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Andromeda galaxy Andromeda galaxy,
by Rob Gendler

Andromeda GalaxyThe closest major spiral galaxy to our own, located about 2.5 million light years away in the constellation of Andromeda. Also known by its catalog number M31, it is the only other large galaxy in the Local Group. See also an extended essay on the Andromeda galaxy in Astronomy Topics.

Apparent MagnitudeHow bright a star or other object in the sky looks from Earth, expressed in the magnitude system. How bright a star looks to us depends on both how bright it really is and how far away it is.

ApertureThe size (diameter) of the light collecting lens or mirror in a telescope.

AsteroidA relatively small rocky object orbiting the Sun.  Most asteroids are found between Mars and Jupiter in a region called the asteroid belt.

AstrologyThe pseudoscientific belief that the positions of the planets at the time of a person's birth influences his or her personality or destiny. Astrology lacks both a coherent theory or how this influence is exerted and observational data to support the (nonexistent) theory. It can be tested, and has been, and fails such tests: Military generals, for instance, are not born when the planets "align" in allegedly militant formations, nor are great composers and artists born under any particular sign.

AstrophotographyImaging of astronomical objects, with film or a digital detector such as a CCD. The resulting image is called an astrophotograph, and the person who made it an astrophotographer.


Big BangThe high-energy, very dense beginning of the universe; the start of its expansion. The Big Bang Theory is the popular name for a whole set of ideas for the origin and early evolution of the universe.

Black HoleThe collapsed remnant of a star (or stars) whose gravity is so great that nothing, not even light, can escape it. More technically, a region of space where gravity has warped space-time so much that straight lines have become circles.


CCD (charge-coupled device)An electronic detector of electromagnetic radiation. For amateur photography, CCDs are light-sensitive arrays that can record the amount of light coming in. They have made digital photography possible, replacing film as the collector of light.

ColorThe colors of astronomical objects can be seen, through ordinary telescopes, only for bright objects such as planets, bright stars, and the brightest nebulae. The colors rendered in galaxies and dimmer nebulae by astrophotography results from long exposure times, and can be glimpsed by the eye only through the largest telescopes, if then.

Comet McNaught over Australia Comet McNaught over Australia,
by Akira Fujii

CometA relatively small body made of ice and dust that orbits the Sun. Most comets remain far away and frozen solid, but when a comet approaches the Sun, its ice can evaporate and its dust can be freed, producing a large cloud of material around the frozen core, and making it much easier to spot.

Constellation Cygnus Constellation Cygnus,
figure superimposed on photo by Akira Fujii

ConstellationIn the old days, a constellation was any recognizable pattern of bright stars in the sky. Today, astronomers have given the word a more precise meaning: it is one of 88 sectors into which astronomers divide the sky (much like the territory of the U.S. is divided into sectors called states.) Each constellation "box" is named after a pattern of stars in it.

Corrector PlateA type of lens at the sky end of a compound telescope, such as a Schmidt-Cassegrain. By combining a corrector lens with a light-gathering mirror (see reflecting telescope), compact telescopes can be made with a focal length nearly three times their physical length, as the light is bounced back and forth inside the tube.


Dark NebulaA dusty cloud of material in the galaxy that blocks the light of more distant stars behind it. Dark nebulae are best seen in regions where there a lot of stars or emission nebulae.

DayThe time a planet takes to spin (turn on its axis) once.

DeclinationA system of measuring the position of objects in the sky that is similar to the latitude system we use on Earth. Declination is measured in degrees north and south of the celestial equator (the circle in the sky that you would see if you extended the Earth's equator out into the sky.)

DegreeA measure of angle equal to one 360th of a full circle. The dome of the sky from a point on the horizon to the opposite point measures 180 degrees.

DensityThe amount of mass per unit volume in a body. A cubic centimeter of lead is a lot more dense than a cubic centimeter of Jell-O.

DogmaWidely believed tenet or doctrine set forth on the basis of authority rather than being supported by observation or experiment.

Doppler shiftThe change in the wavelength (color) of light—or other electromagnetic radiation—caused by motion toward us or away from us.

Double StarTwo or more stars orbiting around their common center of gravity. Also called "binary" stars. Some double stars can be resolved, or "split," with a telescope. Others cannot be resolved but their duplicity can be detected by studying their motions by analyzing their spectrum.

Dwarf planetAn object orbiting the Sun which is large enough for gravity to make it round, but one that shares its general orbit with a number of other objects. Ceres, Pluto, and Eris are dwarf planets.

Dwarf StarAstronomically, any star smaller than a giant. The Sun is a dwarf star, but will grow to giant dimensions when, billions of years from now, it runs low on fuel and becomes unstable, swelling to become a red giant as large as the orbit of Earth.


E=mc²Equation derived from Albert Einstein's special theory of relativity, revealing that matter contains enormous amounts of energy. (E is the energy, m the mass, and c² the energy. Since the speed of light is quite high, that quantity is an extremely large number. This equation, the key to nuclear weapons, demonstrated how stars manage to keep shining for billions of years.)

EclipseWhen one body gets into the shadow of another in space. For example, we get an eclipse of the Moon when the Earth gets right between the Sun and the Moon and casts its shadow on the Moon.

EclipticPath in the sky along which planets and the Sun and Moon appear to move, owing to the fact that they all orbit in roughly the same plane. Known in astrology as the "zodiac."

Electric chargeThe property of some particles of matter that causes them to attract or repel other charged particles. Examples of particles with electric charge are the electron and the proton.

Electromagnetic radiationWaves of energy generated by electric and magnetic changes in matter; these waves travel at the speed of light. Examples include radio waves, infrared waves, visible light, ultraviolet waves, x-rays, and gamma rays.

Emission NebulaA cloud of glowing gas (and dust) in our galaxy or another galaxy. A region of gas excited by the light of hot nearby stars.

ExoplanetAn extrasolar planet.

Extrasolar planetA planet orbiting a star other than the Sun

EyepieceA magnifying lens used to magnify and view the image produced by a telescope.


Focal Length The distance between a telescope's light-gathering mirror or lens and the point where it comes to a focus—a point known as "prime focus." Astrophotography can be accomplished by putting a camera at prime focus, or by adding a magnifying eyepiece ahead of the camera.

FusionA process by which light atomic nuclei come together under tremendously hot conditions to produce energy; fusion is what allows the stars to shine.


GalaxyA great island of millions to hundreds of billions of stars, separated from other galaxies by large gulfs of space. We live in one such island, called the Milky Way Galaxy.

Galaxy Clustera group of galaxies containing dozens to thousands of members

Galileo (Galileo Galilei)(1564-1642) Pioneering scientist who introduced some of the basic methods of experimental science, first turned the telescope to the skies in a systematic way, discovered the moons of Jupiter (the first moons found around another planet), and showed that the Milky Way was made up of stars.

Gamma RayA form of electromagnetic radiation, consisting of waves of the highest energy.

Gamma-ray BurstBrief event which generates a huge amount of gamma-ray energy, lasting from a fraction of a second to a few minutes. Such bursts are now thought to come from rare, extremely violent events in other galaxies. See here for more information.

Giant StarA star much more massive than the Sun. Young giant and "supergiant" stars burn furiously but don't last long. (The rate at which they use up their nuclear fuel rises as the cube of their mass.) Normal stars near the end of the careers swell in size to become giants, too, although they are less massive and a lot cooler.

Global Positioning System (GPS)A receiver that uses signals from artificial Earth satellites to determine its position. Computerized telescopes equipped with GPS receivers can confirm their position, making their digital star-mapping systems more accurate and enabling observers to point the telescope at an object simply by clicking on its name.

Globular cluster M13 Globular cluster M13,
by Rob Gendler

Globular ClusterA crowded group of 100,000 or more stars all traveling together, as part of a galaxy. The Milky Way Galaxy has more than 150 such globular clusters.

GravityThe pull of all matter on all other matter in the universe.


HypernovaThe explosion, at the end of its life, of a very massive star, whose core collapses to be a super-compressed, spinning black hole, stirring and energizing the wreckage around it. Such hypernovae are thought to be the explanation for one kind of gamma-ray burst.


Infrared lightA type of electromagnetic radiation with a longer wavelength than visible light. Infrared light, discovered by William Herschel, is what human beings give off. It is also a way for astronomers to see cooler objects, such as stars that are in the process of forming.

Internet telescopeA robotic telescope that can be operated from afar, over the internet.


Light, velocity ofEquals approximately 300,000 kilometers (186,000 miles) per second, in a vacuum; see light year.

Light YearThe distance that light travels in one year; roughly 9.5 trillion kilometers or 6 trillion miles. The nearest star is a little over 4 light years away.

Local GroupThe group of several dozen galaxies to which our Milky Way Galaxy belongs.

LuminosityThe total energy output of a star or other celestial object. The sum total of all the light and other electromagnetic radiation a star gives off.

Lunar eclipse Lunar eclipse,
by Akira Fujii

Lunar EclipseAn eclipse of the Moon; it happens when the Earth moves between the Moon and the Sun, casting its shadow on the Moon.


MDenotes objects in the Messier Catalog.

MagnitudeA system used in astronomy to indicate the brightness of stars and other objects. The smaller the magnitude, the brighter the star. The brightest star in the sky, Sirius, has a magnitude of -1.42, the star Vega has a magnitude of zero, while dim Barnard's Star, with a magnitude of 9.5, is too faint to be seen with the naked eye.

MassThe total amount of gravitationally-interacting material in a body. Mass is not the same thing as size: Some dwarf stars are smaller than Earth but as massive as the Sun.

Messier CatalogA catalog of fuzzy celestial objects compiled by Charles Messier in the 18th century. He complied the catalog so he would not mistake the objects in it for comets (which were his passion). It was a pioneering list of the most noticeable nebulae, star clusters, and galaxies.

Milky WayGlowing "river of light" crossing the sky; our view of the disk of the Milky Way galaxy from our perspective inside it.

Milky Way GalaxyThe galaxy of stars in which the Sun and the Earth are located; one of the two large spiral galaxies in the Local Group.

MoleculeA combination of two or more atoms bound together. A water molecule, for example, consists of two atoms of hydrogen and one atom of oxygen.

MoonAn object that orbits a planet; a natural satellite.

Rosette nebula Rosette nebula,
by Rob Gendler

NebulaA cloud of gas and dust among the stars in a galaxy. A nebula can be observed in regions where new stars have recently been born and around stars that are dying or have died. Before galaxies were understood to be gigantic islands of stars, they too were classified as nebulae.

Neutron StarThe remnant of a massive star that has exploded at the end of its life as a supernova. These remnants are very dense, because the violence of the star's death has compressed them until electrons and protons have merged to become neutrons.

Newtonian FocusAn arrangement in a reflecting telescope, where the light is reflected by a tilted secondary mirror off to the side, where it can be viewed through an eyepiece. This was invented by Isaac Newton.

NGCDesignates objects in the New General Catalog of star clusters and nebulae, originated by William Herschel in the 18th century and continued by his son, John.


ObservatoryAny structure from which the night sky is observed. Centuries ago, observatories were used for charting the sky with the naked eye, sometimes using sighting tubes and other instruments to map the positions of stars. Today, they usually house telescopes.

Open star cluster M52 Open star cluster M52,
by Rob Gendler

Open ClusterA group of stars within the main disk of a galaxy, containing a few dozen to a few thousand member stars, all of whom were born in roughly the same place at roughly the same time. Another name used for such a group is galactic cluster.

OrbitThe path that an object takes as it revolves around another object due to their mutual gravity. For example, the Earth has an orbit around the Sun, and the Moon and the Hubble Space Telescope both have their orbits around the Earth.

Orion NebulaThe closest region of star formation to the Sun. Found in the constellation of Orion, the nebula is the small visible part of a much larger region of gas and dust. A profile of the Orion Nebula can be found here.

Ozone LayerA region of the Earth's atmosphere (roughly 10 to 20 miles above the surface) which has a higher concentration of ozone (a molecule of oxygen with three oxygen atoms in it). This ozone absorbs harmful ultraviolet radiation from the Sun, contributing to the conditions that make life on the Earth's surface possible.

Mars Mars,
imaged by the Hubble Space Telescope

PlanetAn object of significant size that orbits a star. According to the new definition of a planet adopted by the International Astronomical Union, planets must have enough mass to be spherical and must have their own independent orbits around the Sun. The Sun has 8 such planets. (As discussed here, Pluto is part of the Kuiper Belt and Ceres is part of the Asteroid Belt, making them dwarf planets.)

Helix planetary nebula, Helix planetary nebula,
by Rob Gendler

Planetary NebulaA shell or shells of gas ejected by a relatively low-mass star that is in the process of dying, and becoming a white dwarf. The name is very misleading—this "last gasp" from a star at the end of its life has nothing to do with planets.

Portrait LensCamera lens designed for photographing people; owing to the high quality of 19th-century portrait lenses, some were employed for wide-field astrophotography.

ProminenceAn eruption of hot gas from the surface of the Sun; seen against the darkness of space, it can look like a giant flame. Visible during a solar eclipse, and on any day by amateur or professional astronomers using suitable filters.


RadialOutward from the center, like the radius of a circle, or the spokes of a bicycle wheel. The radius of a planet is the distance from its center to its surface. The total force of gravity experienced at the surface is a product of the planet's mass and radius.

Radio WavesA type of electromagnetic radiation consisting of waves of long wavelength and low energy. Radio waves on Earth travel from the transmitting tower of a radio station to the antenna in your car, for example.

Orion in a Winter sky Orion in a Winter sky,
by Allan Morton

Red GiantA stage in the life of every star, when it runs out of its initial fuel for fusion, and, as part of its readjustment, expands to become much larger. As the hot gas of the star expands, it cools down and becomes reddish in color. A well-known red giant is Betelgeuse, in the constellation figure of Orion, the hunter (seen as the red star to the left on Orion's belt of 3 stars in the accompanying picture.)

Reflecting TelescopeA telescope in which the light is collected (and reflected) by a mirror.

Reflection NebulaA cloud of dust among the stars, which becomes visible to observers by reflecting starlight.

Refracting TelescopeA telescope in which the light is collected (and refracted or bent) by a lens.


Shepherd MoonA small moon that orbits near a ring and helps maintain its structure or smooth-looking edge.

SlewTo move a telescope rapidly from one location in the sky to another.

Solar EclipseAn eclipse of the Sun by the Moon; when the Moon moves exactly in front of the Sun as seen from Earth.

Solar SystemThe Sun and all the planets, moon, and smaller objects that orbit around it. The planetary system which includes the Earth.

SpectrumThe array of wavelength or colors in a beam of light (which can be studied when the white light is spread out using a spectroscope), or a photography of this array of colors. More generally, the array of wavelengths found in any beam of electromagnetic radiation.

SpectroscopeA device that allows scientists (or amateurs) to observe the spectrum of radiation from some source. Using a spectroscope, for example, astronomers spread the light of the Sun out into its component colors and can learn about the Sun's temperature, composition, and motion.

SpectroscopyAnalysis of light by breaking it into a spectrum using a spectroscope.

StarA sphere of hot gas that shines under its own power; the energy that allows a star to shine comes from the process of nuclear fusion. The Sun is our closest example of a star.

Star ChartAn atlas of the night sky. Star charts used to all be printed on paper; now they are mostly digital, as is the star chart on this website.

Star ClusterA group of stars held together by their mutual gravity. See open cluster and globular cluster.

SunspotA cooler region of the Sun's surface, which looks dark in comparison to the hotter material around it.

Supercluster (of galaxies)A group of galaxy clusters; superclusters may contain enough material to make 10,000 or more Milky Way Galaxies and stretch over hundreds of millions of light years.

SupernovaAn explosion that occasions the "death" of a massive star (or, in other cases, signals that a white dwarf in a double star system has re-ignited because a companion star has dumped a critical mass of fresh material onto it.)


TelescopeAn instrument that gathers light (or another type of electromagnetic radiation) and brings it to a focus. Telescopes allow astronomers to see or photograph objects that are too dim to be seen with the naked eye.

TransitWhen a smaller object passes in front of a larger one in space; for example, a planet may transit a star.


UniverseThe sum total of all matter, radiation, and space; everything that is accessible or can become accessible to our observations.


Variable StarA star that changes periodically in brightness. This can happen because the star pulsates, or in some cases when a double star happens to be aligned so that, from our point of view, one star in the system periodically eclipses the other.


White DwarfThe collapsed, hot remnant of a low-mass star at the end of its life. When the Sun becomes a white dwarf, it is expected to be about as big across as two Earths.


ZenithPoint in the sky directly overhead from a given location.

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