Imagine the Universe! Dictionary
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(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 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
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
Show me a picture of Albert Einstein !
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
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 waves (radiation)
The full range of frequencies, from radio waves to gamma rays, that
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.
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 80and 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
A form of the metric unit for power. It is equal to 10-10
kilowatts (see scientific notation).
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
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.
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.
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.
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 !
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)
Gamma-ray Large Area Space Telescope (GLAST)
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.
An international mission planned for launch in 2006, GLAST will study
the universe in the energy range 10 keV - 300 Gev.
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.
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.
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
The third Japanese X-ray mission, also known as Astro-C.
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.
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.)
See event horizon.
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 eminate from the Big Bang.
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.
Goddard Space Flight Center, one of the centers operated by NASA.
The ancient Chinese term for a star that newly appears in the night sky, and
then later disappears. Later, the Europeans called this a
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.
The temperature inferred for a black hole based on the Hawking
radiation detected from it.
The High Energy Astrophysical Observatory satellite series
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 percent 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
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
Show me a picture of Christian
The lightest and most abundant element. A hydrogen atom consists of
one proton and one electron. Hydrogen composes about 75 percent 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.
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.
In astronomy, a picture of the sky.
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
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.
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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.