Black Holes: Gravity's Relentless Pull

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Black Holes: Gravity's Relentless Pull
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accretion disk
The disk around a black hole into which infalling gas settles before being swallowed. Friction and magnetic fields in the disk cause the gas to heat and the resulting X-rays can sometimes be observed.




active black hole
A black hole that is consuming large amounts of material and which produces bright X-rays and radio waves in the process.




Andromeda galaxy
A nearby spiral galaxy; the nearest galaxy that is similar in size and shape to our own Milky Way galaxy.




binary star system (binary star)
A system of two stars that orbit around each other, bound together by their mutual gravitational attraction.




black hole
An object that is so compact (in other words, has enough mass in a small enough volume) that its gravitational force is strong enough to prevent light or anything else from escaping.




Chandra X-ray Observatory
A powerful X-ray telescope that is in orbit around the Earth. Named after the Nobel Prize winning astronomer Subrahmanyan Chandrasekhar (1910-1995).



constellation
An area of the night sky named after the visual impression evoked by the pattern of the brightest stars in that area.



dust
Small grains of material that are spread out between the stars. Similar to grains of sand, but with sizes typically no more than a millionth of an inch (comparable for example to the size of particles in cigarette smoke).




electromagnetic radiation
The technical term for light, which consists of a propagating wave in the electric and magnetic fields that fill space. Includes, among other things, radio waves, infrared light, visible light and X-rays. In each case the radiation is characterized by a different wavelength.



electron
A negatively charged elementary particle; one of the components of an atom.



elliptical galaxy
A galaxy in which the stars are spread out in a featureless oval distribution.




ergosphere
The region around a spinning black hole where you are forced to rotate in the same sense as the black hole, although you can still escape.




event horizon
The imaginary sphere around a black hole that measures how close to the singularity you can safely get. Once you have passed the event horizon, it becomes impossible to escape.




(nuclear) fusion
The process by which two atomic nuclei fuse together to form a heavier nucleus. Stars are powered by the energy that is liberated by this process.




galaxy
A group of a million to a thousand billion stars that are bound together by their mutual gravitational attraction.




general relativity
The theory developed by Albert Einstein in which gravity is described by distortions in the four-dimensional space-time of the Universe.




gravitational lensing
The bending of light rays by the gravitational force of an object.




gravity (gravitational force, gravitational field)
The force by which any two masses attract each other.




gravitational wave
Small periodic variations in the gravitational force that propagate as ripples through the four-dimensional space-time of the Universe.




Hawking radiation
The glow arising from quantum mechanical processes that slowly causes black holes to evaporate. Named after the physicist Stephen Hawking (1942- ).



helium
The second lightest chemical element, after hydrogen. A helium atom consists of two protons, two neutrons, and two electrons. It is a gas at room temperatures and is used for example to float balloons.



hydrogen
The lightest chemical element. A hydrogen atom consists of one proton and one electron. It is a gas at room temperatures. Hydrogen and oxygen combined in the ratio of 2 to 1 make up water.



Hubble Space Telescope
A powerful visible light telescope that is in orbit around the Earth. Named after the astronomer Edwin Hubble (1889-1953).




infrared light
Electromagnetic radiation with a wavelength between that of visible light and radio waves. We encounter infrared light in our daily lives for example in heat lamps, night-vision cameras and remote control devices.




jet
A stream of particles that is sometimes accelerated away from the vicinity of a spinning black hole along its rotation axis. Powerful telescopes can observe radiation such as radio waves from these particles.



lightyear
One lightyear is the distance traveled by light in vacuum in a period of 1 year, and equals almost 6000 billion miles. The quantities lighthour, lightminute and lightsecond are defined similarly.



lightspeed
The speed of light in vacuum, equal to about 186 thousand miles per second.



magnetic field
The collection of forces that are produced by electric currents and which are felt by any charged particle that moves.



Milky Way
The spiral galaxy in which our Solar System resides, apparent as a luminous band across the sky.




neutron star
An object composed entirely of neutrons (neutral elementary particles). The object is little more than 10 miles across but has a mass somewhat larger than the Sun. It is produced when an intermediate-mass star dies in a supernova explosion.




orbit
The trajectory of an object as determined by the gravitational influence of another object.




planet
A large object in orbit around a star that is not massive enough to be a star itself.




planetary nebula
A low-mass star in the final stages of its life that illuminates the outer gas layers that it has previously ejected. Can resemble a planet through a small telescope, but otherwise not related to planets.




pulsar
A rapidly spinning neutron star that emits a bright beam of radiation from its magnetic poles. The name derives from the fact that the star appears to pulsate every time the beam sweeps past us (pulsar is short for 'pulsating star').




quantum gravity
A theory of physics (yet to be discovered) that encompasses both gravity and quantum mechanics, and therefore correctly describes matter on both the largest and the smallest scales.




quantum mechanics (quantum theory)
The theory of physics that describes the behavior of matter on the smallest scales.




quasar
An active supermassive black hole that produces so much energy that it outshines all the stars of the galaxy in which it resides. The name derives from the fact that the object resembles a star in photographs of the night sky (quasar stands for 'quasi-stellar object')




quiescent (black hole)
A black hole that is not producing much energy (for example, bright X-rays and radio waves) as a byproduct of the consumption of nearby material.




radio waves
Electromagnetic radiation with a wavelength much larger than that of visible light. We encounter radio waves in our daily lives for example in communication devices such as radios and cell phones.




red giant
A star that has expanded in the late stages of its life and has become extremely large and bright. The outer gas layers are cool so that the star appears red.




Schwarzschild radius
The size of the event horizon in a non-rotating black hole.




singularity
A region of infinitely small volume and infinitely high density that contains all the matter of a black hole. It is a single point in a non-rotating black hole and a ring in a spinning black hole.




Solar System
Our Sun and all the objects that orbit around it, including planets, moons, comets, asteroids, and meteoroids.



spectrum
The distribution of light (or electromagnetic radiation in general) over its constituent wavelengths. For example, a rainbow shows a spectrum of the visible light from the Sun.



spiral galaxy
A galaxy with the shape of a flat circular disk in which the stars are distributed in a spiral pattern.




spin
The amount of rotation that a black hole possesses.




star
An object made entirely of gas that shines brightly as a result of nuclear fusion in its interior.




static limit
The outer boundary of the region around a spinning black hole that is called the ergosphere.




stellar-mass black hole
A black hole that formed when a massive star died in a supernova explosion and is somewhat more massive than our Sun.




stellar remnant
The compact object that remains after a star has died. Either a white dwarf, neutron star, or black hole.




Sun
The closest star to Earth, and the center of our Solar System.




supermassive black hole
A black hole that formed in the center of a galaxy and is a million to a billion times more massive than our Sun.




supernova explosion
The explosion of a star of intermediate or high mass that has exhausted all its nuclear fuel at the end of its life.




supernova remnant
The glowing gaseous remains of a star that has exploded as a supernova.




telescope
Instrument to focus, collect and detect the light (or electromagnetic radiation in general) from astronomical objects.




visible light
The type of electromagnetic radiation that we can detect with our human eyes.




wavelength
The distance between adjacent wave crests in a wave phenomenon such as electromagnetic radiation. The different wavelengths of visible light are perceived as colors by the human eye.




white dwarf
An object in which gravity is balanced by the pressure exerted by densely packed electrons. The object is about the size of the Earth and about as massive as the Sun. It is produced when a low-mass star dies after it runs out of nuclear fuel.




wormhole
A mysterious solution of the equations of general relativity that resembles a black hole but has no event horizon. May or may not exist in our Universe.




X-rays
Electromagnetic radiation with much smaller wavelength and much higher energy than visible light. We encounter X-rays in our daily lives for example in medical imaging devices and security screening machines.




X-ray binary
A binary star system in which a normal star is in orbit around a stellar remnant. The remnant accretes material from the normal star and produces X-rays in the process.



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