Astronomy Glossary:

[A] [B] [C] [D] [E] [F] [G] [H] [I] [J] [K] [L] [M] [N] [O] [P] [Q] [R] [S] [T] [U] [V] [W] [X] [Y] [Z]

A:

Aberration: When a defective focus, which is caused by a mirror or a lens, inadequately brings the light into a sharp focus.

Absolute Magnitude: The brightness that an object would have if it were 32.6 light years away from Earth.

Absolute Zero: The theoretical temperature that is the coldest, which is equal to 0 Kelvin (-459.69 degrees F of -273.15 degrees C).

Absorption Lines:  In a spectrum dark lines are caused by the absorption of light by atoms or molecules in a star or planet’s atmosphere.

Accretion Disk:  The disk that encloses a black hole or a star that matter gravitationally falls into the central object.

Achromatic Lens: A two element lens that will significantly reduce chromatic aberration.

Active Galactic Nuclei: The unusually bright cores of galaxies.

Active Galaxy: A galaxy that usually gives off large quantities of energy from a small central source.

Airy Disk:  Has a bright disk like quality image of a point source of light, usually from a star.

Albedo: Percentage of light that an object will reflect.

Altazimuth Mount: A telescope mount that allows the telescope to move freely in horizontally and vertically modes for altitude and azimuth.

Andromeda Galaxy: It is the largest galaxy also known as M31.

Annular Eclipse: The moon does not cover the sun, which allows stargazers to see a thin ring of sunlight.

Anti-tail:  The name given to a comet’s tail in this rare occurrence when a comet’s tail points toward the sun.

Antimatter: Matter that has particles of the same mass and properties as their counterparts but with opposite electrical charges.

Aperture: The larger (the diameter) the telescope lens is, the greater light-gathering power it has to see objects clearly.

Aphelion:  The point that is farthest from the sun in an object’s orbit.

Apochromatic Lens: A three element lens that will significantly reduce chromatic aberration, more so than an achromatic lens.

Apogee: The point in which a satellite’s orbit is farthest from the Earth.

Apollo: The US space program that sent astronauts in the 1960s and 1970s to the Moon. And it is also an asteroid with a perihelion less than 1.017 AU.

Apparent Field of View: The size of the circle of light that the eye can see through the eyepiece.

Apparent Magnitude: Measurement of brightness of an object as it can be seen from Earth.

Apparition: The period of time in which an celestial object can been seen in the night sky.

Asterism: A small clusters of stars in the dark night sky.

Asteroid: A tiny, rocky object that orbits a star.

Asteroid Belt: The area that most asteroids orbit the sun located between Jupiter and Mars.

Astrometry: The field of study for the movement of celestial objects and the night sky.

Astronomy: The field of study that deals with objects and celestial objects beyond Earth.

Astrophotography: The usage of photography to photograph astronomical objects.

Astrophysics: The field of study of astronomy that deals with the physical characteristics of celestial objects.

Atmosphere: The gaseous space that surrounds a moon, planet, or a star.

Atom: The main unit of matter, which can be made of protons, neutrons and electrons.

Atomic Nucleus: The center or main part of an atom.

Autoguider: A CCD Camera used to guide the telescope automatically during long exposure shots for astrophotography.

Autumnal Equinox: When the sun crosses the celestial equator heading south, usually around September.

Axis: The straight line in which an object rotates.

B:

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Barlow Lens: The lens attached behind the eyepiece of a telescope to give greater magnification of objects.

Barred Spiral Galaxy: A spiral galaxy that has a central bar that is composed of stars and gas.

Baseline: The line between two observational points or two telescopes.

Big Bang: The theory that a giant explosion created the universe billions of years ago.

Binary Star: (A double star) which consists of two stars that orbit a common center of gravity.

Black Holes: The areas of space where gravity is so powerful that not even light can escape.

Blink Comparator: An instrument that allows stargazers to see two images of the night sky at the same time.

Bok Globule: A tiny and dark nebula that is thought to be the makings of a star.

Bolide: A bright fireball or meteor that burst in mid-air.

Brown Dwarf:  The gaseous object that will form like a star, but does not have the required mass to keep up the nuclear fusion in its core.

Bulge: The central region of a spiral galaxy.

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Carbon Star: A massive red star that has more carbon than oxygen in its surface layers.

Carbonaceous Chondrites: These meteorites and asteroids, which are composed of organic matter are thought to be the primitive samples of the early solar system.

Cassegrain Telescope: A telescope that is a reflecting telescope that allows light to reflect back through a hole in the center of the primary mirror through a secondary mirror.

Catadioptric Telescope: Usually Schmidt-Cassegrain telescopes, a catadioptric telescope combines the primary mirror with a lens placed in front of the mirror that corrects aberrations.

CCD: These chips are made to detect light at a far better rate than using conventional film.

Celestial Sphere: It is often used to plot the coordinates of objects in the night sky, which is an imaginary sphere of large radius centered on Earth.

Cepheid Variable: Luminous stars that can vary in brightness that are used to calibrate distances to galaxies.

Chondrite: Meterite that contains tiny, round silicate granules referred to as chondrules.

Chromosphere: The layer in the star’s atmosphere that lies below the corona and the photosphere.

Clock Drive: Attached to the equatorial mount is a motor that will compensate for the Earth’s rotation and this motor, the clock drive, will keep the telescope pointing in the same area of the night sky.

Collapsar: This giant star will collapse on its own weight at the end of its normal lifespan.

Collimation: Collimation will put a telescope’s optics into alignment.

Coma: A haze and dust of gas that surrounds a comet’s center.

Comet: A small piece of ice and rock that orbits a star in a high elongated orbit. Some comets have orbtil periods longer than 200 years and will reappear.

Comet Nucleus: A solid and tight mass of ice and rock that will heat up when exposed to sunlight and it will release gas and dust.

Conjunction: When two or more bodies appear close together in the night sky.

Constellation: There are 88 patterns of stars in the sky, often named for a mythological god, hero, or animal.

Core: The central zone of a planet, brown dwarf, star, or galaxy.

Corona: The outer atmosphere of a star or the sun.

Cosmology: The field of science that studies the evolution of the universe and its structure.

Cosmos: Another name for the universe.

Crust: Thin, outermost layer of a planet, moon, or asteroid.

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Dark Adaptation: The process of the human eye being well adjusted to seeing objects in the dark night sky.

Dark Energy:  “Negative gravity” that plays a part in the acceleration of universal expansion.

Dark Matter: Matter that does not give off any light or radiation.

Dark Nebula: Clouds of dust grains that are thick and dense enough not to allow light from background stars.

Deep-Sky Objects: Celestial objects that are beyond the solar system (stars, star clusters, galaxies.)

Density: Amount of mass per unit volume of a zone of space or an object.

Diffraction: Spreading of light as it passes the edge of an object.

Dobsonian Telescope: A simple telescope that a very stable mount that moves freely.

Dust: Tiny particles that float throughout space.

Dwarf Galaxy: Small galaxy that contain a few million stars.

Dwarf Star: A small star.

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Eccentric: It is used to describe the shape of an orbit, to stray away from the shape of a circle.

Eclipse: When one object passes in front of another object, thus blocking it fully or partially from view.

Eclipsing Binary: When a binary star passes in front of another star and fully or partially blocks the light from the other star.

Ecliptic: The Earth’s orbit around the sun (all planets but Mercury and Pluto) have orbits in the nearly the same plane.

Elliptical Galaxy: A bound system which is gravitational, that stars have no spiral structure.

Elongation: The separation of on object from the sun.

Emission Nebula: Energetic, young stars illuminate from within a cloud of very hot gas.

Ephemeris: A chart or table that tells the positions of astronomical objects at certain periods of time.

Equatorial Mount: A telescope mount that one axis lies parallel to the Earth’s rotational axis.

Equinox: The sun crosses the celestial equator in which day and night are an equal 12-hour length in all parts of the world.

Evening Star: When Venus appears in the night sky.

Event Horizon: When light can’t escape from inside a black hole.

Extragalactic: Beyond the Milkway galaxy.

Extrasolar: Beyond the sun.

Eye Relief: The distance between the stargazer’s eyeball and the lens in which the stargazer can clearly see the whole field of view.

Eyepiece: A lens that is used to view the image that is produced by a telescope’s primary mirror.

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Field of View: The portion of the sky that is visible through binoculars or a telescope’s eyepiece.

Filter: This object is used by stargazers to reduce the available light of very bright objects.

Finder Scope: This is a smaller telescope that is mounted on a main telescope that allows the stargazer to locate objects easier in the night sky.

Fireball: An extremely bright meteor that has a magnitude –4.

Flare: An extremely violent outburst of energy that comes from the surface of a star.

Focal Length: The distance from the mirror to the point of where it brings the light into focus.

Focal Ratio (f/ratio): The ratio of the focal length of a mirror or a lens to its diameter.  (The size measured across the lens).

Focus: Where rays of light passing through a lens (or that reflect off the mirror) meet.

Focuser: The device on a telescope that holds an eyepiece in place and allows the stargazer to bring light into a sharp focus.

Fork Mount: The equatorial mount which allows a telescope to swing in declination between the prongs of a fork.

Full Moon:  When the moon is halfway around its orbit from the new moon and is opposite from the sun in the sky.

G:

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Galactic Disk: The disk of a spiral galaxy.

Galactic Nucleus: The main zone of a galaxy which will often contain a large amount of stars and gas and an extremely large black hole.

Galactic Plane: The expulsion of the disk of the Milk Way into the night sky.

Galaxy:  An unusually large mass of billions of stars.

Galilean Moons: The four largest moons of Jupiter, Io, Europa, Ganymede and Callisto that were discovered by Galileo in 1610.

Gas Giant: Usually a large planet such as Jupiter, Saturn that is mainly composed of gas.

Giant Molecular Cloud: Clouds that are made up of dust and cold gas and contain thousands or more solar masses.

Globular Cluster: A spherical grouping of thousands of stars that exist in a galaxy’s halo.

Gravitational Lens: A large portioned object that distorts the light from a farther object that falls in the same line of sight.

Gravity: The force that all objects place onto one another (the bigger an object’s weight; the stronger pull is has against another object.)

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Halo: The outer region of a galaxy that holds dark matter, globular clusters and a few stars.

Heliacal Rising: The time of day when a star or another object have been seen for a short period in the eastern sky before dawn and can be no longer hidden from the glare of the sun.

Heliosphere: The large zone that is around the sun that is dominated by the solar wind.

HII Region: A zone that is filled with clouds that contain iodized (the iodized is usually caused by newborn star’s radiation) hydrogen.

HST (The Hubble Telescope): It was manufactured in the 1970s, but it was not until 1990 that was first launched into space to observe the Earth’s atmosphere.

I:

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Igneous Rock: Rock that is made by the solidification of magma.

Inclination: The angle that exist between a planet’s orbit and the ecliptic plane.

Inferior Conjunction: The design on an inferior planet when it lies in the path between the sun and the Earth.

Inferior Planet: When a planet orbits the sun inside the Earth’s orbit (which Mercury and Venus do) it is called an inferior planet.

Inflation: A small and fast period of expansion a fraction of a second the occurred after the Big Bang.

Interacting Galaxies: When galaxies are thrown into one another’s gravitational path, it will often result in an extreme star formation.

Intergalactic: The space between galaxies.

International Space Station: A program between the US and other countries for the development and operation of liveable space station.

Interplanetary: Space between planets.

Interstellar: Space between the stars of a galaxy.

Interstellar Medium: Gas and dust located between stars.

Irregular Galaxy: A galaxy that does not have a clear defined spiral shape.

J:

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Jet: The narrow stream of gas or particles that are ejected from an accretion disk that surrounds a star.

Jet Propulsion Laboratory: A NASA research center located in Pasadena, California, that builds and operates unmanned spacecraft.

Jet stream: A fast flowing, wandering air current located in the troposphere which runs from west to east that affects weather.

Jovian planet: A planet with similarities to Jupiter.

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Kelvin: A unit increment of temperature which is equal to 1.8 degrees on the Fahrenheit scale and one degree on the Celsius scale.

Kuiper Belt: A region of the solar system beyond the planets extending past Neptune’s orbit. It is composed largely of frozen bodies, such as methane, ammonia and water.

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L chondrite: A meteorite that contains relatively low amounts of iron.

Lagrange point: The five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects.

Large Magellanic Cloud: An irregular type galaxy that orbits the Milky Way Galaxy.

Last quarter: A phase of the moon where the western half is illuminated.

Latitude: The location of a place on Earth north or south of the equator.

Lens: A piece of glass that is curved which helps bring objects or light to a focus.

Light pollution: Light, from artificial sources, that reach the night sky which obscures the view of astronomical objects.

Light-year: The distance that light travels in a vacuum in one year.

Limiting magnitude: The faintest apparent magnitude detectable or detected by a given instrument such as a telescope

Local Group: A group of over 30 galaxies that also includes the Milky Way.

Local supercluster:  A galaxy supercluster that contains the Local Group.

Longitude: The location of a place on Earth east or west of the prime meridian running through Greenwich, England.

Luminosity: The amount of light that radiates from an object.

Lunar eclipse:  An event when the moon passes through some portion of the Earth’s shadow.

Lunar month:  The length of time of one complete orbit of the moon around the Earth, which is 29.5 days.

Lunation:  Time between two new moons, which is 29.5 days.

M:

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Magnetograph:  An instrument that maps the strength and direction of magnetic fields of objects such as the sun.

Magnetometer:  An instrument that measures the strength and direction of magnetic fields of objects such as the sun.

Magnitude:  The measurement of the brightness of an object.

Maksutov telescope: A catadioptric telescope design that uses a full diameter meniscus lens as a corrector plate.

Mantle:  A section of a planet’s interior that is between the crust and the core.

Mass:  Measurement of an object’s total matter.

Meridian:  An imaginary circle on the celestial sphere, which connects the zenith to the north (or south) celestial pole.

Messier objects:  A list of  astronomical objects catalogued by French astronomer Charles Messier in 1700s.

Meteor:  The light emitted by a meteorite as it is burned up when entering the Earth’s atmosphere.

Meteor shower:  A large amount of meteor activity in same region of the sky.

Meteorite:  A rock from space that survives the passage through the Earth’s atmosphere and impacts the ground.

Meteoroid:  A small rock or particle in the Solar system.

Milky Way:  A barred spiral galaxy that is part of the Local Group of galaxies.

Minor planet:  Also known as an asteroid, these minor celestial bodies orbit the sun.

Mirror: A piece of glass that is coated with a reflective material.

Molecule:  A stable electrically neutral group of at least two atoms in a definite arrangement held together by strong chemical bonds.

Moon:  A smaller body that orbits a larger body.

Multiple star system:  Systems of more than two stars.

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Naked eye: To view or accomplish without the aid of a telescope, etc.

Nebula:  An interstellar cloud of dust, hydrogen gas and plasma.

Neutron:  A subatomic particle with no electric charge.

New moon:  The lunar phase that occurs when the Moon lies between Earth and the Sun, which makes the moon invisible.

Newtonian telescope: A type of reflecting telescope that uses a parabolic primary mirror and a flat diagonal secondary mirror, which was invented by Isaac Newton.

New General Catalogue (NGC):  Catalogue of deep sky objects in amateur astronomy.

Nova:  A explosion caused by the accretion of hydrogen onto the surface of a white dwarf star.

Nucleus: The central region or core of an atom, comet, or galaxy.

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Objective:  The primary mirror or lens of a telescope that brings the image to focus.

Occultation:  One object is hidden by another object that passes between it and the observer.

Omega Centauri:  A globular cluster of stars seen in the constellation of Centaurus.

Omega nebula:  A region in the constellation Sagittarius area of the Milky Way.

Opposition:  A celestial body is on the opposite of sun in the sky when viewed. This is the best time to view this planet.

Optical double:  Two stars that appear close together, due to line of sight.

Optics:  Lenses or mirrors.

Orbit:  Path that an object makes around another object while under the influence of a force, such as gravity.

Orbital period:  The time it takes one object to travel around another.

Outgassing:  The release of gasses from rocky body.

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Parallax:  The movement of a relatively nearby object against a fixed background, which occurs due to the motion of the observer.

Parsec:  A unit of length used in astronomy. One parsec is defined to be the distance from the Earth to a star that has a parallax of 1 arc second or about 3.262 light-years.

Penumbral eclipse:  A slight shading of the moon occurring when the moon passes through the outer edges of the Earth’s shadow.

Perigee:  The point at which an object’s orbit around the Earth makes its closest approach to the Earth.

Perihelion:  The point at which an object’s orbit makes its closest approach to the sun.

Periodic comet:  A comet that has orbited the sun more than once.

Phase: The regular cycle of an object’s changes of appearance.

Photometer:  Instrument used to measure the intensity of light emitted from an object.

Photons:  The particle composing light.

Photosphere:  The visible portion of the sun.

Photovoltaic:  The process to convert light energy to electricity.

Pixel:  A single point in a graphic image.

Planet:  A celestial body that orbits a star.

Position angle:  The direction in the sky of one object from another.

Power:  Measurement of the capability of an instrument to increase the apparent size of a distant object.

Primary lens:  The main lens of a telescope that collects lights and brings it to a focus.

Primary mirror:  The main mirror of a telescope that collects lights and brings it to a focus.

Prominence:  A large gas eruption off the surface of the sun.

Proper motion:  The measurement of a star’s change in position in the sky over time.

Protostar:  A star that forms from the cloud of gas and dust that is gravitationally collapses.

Proxima Centauri:  The closet star to the sun.

Pulsar:  Highly magnetized neutron stars, which emit a beam of detectable electromagnetic radiation.

Q:

Quasar:  A distant and extremely bright active galactic nucleus.

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Radio galaxy:  A galaxy that emits more than normal amounts of radio waves.

Radio telescope:  A telescope that is able to detect radio waves.

Radio waves:  Form of light with the least energy and the longest wavelength.

Reflecting telescope:  A telescope that gathers light by the use of a curved mirror.

Refracting telescope: A telescope that gathers light by the use of a glass lens.

Resolution:  The ability of any instrument to detect fine detail.

Rich Field telescope:  A telescope with low magnification with a large field of view.

Rotation:  The spin of an object about a central axis.

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Satellite:   A small body that orbits a planet.

Schmidt camera:  A camera designed to take wide view images of the sky.

Schmidt-Cassegrain telescope: A popular telescope that combines a folded optical path with a corrector plate to make a compact instrument.

Secondary mirror:  Small mirror designed to reflect the light from the main mirror.

Small Magellanic Cloud:  A small galaxy that orbits the Milky Way Galaxy.

Solar eclipse:  The moon passing between Earth and the sun.

Solar filter: Filter used to block the sun’s light, so the sun can be viewed safely.

Solar system:  The sun and all the other smaller bodies that are orbiting around it.

Solstice:  Either of the two points on the ecliptic at which its distance from the celestial equator is greatest or least.

Spectrograph:  An instrument on a telescope used to measure the spectrum of an object.

Spectroheliograph:  An instrument that captures an image of the sun at a single wavelength of light.

Star:  A luminous ball of hot gases that is held together by gravity.

Star party:  A gathering of people to view the night sky.

Sunspot:  A cooler than normal spot on the sun, that can be viewed as a temporary dark area.

Supernova:  An explosion of a star.

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Telescope: An instrument designed for the observation of remote objects, which magnifies and brightens the view.

Transparency:  The property of allowing the transmission of light through a material, such as the viewing of the sky.

Tremolite:  A common material found in metamorphic rocks.

Tropical year:  The length of time the sun takes to return to the same position.

U:

Ultraviolet light:  Electromagnetic radiation with more energy than visible light.

Umbra:  The dark area of a sunspot.

UV:Short for ultraviolet.

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Variable star:  A star that varies in brightness.

Vernal equinox:  When the sun crosses the equator heading north.

Visible light:  Portion of light spectrum visible to the human eye.

W:

Waning:  Time period between full moon and new moon.

Wolf-Rayet star:  A very bright and hot star.

X:

X rays:  Radiation less powerful than gamma rays, however stronger than ultraviolet light.

X class flares:  Solar flares that are the most energetic and brightest.

Y:

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Zenith:  The direction point directly above a particular location.

Zodiac:  An annual cycle of twelve stations along the ecliptic.