event horizon, a boundary in spacetime, typically surrounding a black hole, beyond which events cannot affect an exterior observer * Celestial event, an astronomical phenomenon of interest * Extinction event, a sharp decrease in the number of extant species in a short period of time * Impact event, in which an extraterrestrial object impacts planet * Mental event, something that happens in the mind, such as a thought ==In arts and entertainment== * The Event, an American conspiracy thriller television series for NBC * The Event (2003 film), a 2003 film directed by Thom Fitzgerald * Derren Brown: The Events, a Channel 4 television series featuring the illusionist Derren Brown * Event, a literary magazine published by Douglas College ==See also== * Competition, a contest between organisms, animals, individuals, groups, etc.

* Disaster, an event causing significant damage or destruction, loss of life, or change to the environment * Event chain methodology, in project management * Eventing, an equestrian event comprising dressage, cross-country and show-jumping * Eventive (disambiguation) * Grouped Events, in philosophy, the experience of two or more events that occur in sequence or concurrently that can be subsequently categorized * News, new information or information on current events * Phenomenon, any observable occurrence * Portal:Current events (Wikipedia portal) * Sequence of events * Sustainable event management or Event Greening * The Event (disambiguation) Glossary of Astronomy Terms - Astronomy Reference Guide on Sea and Sky Glossary of Astronomy Terms Glossary of Astronomy Terms The universe is a large place full of wondrous and unimaginable things. The amateur astronomy hobby contains words, labels, and technical terms that may seem be a little confusing for both beginners and the more experienced hobbyists. This glossary of astronomy terms contains definitions for some of the most common words used in astronomy, cosmology, astrophysics, and space exploration. You may click on any yellow highlighted word to jump directly to the definition for that astronomy word.

You can also click on any letter of the alphabet below to jump directly to that section in the astronomy glossary listing. Absolute Magnitude A scale for measuring the actual brightness of a celestial object without accounting for the distance of the object. Absolute magnitude measures how bright an object would appear if it were exactly 10 parsecs (about 33 light-years ) away from Earth. On this scale, the Sun has an absolute magnitude of +4.8 while it has an apparent magnitude of -26.7 because it is so close. Asteroid A small planetary body in orbit around the Sun, larger than a meteoroid but smaller than a planet . Most asteroids can be found in a belt between the orbits of Mars and Jupiter. The orbits of some asteroids take them close to the Sun, which also takes them across the paths of the planets. Aurora A glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun. This phenomenon is known as the Aurora Borealis in the Earth's northern hemisphere and the Aurora Australis in the Earth's Southern Hemisphere.

Aurora Australis Also known as the southern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the southern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field. Known as the Aurora Borealis in the northern hemisphere. Aurora Borealis Also known as the northern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the northern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field. Known as the Aurora Australis in the southern hemisphere. Big Bang The theory that suggests that the universe was formed from a single point in space during a cataclysmic explosion about 13.7 billion years ago. This is the current accepted theory for the origin of the universe and is supported by measurements of background radiation and the observed expansion of space. Black Hole The collapsed core of a massive star . Stars that are very massive will collapse under their own gravity when their fuel is exhausted. The collapse continues until all matter is crushed out of existence into what is known as a singularity . The gravitational pull is so strong that not even light can escape.

Black Moon A term used to describe an extra new that occurs in a season. It usually refers to the third new moon in a season with four new moons. The term is sometimes used to describe a second new moon in a single month. Blue Moon A term used to describe an extra full that occurs in a season. It usually refers to the third full moon in a season with four full moons. Note that a blue moon does not actually appear blue in color. It is merely a coinsidence in timing caused by the fact that the lunar month is slightly shorter than a calendar month. More recently, the term has sometimes been used to describe a second full moon in a single month. Steady State Theory The theory that suggests the universe is expanding but exists in a constant, unchanging state in the large scale. The theory states that new matter is being continually being created to fill the gaps left by expansion. This theory has been abandoned by most astronomers in favor of the big bang theory. Stellar Wind The ejection of gas from the surface of a star . Many different types of stars, including our Sun, have stellar winds. The stellar wind of our Sun is also known as the Solar wind.

A star's stellar wind is strongest near the end of its life when it has consumed most of its fuel. Department of Astronomy and Astrophysics, University of Chicago The Main Point Studying neutron stars and black holes gives us access to exotic realms that we can't explore on Earth. Abstract Neutron stars and black holes are among the most exotic objects in the universe. A lump of neutron star matter the size of a sugar cube would weigh as much as all humanity, and the stars have magnetic fields a trillion times Earth's. Since we can't reproduce such conditions in laboratories, we have to observe neutron stars with telescopes to figure out their properties. Recently the Rossi Explorer, a new X-ray satellite, discovered a remarkable new phenomenon of neutron stars that strip matter from their companion stars: their brightness varies almost periodically more than a thousand times per second. I will describe how this phenomenon gives us a sensitive new tool to probe the properties of neutron stars, and how it may even help us search for black holes. Intro to Black Holes A black hole is a region of space in which the matter is so compact that nothing can escape from it, not even light; the "surface" of a black hole, inside of which nothing can escape, is called an event horizon.

The matter that forms a black hole is crushed out of existence. Just as the Cheshire Cat disappeared and left only its smile behind, a black hole represents matter that leaves only its gravity behind. Black holes are usually formed when an extremely massive star dies in a supernova. However, some people think small black holes were formed during the Big Bang, and that the resulting "mini black holes" may be in great abundance in our galaxy. In principle, black holes can have any mass; black holes formed by stellar death have at least twice the mass of our Sun. Unlike ordinary things (e.g., rocks), which have a size roughly proportional to the cube root of their mass, black holes have radii proportional to their mass. The event horizon of a nonrotating black hole the mass of our Sun has a radius about 3~km. Thus, large black holes aren't very dense! A black hole a billion times as massive as our Sun, such as is thought to exist in the center of some galaxies, has an average density just twenty times the density of air. Black holes, like any gravitating objects, exert a tidal force. If you approach a black hole feet first, the gravitational force at your feet is greater than the force at your head.

The tidal force at the event horizon is smaller for larger black holes: you would get torn to shreds far outside a black hole the mass of our sun, but at the event horizon of a billion solar mass black hole the tidal force would only be a millionth of an ounce! Strange Facts About Black Holes Light bends so much near black holes that if you were near one and looking away from the hole, you would see multiple images of every star in the universe, and could actually see the back of your own head! Inside a black hole the roles of time and radius reverse: just as now you can't avoid going into the future, inside a black hole you can't avoid going in to the central singularity. If you stood a safe distance from a black hole and saw a friend fall in, he would appear to slow down and almost stop just outside the event horizon. His image would dim very rapidly. Unfortunately for him, from his point of view he would cross the event horizon just fine, and would meet his doom at the singularity. Black holes are the simplest objects in the universe. You can describe one completely by just its mass, spin rate, and electric charge.

In contrast, to completely describe a dust mote you're have to specify the position and state of all of its atoms, taking at least $10^{16}$ numbers! As Hawking discovered, black holes can evaporate, but only very slowly. Even one the mass of a mountain will last for ten billion years, and one the mass of the Sun will only evaporate after $10^{67}$ years. How Do We Detect Black Holes? Black holes don't radiate light, and an object that falls inside a black hole doesn't emit light either, so detecting them can be challenging. Just as with neutron stars, if a black hole is in a binary and it strips gas from its companion, we can detect X-rays from the resulting accretion disk (see "Observing Neutron Stars" ). The light from accretion disks around black holes looks very similar to the light from disks around neutron stars, and it is not always possible to tell with certainty which object lurks at the center of the disk, although in six cases so far we're sure that the central object is a black hole. You can also infer the presence of a black hole in the center of some galaxies.

This is done by observing stars near the center of the galaxy. If the stars are moving very rapidly around some unseen object, Kepler's laws can be used to estimate the mass in the center. In some cases the mass must be at least a hundred million times our Sun's mass, in a region only a few light years across. Astronomers are virtually certain that the only explanation is a black hole, but we lack direct evidence. The detection of black holes is very difficult and controversial, and it is being studied actively by many research groups. Origin and Scale of Neutron Stars A neutron star has roughly the mass of our Sun crammed in a ball ten kilometers in radius. Its density is therefore a hundred trillion times the density of water; at that density, all the people on Earth could be fit into a teaspoon! Neutron stars are born during supernova, and are held up by neutron degeneracy pressure. These stars are relatively rare: only about 10^8 in our galaxy, or one in a thousand stars, so the nearest one is probably at least 40 light years away. Neutron Stars and Extreme Physics Density at center can be several times the density of an atomic nucleus, so we can't explore this regime in laboratories.

The properties of this matter are unknown, and may include such exotic things as enormous conglomerations of quarks. Magnetic fields are a trillion times Earth's, and more than a million times as strong as can be achieved in laboratories. Matter in the central parts of neutron stars is thought to be a superconductor, even at a hundred million degrees! Neutron stars therefore have states of matter that cannot be duplicated in laboratories. Study of them helps us test our theories, and perhaps discover new physics. But how can we observe neutron stars? Observing Neutron Stars We see a normal star by the light it gives off during fusion. Neutron stars are very hot, more than 100,000 K for most of their lifetimes, so this sounds promising but most of the energy comes out as X-rays (not visible light). Also, neutron stars are so small that at typical distances they are ten billion times fainter than you can see with your naked eye, which is too faint for even the Hubble Space Telescope. We need some other way to see neutron stars. One way is to see them as radio pulsars.

Another way is if the neutron star is one member of a binary, in which case the gravity of the neutron star can strip gas off its companion. The gas from the companion falls onto the neutron star, and emits fantastic power in X-rays: as much as 50,000 times the luminosity the Sun produces. This is a tremendously efficient way to generate energy. Dropping a kilogram of matter onto the surface of a neutron star releases as much energy as a five megaton hydrogen bomb! Since the neutron star is a very small target, astronomically speaking, gas can't fall onto it directly. Instead, gas spirals around the neutron star, and friction with itself releases huge amounts of energy in what is called an accretion disk. Studying the X-rays from accretion disks can give us hints about the star: for example, how does matter behave at extremely high densities? The Equation of State As mentioned above, we want to know the properties of the extremely dense matter in the center of neutron stars. One way to characterize the matter is by its equation of state. The equation of state can be pictured as the relation between the density of matter and its pressure. Consider a glass of water.

The shape of the water in the glass can be changed easily (e.g., by sloshing it around), but the volume, and hence the density, of the water is extremely difficult to change. Even if you apply a huge amount of pressure to the water, for example by a piston, the density changes hardly at all; this is the basis of hydraulic presses. Water may therefore be said to have a stiff equation of state. In contrast, the volume of air in an empty glass can be changed easily, with little pressure, so air may be said to have a soft equation of state. So, a knowledge of the equation of state tells us, essentially, how squeezable the matter is. In the case of a neutron star, knowledge of both the mass and radius of a particular neutron star would tell us the equation of state. This is because gravity squeezes the star, and the more mass the star has the more gravity squeezes it. If the star has a large radius (meaning, say, 15~km!), it was relatively successful in resisting gravity and thus has a very stiff equation of state.

If the star has a small radius (say, 8~km), it was not as successful in resisting gravity and it has a softer equation of state. We therefore need to estimate the mass and radius of neutron stars. Estimating NS Masses and Radii No easy task, this. Astronomical measurements are often challenging, because we can't go to a star and experiment on it. Neutron stars are especially tough, because they are relatively small and far away: even the closest one would appear to be the size of a bacterium on the Moon, so we have to find other ways to determine the mass or radius of a neutron star. One way to do this is to use Kepler's laws. If we can figure out how far two stars in a binary are from each other, and the duration of their orbital period, we know something about their masses. Only for neutron stars in binaries do we have even a rough estimate of the mass, and in only a few of those cases do we know the mass accurately. Estimating the radius is much more difficult than estimating the mass. Unlike the mass, the radius doesn't have any strong effects on what we can observe.

From astronomical observations alone, neutron stars could have radii from 5~km to 30~km (although most of that range, all but about 7~km to 20~km, is ruled out by what we know of nuclear physics). So, we need some kind of breakthrough in the evidence to allow us to further constrain the radii of neutron stars. An Unexpected Discovery We can only discover what our instruments can detect, so many times in astrophysics a breakthrough in our understanding has come from an improvement in instrumental capabilities. Such was the case when the Rossi X-ray Timing Explorer was launched on December 30, 1995. Its many outstanding properties include an unprecedented sensitivity to very rapid variations of the X-ray intensity of accreting neutron stars, i.e., neutron stars stripping mass from their stellar companions. This led to the discovery of a completely unexpected phenomenon: fast intensity oscillations, sometimes more than a thousand times per second! Kilohertz Intensity Oscillations Figure 1 shows the X-ray brightness from one neutron star system, as a function of time. The intensity goes up and down nearly 1000 times per second.

There are at least 10 known neutron stars that show this, and we have discovered that: The intensity variations are fast, up to 1200 times per second For a given neutron star, the frequency of the variations goes up and down with time: in one case, the variation can be anywhere from 500 per second to 1100 per second The dramatic change in frequency means that it can't be something simple like the spin frequency of the neutron star, since the star can't easily be spun up or down. However, the common occurrence of this phenomenon and its other properties mean that it is telling us something fundamental about the flow of matter onto neutron stars. Click to see my proposed explanation of this phenomenon. Implications The frequency with which the clump goes around the star is calculated by Kepler's laws: the higher the frequency, the closer the clump has to be to the star. This limits the radius of the star. When the effects of Einstein's general relativity are included, it turns out that we also get an upper limit to the mass of the star.

The result is that the neutron stars in these systems must have masses less than 2.2 times our Sun's mass, and radii less than 17~km. This is the first convincing observational limit to the radius of neutron stars. Key Points The matter in the center of neutron stars is incredibly dense, and we can't reproduce it on Earth. So, the study of neutron stars can tell us things about the universe that would otherwise remain forever undiscovered. One way to find out about the dense matter of neutron stars is to determine the equation of state of neutron stars, which is the relation between their pressure and density. The equation of state is known if we know both the mass and radius of a neutron star. But, astronomical observations are indirect: we can't experiment on stars. Neutron stars are particularly difficult, since they are relatively small and distant. Luckily for us, the recently-discovered phenomenon of rapid X-ray intensity oscillations may allow us, for the first time, to estimate both the mass and radius of some neutron stars and thus know their equation of state. As it turns out, if we knew the equation of state of neutron stars, we would also know their maximum mass.

This ends up helping in the search for black holes in our galaxy. This phenomenon is still new, and we continue to learn things about it at a rapid pace, both observationally and theoretically; these are exciting times! Glossary Accretion disk: the pattern of flow of matter from a normal star to a neutron star or black hole, which is flattened and thus disk-like. Degeneracy pressure: a quantum-mechanical phenomenon; fermions, such as electrons or neutrons, obey Pauli's exclusion principle, so that no two fermions can occupy the same state. Thus, if fermions are squeezed together they resist even if there is no temperature and no energy generation. This resistance to squeezing is degeneracy pressure. Equation of state: the relation between the pressure and density of a given type of matter, which is an indication of how the matter resists squeezing. If the matter resists squeezing strongly (e.g., water), the equation of state is stiff; if it resists squeezing only weakly (e.g., air), the equation of state is soft. Event horizon: in a black hole, the point beyond which events cannot be detected.

This is the point of no return; an object that falls inside the event horizon can't get out. Kepler's laws: rules for the orbital motion of planets or anything else bound by gravity. The law of most interest here is that the square of the orbital period is proportional to the cube of the orbital separation, and inversely proportional to the mass. Thus, if we see an orbital period, we can estimate the mass or orbital separation and therefore constrain the mass and radius of a neutron star. Singularity: in a black hole, the "center point", at which densities, tidal forces, and other physical quantities become infinite. Our current physical theories break down at this point. Tidal force: BLACK HOLES by Ted Bunn How big is a black hole? ------------------------ There are at least two different ways to describe how big something is. We can say how much mass it has, or we can say how much space it takes up. Let's talk first about the masses of black holes. There is no limit in principle to how much or how little mass a black hole can have. Any amount of mass at all can in principle be made to form a black hole if you compress it to a high enough density.

We suspect that most of the black holes that are actually out there were produced in the deaths of massive stars, and so we expect those black holes to weigh about as much as a massive star. A typical mass for such a stellar black hole would be about 10 times the mass of the Sun, or about 10^{31} kilograms. (Here I'm using scientific notation: 10^{31} means a 1 with 31 zeroes after it, or 10,000,000,000,000,000,000,000,000,000,000.) Astronomers also suspect that many galaxies harbor extremely massive black holes at their centers. These are thought to weigh about a million times as much as the Sun, or 10^{36} kilograms. The more massive a black hole is, the more space it takes up. In fact, the Schwarzschild radius (which means the radius of the horizon) and the mass are directly proportional to one another: if one black hole weighs ten times as much as another, its radius is ten times as large.

A black hole with a mass equal to that of the Sun would have a radius of 3 kilometers. So a typical 10-solar-mass black hole would have a radius of 30 kilometers, and a million-solar-mass black hole at the center of a galaxy would have a radius of 3 million kilometers. Three million kilometers may sound like a lot, but it's actually not so big by astronomical standards. The Sun, for example, has a radius of about 700,000 kilometers, and so that supermassive black hole has a radius only about four times bigger than the Sun. What would happen to me if I fell into a black hole? ---------------------------------------------------- Let's suppose that you get into your spaceship and point it straight towards the million-solar-mass black hole in the center of our galaxy. (Actually, there's some debate about whether our galaxy contains a central black hole, but let's assume it does for the moment.) Starting from a long way away from the black hole, you just turn off your rockets and coast in. What happens? At first, you don't feel any gravitational forces at all.

Since you're in free fall, every part of your body and your spaceship is being pulled in the same way, and so you feel weightless. (This is exactly the same thing that happens to astronauts in Earth orbit: even though both astronauts and space shuttle are being pulled by the Earth's gravity, they don't feel any gravitational force because everything is being pulled in exactly the same way.) As you get closer and closer to the center of the hole, though, you start to feel "tidal" gravitational forces. Imagine that your feet are closer to the center than your head. The gravitational pull gets stronger as you get closer to the center of the hole, so your feet feel a stronger pull than your head does. As a result you feel "stretched." (This force is called a tidal force because it is exactly like the forces that cause tides on earth.) These tidal forces get more and more intense as you get closer to the center, and eventually they will rip you apart. For a very large black hole like the one you're falling into, the tidal forces are not really noticeable until you get within about 600,000 kilometers of the center. Note that this is after you've crossed the horizon.

If you were falling into a smaller black hole, say one that weighed as much as the Sun, tidal forces would start to make you quite uncomfortable when you were about 6000 kilometers away from the center, and you would have been torn apart by them long before you crossed the horizon. (That's why we decided to let you jump into a big black hole instead of a small one: we wanted you to survive at least until you got inside.) What do you see as you are falling in? Surprisingly, you don't necessarily see anything particularly interesting. Images of faraway objects may be distorted in strange ways, since the black hole's gravity bends light, but that's about it. In particular, nothing special happens at the moment when you cross the horizon. Even after you've crossed the horizon, you can still see things on the outside: after all, the light from the things on the outside can still reach you. No one on the outside can see you, of course, since the light from you can't escape past the horizon. How long does the whole process take? Well, of course, it depends on how far away you start from.

Let's say you start at rest from a point whose distance from the singularity is ten times the black hole's radius. Then for a million-solar-mass black hole, it takes you about 8 minutes to reach the horizon. Once you've gotten that far, it takes you only another seven seconds to hit the singularity. By the way, this time scales with the size of the black hole, so if you'd jumped into a smaller black hole, your time of death would be that much sooner. Once you've crossed the horizon, in your remaining seven seconds, you might panic and start to fire your rockets in a desperate attempt to avoid the singularity. Unfortunately, it's hopeless, since the singularity lies in your future, and there's no way to avoid your future. In fact, the harder you fire your rockets, the sooner you hit the singularity. It's best just to sit back and enjoy the ride. My friend Penelope is sitting still at a safe distance, watching me fall into the black hole. What does she see? ------------------------------------------------------------------- Penelope sees things quite differently from you. As you get closer and closer to the horizon, she sees you move more and more slowly.

In fact, no matter how long she waits, she will never quite see you reach the horizon. In fact, more or less the same thing can be said about the material that formed the black hole in the first place. Suppose that the black hole formed from a collapsing star. As the material that is to form the black hole collapses, Penelope sees it get smaller and smaller, approaching but never quite reaching its Schwarzschild radius. This is why black holes were originally called frozen stars: because they seem to 'freeze' at a size just slightly bigger than the Schwarzschild radius. Why does she see things this way? The best way to think about it is that it's really just an optical illusion. It doesn't really take an infinite amount of time for the black hole to form, and it doesn't really take an infinite amount of time for you to cross the horizon. (If you don't believe me, just try jumping in! You'll be across the horizon in eight minutes, and crushed to death mere seconds later.) As you get closer and closer to the horizon, the light that you're emitting takes longer and longer to climb back out to reach Penelope.

In fact, the radiation you emit right as you cross the horizon will hover right there at the horizon forever and never reach her. You've long since passed through the horizon, but the light signal telling her that won't reach her for an infinitely long time. There is another way to look at this whole business. In a sense, time really does pass more slowly near the horizon than it does far away. Suppose you take your spaceship and ride down to a point just outside the horizon, and then just hover there for a while (burning enormous amounts of fuel to keep yourself from falling in). Then you fly back out and rejoin Penelope. You will find that she has aged much more than you during the whole process; time passed more slowly for you than it did for her. So which of these two explanation (the optical-illusion one or the time-slowing-down one) is really right? The answer depends on what system of coordinates you use to describe the black hole. According to the usual system of coordinates, called "Schwarzschild coordinates," you cross the horizon when the time coordinate t is infinity. So in these coordinates it really does take you infinite time to cross the horizon.

But the reason for that is that Schwarzschild coordinates provide a highly distorted view of what's going on near the horizon. In fact, right at the horizon the coordinates are infinitely distorted (or, to use the standard terminology, "singular"). If you choose to use coordinates that are not singular near the horizon, then you find that the time when you cross the horizon is indeed finite, but the time when Penelope sees you cross the horizon is infinite. It took the radiation an infinite amount of time to reach her. In fact, though, you're allowed to use either coordinate system, and so both explanations are valid. They're just different ways of saying the same thing. In practice, you will actually become invisible to Penelope before too much time has passed. For one thing, light is "redshifted" to longer wavelengths as it rises away from the black hole. So if you are emitting visible light at some particular wavelength, Penelope will see light at some longer wavelength. The wavelengths get longer and longer as you get closer and closer to the horizon. Eventually, it won't be visible light at all: it will be infrared radiation, then radio waves. At some point the wavelengths will be so long that she'll be unable to observe them.

Furthermore, remember that light is emitted in individual packets called photons. Suppose you are emitting photons as you fall past the horizon. At some point, you will emit your last photon before you cross the horizon. That photon will reach Penelope at some finite time -- typically less than an hour for that million-solar-mass black hole -- and after that she'll never be able to see you again. (After all, none of the photons you emit *after* you cross the horizon will ever get to her.) If a black hole existed, would it suck up all the matter in the Universe? --------------------------------------------------------------- Heck, no. A black hole has a "horizon," which means a region from which you can't escape. If you cross the horizon, you're doomed to eventually hit the singularity. But as long as you stay outside of the horizon, you can avoid getting sucked in. In fact, to someone well outside of the horizon, the gravitational field surrounding a black hole is no different from the field surrounding any other object of the same mass.

In other words, a one-solar-mass black hole is no better than any other one-solar-mass object (such as, for example, the Sun) at "sucking in" distant objects. What if the Sun became a black hole? ------------------------------------ Well, first, let me assure you that the Sun has no intention of doing any such thing. Only stars that weigh considerably more than the Sun end their lives as black holes. The Sun is going to stay roughly the way it is for another five billion years or so. Then it will go through a brief phase as a red giant star, during which time it will expand to engulf the planets Mercury and Venus, and make life quite uncomfortable on Earth (oceans boiling, atmosphere escaping, that sort of thing). After that, the Sun will end its life by becoming a boring white dwarf star. If I were you, I'd make plans to move somewhere far away before any of this happens. I also wouldn't buy any of those 8-billion-year government bonds. But I digress. What if the Sun *did* become a black hole for some reason? The main effect is that it would get very dark and very cold around here.

The Earth and the other planets would not get sucked into the black hole; they would keep on orbiting in exactly the same paths they follow right now. Why? Because the horizon of this black hole would be very small -- only about 3 kilometers -- and as we observed above, as long as you stay well outside the horizon, a black hole's gravity is no stronger than that of any other object of the same mass. Is there any evidence that black holes exist? --------------------------------------------- Yes. You can't see a black hole directly, of course, since light can't get past the horizon. That means that we have to rely on indirect evidence that black holes exist. Suppose you have found a region of space where you think there might be a black hole. How can you check whether there is one or not? The first thing you'd like to do is measure how much mass there is in that region. If you've found a large mass concentrated in a small volume, and if the mass is dark, then it's a good guess that there's a black hole there.

There are two kinds of systems in which astronomers have found such compact, massive, dark objects: the centers of galaxies (including perhaps our own Milky Way Galaxy), and X-ray-emitting binary systems in our own Galaxy. According to a recent review by Kormendy and Richstone (to appear in the 1995 edition of "Annual Reviews of Astronomy and Astrophysics"), eight galaxies have been observed to contain such massive dark objects in their centers. The masses of the cores of these galaxies range from one million to several billion times the mass of the Sun. The mass is measured by observing the speed with which stars and gas orbit around the center of the galaxy: the faster the orbital speeds, the stronger the gravitational force required to hold the stars and gas in their orbits. (This is the most common way to measure masses in astronomy. For example, we measure the mass of the Sun by observing how fast the planets orbit it, and we measure the amount of dark matter in galaxies by measuring how fast things orbit at the edge of the galaxy.) These massive dark objects in galactic centers are thought to be black holes for at least two reasons.

First, it is hard to think of anything else they could be: they are too dense and dark to be stars or clusters of stars. Second, the only promising theory to explain the enigmatic objects known as quasars and active galaxies postulates that such galaxies have supermassive black holes at their cores. If this theory is correct, then a large fraction of galaxies -- all the ones that are now or used to be active galaxies -- must have supermassive black holes at the center. Taken together, these arguments strongly suggest that the cores of these galaxies contain black holes, but they do not constitute absolute proof. Two very recent discovery has been made that strongly support the hypothesis that these systems do indeed contain black holes. First, a nearby active galaxy was found to have a "water maser" system (a very powerful source of microwave radiation) near its nucleus. Using the technique of very-long-baseline interferometry, a group of researchers was able to map the velocity distribution of the gas with very fine resolution. In fact, they were able to measure the velocity within less than half a light-year of the center of the galaxy. From this measurement they can conclude that the massive object at the center of this galaxy is less than half a light-year in radius.

It is hard to imagine anything other than a black hole that could have so much mass concentrated in such a small volume. (This result was reported by Miyoshi et al. in the 12 January 1995 issue of Nature, vol. 373, p. 127.) A second discovery provides even more compelling evidence. X-ray astronomers have detected a spectral line from one galactic nucleus that indicates the presence of atoms near the nucleus that are moving extremely fast (about 1/3 the speed of light). Furthermore, the radiation from these atoms has been redshifted in just the manner one would expect for radiation coming from near the horizon of a black hole. These observations would be very difficult to explain in any other way besides a black hole, and if they are verified, then the hypothesis that some galaxies contain supermassive black holes at their centers would be fairly secure. (This result was reported in the 22 June 1995 issue of Nature, vol. 375, p. 659, by Tanaka et al.) A completely different class of black-hole candidates may be found in our own Galaxy.

These are much lighter, stellar-mass black holes, which are thought to form when a massive star ends its life in a supernova explosion. If such a stellar black hole were to be off somewhere by itself, we wouldn't have much hope of finding it. However, many stars come in binary systems -- pairs of stars in orbit around each other. If one of the stars in such a binary system becomes a black hole, we might be able to detect it. In particular, in some binary systems containing a compact object such as a black hole, matter is sucked off of the other object and forms an "accretion disk" of stuff swirling into the black hole. The matter in the accretion disk gets very hot as it falls closer and closer to the black hole, and it emits copious amounts of radiation, mostly in the X-ray part of the spectrum. Many such "X-ray binary systems" are known, and some of them are thought to be likely black-hole candidates. Suppose you've found an X-ray binary system. How can you tell whether the unseen compact object is a black hole? Well, one thing you'd certainly like to do is to estimate its mass.

By measuring the orbital speed of visible star (together with a few other things), you can figure out the mass of the invisible companion. (The technique is quite similar to the one we described above for supermassive black holes in galactic centers: the faster the star is moving, the stronger the gravitational force required to keep it in place, and so the more massive the invisible companion.) If the mass of the compact object is found to be very large very large, then there is no kind of object we know about that it could be other than a black hole. (An ordinary star of that mass would be visible. A stellar remnant such as a neutron star would be unable to support itself against gravity, and would collapse to a black hole.) The combination of such mass estimates and detailed studies of the radiation from the accretion disk can supply powerful circumstantial evidence that the object in question is indeed a black hole. Many of these "X-ray binary" systems are known, and in some cases the evidence in support of the black-hole hypothesis is quite strong.

In a review article in the 1992 issue of Annual Reviews of Astronomy and Astrophysics, Anne Cowley summarized the situation by saying that there were three such systems known (two in our galaxy and one in the nearby Large Magellanic Cloud) for which very strong evidence exists that the mass of the invisible object is too large to be anything but a black hole. There are many more such objects that are thought to be likely black holes on the basis of slightly less evidence. Furthermore, this field of research has been very active since 1992, and the number of strong candidates by now is larger than three. How do black holes evaporate? ----------------------------- This is a tough one. Back in the 1970's, Stephen Hawking came up with theoretical arguments showing that black holes are not really entirely black: due to quantum-mechanical effects, they emit radiation. The energy that produces the radiation comes from the mass of the black hole. Consequently, the black hole gradually shrinks. It turns out that the rate of radiation increases as the mass decreases, so the black hole continues to radiate more and more intensely and to shrink more and more rapidly until it presumably vanishes entirely.

Actually, nobody is really sure what happens at the last stages of black hole evaporation: some researchers think that a tiny, stable remnant is left behind. Our current theories simply aren't good enough to let us tell for sure one way or the other. As long as I'm disclaiming, let me add that the entire subject of black hole evaporation is extremely speculative. It involves figuring out how to perform quantum-mechanical (or rather quantum-field-theoretic) calculations in curved spacetime, which is a very difficult task, and which gives results that are essentially impossible to test with experiments. Physicists *think* that we have the correct theories to make predictions about black hole evaporation, but without experimental tests it's impossible to be sure. Now why do black holes evaporate? Here's one way to look at it, which is only moderately inaccurate. (I don't think it's possible to do much better than this, unless you want to spend a few years learning about quantum field theory in curved space.) One of the consequences of the uncertainty principle of quantum mechanics is that it's possible for the law of energy conservation to be violated, but only for very short durations.

The Universe is able to produce mass and energy out of nowhere, but only if that mass and energy disappear again very quickly. One particular way in which this strange phenomenon manifests itself goes by the name of vacuum fluctuations. Pairs consisting of a particle and antiparticle can appear out of nowhere, exist for a very short time, and then annihilate each other. Energy conservation is violated when the particles are created, but all of that energy is restored when they annihilate again. As weird as all of this sounds, we have actually confirmed experimentally that these vacuum fluctuations are real. Now, suppose one of these vacuum fluctuations happens near the horizon of a black hole. It may happen that one of the two particles falls across the horizon, while the other one escapes. The one that escapes carries energy away from the black hole and may be detected by some observer far away. To that observer, it will look like the black hole has just emitted a particle. This process happens repeatedly, and the observer sees a continuous stream of radiation from the black hole. Won't the black hole have evaporated out from under me before I reach it?

--------------------------------------------------------------------- We've observed that, from the point of view of your friend Penelope who remains safely outside of the black hole, it takes you an infinite amount of time to cross the horizon. We've also observed that black holes evaporate via Hawking radiation in a finite amount of time. So by the time you reach the horizon, the black hole will be gone, right? Wrong. When we said that Penelope would see it take forever for you to cross the horizon, we were imagining a non-evaporating black hole. If the black hole is evaporating, that changes things. Your friend will see you cross the horizon at the exact same moment she sees the black hole evaporate. Let me try to describe why this is true. Remember what we said before: Penelope is the victim of an optical illusion. The light that you emit when you're very near the horizon (but still on the outside) takes a very long time to climb out and reach her. If the black hole lasts forever, then the light may take arbitrarily long to get out, and that's why she doesn't see you cross the horizon for a very long (even an infinite) time.

But once the black hole has evaporated, there's nothing to stop the light that carries the news that you're about to cross the horizon from reaching her. In fact, it reaches her at the same moment as that last burst of Hawking radiation. Of course, none of that will matter to you: you've long since crossed the horizon and been crushed at the singularity. Sorry about that, but you should have thought about it before you jumped in. What is a white hole? --------------------- The equations of general relativity have an interesting mathematical property: they are symmetric in time. That means that you can take any solution to the equations and imagine that time flows backwards rather than forwards, and you'll get another valid solution to the equations. If you apply this rule to the solution that describes black holes, you get an object known as a white hole. Since a black hole is a region of space from which nothing can escape, the time-reversed version of a black hole is a region of space into which nothing can fall. In fact, just as a black hole can only suck things in, a white hole can only spit things out.

White holes are a perfectly valid mathematical solution to the equations of general relativity, but that doesn't mean that they actually exist in nature. In fact, they almost certainly do not exist, since there's no way to produce one. (Producing a white hole is just as impossible as destroying a black hole, since the two processes are time-reversals of each other.) What is a wormhole? ------------------- So far, we have only considered ordinary "vanilla" black holes. Specifically, we have been talking all along about black holes that are not rotating and have no electric charge. If we consider black holes that rotate and/or have charge, things get more complicated. In particular, it is possible to fall into such a black hole and not hit the singularity. In effect, the interior of a charged or rotating black hole can "join up" with a corresponding white hole in such a way that you can fall into the black hole and pop out of the white hole. This combination of black and white holes is called a wormhole.

The white hole may be somewhere very far away from the black hole; indeed, it may even be in a "different Universe" -- that is, a region of spacetime that, aside from the wormhole itself, is completely disconnected from our own region. A conveniently-located wormhole would therefore provide a convenient and rapid way to travel very large distances, or even to travel to another Universe. Maybe the exit to the wormhole would lie in the past, so that you could travel back in time by going through. All in all, they sound pretty cool. But before you apply for that research grant to go search for them, there are a couple of things you should know. First of all, wormholes almost certainly do not exist. As we said above in the section on white holes, just because something is a valid mathematical solution to the equations doesn't mean that it actually exists in nature. In particular, black holes that form from the collapse of ordinary matter (which includes all of the black holes that we think exist) do not form wormholes. If you fall into one of those, you're not going to pop out anywhere. You're going to hit a singularity, and that's all there is to it.

Furthermore, even if a wormhole were formed, it is thought that it would not be stable. Even the slightest perturbation (including the perturbation caused by your attempt to travel through it) would cause it to collapse. Finally, even if wormholes exist and are stable, they are quite unpleasant to travel through. Radiation that pours into the wormhole (from nearby stars, the cosmic microwave background, etc.) gets blueshifted to very high frequencies. As you try to pass through the wormhole, you will get fried by these X-rays and gamma rays. Where can I go to learn more about black holes? ----------------------------------------------- Let me begin by acknowledging that I cribbed some of the above material from the article about black holes in the Frequently Asked Questions list for the Usenet newsgroup sci.physics. The sci.physics FAQ is posted monthly to sci.physics and is also available by anonymous ftp from rtfm.mit.edu (and probably other places). The article about black holes, which is excellent, was written by Matt McIrvin. The FAQ contains other neat things too. There are lots of books out there about black holes and related matters.

Kip Thorne's "Black Holes and Time Warps: Einstein's Outrageous Legacy" is a good one. William Kaufmann's "Black Holes and Warped Spacetime" is also worth reading. R. Wald's "Space, Time, and Gravity" is an exposition of general relativity for non-scientists. I haven't read it myself, but I've heard good things about it. Both of these books are aimed at readers without much background in physics. If you want more "meat" (i.e., more mathematics), then you probably start with a book on the basics of relativity theory. The best introduction to the subject is "Spacetime Physics" by E.F. Taylor and J.A. Wheeler. (This book is mostly about special relativity, but the last chapter discusses the general theory.) Taylor and Wheeler have been threatening for about two years now to publish a sequel entitled "Scouting Black Holes," which should be quite good if it ever comes out. "Spacetime Physics" does not assume that you know vast amounts of physics, but it does assume that you're willing to work hard at understanding this stuff.

It is not light reading, although it is more playful and less intimidating than most physics books. Finally, if "Spacetime Physics" isn't enough for you, you could try any of several introductions to general relativity. B. Schutz's "A First Course in General Relativity" and W. Rindler's "Essential Relativity" are a couple of possibilities. And for the extremely valiant reader with an excellent background in physics, there's the granddaddy of all books on general relativity, Misner, Thorne, and Wheeler's "Gravitation." R. Wald's book "General Relativity" is at a comparable level to "Gravitation," although the styles of the two books are enormously different. What little I know about black-hole evaporation comes from Wald's book. Let me emphasize that all of these books, and especially the last two, assume that you know quite a lot of physics. They are not for the faint of heart.

Question: An "event horizon" is the defining feature of which astronomical phenonmenon ?
Answer: BLACK HOLE