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Glossary of Basic Astronomical Terms



Defines the most commonly used terms in introductory astronomy using everyday language.
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Andromeda Galaxy e closest major galaxy to our
own, located about 2.5 million light years away in the
constellation of Andromeda. Also known by its cata-
log number M31, it is the only other large galaxy in
the Local Group of galaxies.
Aperture e size (diameter) of the light-collecting
lens or mirror in a telescope.
Apparent Magnitude How bright a star or other ob-
ject in the sky looks from Earth, expressed in the old-
fashioned magnitude system. e brighter the star
looks, the smaller the magnitude number. e bright-
est star in the sky, Sirius, has a magnitude of –1.42, the
star Vega has a magnitude of zero, while dim Barnards
Star, with a magnitude of 9.5, is too faint to be seen
with the naked eye. (Bear in mind that how bright a
star looks to us depends on both how bright it really
is — its luminosity — and how far away it is.)
Asteroid A relatively small rocky object orbiting the
Sun. Most asteroids are found between Mars and Jupi-
ter in a region called the asteroid belt, although some
asteroids have orbits that cross the orbits of inner plan-
ets (like Earth). ose are the ones we worry about.
Astronomical Unit e average distance between the
Earth and the Sun, about 150 million km or 93 million
miles. Astronomers use this unit to express distances
within the solar system.
Axis An imaginary line drawn through the center of a
body, around which it spins.
Big Bang e high-energy, very dense beginning of
the universe; the start of its expansion. e Big Bang
eory is the popular name for a whole set of ideas for
the origin and early evolution of the universe.
Black Hole e collapsed remnant of a star whose
gravity is so great that nothing, not even light, can es-
cape it. More technically, a region where gravity has
warped space-time so much that straight lines have
become circles.
Brown Dwarf A failed star; a star that cannot (for any
signicant length of time) sustain itself by producing
energy in its core through nuclear reactions (fusion).
CCD (charge-coupled device) An electronic detec-
tor of electromagnetic radiation (including light). In
photography, CCD’s are light-sensitive arrays that re-
cord the amount of light coming in and read them out
as numbers (digits). ey made digital photography
possible, replacing lm as the collector of light.
Comet A relatively small body made of ice and dust
that orbits the Sun. Most comets remain far away and
frozen solid, but when a comet approaches the Sun, its
ice can evaporate and its dust can be freed, producing
a large cloud of material around the frozen core, and
making it much easier to spot. Light and energy from
the Sun can push some of this cloud into a direction
away from the Sun, producing the comet’s tail.
Constellation In the old days, a constellation was any
recognizable pattern of bright stars in the sky. To-
day, astronomers have given the word a more precise
meaning: it is one of 88 sectors into which astrono-
mers divide the sky (much like the territory of the U.S.
is divided into sectors called states.) Each constella-
tion “box” is named aer a pattern of bright stars in
it. For example, in the old system, Orion (the Hunter)
is a pattern of bright stars easily recognized in the
winter sky. In the new system, the “box” in the sky
that includes the hunter pattern is called Orion, and
it includes all the stars and galaxies in the sky that lie
in the box.
Glossary of Basic Astronomical Terms
by Andrew Fraknoi
(Foothill College & ASP)
The Universe at Your Fingertips • Astronomical Society of the Pacic
Astronomy Background 2.8 • Glossary of Basic Astronomical Terms
Page 2
Cosmology e branch of astronomy that deals with
the properties of the universe as a whole — as a single
system. Topics that cosmologists think about include
the organization, origin, and ultimate fate of the uni-
Dark Energy e energy that is causing the expansion
of the universe to accelerate (speed up). Astronomers
have discovered this acceleration by using distant su-
pernovae (exploding stars) as distance markers and
noting that space has not been stretching at a constant
rate over cosmic time. e nature of this energy is un-
known at present.
Dark Matter Material which we cannot detect (by ob-
serving light or other radiation from it) but whose ex-
istence we know about from seeing its gravity aect
the material in the universe that we can see. It appears
that there is more of this mysterious dark matter in
the universe than the regular matter which we see as
stars and galaxies. What dark matter is made of is cur-
rently unknown.
Day In astronomy, the time a planet takes to spin (turn
on its axis) once.
Declination A system of measuring the position of ob-
jects in the sky that is similar to the latitude system
we use on Earth. Declination is measured in degrees
north and south of the celestial equator (the circle in
the sky that you would see if you extended the Earth’s
equator out into the sky.)
Degree A measure of angle equal to one 360th of a full
circle. e dome of the sky from a point on the hori-
zon to the opposite point measures 180 degrees.
Density e amount of mass per unit volume in a
body. A cubic centimeter of lead is a lot more dense
than a cubic centimeter of Jell-O.
Detector A device that registers (detects) light or other
radiation. e human eye and a solar panel are both
examples of detectors; telescopes by themselves are
Doppler shift e change in the wavelength (color) of
light — or other electromagnetic radiation — caused
by motion of its source toward us or away from us.
Dwarf planet An object orbiting the Sun which is
large enough for gravity to make it round, but one that
shares its general orbit with a number of other objects.
Ceres, Pluto, and Eris are examples of dwarf planets.
Eclipse When one body gets into the shadow of an-
other in space. For example, we get an eclipse of the
Moon when the Earth gets right between the Sun and
the Moon and casts its shadow on the Moon.
Electric charge e property of some particles of mat-
ter that causes them to attract or repel other charged
particles. Examples of particles with electric charge
are the electron and the proton.
Electromagnetic radiation Waves of energy gener-
ated by electric and magnetic changes in matter; these
waves travel at the speed of light. Examples include
radio waves, infrared waves, visible light, ultraviolet
waves, x-rays, and gamma rays.
Expansion of the Universe (Expanding Uni-
verse) e obser vation that the groups of galaxies are
all moving away from one another — we now know
that this is because space itself has been “stretching”
since the big bang. Recent observations indicate that
this expansion is speeding up (accelerating).
Extra-solar planet A planet orbiting a star other than
the Sun.
Eyepiece A magnifying lens used to view the image
produced by a telescope.
Frequency e number of cycles (repetitions) per sec-
ond in a wave, such as light. e higher the frequency,
the more energy a wave carries.
Fusion A process by which light atomic nuclei come
together under tremendously hot conditions to pro-
duce energy; fusion is what allows the stars to shine.
When fusion occurs, some mass turns into energy.
Galaxy A great island of millions to hundreds of bil-
lions of stars (plus “raw material” in the form of gas
and dust), separated from other galaxies by large gulfs
of space. We live in one such group called the Milky
Way Galaxy. (Astronomers generally write “Galaxy”
The Universe at Your Fingertips • Astronomical Society of the Pacic
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Page 3
when referring to ours and “galaxy” — lower case —
when referring to others.)
Galaxy Cluster a group of galaxies containing dozens
to thousands of members
Gamma Ray A form of electro-magnetic radiation,
consisting of waves with the highest energy.
Gamma-ray Burst Brief event which generates a huge
amount of gamma-ray energy, lasting from a fraction
of a second to a few minutes. Such bursts are now
known to come from rare, extremely violent events in
other galaxies.
General Theory of Relativity (or General Relativ-
ity) An overarching theory, developed by Albert
Einstein, which relates gravity and the nature of space
and time in the universe. Astronomers use the general
theory of relativity as the basis of modern cosmology.
Its ideas also help to explain the strange properties of
black holes.
Globular Cluster A crowded group of 100,000 or more
stars all traveling together, as part of a galaxy. ese
clusters are spherical in shape, hence their name. e
Milky Way Galaxy has more than 150 such globular
Gravity e pull of all matter on all other matter in the
universe. Gravity is one of the fundamental forces in
the cosmos, and is responsible for pulling structures
like stars together. (A more “sophisticated” or over-
arching theory of gravity is given by the General e-
ory of Relativity.)
Hubble’s Law In an expanding universe, the farther
away a galaxy is from an observer, the faster its speed
of moving away. is principle was rst noted in a
systematic way by the American astronomer Edwin
Hubble (in the 1920s). In the general theory of relativ-
ity, this behavior is expected from the fact that space-
time itself is stretching. e more space there is be-
tween galaxies (or groups of galaxies, to be precise),
the more space there is to stretch, and the faster the
galaxies get “carried away” from each other by that
Hypernova e explosion, at the end of its life, of a
very massive star, whose core collapses to be a super-
compressed, spinning black hole, stirring and en-
ergize the wreckage around it. Such hypernovae are
thought to be the explanation for one kind of gamma-
ray burst.
Infrared light A type of electro-magnetic radiation
with a longer wavelength than visible light. Infrared
light, discovered by William Herschel, is the type of
wave that human beings and chairs give o. (ey also
reect visible light when it is shining on them, but this
is not their radiation; it comes from another source.)
Infrared is a way for astronomers to detect cooler ob-
jects, such as stars that are in the process of forming.
Light Year e distance that light travels in one year;
roughly 9.5 trillion kilometers or 6 trillion miles. e
nearest star is a little over 4 light years away.
Local Group e group of several dozen galaxies to
which our Milky Way Galaxy belongs. Most of the
members of the group are galaxies smaller than the
Milky Way.
Luminosity e total energy output of a star or other
celestial object; a measure of the total of all the visible
light and other electromagnetic radiation an object
gives o.
Lunar Eclipse An eclipse of the Moon; it happens
when the Moon moves directly between the Earth and
the Sun, and thus into the Earth’s shadow.
Magnitude see apparent magnitude
Mass e total amount of material in a body.
Messier Catalog A catalog of “fuzzy” celestial objects
compiled by Charles Messier in the 18th century. He
compiled the catalog so he would not mistake the ob-
jects in it for comets (which were his passion). But the
catalog turned out to be a pioneering list of the most
noticeable nebulae, star clusters, and galaxies, and
is still used today. Astronomers oen call objects by
their number in this catalog: the Andromeda Galaxy
is M31, the Crab Nebula is M1.
Meteor A bit of solid debris from space, which vapor-
izes during its passage through the Earth’s atmosphere.
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Its high-speed interaction the air with air causes a brief
ash of light (which is sometimes called a “shooting
star” — although it has nothing to do with stars.)
Meteorite A rock from space which has survived pas-
sage through the Earths atmosphere.
Milky Way Galaxy e galaxy of stars in which the
Sun and the Earth are located; one of the two large
spiral galaxies in the Local Group.
Molecule A combination of two or more atoms chemi-
cally bound together. A water molecule, for example,
consists of two atoms of hydrogen and one atom of
Moon Any object that orbits a planet (sometimes also
called a satellite.) When astronomers write about the
Earth’s satellite, they call it the Moon, but other plan-
ets’ satellites are written as moons (lower case m).
Nebula A cloud of glowing gas and dust among the
stars. A nebula can oen be observed in regions where
new stars have recently been born and around stars
that are dying or have died. Before galaxies were un-
derstood, they were also classied as nebulae (as in
the term “spiral nebula”) but we don’t use the term
that way any more. e plural is nebulae.
Neutron Star e remains of a massive star that has ex-
ploded at the end of its life as a supernova. ese rem-
nants are very dense, because the violence of the stars
death has compressed them until electrons and protons
have merged to become neutrons. Typically, neutron
stars can contain more than twice the mass of our Sun
in a ball about 10–20 km across. Magnetic, spinning
neutron stars can sometimes be seen as pulsars.
Observatory A place where telescopes and other as-
tronomical instruments are housed and used. Obser-
vatories that have visible-light detecting telescopes
are now built on remote mountain tops, to escape the
lights of civilization.
Open Cluster A group of stars within the main disk
of a galaxy, containing a few dozen to a few thousand
member stars, all of whom were born in roughly the
same place at roughly the same time. Another name
used for such a group is galactic cluster.
Optical Telescope A telescope that is designed to col-
lect visible light (as opposed to other forms of electro-
magnetic radiation, such as radio waves, that are not
visible to the human eye.)
Orbit e path that an object takes as it revolves
around another object due to their mutual gravity. For
example, the Earth has an orbit around the Sun, and
the Moon and the Hubble Space Telescope both have
orbits around the Earth.
Ozone Layer A region of the Earth’s atmosphere
(roughly 10 to 20 miles above the surface) which has
a higher concentration of ozone (a molecule of oxy-
gen with three oxygen atoms in it). is ozone absorbs
harmful ultraviolet radiation coming from the Sun,
contributing to the conditions that make life on the
Earth’s surface possible.
Parsec A unit of distance equal to 3.26 light years (or
206,265 astronomical units.) is unit is still used by
astronomers, but is not so oen heard when astrono-
mers talk to the public.
Phases of the Moon e changing appearance of the
Moon as it revolves around the Earth, caused by the
diering amounts of sunlight we can see reected
from the Moon from our vantage point. (More gener-
ally, phases are the dierent appearances of a planet or
moon as it moves around its orbit.)
Planet An object of signicant size that orbits a star,
but is not itself a star. According to the new denition
of a planet adopted by the International Astronomical
Union, planets must have enough mass to be spherical
and must have their own independent orbits around
the Sun — as opposed to sharing orbits with a number
of other bodies. e Sun has eight planets according
to this denition. (Pluto is part of the Kuiper Belt and
Ceres is part of the Asteroid Belt, making them dwarf
Planetary Nebula A shell or shells of gas ejected by a
relatively low-mass star that is in the process of dying,
and becoming a white dwarf. e name is very mis-
leading — this “last gasp” material from a star at the
end of its life has nothing to do with planets.
Prominence An eruption of hot gas from the surface
of the Sun. Seen against the darkness of space, it can
look like a giant arcing ame.
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Astronomy Background 2.8 • Glossary of Basic Astronomical Terms
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Pulsar Originally, short for “pulsating radio star”.
ese objects, now known to be compact, rotating,
magnetic neutron stars, signal their presence to us by
giving o extremely regular pulses of electromagnetic
Quasar Originally, short for “quasi-stellar object” —
things that rst looked like a dim blue stars, but turned
out to be the bright centers of a distant galaxies. ese
central regions shine with so much energy that they
are much more easily detected than the full galaxy that
surrounds them. Today, we know that their brilliance
is connected with the energy produced by enormous
black holes in the crowded center of each galaxy.
Radial Outward from the center, like the radius of a
circle, or the spokes of a bicycle wheel. Astrono-
mers oen talk about “radial velocity” — the speed
of something in the cosmos in the direction outward
from us.
Radiation Waves of energy that move away (radiate)
from their source at the speed of light. By far the most
common form is electro-magnetic radiation, of which
light is the best-known example.
Radio Waves A type of electro-magnetic radiation
consisting of waves of long wavelength and low ener-
gy. Radio waves on Earth travel from the transmitting
tower of a radio station to the antenna in your car, for
example. Objects in the universe give o natural radio
waves (which would sound like “static” if we were to
translate them into sound.)
Red Giant A stage in the life of every star, when it
runs out of its initial fuel for fusion, and, as part of its
readjustment, expands to become much larger than
its original size. As the hot gas of the star expands,
it cools down and becomes reddish in color. A well-
known red giant is Betelgeuse, in the constellation g-
ure of Orion, the hunter.
Redshift e change in the colors of an astronomi-
cal object caused by its motion away from us. Chris-
tian Doppler discovered in the 19th century that the
motion of a source of light (or other radiation) away
from us or toward us changes its colors in a subtle but
measurable way. e faster an object moves away, the
greater its redshi is. By spreading out the light of a
star or galaxy into a spectrum, astronomers can thus
measure the shi of its colors and thus its speed to-
ward or away from us (objects moving toward us show
a blueshi). Galaxies beyond our immediate group of
neighbors ALL show a redshi, since they are partici-
pating in the expansion of the universe.
Reecting Telescope A telescope in which the light is
collected (and reected) by a mirror.
Refracting Telescope A telescope in which the light is
collected (and refracted or bent) by a lens.
Resolution e ability of a telescope to make out ne
details in an astronomical object or to separate the
image of two objects that are close to each other on
the sky. (Resolution in astronomy is measured in units
of angle on the sky: degrees, minutes of arc, or sec-
onds of arc.)
Right Ascension A system of measuring the position
of objects in the sky that is similar to the longitude
system we use on Earth. Right Ascension is measured
in hours and minutes around the celestial equator (a
circle in the sky that would result if we pushed the
Earth’s equator out into the sky.)
Second of Arc A very small measure of angle, oen
used in astronomy; equal to 1/3600th of a degree. A
U.S. dime, seen from about two miles away, takes up
one second of arc.
SETI e Search for Extra-Terrestrial Intelligence. e
branch of astronomy devoted to nding evidence of
intelligent life among the stars.
Solar Eclipse An eclipse of the Sun by the Moon;
when the Moon moves in front of the Sun as seen
from Earth.
Solar System e Sun and its family of planets, moons,
and assorted smaller chunks of material. Strictly
speaking, this term refers only to the Sun and not to
the other stars; but astronomers oen talk about “oth-
er solar systems”, when they refer to families of planets
around other stars.
Spectrum e array of wavelength or colors in a beam
of light (which can be studied when the white light is
spread out in a spectroscope), or a photograph of this
array of colors. More generally, the array of wavelengths
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Astronomy Background 2.8 • Glossary of Basic Astronomical Terms
Page 6
found in any beam of electro-magnetic radiation.
Spectroscope A device that allows scientists to ob-
serve the spectrum of radiation from some source. Us-
ing a spectroscope, for example, astronomers spread
the light of the Sun out into its component colors and
can learn about the Sun’s temperature, composition,
and motion.
Star A sphere of hot gas that shines under its own pow-
er. e energy that allows a star to shine comes from
the process of nuclear fusion. e Sun is our closest
example of a star.
Sunspot A cooler region of the Sun’s surface, which
looks dark in comparison to the hotter material
around it.
Supercluster (of galaxies) A grouping of galaxy clus-
ters. Such superclusters may contain as much mate-
rial as 10,000 or more Milky Way Galaxies and stretch
over hundreds of millions of light years.
Supernova An enormous explosion that causes the
death” of a massive star. Typically, such an explosion
leaves behind a neutron star, or a black hole. Another
type of supernova is an explosion that occurs because
a white dwarf in a double star system has re-ignited
aer a companion star has dumped a lot of fresh ma-
terial on it. (It is this second kind of supernova, which
always ares up to be roughly the same luminosity,
that has allowed astronomers to measure that the ex-
pansion of the universe is speeding up.)
Telescope An instrument that gathers light (or an-
other type of electromagnetic radiation) and brings
it to a focus. Telescopes allow astronomers to see or
photograph objects that are too dim to be seen with
the naked eye.
Transit When a smaller object passes in front of a larger
one in space; for example, a planet may transit a star.
Universe e sum total of all matter, radiation, and
space; everything that is accessible or can become ac-
cessible to our observations.
Variable Star A star whose luminosity (output of ener-
gy) changes with time — some change regularly, oth-
ers at random intervals. Many amateur astronomers
contribute to astronomy by monitoring such stars
carefully over long periods of time. (One class of vari-
able stars, called Cepheids, has special properties that
allow them to be used to determine distances.)
White Dwarf e collapsed, hot remnant of a low-
mass star at the end of its life. When the Sun becomes
a white dwarf, it is expected to be about as small as
two Earths across. White dwarf are extremely dense
objects, shining only by radiating away their heat en-
ergy. Ultimately, white dwarfs fade away into black
dwarfs — too cool to be seen with visible light.
Year e time it takes a planet to revolve once around
its star. e Earths year is 365 ¼ days long, but the
years of other planets are dierent.
A Few Astronomical Glossaries on the Web
NASA Imagine the Universe Site Dictionary (for
younger readers):
Amazing Space Glossary from the Space Telescope
Science Institute:
Case Western Reserve University, brief denitions of
astronomical terms:
Glossary from the PBS Program Seeing in the Dark:
Catalog of the Cosmos from the PBS Nova Program
Death Star:
NASA/IPAC Extragalactic Database Glossary (a bit
technical, but extremely thorough):
© 2010 Astronomical Society of the Pacific
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