Advanced Dictionary of Astronomical Terms
Radio Emission Produced by cold, low-density hydrogen in
Absolute Bolometeric Magnitude
The Absolute Magnitude we would observe if we could detect
Absolute Visual Magnitude
Intrinsic brightness of a star; the apparent visual
magnitude the star would have if it were 10 Parsecs away from earth.
The lowest possible temperature. The temperature at which
particles in a material, atoms or molecules, contain no energy of motion that can be
extracted from a body.
A dark line in a spectrum. Produced by the absence of
photons absorbed by atoms or molecules.
A Spectrum that contains absorption lines.
A change in velocity
Acceleration of Gravity
Falling objects fall with an increasing acceleration of 9.8
meters per second per second.
The sticking together of solid particles to form a larger
The whirling disk of gas that forms around a compact object
such as a white dwarf, neutron star or black hole as matter is drawn in
A telescope lens composed of two lenses ground from
different types of glass and designed to bring two selected colors to the same focus and
correct for chromatic aberration
A galaxy that is a source of excess radiation, usually
radio waves, X rays, gamma rays or some combination
Active Galaxy Nucleus (AGN)
The central energy source of an active galaxy
Optical elements whose position or shape is continuously
controlled by computers
Computer controlled telescope mirrors that can adjust for
some changes in seeing conditions
A telescope mounting capable of motion parallel to and
perpendicular to the horizon
1 x 10 -10 meters (used to measure wavelengths)
Measure of the rotation of the body around some point.
A type of total eclipse in which the moon is too far from
the earth to totally cover the suns surface. A ring of the photosphere surrounds the moon
during mideclipse. The corona is not visible and neither are the prominence because of the
glare from the photosphere.
Rock of aluminum and calcium silicates found in the lunar
Point in the earth's orbit where the earth is farthest away
from the Sun (summer for us)
Orbital point of greatest distance from the earth
Apparent Relative Orbit
The Orbit of one star in a visual binary with respect to
the other star as seen from earth
Apparent Visual Magnitude (mv)
Is the magnitude of the stars without compensating for
their distance from Earth.
On Venus, one of a number of round networks of fractures in
the crust, resembles spider webs.
The study of astronomy by ancient peoples
Groups of widely scattered stars (10 - 1000) moving
together through space; not gravitationally bound to clusters.
Named group for stars not identified as constellations
Small Rocky worlds, most of which lie between Mars and
Jupiter in the asteroid belt.
A binary star identified by its irregular proper motion
Astronomical Unit (AU)
Is the average distance from the Earth to the Sun. 1 AU =
93 million miles or 1.5 X 1011 meters or 1.5 X 109 km.
Is the study of the universe
Wavelength regions in which our atmosphere is transparent -
at visual wavelengths, infrared and radio.
The glowing light display that results when a planet's
magnetic field guides charged particles toward the north and south magnetic poles, where
they strike the upper atmosphere and excite atoms to emit photons.
The place where the Sun crosses the celestial equator
B Canis Majoris Variable
Short Period variables stars that do not lie in the
Barred Spiral Galaxy
A spiral galaxy with an elongated nuclei resembling a bar
from which the arm originate.
Dark, igneous rock characteristic of solidified lava.
Dark bands of clouds that circle Jupiter parallel to its
equator; generally red, brown, or blue-green; believed to be regions of descending gas.
Big Bang Theory
The theory that the universe began with a violent explosion
from which the expanding universe of galaxies formed.
Pairs of stars that orbit around their common center of
The energy needed to pull an electron away from its atom.
Opposite directed jets of gas ejected by some protostellar
BL Lac Object
Objects resemble quasars; thought to highly luminous cores
of distant galaxies.
Black Body Radiation
Radiation emitted by a hypothetical perfect radiator. The
spectrum is continuous, and the wavelength of maximum emission depends only on the body's
The end state of a white dwarf that has cooled to a low
A mass that has collapsed to such a small volume that its
gravity prevents the escape of all radiation; also, the volume of space from which the
radiation may not escape.
Blue and Red Shifts
Blue shifts: Shorting of wavelengths of observed light when
an object in moving toward the observer. Red Shifts: Lengthening of wavelengths of light
as the object moves away from the observer.
Small, dark clouds only about 1 light year in diameter that
contain 10-1000 solar masses of gas and dust. Believed to be related to star formation.
The boundary between the undisturbed solar wind and the
region being deflected around the planet or comet
A rock composed of fragments of earlier rocks bonded
A very cool, low luminosity star whose mass is not
sufficient to ignite nuclear fusion.
A source of bursts of X-rays or in some cases, gamma rays;
believed to be associated with neutron stars.
The theory that the moon formed elsewhere in the solar
system and was later captured by the earth.
The explosive ignition of carbon burning in some giant
stars. A possible cause of some supernova explosions
A reflecting telescope in which the secondary mirror
reflects light back down the tube through a hole in the center of the primary mirror
An imaginary line around the sky which is directly above
the earth's equator.
Imaginary sphere surrounding the Earth to which the stars,
planets, Sun and moon seem to be attached.
Center of Mass
Two bodies revolve around a common center, called the
balance point of the system.
Cepheid Variable Star
Variable stars with a period of 1-60 days. Their period is
related to luminosity.
The maximum mass of a white dwarf, about 1.4 solar masses.
A white dwarf of greater mass can not support itself and will collapse.
A CCD is an electronic device consisting of an array of
photosensitive elements, used to record images.
A line layer of gas just above the photosphere of the sun,
it is often marked by solar eruptions called solar flares and prominence.
A distortion found in refracting telescopes because lenses
focus different colors at slightly different distances. Images are consequently surrounded
by color fringes
The lateral velocity an object must have to remain in orbit
Constellations which appear around the celestial poles,
which never seem to rise or set.
A circular orbit, one which returns back on itself.
A model universe in which the average density is great
enough to stop the expansion and make the universe contract.
The method of determining the masses of galaxies in a
A series of nuclear reactions that use carbon as a catalyst
to combine four hydrogen atoms to make one helium atom plus energy; effective in stars
more massive than the sun.
The theory that the moon and the earth formed together.
The cloud of gas and dust around a contracting protostar
that conceals it at visible wavelengths.
The smearing out of a spectrum line because of collisions
among the atoms of the gas.
A numerical measure of the color of a star.
One of the small, icy bodies that orbit the sun and produce
tails of gas and dust when they near the sun.
A star that has collapsed to form a white dwarf, neutron
star or black hole.
The study of planets by comparing the characteristics of
The sequence in which different materials condense from the
solar nebula as we move outward from the sun.
Apparent arrangement of stars, usually named after ancient
gods, heroes, animals or mythological beings.
Continuity of Energy Law
One of the basic laws of stellar structure, The amount of
energy flowing out of the top of a shell must equal the amount coming in at the bottom
plus whatever energy is generated within the shell
Continuity of Mass Law
One of the basic laws of stellar structure. The total mass
of the star must equal the sum of the masses of the shells, and the mass must be
distributed smoothly throughout the star.
A spectrum in which there are no absorption or emission
The faint outer atmosphere of the Sun that is exposed
during a total solar eclipse.
On Venus, circular features, not caused by impacts, they
are domed plains caused by the rising plumes of molten rock from below.
A telescope designed to photograph the inner corona of the
An Area of the solar surface that is dark at X-ray
wavelengths; thought to be associated with divergent magnetic fields and the source of the
Atomic nuclei that enter earth's atmosphere at nearly the
speed of light. Some originate in solar flares, and some may come from supernova
explosions, but their true nature is not well understood.
The assumption that any observer in any galaxy sees the
same general features of the universe.
The study of the nature, origin and evolution of the
The focal arrangement of a reflecting telescope in which
mirrors direct the light o a fixed focus beyond the bounds of the telescope's movement.
typically in a separate room, used primarily for spectroscopy.
The average density of the universe needed to make its
The temperature and pressure at which vapor and liquid
phases of a material have the same density.
A nebula consisting of dust and gas blocking our view of
more distant stars.
Radio signals from Jupiter with wavelengths about 10m
Extremely high density matter in which pressure no longer
depends on temperature due to the quantum mechanical effects.
Density Wave Theory
Theory proposed to account for spiral arms as compressions
of the interstellar medium in the disk of the galaxy.
Diamond Ring Effect
Just as totality begins during a solar eclipse a small
portion of the Suns photosphere can peak out from behind the moon through a valley at the
edge of the lunar disk. It is not visible during every solar eclipse.
The rotation of a body in which different parts of the body
have different periods of rotation. This is true of the sun, Jovian planets, and the disk
of the galaxy.
The separation of planetary material according to density.
Blurred fringe surrounding and image caused by wave
properties of light. because of this no image detail smaller than the fringe can be seen
All material confined to the plane of the galaxy.
Objects whose luminosities or diameters are known; used to
find the distance to a star cluster or galaxy.
The difference between the apparent and absolute magnitude
of a star. A measure of how far away the star is.
The apparent daily rotation of the sky.
The smearing of spectral lines because of the motion of
atoms in the gas.
A change in the wavelength of radiation due to relative
radial motion of the source and the observer.
Double Galaxy Method
A method of finding the masses of galaxies from orbiting
pairs of galaxies.
A pair of stars close together in the sky. Not all double
stars are necessarily in orbit around each other.
The theory that double radio lobes are produced by pairs of
jets emitted in opposite directions from the centers of active galaxies.
Double-Line Spectroscopic Binary
A spectroscopic binary star in which spectral lines from
both stars are visible in the spectrum
A star that undergoes novalike explosions every few days or
weeks; believed to be associated with mass transfer onto a white dwarf in a binary system.
The theory that the earth's magnetic field is generated in
the conducting material of its molten core.
An offcenter circular path
Is the season when the Sun is close enough to a node for an
eclipse to occur, An eclipse season is 32 days. Any new moon during this period will cause
a solar eclipse. For Lunar eclipses the period is shorter only about 22 days. A full moon
occurring during this time will cause a lunar eclipse.
The 346.62 days it takes the sun to return to a node
A binary star system in which the stars eclipse each other.
The apparent path of the Sun around the Sky.
Pulverized rock scattered by meteorite impacts on a
The UV radiation produced in the upper atmosphere of
Jupiter, Saturn, Uranus by high-energy particles in the planet's magnetosphere.
Changing electric and magnetic fields that travel through
space and transfer energy from one place to another - light, radio waves and magnetism
A low mass atomic particles carrying a negative charge.
A closed curve enclosing two points (foci) such that the
total distance from one focus to any other point on the curve back to the other focus
equals a constant.
A galaxy that is round or elliptical in outline. It
contains little gas and dust, no disk or spiral arms, and a few hot, bright stars.
A bright line in a spectrum caused by the emission of
photons from atoms.
A cloud of gas excited by UV wavelengths of hot stars,
A spectrum containing emission lines
One of a number of states an electron may occupy in an
atom, depending on its binding energy.
An object that releases energy. Commonly used to refer to
the source of energy in active galactic nuclei
An attempt to explain the retrograde loop in the earth
centered universe, by attaching the planets to epicycles and having them revolve around
that, and in turn around the earth.
Ptolemy placed the earth offcenter and the opposite point
from the deferent on this off center circle is called the equant.
A telescope mounting that allows motion parallel to and
perpendicular to the celestial equator.
The region surrounding a rotating black hole within one
could not resist being dragged around the black hole. It is possible for a particle to
escape from the erosphere and extract energy from the black hole.
The velocity needed to escape from the surface of a body
The boundary of the region of a black hole from which no
radiation may escape. No event that occurs with the event horizon is visible to a distant
An atom in which an electron has moved from a lower to
The dimming of light by intervening material; commonly,
dimming by the interstellar medium.
A short focal length lens used to enlarge the image in a
telescope; the lens nearest the eye
A graphical representation of data in which an images is
colored to reveal additional detail
A way of explaining action at a distance.
An instrument that permits precise measurements at the
telescope of the position of visual binary stars and similar objects.
A photograph (usually of the sun) taken in the light of a
specific region of the spectrum - e.g., an H-alpha filtergram.
The theory that the moon formed by breaking away from the
A violent eruption on the sun's surface.
The emission spectrum of the chromosphere that is visible
for the few seconds during a total solar eclipse when the moon has covered the photosphere
but has not covered the chromosphere.
A model of the universe in which space-time is not curved.
In cosmology the circumstance that the early universe must
have contained almost exactly the right amount of matter to close the space-time (to make
Woolly, fluffy; used to refer to certain galaxies that have
a woolly appearance.
A distance from a lens to a point where it focuses parallel
rays of light.
The points around which an ellipse is drawn
The optical property of finely divided particles to
preferentially direct light in the original direction of the light's travel
The number of times a given event occurs in a given time;
for a wave the number of cycles that pass the observer in one second
The theory that large galaxies absorb smaller galaxies.
The low-density extensions of the halo of a galaxy; now
suspected to extend many times the visible diameter of the galaxy.
The four largest moons of Jupiter, named after their
A unit used to measure the strength of a magnetic field.
Observers can not distinguish locally between inertial
forces due to acceleration and uniform gravitational forces due to the presence of a
massive body. Mass tells space-time how to curve, and the curvature of space-time
(gravity) tells mass how to accelerate
Aristotle believed the universe was divided into two parts,
the earth corrupt and the heavens perfect and immutable. The geocentric universe described
the universe with the earth at the center.
Giant Molecular Cloud
Very large, cool clouds of dense gas in which stars form.
Large, cool, highly luminous stars in the upper right of
the H-R diagram. Typically 10-100 times the diameter of the sun.
Time when glaciers advance and engulf huge sheets of land
A sudden change in the period of a pulsar.
Globular Star Cluster
A star cluster containing 50,000 to 1 million stars in a
sphere about 75 light years in diameter; generally old metal poor and found in the
spherical component of the galaxy.
A linear feature on a planetary surface caused by the
faulting and sinking of portions of the crust.
The fine structure visible on the solar surface caused by
rising currents of hot gas and sinking currents of cool gas below the surface.
Material onto which microscopic parallel lines are
inscribed. used to create a spectrum of colors from light
Gravitational Lens Effect
The focusing of light from a distant galaxy or quasar by an
intervening galaxy to produce multiple images of the distant body.
Gravitational Red Shift
A lengthening of the wavelength of a photon due to its
escape from a gravitational field.
A transport of energy by the motion of waves in a
gravitational field; predicted by general relativity
The process by which a carbon dioxide atmosphere traps heat
and raises the temperature of a planetary surface.
Regions of the surface of Ganymede consisting of parallel
grooves; believed to have formed by repeated fracture and refreezing of the icy crust.
The lowest permitted orbit of an electron in an atom.
H II Region
A region of ionized hydrogen around a hot star.
A plot of the intrinsic brightness
versus the surface temperature of the stars. It separates the effects of temperature and
surface area versus spectral type, but also luminosity versus surface temperature or
The time required for half of the atoms in a radioactive
sample to decay.
The spherical region of a spiral galaxy containing a thin
scattering of stars, star clusters, and small amounts of gas.
Head-Tail Radio Galaxy
A radio galaxy with a contour consisting of a head and a
tail; believed caused by the motion of an active galaxy through the intergalactic medium.
Thermal energy present in a body as agitation (motion)
among its particles (atoms or molecules).
Heat of Formation
In planetology, the heat released by the infall of matter
during the formation of a planetary body.
The Sun is at the center of the universe and everything
revolves around that. Was first proposed by Copernicus.
The study of the interior of the sun by the analysis of its
modes of vibration.
The explosive ignition of helium burning that takes place
in some giant stars.
Small nebula that vary irregularly in brightness; believed
associated with star formation.
The formation of a planet by the accumulation of
planetisimals of different composition - e.g. first iron particles, then silicates
A star with a large space velocity. Such stars are halo
stars passing through the disk of the galaxy at steep angles.
The assumption that, on the large-scale, matter is
uniformly spread throughout the universe.
The formation of a planet by the accumulation of
planetisimals of the same composition.
In the H-R diagram of a globular cluster, the sequence of
stars extending from the red giants toward the blue side of the diagram; includes RR Lyrae
A chart showing the location of heavenly bodies among the
zodiacal signs and with respect to the horizon at the persons birth.
In radio astronomy, a bright spot in a radio lobe.
The linear relation between the distance to a galaxy and
its radial velocity.
Hubble Constant (H)
A measure of the rate of expansion of the universe; the
average value of velocity of recession divided by distance.
The balance between weight of the material pressing
downward on a layer in a star and the pressure in that layer.
A fine network of filaments covering the sky detected in
the far infrared by the IRAS satellite; believed associated with dust in the interstellar
A sudden brightening of an object at infrared wavelengths
Electromagnetic Radiation with wavelengths intermediate
between visible light and radio waves
The region of the H-R diagram in which stars are unstable
to pulsation. A star passing through this strip becomes a variable star.
The relatively smooth terrain on Mercury.
Time when glaciers melt back (cycles are roughly 40,000
Interstellar Absorption Lines
Dark lines in some stellar spectra that are formed by the
The gas and dust distributed between the stars.
Inverse Square Law
Force of gravity decreases as the square of the distance
Io Flux Tube
A tube of magnetic lines and electric currents connecting
Io and Jupiter.
An atom that has lost or gained one or more electrons.
The process in which atoms lose or gain electrons.
A galaxy with a chaotic appearance, large clouds of gas and
dust, and both population I and population II stars, but without spiral arms.
Atoms that have the same number of protons but a different
number of neutrons.
The assumption that in its general properties the universe
looks the same in every direction
A unit of energy roughly equivalent to the energy given up
when an apple falls on the floor. Equivalent to the force of 1 Newton acting over a
distance of 1 meter; 1 joule per second = 1 watt of power.
Jupiterlike planets with large diameters and low densities.
The Julian Day is the number of days since the year -4712.
The Julian Day begins at 12:00 Noon Greenwich mean time.
Strangely disturbed regions of the moon opposite the
locations of the Imbrium basin and Mare Oriental
Kelvin Temperature Scale
The temperature, in Celsius (Centigrade) degrees, measured
above absolute zero.
Orbital motion in accord with Kepler's laws of planetary
Kerr Black Hole
A solution to the equations of general relativity that
describes the properties of a rotating black hole.
A unit of distance equal to 1000 pc or 3260 ly.
A set of laws that describes the Absorption and emission of
light by matter.
Points of stability in the orbital plane of a binary
system, planet, or moon. One is located 60 degrees ahead and one 60 degrees behind the
orbiting bodies. Another is located between the orbiting bodies.
The theory that the moon formed from debris ejected during
a collision between the earth and a large planetisimal.
A graph of brightness versus time commonly used in
analyzing variable stars and eclipsing binaries.
Light Gathering Power
The ability of a telescope to collect light. Proportional
to the are of the telescope objective lens or mirror
Is the distance that light travels in one year.
Abbreviation is ly
A theory that a neutron star produces pulses of radiation
by sweeping radio beams around the sky as it rotates.
The edge of the apparent disk of a body, as in the
"limb of the moon".
The decrease in the brightness of the sun or other body
from its center to its limb.
Line of Nodes
The nodes of the moons orbit are the points where it passes
through the plane of the earth's orbit. An eclipse season occurs whenever the line
connecting these nodes (line of nodes) points toward the sun.
A graph of light intensity verses wavelength showing the
shape of an absorption line
Liquid Metal Hydrogen
A form of hydrogen under high pressure that is a good
A curved cliff such as those found on Mercury.
The theory that quasars are not at great distances but
Long Period Variable
A variable star with a period ranging from 100 days to over
Look Back Time
The amount by which we look into the past when we look at a
distant galaxy; a time equal to the distance to the galaxy in light-years.
The total amount of energy a star radiates in one second.
A category of stars of similar luminosity; determined by
the widths of lines in their spectra.
Lyman, Balmer and Paschen Series
Spectral lines in the UV spectrum of hydrogen produced by
transitions whose lowest orbit of electrons is at ground state (Lyman). Spectral lines in
the visible and near UV of hydrogen produced by transitions whose lowest orbit is second
(Balmer). Spectral lines in the infrared spectrum of hydrogen produced by transitions
whose lowest orbit is the third. (Paschen)
Small irregular galaxies that are companions to the Milky
Way; visible in the southern sky.
The volume of space around a planet within which the motion
of charged particles is dominated by the planetary magnetic field rather than the solar
The ability of a telescope to make an image larger
Method developed by Hipparchus who divided the stars into 6
classes. The brightest stars are first class and those slightly fainter are second class.
The sixth class is the faintest stars that he cold see with the unaided eye. The magnitude
scale is logarithmic like the eye.
The region of the H-R diagram running from upper left to
lower right, which includes roughly 90 percent of all stars.
The layer of dense rock and metal oxides that lies between
the molten core and the surface of the earth; also, similar layers in other planets.
One of the lunar lowlands filled by successive flows of
A measure of the amount of matter in an object
A measure of the ratio of the masses in a single-line
spectroscopic binary. Also includes the inclination, which is unknown for some systems.
Maunder Butterfly Diagram
A graph showing the latitude of sunspots versus time, first
plotted by W.W. Maunder in 1904.
A period of less numerous sunspots and other solar activity
A unit of distance equal to 1 million pc.
IN astronomical usage, all atoms heavier than helium.
A small bit of matter heated by friction to incandescent
vapor as it falls into the atmosphere
A meteor that has survived its passage through the
atmosphere and strikes the ground.
A meteor in space before it enters the earth's atmosphere
Chasms that split the midocean rises where the crustal
plates move apart.
One of the undersea mountain ranges that push up from the
seafloor in the center of the oceans.
Minute of Arc
A measurement of the sky which includes degrees, minutes
and seconds.. There are 60 minutes of arc in one degree.
Unobserved mass in clusters of galaxies believed to provide
sufficient gravity to bind the cluster together.
An intellectual concept of how nature works
Two or more atoms bonded together.
The measurement of the amount of motion. the product of
mass and velocity
Morning and Evening Stars
A planet visible in the Morning shortly before sunrise is a
morning star, a planet visible just before sunset is the Evening Star.
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Objects move toward their proper place. earth and water
downward, fire and air upward
A cloud of gas and dust in space.
A neutral massless atomic particle that travels at the
speed of light.
An atomic particles with no charge and about the same mass
as a proton.
A small highly dense star composed almost entirely of
tightly packed neutrons; radius about 10 km.
A focal arrangement of a reflecting telescope in which a
diagonal mirror reflects light out the side of the telescope for easier access
The point twice a month where the moon crosses the
ecliptic. Once a month the moon crosses heading north and two week later crosses again
North and South Celestial Poles
The north ands south pivots points around which the sky
appears to rotate.
From Latin meaning "new"; a sudden brightening of
a star, making it appear as a new star in the sky; believed associated with eruptions on
white dwarfs in binary systems.
The spherical cloud of stars that lies at the center of
The production of elements heavier than helium by the
fusion of atomic nuclei in stars and during supernova explosions.
The central core of an atom, containing protons and
neutrons; carries a net positive charge.
In a refracting telescope, the long focal length lens that
forms an image of the object viewed; the lens closest to the object
In a reflecting telescope the principle mirror (reflecting
surface) that forms an image of the object viewed
A sphere flattened such that its polar diameter is smaller
than its equatorial diameter.
The flattening of a spherical body; usually caused by
The passage of a larger body in front of a smaller body.
The conflict between observation and theory as to why the
night sky should or should not be dark.
The resistance of a gas to the passage of radiation.
Open (Escape) Orbit
An orbit which leads away from the central body, never to
Open Star Cluster
A cluster of 10 to 10,000 stars with an open, transparent
appearance. The stars are not tightly grouped. Usually relatively young and located in the
disk of the galaxy.
A model universe in which the average density is less than
the critical density needed to halt the expansion of the universe.
A binary star in which the stars are only apparently
associated. One star is nearby and one is more distant.
Oscillating Universe Theory
The theory that the universe begins with a big bang,
expands, slows by its own gravity, collapses to create another big bang.
The release of gas from a planets interior.
In earth's atmosphere, the layer of oxygen ions (O3) lying
15 to 30 km high that protects the surface by absorbing ultraviolet rays.
The apparent change in the position of an object due to a
change in the location of the observer. It was because ancient astronomers did not observe
parallax that they though the earth was the center of the universe and everything in the
heavens revolved around it.
A hypothetical distance to a star whose parallax is one
arcsecond; 1pc = 206,265 A.U. = 3.26 ly.
Partial Eclipse (lunar or Solar)
Partial eclipses are caused when the moon passes through
only part of the umbra shadow of the earth. Or when the earth passes through the only part
of the umbra shadow of the moon.
Path of Totality
The path of a total eclipse that is swept out by the umbra
shadow of the moon on the earth.
A Partial blocking of the Sun by the Earth creates an
Penumbra Shadow. The Sunlight is dimmed but not extinguished.
When the moon passes only through the penumbra shadow of
the earth or when the earth passes through only the penumbra shadow of the moon.
Point of closet approach to the earth
Point in earth's orbit around the sun where it is closest
to the Sun (winter for us)
A graph showing the relation-between period of pulsation
and intrinsic brightness among Cepheid variable stars.
A device used to measure the intensity and color of light
A quantum of electromagnetic energy Carries an amount of
energy that depends inversely on its wavelength
The bright disk of the sun that is covered completely by a
total Solar eclipse.
Always found near the ecliptic (except Pluto)
An expanding shell of gas ejected by a star in the latter
stages of its evolution.
One of the small bodies that formed from the solar nebula
and eventually grew into protoplanets.
A material with properties of a solid but capable of
flowing under pressure.
The constant destruction and renewal of the earth's surface
by the motions of sections of the crust.
The axis of a telescope around which the celestial sphere
Poor Galaxy Cluster
An irregularly shaped cluster that contains fewer than 1000
galaxies, many spiral, and no giant ellipticals.
Stars rich in atoms heavier than helium; nearly always
relatively young stars found in the disk of the galaxy.
Stars poor in atoms heavier than helium; nearly always
relatively old stars found in the halo, globular clusters, or the nuclear bulge.
The angular direction of one body with respect to another;
measured from north toward the east; typically used in the study of visual binaries.
A wobbling of the Earth's Axis. It takes 26,000 years for
the earth to complete one wobble.
Pressure (P) Waves
In geophysics, mechanical waves of compression and
rarefaction that travel through the earth's interior
In the light curve of an eclipsing binary, the deeper
The point at which an object mirror forms an image in a
Earth's first air, composed of gases from the solar nebula.
Primordial Background Radiation
Radiation from the hot clouds of the big-bang explosion.
Because of its large red shift it appears to come from a body whose temperature is only
A sphere stretched along its polar axis so its polar
diameter is greater than its equatorial diameter.
Eruptions of the solar surface. Visible during total solar
The rate at which a star moves across the sky. Measured in
arc seconds per year.
A positively charged atomic particle contained in the
nucleus of an atom. The nucleus of hydrogen atom.
A series of three nuclear reactions that build a helium
atom by adding together protons. The main energy source in our sun.
Massive object resulting from the coalescence of
planetisimals in the solar nebula and destined to become a planet.
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A collapsing cloud of gas and dust destined to become a
A source of short, precisely times radio bursts believed to
be spinning neutron stars.
The study of behavior of atoms and atomic particles.
Small powerful source of energy believed to be the active
core of very distant galaxies.
Quasi-Periodic Object (QPO)
Certain X-rays sources that "flicker" rapidly for
(Vr) That component of an object's velocity directed away
from or toward the earth.
Radial Velocity Curve
A graph of the velocity of recession or approach of the
stars in a spectroscopic binary.
The force exerted on the surface of a body by its
absorption of light. Small particles floating in the solar system can be blown outward by
the pressure of sunlight.
A galaxy that is strong source of radio signals.
Two or more radio telescopes that combine their signals to
achieve the resolving power of a larger telescope
Ejecta from meteorite impacts forming white streamers
radiating from some lunar craters.
The stage within 1 million years of the big bang when the
gas became transparent to radiation
Stars that erupt as nova every few dozen years.
Cool, low mass stars on the lower main sequence
A telescope which uses a concave mirror to focus light into
A nebula produced by starlight reflecting off dust
particles in the interstellar medium.
A telescope that forms images by bending light through an
A soil made up of crushed rock fragments.
The age of a geological feature referred to other features.
For example, relative ages tell us the lunar maria and younger than the highlands.
Relativistic Jet Model
An explanation of superluminal expansion based on a high
velocity jet from a quasar directed approximately toward the earth.
Relativistic Red Shift
The red shift due to Doppler effect for objects traveling
near the speed of light.
The ability of a telescope to reveal fine detail. depends
on the diameter of the telescope objective
The coincidental agreement between two periodic phenomena;
commonly applied to agreements between orbital periods, which can make orbits more or less
The planets sometimes speed up in their movements, slow
down, stop and even reverse direction. The motion traces out a retrograde loop.
Rich Galaxy Cluster
A cluster containing over 1000 galaxies, mostly elliptical,
scattered over a volume about 3 Mpc in diameter.
A long, straight, deep valley produced By the separation of
A galaxy that resembles a ring around a bright nucleus;
believed to be the result of a head-on collision of two galaxies.
The minimum distance between a planet and a satellite that
holds itself together by its own gravity. If a satellites orbit brings it inside the Roche
limit, tidal forces will break the satellite up.
The most common type of terrain on Venus.
A graph of orbital velocity versus radius in the disk of a
RR Lyrae Variable
Variable stars with periods of 12-24 hours, common in some
The powerful radio source located at the core of the Milky
After 18 years and 11 1/3 days the eclipse pattern repeats
The saros cycle does not mean the eclipse will occur in the same place. Sine the cycle
take one third of a day. The earth will have rotated 8 hours westward from the original
location. It takes 3 saros cycles for the eclipse to repeat in the exact same place or 54
years and 1 month.
A photographic telescope that take wide angle photographs
Schmidt Cassegrain Telescope
A cassegrain telescope that uses a thin correcting lens as
in a schmidt camera
The radius of the event horizon around a black hole.
A method of writing large numbers in a simple way. For
example 380,000 is written 3.8 x106. In scientific notation a number is always written as
a x 10 h Where a >= 1 and a <10.
Caused by the tilt of the earth's Axis 23.5 degrees
Second of Arc
See Minute of Arc. 60 seconds of arc make up one minute.
The gases outgassed from a planet's interior; rich in
In the light curve of an eclipsing binary, the shallower
In a reflecting telescope, the mirror that reflects the
image to a point for easy observation
Atmospheric conditions on a given night When the atmosphere
is unsteady, producing blurred images the seeing is considered poor
A mechanical vibration that travels through the earth.
Usually caused by an earthquake.
An instrument that records seismic waves.
Self-Sustaining Star Formation
The process by which the birth of stars compress the
surrounding gas clouds and triggers the formation of more stars, proposed to explain
Half the long diameter of an ellipse
An otherwise normal spiral galaxy with an unusually bright,
small core that fluctuates in brightness; believed to indicate the core is erupting.
Shear (S) Waves
Mechanical waves that travel through earth's interior by
the vibration of particles perpendicular to the direction of the wave travel.
A satellite that, by its gravitational field, confines
particles to a planetary ring.
Wide, low profile volcanic cones produced by highly liquid
A sudden change in pressure that travels as an intense
The motion and gears on a telescope that turn westward to
keep it pointing at a star
(Sidereal Month) 27.5 Days - Actual Lunar orbit around the
earth. 1 revolution with respect to the stars. Moon drifts eastward by 13 degrees per day.
Single-Line Spectroscopic Binary
A spectroscopic binary in which lines of one star are
visible in the spectrum.
The object of zero radius into which the matter of a black
hole is believed to fall.
A narrow, winding valley on the moon caused by ancient lava
flows along narrow channels.
Apparently young plains on Mercury formed by lava flows at
or soon after the formation of the Caloris Basin
A measure of the energy output of the sun. The total solar
energy striking 1 sq. meter just above the earth's atmosphere in 1 second.
Solar Nebula Theory
The theory that the planets formed from the same cloud of
gas and dust that formed the sun.
Rapidly moving atoms and ions that escape from the solar
corona and blow outward through the solar system.
Observers can never detect their uniform motion except
relative to other objects. The velocity of light is constant for all observers
A star's position in the temperature classification system
O,B,A,F,G,K, and M. Based on the appearance of the star's spectrum.
The arrangement of spectral classes (O,B,A,F,G,K, and M)
ranging from hot to cool stars.
Device that separates light by wavelengths to produce a
Supernova Type I
The explosion of a star believed to be caused by mass
transfer to a white dwarf.
Supernova Type II
The explosion of a star believed to be caused by the
collapse of a massive star.
Radiation emitted when high speed electrons move through a
(Synodic Month) - 29.5 days - One revolution with respect
to the Sun. This is the time frame that determines lunar phases. Used as the basis for the
first Roman Calendar.
T Tauri Stars
Young stars surrounding by gas and dust. Believed to be
contracting toward the main sequence.
A measure of the velocity of random motions among atoms or
molecules in a material.
The dividing line between daylight and darkness on a planet
Earthlike planets - small, dense, rocky.
The locking of the rotation of a body to its revolution
around another body.
The heating of a planet or satellite because of friction
caused by tides.
The slowing of time in curved space time, believed to occur
as one approaches the speed of light or crosses the even horizon of a black hole.
A simple series of steps that produces numbers
approximately matching the sizes of the planetary orbits.
Total Eclipse (lunar or Solar)
When the umbra part of the Earth's Shadow (lunar eclipse)
totally blocks the light being received by the moon, these can only occur during a Full
Moon. A Total Solar Eclipse is the earth passing through the umbra shadow of the moon and
can only occur during a New moon.
The movement of an electron from one atomic orbit to
The velocity of a star perpendicular to the line of sight.
A geometrical solid whose three axes are equal.
Triple Alpha Process
The nuclear fusion process that combines three helium
nuclei (alpha particles) to make one carbon nucleus.
True Relative Orbit
The orbit of one star in a visual binary with respect to
the other star after correction for orbital inclination.
Tuning Fork Diagram
A system of classification for elliptical, spiral and
The point on the H-R diagram where a cluster's stars turn
off the main sequence and move toward the red giant region revealing the approximate age
of the cluster.
Electromagnetic Radiation with wavelengths shorter than
visible light but longer than X-rays
Is the Earth shadow that is total. No part of the Sun can
be seen when in the umbra shadow
The density a planet would have if its gravity did not
Uniform circular motion
Plato argued that the most perfect form should be a circle
and therefore motions of the heavens should be made of up combinations of circular motion.
The most perfect motion would be uniform circular motion, so astronomers of ancient times
tried to describe the motions of the heavens in these terms
The assumption that the physical laws observed on earth
apply throughout the universe.
Van Allen Belts
Radiation belts of high-energy particles trapped in the
A star whose brightness changes periodically.
A directed rate of motion
Velocity Dispersion Method
A method of finding a galaxy's mass by observing the range
of velocities within the galaxy.
The place where the Sun crosses the celestial equator
Very Long Baseline
The use of radio telescopes located thousands of miles
apart to resolve detail in radio sources.
A porous rock formed by solidified lava with trapped
Motion other than natural motion
A binary star system in which two stars are separately
visible in a telescope.
The distance between successive peaks or troughs of a wave
Wavelength of Maximum
The wavelength at which a perfect radiator emits the
maximum amount of energy; depends only on the object's temperature.
White Dwarf Stars
Dying stars that have collapsed to the size of the earth
and are slowly cooling off; at the lower left of the H-R diagram.
The point on the ecliptic where the Sun reaches it most
The splitting of spectral lines into multiple components
when the atoms are in a magnetic field.
Point in the Sky that is directly overhead.
Zero-Age Main Sequence (ZAMS)
The locus in the H-R- diagram where stars first reach
stability as hydrogen burning stars.
The 12 constellations near the ecliptic through which the
Zone of Avoidance
A region around the Milky Way where almost no galaxies are
visible because our view is blocked by dust in our galaxy.
Yellow-white regions that circle Jupiter parallel to its
equator; believed to be areas of rising gas.
Compiled by Don Ware