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Skyview NASA - Astrophysics Dictionary
Category: Sciences > Astophysics
Date & country: 11/09/2007, USA
Words: 223

American Astronomical Society

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

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.

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

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

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 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. 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 …

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.

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. (132 k GIF)

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

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.) (125 k GIF).

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

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 t…

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.

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. (32 k GIF)

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. (14 k GIF)

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

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.

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.

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
See event horizon.

A mutual physical force attracting two bodies.

Goddard Space Flight Center

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.

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. (58 k GIF)

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. (88 k GIF)

International Astronomical Union

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

Space Research Institute (Russia)

In astronomy, a picture of the sky.

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

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

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.

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.