High-Energy Astrophysics dictionary

Listed below is the High-Energy Astrophysics 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 ]


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

American Astronomical Society

active galactic nuclei (AGN)
It is believed that these are normal galaxies with a massive black hole accreting gas at its center, thus producing enormous amounts of energy at all wavelengths of the electromagnetic spectrum.

A unit of length equal to 0.00000001 centimeters. Scientists sometimes write this as 1 × 10-8 cm (see
scientific notation).

anonymous FTP
A widely used convention for allowing users electronic access to public data without requiring passwords or prior registration.

The point of greatest separation of two stars, as in a binary star orbit.

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

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

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

The Japanese Asuka spacecraft (formerly ASTRO-D)

All Sky Monitor. Many high-energy satellites have carried ASM detectors, including the ASM on Vela 5B, Ariel V, and the Rossi X-ray Timing Explorer.

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

The study of the chemical interactions between the gases and dust interspersed between the stars.

atomic gas
Gas that is composed of individual atoms (such as hydrogen or carbon) that are not bound to each other as molecules. Atomic gas may be ionized or mixed with molecular gas.


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

Balmer series (J. Balmer; 1885)
An equation which describes the emission spectrum of hydrogen when an electron is jumping to the second orbital. Four of the lines are in the visible spectrum; the remainder are in the ultraviolet.

Broad Band X-Ray Telescope on ASTRO-1 shuttle flight (Dec. 1990)

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. Matter is stripped from the normal star and falls onto the collapsed star, producing X-rays.

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
The radiation -- the radiance at particular frequencies all across the spectrum -- produced by a blackbody -- that is, a perfect radiator (and absorber) of heat. Physicists had difficulty explaining it until Planck introduced his quantum of action.

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.

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 × 10-23 J/K (see
scientific notation).

Brahe, Tycho 1546-1601
(a.k.a Tyge Ottesen) Danish astronomer whose accurate astronomical observations formed the basis for Johannes Kepler's laws of planetary motion.

The HEASARC's user interface/data browser


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.

A pulsating variable star. This type of star undergoes a rhythmic pulsation as indicated by its regular pattern of changing brightness as a function of time. The period of pulsation has been demonstrated to be directly related to a Cepheid's intrinsic brightness making observations of these stars one of the most powerful tools for determining distance known to modern day astronomy.

The Compton Gamma Ray Observatory

Chandrasekhar limit (S. Chandrasekhar; 1930)
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 sensitive it is to 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

Copernicus, Nicolaus 1473-1543
Polish astronomer who advanced the heliocentric theory that the Earth and other planets revolve around the Sun. This was highly controversial at the time as the Ptolemaic view of the universe, which was the prevailing theory for over 1000 years, was deeply ingrained in the prevailing philosophy and religion. (It should be noted, however, that the heliocentric idea was first put forth by Aristarcus of Samos in the 3rd century BC, a fact known to Copernicus but long ignored.)

The uppermost level of the solar atmosphere, characterized by low densities and high temperatures (> 1,000,000 degrees K).

cosmic background radiation; primal glow
The background of radiation mostly in the frequency range 3 × 108 to 3 × 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 ray
Atomic nuclei (mostly protons) that are observed to strike the Earth's atmosphere with exceedingly high energies.

cosmological constant; Lambda
The constant introduced to the Einstein field equation, intended to admit static cosmological solutions. At the time the current philosophical view was the steady-state model of the Universe, where the Universe has been around for infinite time. Early analysis of the field equation indicated that general relativity allowed dynamic cosmological models only (ones that are either contracting or expanding), but no static models. Einstein introduced the most natural abberation to the field equation that he could think of: the addition of a term proportional to the spacetime metric tensor, g, with the constant of proportionality being the cosmological constant:

G + Lambda g = 8 pi T.
Hubble's later discovery of the expansion of the Universe indicated that the introduction of the cosmological constant was unnecessary; had Einstein believed what his field equation was 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 red shift.

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


de Broglie wavelength (L. de Broglie; 1924)
The prediction that particles also have wave characteristics, where the effective wavelength of a particle would be inversely proportional to its momentum; 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.

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.

Measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0; iron is 7.9; lead is 11.3.

The visible surface of the Sun (or any heavenly body) projected against the sky.

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.


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 cutting off, or blocking, of light from one celestial body by another.

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.

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

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

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. A keV (or kiloelectron volt) is equal to 1000 electron volts. An MeV is equal to one million electron volts.

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

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

event horizon
The radius that a spherical mass must be compressed to in order to transform it into a black hole, or the radius at which time and space switch responsibilities. Once inside the event horizon, it is fundamentally impossible to escape to the outside. Furthermore, 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

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 accelerations
The resultant accelerations of a particle which undergoes a multi-collisional process. Based on a model developed by Enrico Fermi, it is found that a particle which has a head-on collision is accelerated (the "first-order" process) and that a particle decelerates from a receding collision (the "second-order" process).

The Flexible Image Transport System format -- the IAU standard for astronomical data.

A portable suite of subroutines developed to provide convenient access to FITS files.

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 the OGIP for general and mission-specific manipulation of FITS files.

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


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.

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 his own version of 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.

The highest energy, shortest wavelength electromagnetic radiations. Usually, they are thought of as any photons having energies greater than about 100 keV.

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 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 or even their distance.

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.

globular cluster
A spherically symmetric star cluster, containing over 100,000 individual stars, which are in a roughly spherical distribution about the main hub of a galaxy. They form more or less a spherical halo around the main body of the galaxy, like bees around a hive.

Grand Unified Theory (GUT)
A single theory of physics which will unite the 4 known forces of nature -- gravity, electromagnetism, weak-interaction, and strong nuclear forces. The pursuit of such a theory has been the focus of much effort in the 20th century. However, a successful GUT has yet to be achieved.

gravitational radius
event horizon.

A mutual physical force attracting two bodies.

Goddard Space Flight Center


Hawking radiation (S.W. Hawking; 1973)
The theory that black holes emit radiation like any other hot body. Virtual particle-antiparticle pairs are constantly being created in supposedly empty space. Occasionally, a pair will be created just outside the event horizon of a black hole. There are three possibilities:
  1. both particles are captured by the hole;
  2. both particles escape the hole;
  3. one particle escapes while the other is captured.
The first two cases are straightforward; the virtual particle-antiparticle pair recombine and return their energy back to the void via the uncertainty principle.
It is the third case that interests us. In this case, one of the particles has escaped (and is speeding away to infinity), while the other has been captured by the hole. The escapee becomes real and can now be detected by distant observers. But the captured particle is still virtual; because of this, it has to restore conservation of energy by assigning itself a negative mass-energy. Since the hole has absorbed it, the hole loses mass and thus appears to shrink. From a distance, it appears as if the hole has emitted a particle and reduced in mass.
The rate of power emission is proportional to the inverse square of the hole's mass; thus, the smaller a hole gets, the faster and faster it emits Hawking radiation. This leads to a runaway process; what happens when the hole gets very small is unclear; quantum theory seems to indicate that some kind of "remnant" might be left behind after the hole has emitted away all its mass-energy.

Hawking temperature
The temperature of a black hole caused by the emission of Hawking radiation.

High Energy Astrophysical Observatory

High Energy Astrophysics Science Archive Research Center

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 s.

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 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.

Hubble constant; H_0 (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. Since the Universe is self-gravitating, it is not truly constant. In cosmology, it is defined as

H = (da/dt)/a,
where a is the 4-radius of the Universe. When evaluated for the present, it is written H0 = Hnow.
The Hubble constant is not known to great accuracy (only within about a factor of 2), but is believed to lie somewhere between 50 and 100 km/s/Mpc.

Hubble's law (E.P. Hubble; 1925)
A relationship discovered between distance and radial velocity. The further away a galaxy is from us, the faster it is receding from us. The constant of proportionality is the Hubble constant, H_0. The cause is interpreted as the expansion of spacetime itself.

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.


International Astronomical Union

Interpretive Data Language: a proprietary data analysis system of Research Systems International

Space Research Institute (Russia)

The property of matter that requires a force to act on it to change the way it is moving; momentum is a measure of inertia.

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.

In astronomy, a picture of the sky.

A type of telescope in which signals from two or more small telescopes are combined to produce an image with the resolution of a much larger telescope. The larger the separation between the individual telescopes, the higher the resolution of the resulting image.

interstellar medium
The gas and dust that exists in the space between the stars

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.

Image Reduction and Analysis Facility -- A large astronomical analysis system developed at Kitt Peak National Observatory (KPNO).

International Ultraviolet Explorer



kelvin (after Lord Kelvin, 1824-1907)
A temperature scale often used in sciences such as astronomy. The fundamental SI unit of thermodynamic temperature defined as 1/273.16 of the thermodynamic temperature of the triple point of water. The Kelvin temperature scale is just like the Celsius scale except that the freezing point of water, zero degrees Celsius, is equal to 273 degrees Kelvin. [ K = C + 273° => F = 9/5C + 32°]

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 ray directed from the Sun to a planet sweeps out equal areas in equal times.

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)
One kilogram is equivalent to 1,000 grams or 2.2 pounds; the mass of a liter of water. The fundamental SI unit of mass, it 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.

kilometer (km)
One kilometer is equivalent to 1,000 meters or 0.62 miles.

A distance equal to 1000 parsecs.

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.


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.

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 1/25.96 the mass of the primary. These points are stable because centrifugal pseudoforces work against gravity to cancel it out.

Los Alamos National Laboratory

large scale structure
The largest spatial features in an image.

Laboratory for High Energy Astrophysics (GSFC, Code 660)

Electromagnetic radiation that is visible to the human eye.

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 × 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.

Lyman series
The series which describes the emission spectrum of hydrogen when electrons are jumping to the ground state. All of the lines are in the ultraviolet.


magnetic field
A condition found in the region around a magnet or an electric current, characterized by the existence of a detectable magnetic force at every point in the region and by the existence of magnetic poles.

magnetic monopole
A hypothetical particle which constitutes sources and sinks of the magnetic field. Magnetic monopoles have never been found, but would only cause fairly minor modifications to Maxwell's equations. They also seem to be predicted by some grand-unified theories. If magnetic monopoles do exist, they do not seem to be very common in our Universe.

magnetic pole
Either of two limited regions in a magnet at which the magnet's field is most intense.

the region of space in which a planet's magnetic field dominates that of the solar wind.

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 visible star has magnitude 6, with the scale rule such that a decrease of one unit 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.

megabits per second
A unit for measuring how fast data can be sent through a computer network equal to one million bits per second.

Used by astrophysicists to refer to all elements except hydrogen and helium, as in: "the universe is composed of hydrogen, helium and traces of metals".

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.

1/1000 of a bar. Standard sea-level pressure is about 1013 millibars.

molecular gas
Gas that is composed of atoms that are bound to each other as molecules. The most abundant molecule in space is molecular hydrogen (two hydrogen atoms bound to each other) followed by carbon monoxide (CO, or a carbon and oxygen atom bound together). Molecular gas may be mixed with atomic gas.

A measure of the state of motion of a body; mathematically, it is equal to the product of its mass and velocity.

A non-proprietary software tool using hypertext links to navigate and retrieve data from the Internet -- developed by the National Center for Supercomputing Activities.


A diffuse mass of interstellar dust and gas.

A fundamental particle supposedly 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.

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; N (after Sir I. Newton, 1642-1727)
The derived SI unit of force, defined as the force required to give a mass of 1 kg an acceleration of 1 m/s/s.

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

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.

A start 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.

NASA Research Announcement

National Space Science Data Center (GSFC, Code 633)

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.


The angle between a body's equatorial plane and orbital plane.

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

Office of Guest Investigator Programs (GSFC, Code 668)

A property of matter that prevents light from passing through it; non-transparent. The opacity or opaqueness of something depends on the frequency of the light. For instance, the atmosphere of Venus is transparent to ultra-violet light, but is opaque to visual light.

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

Orbiting Solar Observatory 8


A large distance often used in astronomy, it is equal to 3.26 light years, or 3.1 × 1018 cm (see scientific notation).

The point in the orbit closest to the planet.

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

The point in the orbit closest to the Earth.

The point in its orbit where a planet is closest to the Sun. when referring to objects orbiting the Earth the term perigee is used; the term periapsis is used for orbits around other bodies. (opposite of aphelion)

To cause a planet or satellite to deviate from a theoretically regular orbital motion.

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.

Principal Investigator

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 × 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 nu.

planetary nebula
A shell of gas ejected from, and expanding about, a certain kind of extremely hot star.

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.

ROSAT (and X-ray) analysis system developed within IRAF

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.

A rotating neutron star whose light appears to pulse. These pulses are the result of beams of radiation sweeping through the direction to the earth, much like a lighthouse beacon. Pulsars were discovered by observations at radio wavelengths but have since been observed at optical, x-ray, and gamma-ray energies.

Pulsars are rapidly rotating highly magnetized neutron stars. Highly energetic electrons spiraling in the magnetic field emit radio beams along the magnetic axis. The pulses are detected when the radio beam


A specific type of quasi-stellar source.

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.

Energy radiated in the form of waves or particles; photons.

radiation belt
Regions of charged particles in a magnetosphere.

Rayleigh criterion; resolving power
A criterion for how finely a set of optics may be able to distinguish. 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 lambda/d.

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

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 qualitatively, in all frames. That is, there is no frame that is better (or qualitatively any different) from any other. This principle, along with the constancy principle, constitute the founding principles of special relativity.

relativity, Theory of
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.

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 primary would break up the secondary.

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

Röntgen Satellite


A body that revolves around a larger body.

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 × 104 means 1.23 times 10 to the fourth power or 12,300; 5.67 × 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.

semimajor axis
The semimajor axis of an ellipse (e.g. a planetary orbit) is 1/2 the length of the major axis which is a segment of a line passing thru 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.

of, relating to, or concerned with the stars. Sidereal rotation is that measured with respect to the stars rather than with respect to the Sun or the primary of a satellite.

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. Although singularities generally predict inconsistencies in theory, singularities within black holes do not necessarily imply that general relativity is incomplete so long as singularities are always surrounded by event horizons.
A proper formulation of quantum gravity may well avoid the classical singularity at the centers of black holes.

small scale structure
The smallest spatial features in an image.

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

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 its 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.

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.

A plot of the intensity of light at different frequencies. Or the distribution of wavelengths and frequencies.

speed of light (in vacuo); c
The speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299 792 458 m/s (186,000 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light.

Structured Query Language -- The ANSI standard database language

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 × 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.

The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, supernova explosions 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).

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

The Spectrum X-Gamma mission

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.

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.


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

time dilation
Stretching of time produced by relativity. Time dilation is a predicted effect of the cosmological paradigm.

A suite of software tools developed at the OGIP to transform non-FITS information into FITS.

21-cm Line
The spectral line given off by atomic hydrogen with a wavelength of 21 cm ( or frequency of 1.4 Gigahertz). Since hydrogen is the most abundant atom in the universe, the 21-cm line of hydrogen is an extremely useful tool for radio astronomers.


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 6.664 × 10-11 newtons per square meter per kilogram squared (see
scientific notation).


Van Allen, James A.
American physicist who discovered the Earth's radiation belts (that now bear his name) with an instrument aboard the first successful American satellite, Explorer 1.

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


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.

Portions of the electromagnetic spectrum. Examples of different wavebands include the infrared, visual, and radio wavebands.

A property of a wave that gives the length between two peaks of the wave.

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.

Write-Once-Read-Many. A CD-ROM is a typical WORM medium: the CD-ROM is written once when it is etched and it cannot be written to again. However, it can be read any number of times.

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


X-ray analysis system maintained at the HEASARC

Image analysis program in XANADU

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

Temporal analysis program in XANADU

A high-level tool to manage the FTOOLs

X-ray/gamma-ray spectral analysis package in XANADU

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



Zeeman effect; Zeeman line splitting (P. Zeeman; 1896)
The splitting of the lines in a spectrum when the source is exposed to a magnetic field.