Advanced Dictionary of Astronomical Terms




21-cm Radiation

Radio Emission Produced by cold, low-density hydrogen in interstellar space.


Absolute Bolometeric Magnitude

The Absolute Magnitude we would observe if we could detect all wavelengths

Absolute Visual Magnitude

Intrinsic brightness of a star; the apparent visual magnitude the star would have if it were 10 Parsecs away from earth.

Absolute Zero

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.

Absorption Line

A dark line in a spectrum. Produced by the absence of photons absorbed by atoms or molecules.

Absorption Spectrum

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 particle.

Accretion Disk

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

Achromatic Lens

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

Active Galaxy

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

Active Optics

Optical elements whose position or shape is continuously controlled by computers

Adaptive Optics

Computer controlled telescope mirrors that can adjust for some changes in seeing conditions

Alt-Azimuth Mounting

A telescope mounting capable of motion parallel to and perpendicular to the horizon


1 x 10 -10 meters (used to measure wavelengths)

Angular Momentum

Measure of the rotation of the body around some point.

Annual Eclipse

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 highlands.


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.

Astrometric Binary

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

Atmospheric Window

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.

Autumnal Equinox

The place where the Sun crosses the celestial equator heading south.


B Canis Majoris Variable

Short Period variables stars that do not lie in the instability strip

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.

Binary Stars

Pairs of stars that orbit around their common center of mass

Binding Energy

The energy needed to pull an electron away from its atom.

Bipolar Flow

Opposite directed jets of gas ejected by some protostellar objects.

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 temperature.

Black Dwarf

The end state of a white dwarf that has cooled to a low temperature

Black Hole

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.

Bok Globules

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.

Bow Shock

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 together.

Brown Dwarf

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.


Capture Hypothesis

The theory that the moon formed elsewhere in the solar system and was later captured by the earth.

Carbon Detonation

The explosive ignition of carbon burning in some giant stars. A possible cause of some supernova explosions

Cassegrain Telescope

A reflecting telescope in which the secondary mirror reflects light back down the tube through a hole in the center of the primary mirror

Celestial Equator

An imaginary line around the sky which is directly above the earth's equator.

Celestial Sphere

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.

Chandrasekhar Limit

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.

Charge-Coupled Device

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.

Chromatic Aberration

A distortion found in refracting telescopes because lenses focus different colors at slightly different distances. Images are consequently surrounded by color fringes

Circular Velocity

The lateral velocity an object must have to remain in orbit

Circumpolar Constellation

Constellations which appear around the celestial poles, which never seem to rise or set.

Closed Orbit

A circular orbit, one which returns back on itself.

Closed Universe

A model universe in which the average density is great enough to stop the expansion and make the universe contract.

Cluster Method

The method of determining the masses of galaxies in a cluster.

CNO Cycle

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.

Co-Accretion Hypothesis

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.

Collisional Broadening

The smearing out of a spectrum line because of collisions among the atoms of the gas.

Color Index

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.

Compact Object

A star that has collapsed to form a white dwarf, neutron star or black hole.

Comparative Planetology

The study of planets by comparing the characteristics of different examples.

Condensation Sequence

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.

Continuous Spectrum

A spectrum in which there are no absorption or emission lines.


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 sun.

Coronal Hole

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 solar wind.

Cosmic Ray

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.

Cosmological Principle

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 universe.

Coude Focus

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.

Critical Density

The average density of the universe needed to make its curvature flat.

Critical Point

The temperature and pressure at which vapor and liquid phases of a material have the same density.


Dark Nebula

A nebula consisting of dust and gas blocking our view of more distant stars.

Decameter Radiation

Radio signals from Jupiter with wavelengths about 10m

Decimeter Radiation

Radio signals from Jupiter with wavelengths about 0.1m


The larger circle of which an epicycle revolves (the earth in this case).

Degenerate Matter

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.

Differential Rotation

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.

Diffraction Fringe

Blurred fringe surrounding and image caused by wave properties of light. because of this no image detail smaller than the fringe can be seen

Dirty Snowball Theory

The hypothesis that comets are kilometer-size balls of ices with embedded impurities.

Disk Component

All material confined to the plane of the galaxy.

Distance Indicator

Objects whose luminosities or diameters are known; used to find the distance to a star cluster or galaxy.

Distance Modulus

The difference between the apparent and absolute magnitude of a star. A measure of how far away the star is.

Diurnal Motion

The apparent daily rotation of the sky.

Doppler Broadening

The smearing of spectral lines because of the motion of atoms in the gas.

Doppler Effect

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.

Double Stars

A pair of stars close together in the sky. Not all double stars are necessarily in orbit around each other.

Double-Exhaust Model

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

Dwarf Nova

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.

Dynamo Effect

The theory that the earth's magnetic field is generated in the conducting material of its molten core.



An offcenter circular path

Eclipse Season

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.

Eclipse Year

The 346.62 days it takes the sun to return to a node

Eclipsing Binary

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 planetary surface.


The UV radiation produced in the upper atmosphere of Jupiter, Saturn, Uranus by high-energy particles in the planet's magnetosphere.

Electromagnetic Radiation

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.

Elliptical Galaxy

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.

Emission Line

A bright line in a spectrum caused by the emission of photons from atoms.

Emission Nebula

A cloud of gas excited by UV wavelengths of hot stars,

Emission Spectrum

A spectrum containing emission lines

Energy Level

One of a number of states an electron may occupy in an atom, depending on its binding energy.

Energy Machine

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.

Equatorial Mounting

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.

Escape Velocity

The velocity needed to escape from the surface of a body

Event Horizon

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 observer.

Excited Atom

An atom in which an electron has moved from a lower to higher orbit.


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


False Color

A graphical representation of data in which an images is colored to reveal additional detail


A way of explaining action at a distance.

Filar Micrometer

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.

Fission Hypothesis

The theory that the moon formed by breaking away from the earth.


A violent eruption on the sun's surface.

Flash Spectrum

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.

Flat Universe

A model of the universe in which space-time is not curved.

Flatness Problem

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 space-time flat)


Woolly, fluffy; used to refer to certain galaxies that have a woolly appearance.

Focal Length

A distance from a lens to a point where it focuses parallel rays of light.


The points around which an ellipse is drawn

Forward Scattering

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


Galactic Cannibalism

The theory that large galaxies absorb smaller galaxies.

Galactic Corona

The low-density extensions of the halo of a galaxy; now suspected to extend many times the visible diameter of the galaxy.

Galilean Satellites

The four largest moons of Jupiter, named after their discoverer Galileo.


A unit used to measure the strength of a magnetic field.

General Relativity

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

Geocentric Universe

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.

Giant Stars

Large, cool, highly luminous stars in the upper right of the H-R diagram. Typically 10-100 times the diameter of the sun.

Glacial Period

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.

Graben Rille

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.

Gravitational Wave

A transport of energy by the motion of waves in a gravitational field; predicted by general relativity

Greenhouse Effect

The process by which a carbon dioxide atmosphere traps heat and raises the temperature of a planetary surface.

Grooved Terrain

Regions of the surface of Ganymede consisting of parallel grooves; believed to have formed by repeated fracture and refreezing of the icy crust.

Ground State

The lowest permitted orbit of an electron in an atom.


H II Region

A region of ionized hydrogen around a hot star.

H-R Diagram

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 color.


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.

Heliocentric Universe

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.

Helium Flash

The explosive ignition of helium burning that takes place in some giant stars.

Herbig-Haro Object

Small nebula that vary irregularly in brightness; believed associated with star formation.

Heterogeneous Accretion

The formation of a planet by the accumulation of planetisimals of different composition - e.g. first iron particles, then silicates

High-Velocity Star

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.

Homogeneous Accretion

The formation of a planet by the accumulation of planetisimals of the same composition.

Horizontal Branch

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 stars.


A chart showing the location of heavenly bodies among the zodiacal signs and with respect to the horizon at the persons birth.

Hot Spot

In radio astronomy, a bright spot in a radio lobe.

Hubble's Law

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.

Hydrostatic Equilibrium

The balance between weight of the material pressing downward on a layer in a star and the pressure in that layer.


Infrared Cirrus

A fine network of filaments covering the sky detected in the far infrared by the IRAS satellite; believed associated with dust in the interstellar medium.

Infrared Outburst

A sudden brightening of an object at infrared wavelengths

Infrared Radiation

Electromagnetic Radiation with wavelengths intermediate between visible light and radio waves

Instability Strip

The region of the H-R diagram in which stars are unstable to pulsation. A star passing through this strip becomes a variable star.

Intercrater Plain

The relatively smooth terrain on Mercury.

Interglacial Period

Time when glaciers melt back (cycles are roughly 40,000 years)

Interstellar Absorption Lines

Dark lines in some stellar spectra that are formed by the interstellar gas.

Interstellar Medium

The gas and dust distributed between the stars.

Interstellar Reddening 

The process in which dust scatters blue light out of starlight and makes the stars look redder than they actually are.

Inverse Square Law

Force of gravity decreases as the square of the distance increases

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.

Irregular Galaxy

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


Joule (J)

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.

Jovian Planets

Jupiterlike planets with large diameters and low densities.

Julian Day

The Julian Day is the number of days since the year -4712. The Julian Day begins at 12:00 Noon Greenwich mean time.

Jumbled Terrain

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.

Keplerian Motion

Orbital motion in accord with Kepler's laws of planetary motion.

Kerr Black Hole

A solution to the equations of general relativity that describes the properties of a rotating black hole.

Kiloparsec (kpc)

A unit of distance equal to 1000 pc or 3260 ly.

Kirchoff's Laws

A set of laws that describes the Absorption and emission of light by matter.


Lagrangian Point

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.

Large-Impact Hypothesis

The theory that the moon formed from debris ejected during a collision between the earth and a large planetisimal.

Light Curve

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

Lighthouse Theory

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".

Limb Darkening

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.

Line Profile

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 electrical conductor.

Lobate Scarp

A curved cliff such as those found on Mercury.

Local Hypothesis

The theory that quasars are not at great distances but relatively nearby.

Long Period Variable

A variable star with a period ranging from 100 days to over 400 days.

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.

Luminosity Class

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)


Magellanic Cloud

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 wind.

Magnifying Power

The ability of a telescope to make an image larger

Magnitude Scale

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.

Main Sequence

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 dark lava.


A measure of the amount of matter in an object

Mass Function

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.

Maunder Minimum

A period of less numerous sunspots and other solar activity from 1645-1715.

Megaparsec (Mpc)

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

Mid-ocean Rift

Chasms that split the midocean rises where the crustal plates move apart.

Midocean Rise

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.

Missing Mass

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.



(nm) 1 x 10-9 meters

Natural Motion

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.

Neutron Star

A small highly dense star composed almost entirely of tightly packed neutrons; radius about 10 km.

Newtonian Focus

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 heading south.

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.

Nuclear Bulge

The spherical cloud of stars that lies at the center of spiral galaxies.


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.


Objective Lens

In a refracting telescope, the long focal length lens that forms an image of the object viewed; the lens closest to the object

Objective Mirror

In a reflecting telescope the principle mirror (reflecting surface) that forms an image of the object viewed

Oblate Spheroid

A sphere flattened such that its polar diameter is smaller than its equatorial diameter.


The flattening of a spherical body; usually caused by rotation


The passage of a larger body in front of a smaller body.

Olber's Paradox

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 return

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.

Open Universe

A model universe in which the average density is less than the critical density needed to halt the expansion of the universe.

Optical Binary

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.

Ozone Layer

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.

Penumbra Eclipse

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)

Period-Luminosity Diagram

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.

Planet Motion

Always found near the ecliptic (except Pluto)

Planetary Nebula

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.

Plate Tectonics

The constant destruction and renewal of the earth's surface by the motions of sections of the crust.

Polar Axis

The axis of a telescope around which the celestial sphere rotates

Poor Galaxy Cluster

An irregularly shaped cluster that contains fewer than 1000 galaxies, many spiral, and no giant ellipticals.

Population I

Stars rich in atoms heavier than helium; nearly always relatively young stars found in the disk of the galaxy.

Population II

Stars poor in atoms heavier than helium; nearly always relatively old stars found in the halo, globular clusters, or the nuclear bulge.

Position Angle

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

Primary Minimum

In the light curve of an eclipsing binary, the deeper eclipse.

Prime Focus

The point at which an object mirror forms an image in a reflecting telescope

Primeval Atmosphere

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 2.7K

Prolate Spheroid

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 eclipses.

Proper Motion

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.

Proton-Proton Chain

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.


A collapsing cloud of gas and dust destined to become a star.


A source of short, precisely times radio bursts believed to be spinning neutron stars.


Quantum Mechanics

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 short intervals.


Radial Velocity

(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.

Radiation Pressure

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.

Radio Galaxy

A galaxy that is strong source of radio signals.

Radio Interferometer

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

Recurrent Nova

Stars that erupt as nova every few dozen years.

Red Dwarf

Cool, low mass stars on the lower main sequence

Reflecting Telescope

A telescope which uses a concave mirror to focus light into an image

Reflection Nebula

A nebula produced by starlight reflecting off dust particles in the interstellar medium.

Refracting Telescope

A telescope that forms images by bending light through an objective lens


A soil made up of crushed rock fragments.

Relative Age

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.

Resolving Power

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 stable.

Retrograde loop

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.

Rift Valley

A long, straight, deep valley produced By the separation of crustal plates.

Ring Galaxy

A galaxy that resembles a ring around a bright nucleus; believed to be the result of a head-on collision of two galaxies.

Roche Limit

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.

Rolling Plains

The most common type of terrain on Venus.

Rotation Curve

A graph of orbital velocity versus radius in the disk of a galaxy.

RR Lyrae Variable

Variable stars with periods of 12-24 hours, common in some globular clusters.


Sagittarius A

The powerful radio source located at the core of the Milky Way.

Saros Cycle

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.

Schmidt Camera

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

Schwartzchild Radius

The radius of the event horizon around a black hole.

Scientific Notation

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.

Secondary Atmosphere

The gases outgassed from a planet's interior; rich in carbon dioxide.

Secondary Minimum

In the light curve of an eclipsing binary, the shallower eclipse

Secondary Mirror

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

Seismic Waves

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 spiral arms

Semimajor Axis

Half the long diameter of an ellipse

Seyfert Galaxy

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.

Shepherd Satellite

A satellite that, by its gravitational field, confines particles to a planetary ring.

Shield Volcanoes

Wide, low profile volcanic cones produced by highly liquid lava.

Shock Wave

A sudden change in pressure that travels as an intense sound wave.

Sidereal Drive

The motion and gears on a telescope that turn westward to keep it pointing at a star

Sidereal Period

(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.

Sinuous Rille

A narrow, winding valley on the moon caused by ancient lava flows along narrow channels.

Smooth Plain

Apparently young plains on Mercury formed by lava flows at or soon after the formation of the Caloris Basin

Solar Constant

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.

Solar Wind

Rapidly moving atoms and ions that escape from the solar corona and blow outward through the solar system.

Special Relativity

Observers can never detect their uniform motion except relative to other objects. The velocity of light is constant for all observers

Spectral Class

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.

Spectral Sequence

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 spectrum

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.

Synchrotron Radiation

Radiation emitted when high speed electrons move through a magnetic field.

Synodic Period

(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 or moon.

Terrestrial Planets

Earthlike planets - small, dense, rocky.

Tidal Coupling

The locking of the rotation of a body to its revolution around another body.

Tidal Heating

The heating of a planet or satellite because of friction caused by tides.

Time Dilation

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.

Titius-Bode Rule

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 another.

Transverse Velocity

The velocity of a star perpendicular to the line of sight.

Triaxial Ellipsoid

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 irregular galaxies.

Turnoff Point

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.


Ultraviolet Radiation

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

Uncompressed Density

The density a planet would have if its gravity did not compress it.

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 earth's magnetosphere.

Variable Star

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.

Vernal Equinox

The place where the Sun crosses the celestial equator heading North.

Very Long Baseline

The use of radio telescopes located thousands of miles apart to resolve detail in radio sources.

Vesicular Basalt

A porous rock formed by solidified lava with trapped bubbles.

Violent Motion

Motion other than natural motion

Visual Binary

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.

Winter Solstice

The point on the ecliptic where the Sun reaches it most southern point.


Zeeman Effect

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 Sun passes.

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