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Imagine the Universe! Dictionary

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[A B C D E F G H I J K L M N O P Q R S T U V W X Y Z ]

(Note - Greek letters are written out by name - alpha, beta etc.)


The process in which light or other electromagnetic radiation gives up its energy to an atom or molecule.

absorption line spectrum
A spectrum showing dark lines at some narrow color regions (wavelengths). The lines are formed by atoms absorbing light, which lifts their electrons to higher orbits.

Accumulation of dust and gas onto larger bodies such as stars, planets and moons.

accretion disk
A relatively flat sheet of gas and dust surrounding a newborn star, a black hole, or any massive object growing in size by attracting material.

active galactic nuclei (AGN)
A class of galaxies which spew massive amounts of energy from their centers, far more than ordinary galaxies. Many astronomers believe supermassive black holes may lie at the center of these galaxies and power their explosive energy output.

A unit of length equal to 0.00000001 centimeters. This may also be written as 1 x 10-8 cm (see scientific notation).

angular momentum
A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit. According to the conservation laws of physics, the angular momentum of any orbiting body must remain constant at all points in the orbit, i.e., it cannot be created or destroyed. If the orbit is elliptical the radius will vary. Since the mass is constant, the velocity changes. Thus planets in elliptical orbits travel faster at perihelion and more slowly at aphelion. A spinning body also possesses spin angular momentum.

The point of greatest separation between two stars which are in orbit around each other. See binary stars. Opposite of periastron.

The point in its orbit where a planet is farthest from the Sun. Opposite of perihelion.

The point in an orbit when the two objects are farthest apart. Special names are given to this orbital point for commonly used systems: see apastron, aphelion, and apogee.

The point in its orbit where an Earth satellite is farthest from the Earth. Opposite of perigee.

arc minute
An angular measurement equal to 1/60th of a degree.

arc second
An angular measurement equal to 1/60th of an arc minute or 1/3600th of a degree.

Ariel V
A UK X-ray mission, also known as UK-5

    * Tell me more about Ariel V. (

The Japanese Asuka spacecraft (formerly Astro-D), an X-ray mission
Astrophysics Science Division, located at NASA's Goddard Space Flight Center. The scientists, programmers and technicians working here study the astrophysics of objects which emit cosmic ray, x-ray and gamma-ray radiation.

All Sky Monitor. An instrument designed to observe large areas of the sky for interesting astronomical phenomena. An ASM measures the intensity of many sources across the sky and looks for new sources. Many high-energy satellites have carried ASM detectors, including the ASM on Vela 5B, Ariel V, and the Rossi X-ray Timing Explorer.

A X-ray/gamma-ray mission built jointly by the United States and Japan. Astro E was destroyed in February 2000, when a Japanese M-5 rocket failed to lift the instrument into orbit. A replacement mission, Astro-E2, was succesfully launched in July 2005, and subsequently renamed Suzaku.

    * Tell me more about Astro-E. (

    * Tell me more about Astro-E2/Suzaku. (

astronomical unit (AU)
149,597,870 km; the average distance from the Earth to the Sun.

The scientific study of matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena.

The part of astronomy that deals principally with the physics of the universe, including luminosity, density, temperature, and the chemical composition of stars, galaxies, and the interstellar medium.

The gas that surrounds a planet or star. The Earth's atmosphere is made up of mostly nitrogen, while the Sun's atmosphere consists of mostly hydrogen.

The Advanced X-ray Astrophysics Facility. AXAF was renamed Chandra X-ray Observatory, CXO, and launched in July 1999.

    * Tell me more about AXAF. (


Balmer lines (J. Balmer)
Emission or absorption lines in the spectrum of hydrogen that arise from transitions between the second (or first excited) state and higher energy states of the hydrogen atom. They were discovered by Swiss physicist J. J. Balmer.

Any of the subatomic particles which interact via the strong nuclear force. Most commonly, these are protons and neutrons. Their presence in the universe is determined through their gravitational and electromagnetic interactions.

BATSE (Burst and Transient Source Experiment) was an instrument aboard the Compton Gamma Ray Observatory that detected and located gamma-ray bursts in the sky.

The Broad Band X-Ray Telescope, which was flown on the Astro-1 space shuttle flight (Dec. 1990)

    * Tell me more about BBXRT. (

Be star
A spectral type "B" star that shows emission lines in its spectrum. Be stars are also rapidly rotating and losing mass. The emission lines result from ultraviolet light from the star being reprocessed in the ejected material.

A major satellite program of the Italian Space Agency with partcipation from the Netherlands Agency for Aerospace Programs. BeppoSAX operated from 1996 to 2002, and covered more than three decades of energy (from 0.1 to 300 keV) with relatively large effective area, medium energy resolution and imaging capabilities from 0.1 - 10 keV. Among BeppoSAX's claims to fame is its first detection of an afterglow from a gamma-ray burst in 1997. BeppoSAX was named after Italian physicist Giuseppe Occhialini, whose nickname was Beppo.

Big Bang
A widely accepted model of the Universe that assumes that the observed expansion of the Universe originated about 13.7 billion years ago, when the Universe was very hot and very dense. It successfully explains the cosmic microwave background and the ratio of hydrogen, helium, and other light elements, as well as the expansion of the Universe.

binary stars
Binary stars are two stars that orbit around a common center of mass. An X-ray binary is a special case where one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole, and the separation between the stars is small enough so that matter is transferred from the normal star to the compact star star, producing X-rays in the process.

black dwarf
A non-radiating ball of gas resulting from a white dwarf that has radiated all its energy.

black hole
An object whose gravity is so strong that not even light can escape from it.

black-hole dynamic laws; laws of black-hole dynamics
  1. First law of black hole dynamics:
    For interactions between black holes and normal matter, the conservation laws of mass-energy, electric charge, linear momentum, and angular momentum, hold. This is analogous to the first law of thermodynamics.
  2. Second law of black hole dynamics:
    With black-hole interactions, or interactions between black holes and normal matter, the sum of the surface areas of all black holes involved can never decrease. This is analogous to the second law of thermodynamics, with the surface areas of the black holes being a measure of the entropy of the system.

blackbody radiation
Blackbody radiation is produced by an object which is a perfect absorber of heat. Perfect absorbers must also be perfect radiators. For a blackbody at a temperature T, the intensity of radiation emitted I at a particular energy E is given by Plank's law:

I(E,T) = 2 E3[h2c2(eE/kT - 1)]-1

where h is Planck's constant, k is Boltzmann's constant, and c is the the speed of light.

blackbody temperature
The temperature of an object if it is re-radiating all the thermal energy that has been added to it; if an object is not a blackbody radiator, it will not re-radiate all the excess heat and the leftover will go toward increasing its temperature.

An apparent shift toward shorter wavelengths of spectral lines in the radiation emitted by an object caused by motion between the object and the observer which decreases the distance between them. See also Doppler effect.

bolometric luminosity
The total energy radiated by an object at all wavelengths, usually given in joules per second (identical to watts).

Boltzmann constant; k (L. Boltzmann)
A constant which describes the relationship between temperature and kinetic energy for molecules in an ideal gas. It is equal to 1.380622 x 10-23 J/K (see scientific notation).

Brahe, Tycho (1546 - 1601)
(a.k.a Tyge Ottesen) Danish astronomer whose accurate astronomical observations of Mars in the last quarter of the 16th century formed the basis for Johannes Kepler's laws of planetary motion. Brahe lost his nose in a duel in 1566 with Manderup Parsberg (a fellow student and nobleman) at Rostock over who was the better mathematician. He died in 1601, not of a burst bladder as legend suggests, but from high levels of mercury in his blood (which he may have taken as medication after falling ill from the infamous meal). Show me a picture of Tycho Brahe !

"Braking radiation", the main way very fast charged particles lose energy when traveling through matter. Radiation is emitted when charged particles are accelerated. In this case, the acceleration is caused by the electromagnetic fields of the atomic nuclei of the medium.


A process for translating the signals produced by a measuring instrument (such as a telescope) into something that is scientifically useful. This procedure removes most of the errors caused by environmental and instrumental instabilities.

An instrument that measures the energy of a particle or photon through the amount of heat the particle or photon deposits in a material.

cataclysmic variable (CV)
Binary star systems with one white dwarf star and one normal star, in close orbit about each other. Material from the normal star falls onto the white dwarf, creating a burst of X-rays.

Cepheid Variable
A type of variable star which exhibits a regular pattern of changing brightness as a function of time. The period of the pulsation pattern is directly related to the star's intrinsic brightness. Thus, Cepheid variables are a powerful tool for determining distances in modern astronomy.

The Compton Gamma Ray Observatory
    * Tell me more about CGRO. (

Chandra X-ray Observatory (CXO)
One of NASA's Great Observatories in Earth orbit, launched in July 1999, and named after S. Chandrasekhar. It was previously named the Advanced X-ray Astrophysics Facility (AXAF).

Chandrasekhar, S. (1910 - 1995)
Indian astrophysicist reknowned for creating theoretical models of white dwarf stars, among other achievements. His equations explained the underlying physics behind the creation of white dwarfs, neutron stars and other compact objects.

Chandrasekhar limit
A limit which mandates that no white dwarf (a collapsed, degenerate star) can be more massive than about 1.4 solar masses. Any degenerate object more massive must inevitably collapse into a neutron star.

cluster of galaxies
A system of galaxies containing from a few to a few thousand member galaxies which are all gravitationally bound to each other.

collecting area
The amount of area a telescope has that is capable of collecting electromagnetic radiation. Collecting area is important for a telescope's sensitivity: the more radiation it can collect (that is, the larger its collecting area), the more likely it is to detect dim objects.

Compton effect (A.H. Compton; 1923)
An effect that demonstrates that photons (the quantum of electromagnetic radiation) have momentum. A photon fired at a stationary particle, such as an electron, will impart momentum to the electron and, since its energy has been decreased, will experience a corresponding decrease in frequency.

NASA ultraviolet/X-ray mission, also known as OAO-3
    * Tellme more about the Copernicus mission. (

Copernicus, Nicolaus (1473 - 1543)
Polish astronomer who advanced the theory that the Earth and other planets revolve around the Sun (the "heliocentric" theory). This was highly controversial at the time, since the prevailing Ptolemaic model held that the Earth was the center of the universe, and all objects, including the sun, circle it. The Ptolemaic model had been widely accepted in Europe for 1000 years when Copernicus proposed his model. (It should be noted, however, that the heliocentric idea was first put forth by Aristarcus of Samos in the 3rd century B.C., a fact known to Copernicus but long ignored by others prior to him.). Show me a picture of Nicholas Copernicus !

corona (plural: coronae)
The uppermost level of a star's atmosphere. In the sun, the corona is characterized by low densities and high temperatures (> 1,000,000 degrees K).

A satellite launched in August 1975 to study extraterrestrial sources of gamma-ray emission.
    * Tell me more about COS-B. (

cosmic background radiation; primal glow
The background of radiation mostly in the frequency range 3 x 108 to 3 x 1011 Hz (see scientific notation) discovered in space in 1965. It is believed to be the cosmologically redshifted radiation released by the Big Bang itself.

cosmic rays
Atomic nuclei (mostly protons) and electrons that are observed to strike the Earth's atmosphere with exceedingly high energies.

cosmological constant; Lambda
A constant term (labeled Lambda) which Einstein added to his general theory of relativity in the mistaken belief that the Universe was neither expanding nor contracting. The cosmological constant was found to be unnecessary once observations indicated the Universe was expanding. Had Einstein believed what his equations were telling him, he could have claimed the expansion of the Universe as perhaps the greatest and most convincing prediction of general relativity; he called this the "greatest blunder of my life".

cosmological distance
A distance far beyond the boundaries of our Galaxy. When viewing objects at cosmological distances, the curved nature of spacetime could become apparent. Possible cosmological effects include time dilation and redshift.

cosmological redshift
An effect where light emitted from a distant source appears redshifted because of the expansion of spacetime itself. Compare Doppler effect.

The astrophysical study of the history, structure, and dynamics of the universe.

The Chandra X-ray Observatory. CXO was launched by the Space Shuttle in July 1999, and named for S. Chandrasekhar.

    * Tell me more about CXO. (


dark matter
Name given to the amount of mass whose existence is deduced from the analysis of galaxy rotation curves but which until now, has escaped all detections. There are many theories on what dark matter could be. Not one, at the moment is convincing enough and the question is still a mystery.

de Broglie wavelength (L. de Broglie; 1924)
The quantum mechanical "wavelength" associated with a particle, named after the scientist who discovered it. In quantum mechanics, all particles also have wave characteristics, where the wavelength of a particle is inversely proportional to its momentum and the constant of proportionality is the Planck constant.

A coordinate which, along with Right Ascension, may be used to locate any position in the sky. Declination is analogous to latitude for locating positions on the Earth, and ranges from +90 degrees to -90 degrees.

An image processing technique that removes features in an image that are caused by the telescope itself rather than from actual light coming from the sky. For example, the optical analog would be to remove the spikes and halos which often appear on images of bright stars because of light scattered by the telescope's internal supports.

The ratio between the mass of an object and its volume. In the metric system, density is measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0 gm/cm3; iron is 7.9 gm/cm3; lead is 11.3 gm/cm3.

A container (akin to a thermos bottle) that keeps cold material cold. In astronomy, these are often used for liquid nitrogen (at 77K), but can also be used for solid neon (17K) or liquid helium (4.2K). Some astronomical detectors work better at cold temperatures.

(a) A flattened, circular region of gas, dust, and/or stars. It may refer to material surrounding a newly-formed star; material accreting onto a black hole or neutron star; or the large region of a spiral galaxy containing the spiral arms. (b) The apparent circular shape of the Sun, a planet, or the moon when seen in the sky or through a telescope.

Doppler effect (C.J. Doppler)
The apparent change in wavelength of sound or light caused by the motion of the source, observer or both. Waves emitted by a moving object as received by an observer will be blueshifted (compressed) if approaching, redshifted (elongated) if receding. It occurs both in sound and light. How much the frequency changes depends on how fast the object is moving toward or away from the receiver. Compare cosmological redshift.

Not the dust one finds around the house (which is typically fine bits of fabric, dirt, and dead skin cells). Rather, irregularly shaped grains of carbon and/or silicates measuring a fraction of a micron across which are found between the stars. Dust is most evident by its absorption, causing large dark patches in regions of our Milky Way Galaxy and dark bands across other galaxies.

dust tail
A stream of dust particles emitted from the nucleus of a comet. It is the most visible part of a comet.


Non-circular; elliptical (applied to an orbit).

A value that defines the shape of an ellipse or planetary orbit. The eccentricity of an ellipse (planetary orbit) is the ratio of the distance between the foci and the major axis. Equivalently the eccentricity is (ra-rp)/(ra+rp) where ra is the apoapsis distance and rp is the periapsis distance.

The passage of one celestial body in front of another, cutting off the light from the second body (e.g. an eclipse of the sun by the moon, or one star in a binary system eclipsing the other). It may also be the passage of all or part of one body through the shadow of another (e.g. a lunar eclipse in which the moon passes through the Earth's shadow).

The plane of Earth's orbit about the Sun.

Eddington limit (Sir A. Eddington)
The theoretical limit at which the photon pressure would exceed the gravitational attraction of a light-emitting body. That is, a body emitting radiation at greater than the Eddington limit would break up from its own photon pressure.

Einstein, Albert (1879 - 1955)
German-American physicist; developed the Special and General Theories of Relativity which along with Quantum Mechanics is the foundation of modern physics. Show me a picture of Albert Einstein !

Einstein Observatory,
The first fully imaging x-ray telescope in space, launched by NASA in 1978. Originally named "HEAO-2" (High Energy Astrophysics Observatory 2), it was renamed for Albert Einstein upon launch. Also see HEAO.

    * Tell memore about HEAO-2 (Einstein Observatory). (
Material that is ejected. Used mostly to describe the content of a massive star that is propelled outward in a supernova explosion. Also used to describe the material that is blown radially outward in a meteor impact on the surface of a planet or moon.

electromagnetic spectrum
The full range of frequencies, from radio waves to gamma rays, that characterizes light.

electromagnetic waves (radiation)
Another term for light. Light waves are fluctuations of electric and magnetic fields in space.

A negatively charged particle commonly found in the outer layers of atoms. The electron has only 0.0005 the mass of the proton.

electron volt
The change of potential energy experienced by an electron moving from a place where the potential has a value of V to a place where it has a value of (V+1 volt). This is a convenient energy unit when dealing with the motions of electrons and ions in electric fields; the unit is also the one used to describe the energy of X-rays and gamma rays. A keV (or kiloelectron volt) is equal to 1000 electron volts. An MeV is equal to one million electron volts. A GeV is equal to one billion (109) electron volts. A TeV is equal to a million million (1012) electron volts.

The fundamental kinds of atoms that make up the building blocks of matter, which are each shown on the periodic table of the elements. The most abundant elements in the universe are hydrogen and helium. These two elements make up about 80% and 20% of all the matter in the universe respectively. Despite comprising only a very small fraction the universe, the remaining heavy elements can greatly influence astronomical phenomena. About 2% of the Milky Way's disk is comprised of heavy elements.

Oval. That the orbits of the planets are ellipses, not circles, was first discovered by Johannes Johannes Kepler the careful observations by Tycho Brahe

The production of light, or more generally, electromagnetic radiation by an atom or other object.

emission line spectrum
A spectrum consisting of bright lines at certain wavelengths separated by dark regions in which there is no light.

A form of the metric unit for power. It is equal to 10-10 kilowatts (see scientific notation).

Exploration of the Universe Division, located at NASA's Goddard Space Flight Center. The scientists, programmers and technicians working here study the astrophysics of objects which emit cosmic ray, x-ray and gamma-ray radiation.

event horizon
The distance from a black hole within which nothing can escape. In addition, nothing can prevent a particle from hitting the singularity in a very short amount of proper time once it has entered the horizon. In this sense, the event horizon is a "point of no return". See Schwarzschild radius.

evolved star
A star near the end of its lifetime when most of its fuel has been used up. This period of the star's life is characterized by loss of mass from its surface in the form of a stellar wind.

European Space Agency's X-ray Observatory

    * Tell me more about EXOSAT. (

Outside of, or beyond, our own galaxy.


Fast Fourier Transformation (FFT)
A Fourier Transform is the mathematical operation that takes measurements made with a radio interferometer and transforms them into an image of the radio sky. The Fast Fourier Transform is technique used by computer programs that allows the Fourier Transform to be computed very quickly.

Fermi acceleration
In order to explain the origins of cosmic rays, Enrico Fermi (1949) introduced a mechanism of particle acceleration, whereby charged particles bounce off moving interstellar magnetic fields and either gain or lose energy, depending on whether the "magnetic mirror" is approaching or receding. In a typical environment, he argued, the probability of a head-on collision is greater than a head-tail collision, so particles would be accelerated on average. This random process is now called 2nd order Fermi acceleration, because the mean energy gain per "bounce" is dependent on the "mirror" velocity squared.
Bell (1978) and Blandford and Ostriker (1978) independently showed that Fermi acceleration by supernova remnant (SNR) shocks is particularly efficient, because the motions are not random. A charged particle ahead of the shock front can pass through the shock and then be scattered by magnetic inhomogeneities behind the shock. The particle gains energy from this "bounce" and flies back across the shock, where it can be scattered by magnetic inhomogeneities ahead of the shock. This enables the particle to bounce back and forth again and again, gaining energy each time. This process is now called 1st order Fermi acceleration, because the mean energy gain is dependent on the shock velocity only to the first power.

Fermi Gamma-ray Telescope
An international mission launched on June 11, 2008, the Fermi Gamma-ray Telescope studies the universe in the energy range 10 keV - 300 Gev.

A measure of the amount of energy given off by an astronomical object over a fixed amount of time and area. Because the energy is measured per time and area, flux measurements make it easy for astronomers to compare the relative energy output of objects with very different sizes or ages.

A property of a wave that describes how many wave patterns or cycles pass by in a period of time. Frequency is often measured in Hertz (Hz), where a wave with a frequency of 1 Hz will pass by at 1 cycle per second.

A suite of software tools developed at NASA's Goddard Space Flight Center for analyzing high-energy astronomy data.

File Transfer Protocol -- A widely available method for transferring files over the Internet.

The process in which atomic nuclei collide so fast that they stick together and emit a large amount of energy. In the center of most stars, hydrogen fuses into helium. The energy emitted by fusion supports the star's enormous mass from collapsing in on itself, and causes the star to glow.


galactic halo
A spherical region surrounding the center of a galaxy. This region may extend beyond the luminous boundaries of the galaxy and contain a significant fraction of the galaxy's mass. Compared to cosmological distances, objects in the halo of our galaxy would be very nearby.

A component of our universe made up of gas and a large number (usually more than a million) of stars held together by gravity. When capitalized, Galaxy refers to our own Milky Way Galaxy.

Galilei, Galileo (1564 - 1642)
An Italian scientist, Galileo was renowned for his epoch making contribution to physics, astronomy, and scientific philosophy. He is regarded as the chief founder of modern science. He developed the telescope, with which he found craters on the Moon and discovered the largest moons of Jupiter. Galileo was condemned by the Catholic Church for his view of the cosmos based on the theory of Copernicus. Show me a picture of Galileo !

gamma ray
The highest energy, shortest wavelength electromagnetic radiations. Usually, they are thought of as any photons having energies greater than about 100 keV. (It's "gamma-ray" when used as an adjective.)

Gamma-Ray Burst (GRB)
Plural is GRBs. A burst of gamma rays from space lasting from a fraction of a second to many minutes. There is no clear scientific consensus as to their cause. Recently, their distances were determined to be large, placing the origins of the bursts in other galaxies.

Gamma-ray Large Area Space Telescope (GLAST)
An international mission launched on June 11, 2008, GLAST studies the universe in the energy range 10 keV - 300 Gev. In August 2008, NASA renamed the mission the Fermi Gamma-ray Space Telescope. Gamma Ray Imaging Platform (GRIP)
A balloon-borne gamma-ray telescope made by a group at the California Institute of Technology. It has had many successful flights.

Gamma Ray Imaging Spectrometer (GRIS)
A balloon-borne instrument which uses germanium detectors for high resolution gamma-ray spectroscopy.

    * Tell me more about GRIS. (

One of the three states of matter, in which atoms, molecules, or ions move freely and are not bound to each other. In astronomy, it usually refers to hydrogen or helium.

general relativity
The geometric theory of gravitation developed by Albert Einstein, incorporating and extending the theory of special relativity to accelerated frames of reference and introducing the principle that gravitational and inertial forces are equivalent. The theory has consequences for the bending of light by massive objects, the nature of black holes, and the fabric of space and time.

Giant Molecular Cloud (GMC)
Massive clouds of gas in interstellar space composed primarily of hydrogen molecules (two hydrogen atoms bound together), though also containing other molecules observable by radio telescopes. These clouds can contain enough mass to make several million stars like our Sun and are often the sites of star formation.

The third Japanese X-ray mission, also known as Astro-C.

    * Tell me more about Ginga. (

globular cluster
A spherically symmetric collection of stars which shared a common origin. The cluster may contain up to millions of stars spanning up to 50 parsecs.

gravitational collapse
When a massive body collapses under its own weight. (For example, interstellar clouds collapse to become stars until the onset of nuclear fusion stops the collapse.)

gravitational radius
See event horizon.

gravitational waves
Ripples in space-time caused by the motion of objects in the universe. The most notable sources are orbiting neutron stars, merging black holes, and collapsing stars. Gravitational waves are also thought to emanate from the Big Bang.

gravitationally bound
Objects held in orbit about each other by their gravitational attraction. For example, satellites in orbit around the earth are gravitationally bound to Earth since they can't escape Earth's gravity. By contrast, the Voyager spacecraft, which explored the outer solar system, was launched with enough energy to escape Earth's gravity altogether, and hence it is not gravitationally bound.

A mutual physical force attracting two bodies.

Gravity and Extreme Magnetism SMEX (GEMS)
A NASA mission which will utilize the polarization properties of X-rays to characterize the geometry and behavior of X-ray sources. Proposed research includes exploring the shape of space that has been distorted by a spinning black hole's gravity, and characterizing the magnetic fields around pulsars and magnetars. GEMS is scheduled for launch in 2014.

Goddard Space Flight Center, one of the centers operated by NASA.

guest star
The ancient Chinese term for a star that newly appears in the night sky, and then later disappears. Later, the Europeans called this a nova.


hard x-ray
High energy x-rays, often from about 10 keV to nearly 1000 keV. The dividing line between hard and soft x-rays is not well defined and can depend on the context.

Hawking radiation (S.W. Hawking; 1973)
A theory first proposed by British physicist Stephen Hawking, that due to a combination of properties of quantum mechanics and gravity, under certain conditions black holes can seem to emit radiation.

Hawking temperature
The temperature inferred for a black hole based on the Hawking radiation detected from it.

The High Energy Astrophysical Observatory satellite series

    * Tell me more about HEAO-1. (
    * Tell me more about HEAO-2 (Einstein Observatory). (
    * Tell me more about HEAO-3. (

High Energy Astrophysics Science Archive Research Center, located at NASA's Goddard Space Flight Center. The HEASARC creates and maintains archives of data from ultraviolet, x-ray and gamma-ray satellites for use by astronomers around the world.

The second lightest and second most abundant element. The typical helium atom consists of a nucleus of two protons and two neutrons surrounded by two electrons. Helium was first discovered in our Sun. Roughly 25% of the mass of our Sun is helium.

Herschel, Sir William (1738 - 1822)
Sir William Herschel was a renowned astronomer who first detected the infrared region of the electromagnetic spectrum in 1800.

Hertz, Heinrich (1857 - 1894)
A German physics professor who did the first experiments with generating and receiving electromagnetic waves, in particular radio waves. In his honor, the units associated with measuring the cycles per second of the waves (or the number of times the tip-tops of the waves pass a fixed point in space in 1 second of time) is called the hertz.

hertz; Hz (after H. Hertz, 1857 - 1894)
The derived SI unit of frequency, defined as a frequency of 1 cycle per second.

Hubble Space Telescope

Hubble, Edwin P. (1889 - 1953)
American astronomer whose observations proved that galaxies are "island universes", not nebulae inside our own galaxy. His greatest discovery, called "Hubble's Law", was the linear relationship between a galaxy's distance and the speed with which it is moving. The Hubble Space Telescope is named in his honor. Show me a picture of Edwin Hubble! (Image Credit: The Huntington Library, San Marino, California)

Hubble constant; Ho (E.P. Hubble; 1925)
The constant which determines the relationship between the distance to a galaxy and its velocity of recession due to the expansion of the Universe. After many years in which the Hubble constant was only known to be somewhere between 50 and 100 km/s/Mpc, it has been determined to be 70 km/s/Mpc ± 7 km/s/Mpc by the Hubble Space Telescope's Key Project team. (Advances in cosmology have shown that since the Universe is self gravitating, Ho is not truly constant. Astronomers thus seek its present value.)

Hubble's law (E.P. Hubble; 1925)
A relationship between a galaxy's distance from us and its velocity through space. The farther away a galaxy is from us, the faster it is receding from us. The constant of proportionality is the Hubble constant, Ho, named after Edwin P. Hubble who discovered the relationship. Hubble's Law is interpreted as evidence that the Universe is expanding.

Huygens, Christiaan (1629 - 1695)
A Dutch physicist who was the leading proponent of the wave theory of light. He also made important contributions to mechanics, stating that in a collision between bodies, neither loses nor gains ``motion'' (his term for momentum). In astronomy, he discovered Titan (Saturn's largest moon) and was the first to correctly identify the observed elongation of Saturn as the presence of Saturn's rings. Show me a picture of Christian Huygens !

The lightest and most abundant element. A hydrogen atom consists of one proton and one electron. Hydrogen composes about 75% of the mass of the Sun, but only a tiny fraction of the Earth.


The Space Research Institute in Russia. It is the equivalent of NASA in the U.S.

In astronomy, a picture of the sky.

A violent inward collapse. An inward explosion.

Electromagnetic radiation at wavelengths longer than the red end of visible light and shorter than microwaves (roughly between 1 and 100 microns). Almost none of the infrared portion of the electromagnetic spectrum can reach the surface of the Earth, although some portions can be observed by high-altitude aircraft (such as the Kuiper Observatory) or telescopes on high mountaintops (such as the peak of Mauna Kea in Hawaii).

The inclination of a planet's orbit is the angle between the plane of its orbit and the ecliptic; the inclination of a moon's orbit is the angle between the plane of its orbit and the plane of its primary's equator.

The INTErnational Gamma-Ray Astrophysics Laboratory, a project of the European Space Agency. It was the first space observatory to have the capability to observe simultaneously in the gamma-ray, X-ray and visual regions of the em spectrum. Targets include gamma-ray bursts, supernovae and black holes.

International X-ray Observatory (IXO)
A joint mission by NASA, the European Space Agency (ESA), and Japenese Aeropsace Exploration Agency (JAXA), which will combine a large X-ray mirror with new instrumentation. IXO will study the formation of structure in the universe, matter under extreme conditions in black holes and neutron stars, and the life cycles of matter and energy in the universe. Launch is planned for the 2020's.

interstellar medium
The gas and dust between stars, which fills the plane of the Galaxy much like air fills the world we live in. For centuries, scientists believed that the space between the stars was empty. It wasn't until the eighteenth century, when William Herschel observed nebulous patches of sky through his telescope, that serious consideration was given to the notion that interstellar space was something to study. It was only in the last century that observations of interstellar material suggested that it was not even uniformly distributed through space, but that it had a unique structure.

An atom with one or more electrons stripped off, giving it a net positive charge.

ionic (or ionized) gas
Gas whose atoms have lost or gained electrons, causing them to be electrically charged. In astronomy, this term is most often used to describe the gas around hot stars where the high temperature causes atoms to lose electrons.

International Ultraviolet Explorer, an ultraviolet space observatory launch in 1978. Originally designed for a 3 year mission, IUE exceeded all expectations and functioned for over 18 years, finally ceasing operation in September 1996.

    * Tell me more about IUE. (


Beams of particles, usually coming from an active galactic nucleus or a pulsar. Unlike a jet airplane, when the stream of gas is in one direction, astrophysical jets come in pairs with each jet aiming in opposite directions.


kelvin (after Lord Kelvin, 1824 - 1907)
The fundamental SI unit of thermodynamic temperature defined as 1/273.16 of the thermodynamic temperature of the triple point of water. More practically speaking, the Kelvin temperature scale measures an object's temperature above absolute zero, the theoretical coldest possible temperature. On the Kelvin scale the freezing point of water is 273 ( = 0o C = 32o F) [ K = 273 + C = 273 + 5/9 * (F-32)]. The Kelvin temperature scale is often used in sciences such as astronomy.

Kepler, Johannes (1571 - 1630)
German astronomer and mathematician. Considered a founder of modern astronomy, he formulated the famous three laws of planetary motion. They comprise a quantitative formulation of Copernicus's theory that the planets revolve around the Sun.

Kepler's laws (J. Kepler)
Kepler's first law
A planet orbits the Sun in an ellipse with the Sun at one focus.

Kepler's second law
A line directed from the Sun to a planet sweeps out equal areas in equal times as the planet orbits the Sun.

Kepler's third law
The square of the period of a planet's orbit is proportional to the cube of that planet's semimajor axis; the constant of proportionality is the same for all planets.

kilogram (kg)
The fundamental SI unit of mass. The kilogram is the only SI unit still maintained by a physical artifact (a platinum-iridium bar) kept in the International Bureau of Weights and Measures at Sevres, France. One kilogram is equivalent to 1,000 grams or about 2.2 pounds; the mass of a liter of water.

Refers to the calculation or description of the underlying mechanics of motion of an astronomical object. For example, in radioastronomy, spectral line graphs are used to determine the kinematics or relative motions of material at the center of a galaxy or surrounding a star as it is born.

Kirchhoff's law of radiation (G.R. Kirchhoff)
The emissivity of a body is equal to its absorbance at the same temperature.

Kirchhoff's laws (G.R. Kirchhoff)
Kirchhoff's first law
An incandescent solid or gas under high pressure will produce a continuous spectrum.

Kirchhoff's second law
A low-density gas will radiate an emission-line spectrum with an underlying emission continuum.

Kirchhoff's third law
Continuous radiation viewed through a low-density gas will produce an absorption-line spectrum.


A representation of the luminosity of an object in terms of Solar luminosity. The average luminosity of the Sun is about 4x1033 erg/sec. Astronomers often express units for other objects in terms of solar units, which makes the resulting numbers smaller and easier to deal with.

Lagrange, Joseph (1736 - 1813)
A French mathematician of the eighteenth century. His work Mecanique Analytique (Analytical Mechanics; 1788) was a mathematical masterpiece. It contained clear, symmetrical notation and covered almost every area of pure mathematics. Lagrange developed the calculus of variations, established the theory of differential equations, and provided many new solutions and theorems in number theory. His classic Theorie des fonctions analytiques laid some of the foundations of group theory. Lagrange also invented the method of solving differential equations known as variation of parameters. Show me a picture of Joseph Lagrange !

Lagrange points
Points in the vicinity of two massive bodies (such as the Earth and the Moon) where each others' respective gravities balance. There are five, labeled L1 through L5. L1, L2, and L3 lie along the centerline between the centers of mass between the two masses; L1 is on the inward side of the secondary, L2 is on the outward side of the secondary; and L3 is on the outward side of the primary. L4 and L5, the so-called Trojan points, lie along the orbit of the secondary around the primary, sixty degrees ahead and behind of the secondary.

L1 through L3 are points of unstable equilibrium; any disturbance will move a test particle there out of the Lagrange point. L4 and L5 are points of stable equilibrium, provided that the mass of the secondary is less than about 1/24.96 the mass of the primary. These points are stable because centrifugal pseudo-forces work against gravity to cancel it out.

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. It's a device that produces a coherent beam of optical radiation by stimulating electronic, ionic, or molecular transitions to higher levels so that when they return to lower energy levels they emit energy.

Laboratory for High Energy Astrophysics, located at NASA's Goddard Space Flight Center. The scientists, programmers and technicians working here study the astrophysics of objects which emit cosmic ray, x-ray and gamma-ray radiation.

The common term for electromagnetic radiation, usually referring to that portion visible to the human eye. However, other bands of the e-m spectrum are also often referred to as different forms of light.

light curve
A graph showing how the radiation from an object varies over time.

light year
A unit of length used in astronomy which equals the distance light travels in a year. At the rate of 300,000 kilometers per second (671 million miles per hour), 1 light-year is equivalent to 9.46053 x 1012 km, 5,880,000,000,000 miles or 63,240 AU (see scientific notation).

The outer edge of the apparent disk of a celestial body.

LISA (Laser Interferometer Space Antenna)
A NASA mission which will detect gravitational waves. LISA will consist of three satellites that use laser interferometry to monitor their positions relative to each other. Gravitational waves passing by the satellites cause small changes in the distances between the satellites.

The rate at which a star or other object emits energy, usually in the form of electromagnetic radiation.


A representation of the mass of an object in terms of Solar mass. The average mass of the Sun is about 2x1033 grams. Astronomers often express units for other objects in terms of solar units, since it makes the resulting numbers smaller and easier to deal with.

magnetic field
A description of the strength of the magnetic force exerted by an object. Bar magnets have "di-polar" fields, as the force is exerted from the two ends of the bar. In simple terms, the earth, the sun, stars, pulsars all have dipolar magnetic fields.

magnetic pole
Either of two limited regions in a magnet at which the magnet's field is most intense. The two regions have opposing polarities, which we label "north" and "south", after the two poles on the Earth.

The region of space in which the magnetic field of an object (e.g., a star or planet) dominates the radiation pressure of the stellar wind to which it is exposed.

The portion of a planetary magnetosphere which is pushed in the direction of the solar wind.

The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest star visible without a telescope has magnitude 6. A decrease of one magnitude represents an increase in apparent brightness by a factor of 2.512; also called apparent magnitude.

A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.

A word used for any kind of stuff which contains mass.

A unit of energy used to describe nuclear warheads. The same amount energy as 1 million tons of TNT.

1 mega-ton = 4 x 1022 ergs = 4 x 1015 joules.

Messier, Charles (1730 - 1817)
The 18th century French astronomer who compiled a list of approximately 100 fuzzy, diffuse looking objects which appeared at fixed positions in the sky. Being a comet-hunter, Messier compiled this list of objects which he knew were not comets. His list is now well known to professional and amateur astronomers as containing the brightest and most striking nebulae, star clusters, and galaxies in the sky.

meter; m
The fundamental SI unit of length, defined as the length of the path traveled by light in vacuum during a period of 1/299 792 458 s. A unit of length equal to about 39 inches. A kilometer is equal to 1000 meters.

metric system
See SI.

Microquasars are stellar mass black holes, that display characteristics of the supermassive black holes found at the centers of some galaxies. For instance, they have radio jets - something not every black hole has.

Electromagnetic radiation which has a longer wavelength (between 1 mm and 30 cm) than visible light. Microwaves can be used to study the Universe, communicate with satellites in Earth orbit, and cook popcorn.


The National Aeronautics and Space Administration, founded in 1958 as the successor to the National Advisory Committee for Aeronautics.

nebula (pl. nebulae)
A diffuse mass of interstellar dust and gas. A reflection nebula shines by light reflected from nearby stars. An emission nebula shines by emitting light as electrons recombine with protons to form hydrogen. The electrons were made free by the ultraviolet light of a nearby star shining on a cloud of hydrogen gas. A planetary nebula results from the explosion of a solar-like type star.

A fundamental particle produced in massive numbers by the nuclear reactions in stars; they are very hard to detect because the vast majority of them pass completely through the Earth without interacting.

A particle with approximately the mass of a proton, but zero charge, commonly found in the nucleus of atoms .

neutron star
The imploded core of a massive star produced by a supernova explosion. (typical mass of 1.4 times the mass of the Sun, radius of about 5 miles, density of a neutron.) According to astronomer and author Frank Shu, "A sugar cube of neutron-star stuff on Earth would weigh as much as all of humanity!" Neutron stars can be observed as pulsars.

Newton, Isaac 1642 - 1727
English cleric and scientist; discovered the classical laws of motion and gravity; the bit with the apple is probably apocryphal. Show me a picture of Isaac Newton !

Newton's law of universal gravitation (Sir I. Newton)
Two bodies attract each other with equal and opposite forces; the magnitude of this force is proportional to the product of the two masses and is also proportional to the inverse square of the distance between the centers of mass of the two bodies.

Newton's laws of motion (Sir I. Newton)
Newton's first law of motion
A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.

Newton's second law of motion
For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.

Newton's third law of motion
In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction

The random fluctuations that are always associated with a measurement that is repeated many times over. Noise appears in astronomical images as fluctuations in the image background. These fluctuations do not represent any real sources of light in the sky, but rather are caused by the imperfections of the telescope. If the noise is too high, it may obscure the dimmest objects within the field of view.

nova (plural: novae)
A star that experiences a sudden outburst of radiant energy, temporarily increasing its luminosity by hundreds to thousands of times before fading back to its original luminosity.

nuclear fusion
A nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. The difference in mass is converted to energy by Einstein's famous equivalence "Energy = Mass times the Speed of Light squared". This is the source of the Sun's energy.


Orbiting Astronomical Observatory 3 - NASA ultraviolet/X-ray mission, also known as Copernicus.
    * Tell me more aboutOAO 3. (

The blockage of light by the intervention of another object; a planet can occult (block) the light from a distant star.

A property of matter that prevents light from passing through it. The opacity or opaqueness of something depends on the frequency of the light. For instance, the atmosphere of Venus is transparent to ultraviolet light, but is opaque to visible light.

The path of an object that is moving around a second object or point.

Orbiting Solar Observatory 3

    * Tell me more about OSO 3 (

Orbiting Solar Observatory 8
    * Tell me more about OSO 8 (


pair production
The physical process whereby a gamma-ray photon, usually through an interaction with the electromagnetic field of a nucleus, produces an electron and an anti-electron (positron). The original photon no longer exists, its energy having gone to the two resulting particles. The inverse process, pair annihilation, creates two gamma-ray photons from the mutual destruction of an electron/positron pair.
The apparent motion of a relatively close object compared to a more distant background as the location of the observer changes. Astronomically, it is half the angle which a a star appears to move as the earth moves from one side of the sun to the other.

The distance to an object which has a parallax of one arc second. It is equal to 3.26 light years, or 3.1 x 1018 cm (see scientific notation). A kiloparsec (kpc) is equal to 1000 parsecs. A megaparsec (Mpc) is equal to a million (106) parsecs.

The point in an orbit when two objects are closest together. Special names are given to this point for commonly used systems: see periastron, perihelion, and perigee. The opposite of apoapsis.

The point of closest approach of two stars, as in a binary star orbit. Opposite of apastron.

The point in its orbit where an Earth satellite is closest to the Earth. Opposite of apogee.

The point in its orbit where a planet is closest to the Sun. Opposite of aphelion.

The transfer of a photon's energy to an atom, molecule or nucleus.

photoelectric effect
An effect explained by A. Einstein which demonstrates that light seems to be made up of particles, or photons. Light can excite electrons (called photoelectrons in this context) to be ejected from a metal. Light with a frequency below a certain threshold, at any intensity, will not cause any photoelectrons to be emitted from the metal. Above that frequency, photoelectrons are emitted in proportion to the intensity of incident light.
The reason is that a photon has energy in proportion to its wavelength, and the constant of proportionality is the Planck constant. Below a certain frequency -- and thus below a certain energy -- the incident photons do not have enough energy to knock the photoelectrons out of the metal. Above that threshold energy, called the work function, photons will knock the photoelectrons out of the metal, in proportion to the number of photons (the intensity of the light). At higher frequencies and energies, the photoelectrons ejected obtain a kinetic energy corresponding to the difference between the photon's energy and the work function.

The smallest (quantum) unit of light/electromagnetic energy. Photons are generally regarded as particles with zero mass and no electric charge.

The constant equal to the ratio of the circumference of a circle to its diameter, which is approximately 3.141593.

Planck constant; h
The fundamental constant equal to the ratio of the energy of a quantum of energy to its frequency. It is the quantum of action. It has the value 6.626196 x 10-34 J s (see scientific notation).

Planck equation
The quantum mechanical equation relating the energy of a photon E to its frequency nu:

E = h x nu

planetary nebula
A shell of gas ejected from stars like our Sun at the end of their lifetime. This gas continues to expand out from the remaining white dwarf.

A low-density gas in which the individual atoms are ionized (and therefore charged), even though the total number of positive and negative charges is equal, maintaining an overall electrical neutrality.

The direction in the sky to which the telescope is pointed. Pointing also describes how accurately a telescope can be pointed toward a particular direction in the sky.

A special property of light; light has three properties, brightness, color and polarization. Polarization is a condition in which the planes of vibration of the various rays in a light beam are at least partially aligned.

The antiparticle to the electron. The positron has most of the same characteristics as an electron except it is positively charged.

A particle with a positive charge commonly found in the nucleus of atoms.

Very dense regions (or cores) of molecular clouds where stars are in the process of forming.

Ptolemy (ca. 100-ca. 170)
A.k.a. Claudius Ptolemaeus. Ptolemy believed the planets and Sun to orbit the Earth in the order Mercury, Venus, Sun, Mars, Jupiter, Saturn. This system became known as the Ptolemaic system and predicted the positions of the planets accurately enough for naked-eye observations (although it made some ridiculous predictions, such as that the distance to the moon should vary by a factor of two over its orbit). He authored a book called Mathematical Syntaxis (widely known as the Almagest). The Almagest included a star catalog containing 48 constellations, using the names we still use today. Show me a picture of Ptolemy !

A rotating neutron star which generates regular pulses of radiation. Pulsars were discovered by observations at radio wavelengths but have since been observed at optical, X-ray, and gamma-ray energies.

Pioneer Venus Orbiter
    * Tell me more about PVO (


An enormously bright object at the edge of our universe which emits massive amounts of energy. In an optical telescope, they appear point-like, similar to stars, from which they derive their name (quasar = quasi-stellar). Current theories hold that quasars are one type of AGN.

quasi-stellar source (QSS)
Sometimes also called quasi-stellar object (QSO); A stellar-appearing object of very large redshift that is a strong source of radio waves; presumed to be extragalactic and highly luminous.


radial velocity
The speed at which an object is moving away or toward an observer. By observing spectral lines, astronomers can determine how fast objects are moving away from or toward us; however, these spectral lines cannot be used to measure how fast the objects are moving across the sky.

radian; rad
The supplementary SI unit of angular measure, defined as the central angle of a circle whose subtended arc is equal to the radius of the circle. One radian is approximately 57o.

Energy emitted in the form of waves (light) or particles (photons).

radiation belt
Regions of charged particles in a magnetosphere.

Electromagnetic radiation which has the lowest frequency, the longest wavelength, and is produced by charged particles moving back and forth; the atmosphere of the Earth is transparent to radio waves with wavelengths from a few millimeters to about twenty meters.

Rayleigh criterion; resolving power
A criterion for how finely a set of optics may be able to distinguish the location of objects which are near each other. It begins with the assumption that the central ring of one image should fall on the first dark ring of another image; for an objective lens with diameter d and employing light with a wavelength lambda (usually taken to be 560 nm), the resolving power is approximately given by

1.22 x lambda/d

Rayleigh-Taylor instabilities
Rayleigh-Taylor instabilities occur when a heavy (more dense) fluid is pushed against a light fluid -- like trying to balance water on top of air by filling a glass 1/2 full and carefully turning it over. Rayleigh-Taylor instabilities are important in many astronomical objects, because the two fluids trade places by sticking "fingers" into each other. These "fingers" can drag the magnetic field lines along with them, thus both enhancing and aligning the magnetic field. This result is evident in the example of a supernova remnant in the diagram below, from Chevalier (1977):

Rayleigh-Taylor instabilities

red giant
A star that has low surface temperature and a diameter that is large relative to the Sun.

An apparent shift toward longer wavelengths of spectral lines in the radiation emitted by an object caused by the emitting object moving away from the observer. See also Doppler effect.

reflection law
For a wavefront intersecting a reflecting surface, the angle of incidence is equal to the angle of reflection, in the same plane defined by the ray of incidence and the normal.

relativity principle
The principle, employed by Einstein's relativity theories, that the laws of physics are the same, at least locally, in all coordinate frames. This principle, along with the principle of the constancy of the speed of light, constitutes the founding principles of special relativity.

relativity, theory of
Theories of motion developed by Albert Einstein, for which he is justifiably famous. Relativity More accurately describes the motions of bodies in strong gravitational fields or at near the speed of light than Newtonian mechanics. All experiments done to date agree with relativity's predictions to a high degree of accuracy. (Curiously, Einstein received the Nobel prize in 1921 not for Relativity but rather for his 1905 work on the photoelectric effect.)

resolution (spatial)
In astronomy, the ability of a telescope to differentiate between two objects in the sky which are separated by a small angular distance. The closer two objects can be while still allowing the telescope to see them as two distinct objects, the higher the resolution of the telescope.

resolution (spectral or frequency)
Similar to spatial resolution except that it applies to frequency, spectral resolution is the ability of the telescope to differentiate two light signals which differ in frequency by a small amount. The closer the two signals are in frequency while still allowing the telescope to separate them as two distinct components, the higher the spectral resolution of the telescope.

A relationship in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5.

The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic; moving in the opposite sense from the great majority of solar system bodies.

The movement of one celestial body which is in orbit around another. It is often measured as the "orbital period."

Right Ascension
A coordinate which, along with declination, may be used to locate any position in the sky. Right ascension is analogous to longitude for locating positions on the Earth.

Ritter, Johann Wilhelm (1776 - 1810)
Ritter is credited with discovering and investigating the ultraviolet region of the electromagnetic spectrum.

Roche limit
The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary. At less than this distance the tidal forces of the larger object would break up the smaller object.

Roche lobe
In a binary star system, the volume around a star within which matter is gravitationally bound to that star. That is, if you were to release a particle within the Roche lobe, it would fall back onto the surface of that star. The point at which the Roche lobes of the two stars touch is called the inner Lagrangian or L1 point. If a star in a close binary system evolves to the point at which it `fills' its Roche lobe, material from this star will overflow onto the companion star (via the L1 point) and into the environment around the binary system.

Röntgen, Wilhelm Conrad (1845 - 1923)
A German scientist who fortuitously discovered X-rays in 1895.

Röntgen Satellite
    * Tell me more about ROSAT (

The spin of a celestial body on its own axis. In high energy astronomy, this is often measured as the "spin period."


The second Small Astronomy Satellite: a NASA satellite launched November 1972 with a mission dedicated to gamma-ray astronomy.
    * Tell me more about SAS-2 (

The third Small Astronomy Satellite: a NASA satellite launched May 1975 to determine the location of bright X-ray sources and search for X-ray novae and other transient phenomena.
    * Tell me more about SAS-3 (

A body that revolves around a larger body. For example, the moon is a satellite of the earth.

Schwarzschild black hole
A black hole described by solutions to Einstein's equations of general relativity worked out by Karl Schwarzschild in 1916. The solutions assume the black hole is not rotating, and that the size of its event horizon is determined solely by its mass.

Schwarzschild radius
The radius r of the event horizon for a Schwarzschild black hole.

scientific notation
A compact format for writing very large or very small numbers, most often used in scientific fields. The notation separates a number into two parts: a decimal fraction, usually between 1 and 10, and a power of ten. Thus 1.23 x 104 means 1.23 times 10 to the fourth power or 12,300; 5.67 x 10-8 means 5.67 divided by 10 to the eighth power or 0.0000000567.

second; s
The fundamental SI unit of time, defined as the period of time equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. A nanosecond is equal to one-billionth (10-9) of a second.

semimajor axis
The semimajor axis of an ellipse (e.g. a planetary orbit) is half the length of the major axis, which is the line segment passing through the foci of the ellipse with endpoints on the ellipse itself. The semimajor axis of a planetary orbit is also the average distance from the planet to its primary. The periapsis and apoapsis distances can be calculated from the semimajor axis and the eccentricity by

rp = a(1-e) and ra = a(1+e).

A measure of how bright objects need to be in order for that telescope to detect these objects. A highly sensitive telescope can detect dim objects, while a telescope with low sensitivity can detect only bright ones.

Seyfert galaxy
A spiral galaxy whose nucleus shows bright emission lines; one of a class of galaxies first described by C. Seyfert.

shock wave
A strong compression wave where there is a sudden change in gas velocity, density, pressure and temperature.

In astronomy, a term often used to refer to the center of a black hole, where the curvature of spacetime is maximal. At the singularity, the gravitational tides diverge; no solid object can even theoretically survive hitting the singularity. Mathematically, a singularity is a condition when equations do not give a valid value, and can sometimes be avoided by using a different coordinate system.

soft x-ray
Low energy x-rays, often from about 0.1 keV to 10 keV. The dividing line between soft and hard x-rays is not well defined and can depend on the context.

solar flares
Violent eruptions of gas on the Sun's surface.

solar mass
A unit of mass equivalent to the mass of the Sun. 1 solar mass = 1 Msun = 2 x 1033 grams.

special relativity
The physical theory of space and time developed by Albert Einstein, based on the postulates that all the laws of physics are equally valid in all frames of reference moving at a uniform velocity and that the speed of light from a uniformly moving source is always the same, regardless of how fast or slow the source or its observer is moving. The theory has as consequences the relativistic mass increase of rapidly moving objects, time dilatation, and the principle of mass-energy equivalence. See also general relativity.

spectral line
Light given off at a specific frequency by an atom or molecule. Every different type of atom or molecule gives off light at its own unique set of frequencies; thus, astronomers can look for gas containing a particular atom or molecule by tuning the telescope to one of the gas's characteristic frequencies. For example, carbon monoxide (CO) has a spectral line at 115 Gigahertz (or a wavelength of 2.7 mm).

The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum.

A Dispersive Spectrometer is like a prism. It scatters light of different energies to different places. We measure the energy by noting where the X-rays go. A Non-Dispersive Spectrometer measures the energy directly.

The study of spectral lines from different atoms and molecules. Spectroscopy is an important part of studying the chemistry that goes on in stars and in interstellar clouds.

spectrum (plural: spectra)
A plot of the intensity of light at different frequencies. Or the distribution of wavelengths and frequencies.

speed of light (in vacuum)
The speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299 792 458 m/s (186,282 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light.

A large ball of gas that creates and emits its own radiation.

star cluster
A bunch of stars (ranging in number from a few to hundreds of thousands) which are bound to each other by their mutual gravitational attraction.

Stefan-Boltzmann constant; sigma (Stefan, L. Boltzmann)
The constant of proportionality present in the Stefan-Boltzmann law. It is equal to 5.6697 x 10-8 Watts per square meter per degree Kelvin to the fourth power (see scientific notation).

Stefan-Boltzmann law (Stefan, L. Boltzmann)
The radiated power P (rate of emission of electromagnetic energy) of a hot body is proportional to the radiating surface area, A, and the fourth power of the thermodynamic temperature, T. The constant of proportionality is the Stefan-Boltzmann constant.

stellar classification
Stars are given a designation consisting of a letter and a number according to the nature of their spectral lines which corresponds roughly to surface temperature. The classes are: O, B, A, F, G, K, and M; O stars are the hottest; M the coolest. The numbers are simply subdivisions of the major classes. The classes are oddly sequenced because they were assigned long ago before we understood their relationship to temperature. O and B stars are rare but very bright; M stars are numerous but dim. The Sun is designated G2.

stellar wind
The ejection of gas off the surface of a star. Many different types of stars, including our Sun, have stellar winds; however, a star's wind is strongest near the end of its life when it has consumed most of its fuel.

steradian; sr
The supplementary SI unit of solid angle defined as the solid central angle of a sphere that encloses a surface on the sphere equal to the square of the sphere's radius.

supernova (plural: supernovae)
(a)The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, these type of supernova explosions (called Type II supernovae) can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant (SNR).
(b) The explosion of a white dwarf which has accumulated enough material from a companion star to achieve a mass equal to the Chandrasekhar limit. These types of supernovae (called Type Ia) have approximate the same intrinsic brightness, and can be used to determine distances.

Cooler (and thus darker) regions on the sun where the magnetic field loops up out of the solar surface.

A Japanese X-ray satellite observatory for which NASA provided X-ray mirrors and an X-ray Spectrometer using a calorimeter design. Suzaku (formerly known as Astro-E2) was successfully launched in July 2005.

The Spectrum X-Gamma mission
    * Tell me more about SXG (

Swift is a NASA mid-sized mission whose primary goal is to study gamma-ray bursts and address the mysteries surrounding their nature, origin, and causes. Swift launched November 20, 2004.
    * Tell me more about Swift (

synchronous rotation
Said of a satellite if the period of its rotation about its axis is the same as the period of its orbit around its primary. This implies that the satellite always keeps the same hemisphere facing its primary (e.g. the Moon). It also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite's motion while the other (trailing) one always faces backward.

synchrotron radiation
Electromagnetic radiation given off when very high energy electrons encounter magnetic fields.

Systéme Internationale d'Unités (SI)
The coherent and rationalized system of units, derived from the MKS system (which itself is derived from the metric system), in common use in physics today. The fundamental SI unit of length is the meter, of time is the second, and of mass is the kilogram.


The second Japanese X-ray mission, also known as Astro-B.
    * Tell me more about Tenma (

Thomson, William 1824 - 1907
Also known as Lord Kelvin, the British physicist who developed the Kelvin temperature scale and who supervised the laying of a trans-Atlantic cable. Show me a picture of Lord Kelvin!

time dilation
The increase in the time between two events as measured by an observer who is outside of the reference frame in which the events take place. The effect occurs in both special and general relativity, and is quite pronounced for speeds approaching the speed of light, and in regions of high gravity.


NASA's first Small Astronomy Satellite, also known as SAS-1. Uhuru was launched from Kenya on 12 December, 1970; The seventh anniversary of Kenya's independence. The satellite was named "Uhuru" (Swahili for "freedom") in honor of its launch date.
    * Tell me more about Uhuru (

Electromagnetic radiation at wavelengths shorter than the violet end of visible light; the atmosphere of the Earth effectively blocks the transmission of most ultraviolet light.

universal constant of gravitation; G
The constant of proportionality in Newton's law of universal gravitation and which plays an analogous role in A. Einstein's general relativity. It is equal to 6.67428 x 10-11 m3 / kg-sec2, a value recommended in 2006 by the Committee on Data for Science and Technology. (Also see scientific notation.)

Everything that exists, including the Earth, planets, stars, galaxies, and all that they contain; the entire cosmos.


Vela 5B
US Atomic Energy Commission (now the Department of Energy) satellite with an all-sky X-ray monitor
    * Tell me more about Vela 5B (

The Venera satellite series
The Venera satellites were a series of probes (fly-bys and landers) sent by the Soviet Union to the planet Venus. Several Venera satellites carried high-energy astrophysics detectors.
    * Tell me more about Venera 11 & 12 (
    * Tell me more about Venera 13 & 14 (

Electromagnetic radiation at wavelengths which the human eye can see. We perceive this radiation as colors ranging from red (longer wavelengths; ~ 700 nanometers) to violet (shorter wavelengths; ~400 nanometers.)


wave-particle duality
The principle of quantum mechanics which implies that light (and, indeed, all other subatomic particles) sometimes act like a wave, and sometimes act like a particle, depending on the experiment you are performing. For instance, low frequency electromagnetic radiation tends to act more like a wave than a particle; high frequency electromagnetic radiation tends to act more like a particle than a wave.

The distance between adjacent peaks in a series of periodic waves. Also see electromagnetic spectrum.

white dwarf
A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size. Typically, a white dwarf has a radius equal to about 0.01 times that of the Sun, but it has a mass roughly equal to the Sun's. This gives a white dwarf a density about 1 million times that of water!

Wien's displacement law
For a blackbody, the product of the wavelength corresponding to the maximum radiancy and the thermodynamic temperature is a constant. As a result, as the temperature rises, the maximum of the radiant energy shifts toward the shorter wavelength (higher frequency and energy) end of the spectrum.

WIMP (weakly interacting massive particle)
Theoretical subatomic particles that do not respond to electromagnetic force or interact through strong nuclear force, but would interact only through weak nuclear force and gravity. Because of these properties, they are difficult to detect, and are therefore considered "dark" — hence, WIMPs are a possible form of dark matter.

WMAP (Wilkinson Microwave Anisotropy Probe)
A NASA satellite designed to detect fluctuations in the cosmic microwave background. From its initial results published in Feb 2003, astronomers pinpointed the age of the universe, its geometry, and when the first stars appeared.

The World Wide Web -- a loose linkage of Internet sites which provide data and other services from around the world.


The X-ray Multi-Mirror Mission, launched by the European Space Agency in 1999. Observation targets include quasars, gamma-ray bursts, galaxy clusters and comets. The telescope's field of view is 30 arcmin, in the energy range from 0.15 to 15 keV.

Electromagnetic radiation of very short wavelength and very high-energy; X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than gamma rays.

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A software tools used by astrophysicists in conjunction with the FTOOLS software to analyze certain types of astronomical data.

X-ray Timing Explorer, also known as the Rossi X-ray Timing Explorer (RXTE)

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The ratio of the observed change in wavelength of light emitted by a moving object to the rest wavelength of the emitted light. See Doppler Effect. This ratio is related to the velocity of the object. In general, with v = velocity of the object, c is the speed of light, lambda is the rest wavelength, and delta-lambda is the observed change in the wavelength, z is given by
    z = (delta-lambda)/lambda = (sqrt(1+v/c) / sqrt(1-v/c)) - 1.
If the velocity of the object is small compared to the speed of light, then
    z = (delta-lambda)/lambda = v/c
Objects at the furthest reaches of the known universe have values of z = 5 or slightly greater.

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Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

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