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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.)


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. (

[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 ]

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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