Imagine the Universe! Dictionary
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(Note - Greek letters are written out by name - alpha, beta etc.)
An enormously bright object at the edge of our universe which emits
massive amounts of energy. In an optical telescope, they appear
point-like, similar to stars, from which they derive their name
(quasar = quasi-stellar). Current theories hold that quasars are one
type of AGN.
quasi-stellar source (QSS)
Sometimes also called quasi-stellar object (QSO); A stellar-appearing object of
very large redshift that is a strong source of radio waves; presumed to be
extragalactic and highly luminous.
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.
The supplementary SI unit of angular measure, defined as the central angle
of a circle whose subtended arc is equal to the radius of the circle.
One radian is approximately 57o.
Energy emitted in the form of waves (light) or particles (photons).
Regions of charged particles in a magnetosphere.
Electromagnetic radiation which has the lowest frequency, the longest
wavelength, and is produced by charged particles moving back and forth; the
atmosphere of the Earth is transparent to radio waves with wavelengths from
a few millimeters to about twenty meters.
Rayleigh criterion; resolving power1.22 x lambda/d
A criterion for how finely a set of optics may be able to distinguish
the location of objects which are near each other. It begins with the
assumption that the central ring of one image should fall on the first
dark ring of another image; for an objective lens with diameter d and
employing light with a wavelength lambda (usually taken to be 560 nm), the
resolving power is approximately given by
Rayleigh-Taylor instabilities occur when a heavy (more dense) fluid is
pushed against a light fluid -- like trying to balance water on top of air by
filling a glass 1/2 full and carefully turning it over. Rayleigh-Taylor
instabilities are important in many astronomical objects, because the two
fluids trade places by sticking "fingers" into each other. These
"fingers" can drag the magnetic field lines along with them, thus
both enhancing and aligning the magnetic field. This result is evident in the
example of a supernova remnant in the diagram below,
from Chevalier (1977):
A star that has low surface temperature and a diameter that is large
relative to the Sun.
An apparent shift toward longer wavelengths of spectral lines in the radiation
emitted by an object caused by the emitting object moving away from the
observer. See also Doppler effect.
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.
The principle, employed by Einstein's relativity theories, that the laws of
physics are the same, at least locally, in all coordinate frames. This
principle, along with the principle of the constancy of the speed of light,
constitutes the founding principles of special relativity.
relativity, theory of
Theories of motion developed by Albert
Einstein, for which he is justifiably famous. Relativity More
accurately describes the motions of bodies in strong gravitational
fields or at near the speed of light than Newtonian mechanics. All
experiments done to date agree with relativity's predictions to a high
degree of accuracy. (Curiously, Einstein received the Nobel prize in
1921 not for Relativity but rather for his 1905 work on the
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
A relationship in which the orbital period of one body is related to that
of another by a simple integer fraction, such as 1/2, 2/3, 3/5.
The rotation or orbital motion of an object in a clockwise direction when
viewed from the north pole of the ecliptic; moving in the opposite
sense from the great majority of solar system bodies.
The movement of one celestial body which is in orbit around another. It is often measured as the "orbital period."
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.
The smallest distance from a planet or other body at which purely
gravitational forces can hold together a satellite or secondary body of the
same mean density as the primary. At less than this distance the tidal forces
of the larger object would break up the smaller object.
The volume around a star in a binary system in which, if you were to
release a particle, it would fall back onto the surface of that star.
A particle released above the Roche lobe of either star will, in general,
occupy the `circumbinary' region that surrounds both stars. The point
at which the Roche lobes of the two stars touch is called the inner
Lagrangian or L1 point. If a star in a close
binary system evolves to the point at which it `fills' its Roche lobe,
theoretical calculations predict that material from this star will overflow
both onto the companion star (via the L1 point) and into the
environment around the binary system.
Röntgen, Wilhelm Conrad (1845 - 1923)
A German scientist who fortuitously discovered X-rays in 1895.
The spin of a celestial body on its own axis. In high energy astronomy, this is often measured as the "spin period."
The second Small Astronomy Satellite: a NASA satellite launched November 1972
with a mission dedicated to gamma-ray astronomy.
The third Small Astronomy Satellite: a NASA satellite launched May 1975 to
determine the location of bright X-ray sources and search for X-ray
novae and other transient phenomena.
A body that revolves around a larger body. For example, the moon is a
satellite of the earth.
A black hole described by solutions to Einstein's equations of general relativity worked out by Karl
Schwarzschild in 1916. The solutions assume the black hole is not
rotating, and that the size of its event
horizon is determined solely by its mass.
The radius r of the event horizon for a Schwarzschild black hole.
A compact format for writing very large or very small numbers, most often used
in scientific fields. The notation separates a number into two parts: a
decimal fraction, usually between 1 and 10, and a power of ten. Thus
1.23 x 104 means 1.23 times 10 to the fourth power or 12,300;
5.67 x 10-8 means 5.67 divided by 10 to the eighth power or
The fundamental SI unit of time, defined as the
period of time equal to the duration of 9,192,631,770 periods of the
radiation corresponding to the transition between two hyperfine levels
of the ground state of the cesium-133 atom. A nanosecond is
equal to one-billionth (10-9) of a second.
semimajor axis rp =
a(1-e) and ra = a(1+e).
The semimajor axis of an ellipse (e.g. a planetary orbit) is half the
length of the major axis, which is the line segment passing through
the foci of the ellipse with endpoints on the ellipse itself. The
semimajor axis of a planetary orbit is also the average distance from
the planet to its primary. The periapsis and
apoapsis distances can be calculated from the
semimajor axis and the eccentricity by
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.
A spiral galaxy whose nucleus shows bright emission lines; one of a class of
galaxies first described by C. Seyfert.
A strong compression wave where there is a sudden change in
gas velocity, density, pressure and temperature.
In astronomy, a term often used to refer to the center of a black hole, where the curvature of spacetime is maximal. At
the singularity, the gravitational tides diverge; no solid object can even
theoretically survive hitting the singularity.
Mathematically, a singularity is a condition when equations do not
give a valid value, and can sometimes be avoided by using a different
Low energy x-rays, often from about 0.1 keV to 10 keV. The dividing line between soft and hard x-rays is not well defined and can depend on the context.
Violent eruptions of gas on the Sun's surface.
A unit of mass equivalent to the mass of the Sun. 1 solar mass =
1 Msun = 2 x 1033 grams.
The physical theory of space and time developed by Albert Einstein,
based on the postulates that all the laws of physics are equally valid
in all frames of reference moving at a uniform velocity and that the
speed of light from a uniformly moving source is always the same,
regardless of how fast or slow the source or its observer is moving.
The theory has as consequences the relativistic mass increase of
rapidly moving objects, time dilatation,
and the principle of mass-energy equivalence. See also general relativity.
Light given off at a specific frequency by an atom or molecule. Every
different type of atom or molecule gives off light at its own unique set of
frequencies; thus, astronomers can look for gas containing a particular
atom or molecule by tuning the telescope to one of the gas's characteristic
frequencies. For example, carbon monoxide (CO) has a spectral line at
115 Gigahertz (or a wavelength of 2.7 mm).
The instrument connected to a telescope that separates the light signals
into different frequencies, producing a spectrum.
A Dispersive Spectrometer is like a prism. It scatters
light of different energies to different places. We measure the
energy by noting where the X-rays go. A Non-Dispersive
Spectrometer measures the energy directly.
The study of spectral lines from different atoms and molecules.
Spectroscopy is an important part of studying the chemistry that goes on
in stars and in interstellar clouds.
spectrum (plural: spectra)
speed of light (in vacuum)
A plot of the intensity of light at different frequencies. Or
the distribution of wavelengths and frequencies.
The speed at which electromagnetic radiation propagates in a vacuum; it is
defined as 299 792 458 m/s (186,212 miles/second). Einstein's
Theory of Relativity implies that nothing can go
faster than the speed of light.
A large ball of gas that creates and emits its own radiation.
A bunch of stars (ranging in number from a few to hundreds of thousands) which
are bound to each other by their mutual gravitational attraction.
Stefan-Boltzmann constant; sigma (Stefan, L. Boltzmann)
The constant of proportionality present in the Stefan-Boltzmann law. It is
equal to 5.6697 x 10-8 Watts per square meter per degree Kelvin
to the fourth power (see scientific
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
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
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.
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
supernova (plural: supernovae)
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
Swift is a NASA mid-sized mission whose primary goal is to study
gamma-ray bursts and address the mysteries surrounding their nature,
origin, and causes. Swift is expected to launch in 2003.
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.
Electromagnetic radiation given off when very high energy electrons
encounter magnetic fields.
Systéme Internationale d'Unités (SI)
The coherent and rationalized system of units, derived from
the MKS system (which itself is derived from the metric system),
in common use in physics today. The fundamental SI unit of length is the
meter, of time is the second, and
of mass is the kilogram.
The second Japanese X-ray mission, also known as Astro-B.
Thomson, William 1824 - 1907
Also known as Lord Kelvin, the British physicist who developed the Kelvin temperature scale
and who supervised the laying of a trans-Atlantic cable.
Show me a picture of Lord Kelvin!
The increase in the time between two events as measured by an observer
who is outside of the reference frame in which the events take place.
The effect occurs in both special
and general relativity, and is quite
pronounced for speeds approaching the speed of light, and in regions
of high gravity.
NASA's first Small Astronomy Satellite, also known as SAS-1. Uhuru was
launched from Kenya on 12 December, 1970; The seventh anniversary of Kenya's
independence. The satellite was named "Uhuru" (Swahili for
"freedom") in honor of its launch date.
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 x
10-11 m3 / kg-sec2 (see scientific notation).
Everything that exists, including the Earth, planets, stars, galaxies, and all that they contain; the entire cosmos.
US Atomic Energy Commission (now the Department of Energy) satellite with an
all-sky X-ray monitor
The Venera satellite series
The Venera satellites were a series of probes (fly-bys and landers) sent
by the Soviet Union to the planet Venus. Several Venera satellites
carried high-energy astrophysics detectors.
Electromagnetic radiation at wavelengths which the human eye can see. We
perceive this radiation as colors ranging from red (longer wavelengths;
~ 700 nanometers) to violet (shorter wavelengths; ~400 nanometers.)
The principle of quantum mechanics which implies that light (and, indeed, all
other subatomic particles) sometimes act like a wave, and sometimes act like a
particle, depending on the experiment you are performing. For instance, low
frequency electromagnetic radiation tends to act more like a wave than a
particle; high frequency electromagnetic radiation tends to act more like a
particle than a wave.
The distance between adjacent peaks in a series of periodic waves.
Also see electromagnetic spectrum.
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.
WMAP (Wilkinson Microwave Anistropy Probe)
A NASA satellite designed to detect fluctuations in the cosmic microwave background. From its initial results published in Feb 2003, astronomers pinpointed the age of the universe, its geometry, and when the first stars appeared.
The World Wide Web -- a loose linkage of Internet sites which provide
data and other services from around the world.
Electromagnetic radiation of very short wavelength and very high-energy;
X-rays have shorter wavelengths than ultraviolet light but longer wavelengths
than gamma rays.
A software tools used by astrophysicists in conjunction with the FTOOLS software to analyze certain types of
X-ray Timing Explorer, also known as the Rossi X-ray Timing Explorer (RXTE)
The ratio of the observed change in wavelength of light emitted by a
moving object to the rest wavelength of the emitted light. See Doppler Effect. This ratio is related to
the velocity of the object. In general, with v = velocity of the
object, c is the speed of light, lambda is the rest wavelength, and
delta-lambda is the observed change in the wavelength, z is given by
z = (delta-lambda)/lamda = (sqrt(1+v/c) / sqrt(1-v/c)) - 1.
If the velocity of the object is small compared to the speed of
z = (delta-lambda)/lamda = v/c
Objects at the furthest reaches of the known universe have values
of z = 5 or slightly greater.
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