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Black Holes Encyclopedia


accretion disk

A swirling disk of gas and dust orbiting a star or black hole. The material within the disk may generate heat from friction; the hottest accretion disks produce enormous amounts of X-rays.

active galaxy

A galaxy that emits much more energy than the sum of the individual stars within it. Such galaxies probably are powered by large, hot accretion disks that encircle supermassive black holes in their cores.

Chandrasekhar Limit

The maximum mass of a white dwarf, 1.4 solar masses. Beyond this mass, the star collapses into a neutron star or black hole depending on the mass of the collapsing core. Subrahamanyan Chadrasekhar, at age 19 in 1930, worked out this limiting mass while on a steamship to England.

escape velocity

The minimum speed necessary to escape the gravitational pull of a celestial body. A rocket launched from Kennedy Space Center must accelerate to about 17,500 mph to enter orbit, or about 25,000 mph (11.2 km/second) to escape Earth's gravitational pull and travel to another planet. For a black hole, the escape velocity is greater than the speed of light. Since lightspeed is the ultimate cosmic speed limit, nothing can achieve the speed needed to escape from a black hole.

event horizon

A black hole's point of no return. Any light or matter crossing within this boundary is doomed by the hole's gravity. Beyond this point, the escape velocity is greater than the speed of light, the ultimate speed limit. In essence, although it is not a physical boundary, the event horizon marks the black hole's "surface."

galactic bulge

The bulge is a massive, densely packed region at the center of a galaxy. A true bulge formed at the same time as the supermassive black hole. Astronomers theorize that the black hole pulls in the gas and dust that gives birth to stars in the bulge. Some of the material falls toward the black hole, however, encircling it with a bright, hot disk. Radiation and "winds" from the disk can stop the starbirth process by blowing away the remaining gas, leaving nothing to make stars. A tight relationship between the masses of supermassive black holes and their surrounding bulges would support this scenario.


The most feeble of the four fundamental forces in the universe that affect all matter. The magnitude of gravitational attraction depends directly on mass and inversely on distance squared. For instance, the gravitational attraction between you and Earth is much greater than that between you and the Sun, even though the Sun is 333,000 times more massive than Earth. The distance separating you from the Sun weakens the mutual gravitational attraction, so as you stand on Earth's surface, Earth's gravitational pull on you is 1,650 times greater than the Sun's.


1,000 parsecs or 3,260 light-years


Electromagnetic radiation of all wavelengths and frequencies. The familiar "rainbow" of light spans a narrow slit in the electromagnetic spectrum, from 700 nanometers (red) to 400 nanometers (blue). The wavelengths of red and blue light differ by less than a factor of two. The electromagnetic spectrum range spans beyond a factor of 10^18, (1 followed by 18 zeroes) from radio to gamma ray wavelengths. Radio wavelengths can be the size of mountains while gamma ray wavelengths are the size of an atomic nucleus.


Electromagnetic radiation of all wavelengths and frequencies. The familiar "rainbow" of light spans a narrow slit in the electromagnetic spectrum, from 700 nanometers (red) to 400 nanometers (blue). The wavelengths of red and blue light differ by less than a factor of two. The electromagnetic spectrum range spans beyond a factor of 10^18, (1 followed by 18 zeroes) from radio to gamma ray wavelengths. Radio wavelengths can be the size of mountains while gamma ray wavelengths are the size of an atomic nucleus.


The total matter content of an object. Also a physical measure of inertia. Newton's law states that mass is related to force and acceleration: m = F/a. Einstein says that mass and space are related, because mass warps space and space directs the motion of mass.


One million parsecs or 3.26 million light-years. The megaparsec is a standard unit for measuring the distances to other galaxies.

neutron star

Stars born with about 8 to 20 times the mass of the Sun blast most of their material into interstellar space in titanic explosions, leaving only their crushed, dense cores, called neutron stars. Neutron stars are named after their composition: neutrons. In a star with a core that is 1.4 to 3 times the mass of the Sun, the core collapses so completely that electrons and protons combine to form neutrons. A full bathtub of neutron-star material (instead of water) would weigh as much as two Mount Everests. A neutron star is about 10-15 miles (16-24 km) in diameter, with a liquid neutron core and a crust of solid iron. Some neutron stars, called pulsars, spin rapidly (from once a second to several hundred times per second) and generate powerful magnetic fields.


A unit of distance equal to 3.26 light-years. The name means "PARallax-SECond," and it refers to a way to measure the distances to other stars. The most accurate way to measure the distances to close stars is to use basic geometry. Astronomers measure the position of a star in the sky at six-month intervals, when Earth is on opposite sides of the Sun. If the star is close, then it will appear to shift a bit compared to the background stars. It's the same effect you see if you hold your finger in front of your face and look at it with first one eye, then the other: the finger appears to move against the background of objects. This effect is called parallax. If a star has a parallax of one second -- in other words, it appears to shift back and forth across the sky by exactly one second of arc (1/3600 of a degree), then its distance is one parsec.


The most luminous and some of the most distant of all objects in the universe. They radiate between 10 and 100,000 times as much energy as our entire galaxy from an energy source that measures less than one light-year in diameter. Such a compact source may be a supermassive black hole surrounded by an accretion disk of matter falling into the hole. The matter is heated to millions of degrees, making the accretion disk glow brightly.

Relativity, General

"Space tells mass how to move" while "mass tells space how to curve" -- J.A. Wheeler. Einstein created this model, which describes gravity as curvature in space-time, the four-dimensional fabric of our universe. His theory is the best model for gravity so far, and has been confirmed in experiments and observations. According to the theory, regardless of one's point of view (as measured by speed and direction), physical law and the speed of light are unchanged. This implies that measurements made in time and space are not absolute, but relative to your particular point of view or reference frame. General relativity led to concepts and theories such as black hole, parallel universes, worm holes, and space-time.

Relativity, Special

Einstein's rejection of the notion that space and time are absolute, based on the observation that the speed of light is independent of the motion of an observer. No matter how fast someone runs toward you with a flashlight, the speed of the light that flashlight emits will always remain the same. From this foundation, Einstein constructed a revolutionary model of gravity and a universe full of unexpected surprises like black holes, gravity waves, time dilation, and the equivalence of mass and energy: E=mc2. Astronomers and astrophysicists regularly use the theoretical tools of special relativity to interpret and analyze light.

Schwarzschild Radius

The distance between the central singularity and event horizon of a black hole. The length of the Schwarzschild Radius depends on the mass of the black hole. Anything inside this radius can not escape the black hole.


A theoretical point at the core of a black hole where all of the black hole's mass is concentrated. The singularity is compressed so tightly that it is almost infinitely dense. The curvature of space-time is infinite, and the normal laws of physics break down.

speed of light

The maximum velocity for everything in the universe; 186,282.397 miles (299,792.458 km) per second, or fast enough to go to the Moon and back in less than three seconds.


A dense, glowing ball of hydrogen, helium, and perhaps heavier elements that shines with the energy released from thermonuclear fusion reactions in its core. Stars appear in colors that range from red, orange, and yellow to white and blue. The surface temperature depends on the star's mass and its stage in life. In general, the most massive stars are the hottest, so they shine blue or white, while the least massive stars are coolest, so they shine orange or red. Stars are born, live and die within a metropolis populated with billions of stars called a galaxy. They may live for millions or billions of years depending on their mass.


A violent stellar explosion that can shine as brightly as an entire galaxy of billions of normal stars. Astronomers divide supernovae into two groups: Type I and Type II. Type I supernovae most likely form as a white dwarf "steals" hot gas from a companion star. If enough gas piles up on the surface of the white dwarf, a runaway thermonuclear explosion blasts the star to bits, leaving nothing behind. These are the brightest supernovae, and can be used to measure the distances to other galaxies. Type II supernovae are the final stage in the evolution of stars that are at least eight times as massive as the Sun. Such a star reaches a point where it can no longer produce nuclear energy in its core. Without the outward pressure created by this energy, gravity wins out and causes the star's core to collapse to form a neutron star or black hole. The star's outer layers "rebound" violently, blasting into space at several percent of the speed of light.

thermonuclear fusion

The process that powers a star. In the simplest reaction, the nuclei of hydrogen atoms "fuse" to form helium. About 0.7 percent of the mass of the hydrogen is converted to energy, which makes the star shine. Later in life, a star may "burn" the helium to make carbon and other elements. The most massive will continue this process of burning the "ashes" of the last reaction until their cores are converted to iron. At that point, such a star's core collapses and its outer layers are blasted into space.

white dwarf

The hot, dense core of a once-normal star like the Sun. At the end of such a star's life, it can no longer produce the nuclear-fusion reactions that power it. Its outer layers drift away into space, while its core collapses into a ball that is as about as massive as the Sun but no bigger than Earth. This is the fate of stars that do not exceed about four to eight times the mass of the Sun. The Sun will reach this stage in several billion years. A white dwarf spins rapidly, is extremely hot, and may generate a strong magnetic field.


A theoretical "shortcut" between two points in space-time made possible by a singularity. In science fiction, wormholes allow people and starships to travel from one part of the galaxy to another almost instantaneously. While theory allows wormholes to exist, they would make poor passageways because they should close up as soon as anything tries to enter them.