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Glossary Terms

A

aberration

  in optics, an imperfect focus caused
when a mirror or lens fails to bring light to a sharp focus 

absolute magnitude

  the apparent brightness an object would have if it were 10 parsecs (32.6 light-years) from Earth 

absolute zero

  the coldest theoretical temperature, equal to 0 kelvin (-459.67 F or
-273.15 C) 

absorption lines

  dark lines in a spectrum caused by the absorption of light by atoms or molecules in a star or planets atmosphere 

accretion disk

  a disk surrounding a black hole or star in which matter gravitationally falls onto the central object 

achromatic lens

  a two-element lens, or doublet, that significantly reduces chromatic aberration 

active galactic nuclei

  the exceptionally bright cores of some galaxies, thought to be fueled by matter falling into supermassive black holes 

adaptive optics

  a system of telescopes, computers, and deformable mirrors used to compensate for atmospheric blurring 

Airy disk

  the bright disk-like image of a point source of light, such as a star, as seen in an optical system with a circular aperture 

albedo

  the percentage of light that an object reflects 

altazimuth mount

  a mount that enables a telescope to move freely both vertically (in altitude) and horizontally (in azimuth) 

altitude

  1. the height above sea level
2. the angle between an objects position on the celestial sphere and the horizon 

anaglyph

  a stereoscopic, composite image in which the right component (usually red in color) is superimposed on the left component (usually blue) to
produce a three dimensional effect when viewed through correspondingly colored filters 

Andromeda Galaxy

  the largest member of the Local Group of galaxies; roughly twice the size of the Milky Way; also known as M31 

angular size

  the apparent width of an object as seen by an observer, usually expressed in degrees, arcminutes, or arcseconds 

anisotropy

  the variation of a physical property depending on direction 

annular eclipse

  a solar eclipse in which the moon does not fully cover the suns disk, allowing observers to see a thin ring of sunlight 

antimatter

  matter consisting of particles that have the same mass and properties as their matter counterparts, only with the opposite electrical charges 

aperture

  the diameter of a telescopes primary lens or mirror; the larger the aperture, the greater the telescopes light-gathering power 

aphelion

  the point farthest from the sun in an objects orbit 

apochromatic lens

  a lens with three or more elements that reduces chromatic aberration even more than an achromatic lens 

apogee

  the point in a satellites orbit when it is farthest from Earth 

Apollo

  1. the U.S. space program that sent astronauts to the moon in the 1960s and '70s
2. an asteroid with a perihelion less than 1.017 AU (and thus comes within the orbit of Earth) 

apparent field of view

  the angular diameter of the circle of light that the eye sees through an eyepiece 

apparent magnitude

  the measure of the brightness of an object as seen from Earth 
 

Definition

apparition

  the period of time during which a particular celestial object can be seen 

archeoastronomy

  the study of physical artifacts with astronomical connections 

arcminute

  a unit of angular size equal to 1/60 of a degree 

arcsecond

  a unit of angular size equal to 1/3,600 of a degree (or 1/60 of an arcminute). 

asterism

  a small grouping of stars in the night sky 

asteroid

  a small, rocky body that orbits a star 

asteroid belt

  the zone in which most asteroids orbit the sun, located between the orbits of Mars and Jupiter 

astrometry

  the study of the positions and motions of celestial objects 

astronomical unit (AU)

  the average distance from Earth to the sun, equal to about 93,000,000 miles (150,000,000 km) 

astronomical units

  a measurement used by astronomers within the solar system; one astronomical unit (AU) is the average distance between Earth and the sun (about 93,000,000 miles or 150,000,000 kilometers) 

astronomy

  the branch of science concerned with objects beyond Earth 

astrophotography

  the photography of astronomical objects 

astrophysics

  the branch of astronomy that deals with the physical characteristics of celestial objects 

atmosphere

  a gaseous envelope surrounding a moon, planet, or star 

atom

  the fundamental unit of matter; can consist of protons, neutrons, and electrons 

atomic nucleus

  the central region of an atom; can consist of protons and neutrons 

attitude

  the orientation of a spacecraft relative to the direction of its motion 

AU

  a measurement used by astronomers within the solar system; one astronomical unit (AU) is the average distance between Earth and the sun (about 93,000,000 miles or 150,000,000 kilometers) 

aurora

  the emission of light when charged particles from the solar wind slam into and excite atoms and molecules in a planets upper atmosphere 

aurora australis

  the southern lights; see definition for aurora 

aurora borealis

  the northern lights; see definition for aurora 

aurorae

  the plural of aurora (the emission of light when charged particles from the solar wind slam into and excite atoms and molecules in a planets upper atmosphere) 

autoguider

  a CCD camera used to automatically guide a telescope during long-exposure photography 

autumnal equinox

  the time of year around September 23 when the sun crosses the celestial equator heading south 

averted vision

  a technique that uses the more light-sensitive rods in the eye to better see a faint object by looking at it indirectly 

 

  


 

 

axis

  a straight line about which an object rotates 

azimuth

  the angle along the horizon measured eastward from due north to the point on the horizon directly below an object 

B

Barlow lens

  a lens attached behind the eyepiece of a telescope that increases magnification 

barred spiral galaxy

  a spiral galaxy with a central bar consisting of stars and gas 

baryonic

  made up of baryons (elementary particles such as protons and electrons) 

baryonic matter

  "normal" matter composed of elementary particles called baryons 

baryons

  elementary particles such as protons and neutrons composed of three quarks 

baseline

  the line between two observational points or two telescopes of an interferometer 

Big Bang

  the giant explosion that is theorized to have created the universe 10 billion to 20 billion years ago 

billion

  1,000,000,000 (in American usage) 

binary star

  a system of two stars that orbit a common center of gravity; also known as a double star 

binoculars

  a small, usually hand-held instrument with two tubes that is used to magnify the view of astronomical objects; the two numbers used to describe the binoculars refer to its magnification and its aperture in millimeters, respectively 

black hole

  a region of space where gravity is so powerful that not even light can escape; black holes can form either from the death of high-mass stars or in the cores of galaxies 

blazar

  a high-energy, variable type of quasar which astronmers believe has a jet of material aimed in our direction that causes it to appear more energetic than other quasars 

blueshift

  a decrease in the wavelength of light coming from an object due to its motion toward Earth. 

Bok globule

  a small, dark nebula thought to be a region of star formation 

bolide

  a brilliant meteor or fireball that explodes in mid-air 

bolides

  brilliant meteors or fireballs that explode in mid-air 

Bose-Einstein condensate

  atoms crowded close together in ultra-low temperatures that behave as if they were one fluid-like superatom 

brown dwarf

  a gaseous object that forms like a star but lacks the necessary mass to sustain nuclear fusion in its core; a body intermediate in mass between a star and planet 

buckyball

  a naturally occurring form of carbon known as C-60, its molecular structure resembles the geodesic domes once designed by Buckminster Fuller 

bulge

  the generally spherical, central region of a spiral galaxy 

C

carbon star

  a red giant star with much more carbon than oxygen in its surface layers 

Cassegrain telescope

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

cataclysmic variable

  a close binary system which includes a white dwarf accreting matter from a less massive companion 

catadioptric telescope

  a telescope that combines the primary mirror of a reflector with a lens placed in front of the mirror that corrects for aberrations; most catadioptric telescopes for amateurs are Schmidt-Cassegrain telescopes 

CCD

  a silicon chip used to detect light; charge-coupled devices (CCDs) are far more efficient at collecting light than conventional film 

celestial pole

  the imaginary projection of Earths rotational axis onto the celestial sphere 

celestial sphere

  the apparent sphere of the sky; an imaginary sphere of immense radius centered on Earth often used to plot the coordinates of objects in the sky 

Cepheid variable

  a class of luminous stars that vary in brightness; used to calibrate distances to galaxies 

Chandrasekhar limit

  the maximum mass of a white dwarf star, equivalent to 1.4 solar masses 

charge-coupled device

  a silicon chip used to detect light; charge-coupled devices (CCDs) are far more efficient at collecting light than conventional film 

Charles Messier

  A French astronomer and comet hunter who discovered 13 comets independently and codiscovered a half-dozen others. While hunting for comets, Messier compiled a list of fuzzy objects that were not comets in order to avoid them. These catalog entries were later identified as star clusters, nebulae, and galaxies and became the Messier Catalog. Published in various versions beginning in 1771, the catalog grew to 103 objects by 1781. Charles Messier lived from June 26, 1730 to April 12, 1817. 

chondrite

  a stony meteorite containing small, round, silicate granules called chondrules 

chromosphere

  a layer in a stars atmosphere lying below the corona and above the photosphere 

circumpolar

  Circumpolar stars are permanently above the horizon from a given observing point on Earth; that is to say, they never set. At Earth's Geographical North Pole (90 north latitude), all stars in the sky are cirumpolar. On Earth's equator, no stars are circumpolar. 

clock drive

  a motor attached to an equatorial mount that compensates for Earths rotation and thus keeps the telescope pointing at the same area of sky 

coated optics

  optics treated with a thin, uniform coating that greatly reduces scattered light and thus makes the image brighter 

collimation

  the act of putting a telescopes optics into perfect alignment 

coma

  the bright shroud of gas that surrounds a comets nucleus 

comet

  a small piece of ice and rock that orbits a star usually in a highly elongated orbit; long-period comets have orbital periods longer than 200 years, short-period comets have orbital periods less than 200 years 

comet nucleus

  a solid, compact mass of rock and ice that heats up when exposed to sunlight and releases gas and dust 

conjunction

  a time when two or more bodies appear close together in the sky 

constellation

  one of the 88 patterns of stars in the sky, often named for a mythological god, hero, or animal 

convection

  the transfer of heat energy by moving currents of material 

core

  the central region of a planet, brown dwarf, star, or galaxy 

corona

  the outer atmosphere of the sun or a star 

 

coronagraph

 

 

an instrument designed to block light from the solar disk, allowing the corona to be observed 


coronagraphic mask

  an disk-shaped instrument designed to block light from the disk of a star, allowing the region very close to a target star to be studied 

coronal mass ejections

  huge eruptions of gas from the solar corona; CMEs can produce geomagnetic storms and auroral displays on Earth 

cosmic background radiation

  microwave radiation that permeates the universe and represents the still-cooling heat generated from the Big Bang 

cosmic microwave background

  microwave radiation that permeates the universe and represents the still-cooling heat generated from the Big Bang 

cosmic ray

  an atomic nucleus (most are protons) moving at a speed approaching that of light 

cosmological constant

  a term in the equations of general relativity that represents a repulsive force in the universe 

cosmology

  the branch of science concerned with the structure and evolution of the universe 

cosmos

  a synonym for universe 

crescent

  the phase of a planet or moon during which less than half the surface is illuminated 

critical density

  the density of the universe that provides just enough gravity to bring the expansion to a halt after an infinite time 

crust

  the thin, outermost geological layer of a planet, moon, or asteroid 

D

Damocloid

  a rare type of asteroid with an elliptical, comet-like orbit; named for the first one discovered, asteroid 5335 Damocles 

dark adaptation

  the process by which the human eye becomes well adjusted to seeing dim objects in the dark. 

dark energy

  a type of "negative gravity" that seems to play a role in the acceeleration of universal expansion 

dark matter

  matter that exerts gravitational force but does not emit any detectable light or radiation; dark matter comprises most of the mass of the universe but its exact nature remains unknown 

dark nebula

  a cloud of dust grains that is thick enough to obscure the light from background stars 

declination

  the angular distance of a celestial object above or below the celestial equator; the celestial sphere equivalent of latitude 

deep-sky objects

  objects located beyond the solar system; consist of stars, nebulae, star clusters, and galaxies. 

degree

 

1. a unit of angular size equal to 1/360 the circumference of the celestial sphere; the sun and full moon both appear about half a degree wide

2. a unit of measure for temperature along a graded scale

 

density

  the amount of mass per unit volume of an object or region of space 

deuterium

  an isotope of hydrogen; its nucleus, consisting of one proton and one neutron, has double the mass of the nucleus of ordinary hydrogen 

diffraction

  the spreading out of light as it passes the edge of an obstacle 

Dobsonian telescope

  a telescope with a simple but stable altazimuth mount that rotates easily 

Doppler effect

  the change in wavelength of radiation coming from a source thats moving toward or away from an observer; produces either a blueshift or redshift 

dust

  tiny particles floating in space 

dwarf galaxy

  a small galaxy containing a few million stars; the most common type of galaxy in the universe 

dwarf star

  a main-sequence or smaller star 

E

eccentric

  deviating from a circle (used to describe the shape of an orbit) 

eccentricity

  the extent to which a bodys elliptical orbit deviates from a circle 

eclipse

  an event in which one body passes in front of another, blocking it partially or completely from view; a specific type of occultation 

ecliptic

  the plane of Earths orbit around the sun; all the planets except Mercury and Pluto have orbits in nearly the same plane 

ejecta

  material thrown about by an impact or volcano 

electromagnetic radiation

  the various forms of light; includes radio waves, infrared light, visible light, ultraviolet light, x rays, and gamma rays 

electromagnetic spectrum

  the spectrum encompassing the entire range of electromagnetic radiation (light) 

electron

  a subatomic particle with a negative electric charge; electrons surround the atomic nucleus and are much less massive than protons or neutrons 

element

  a fundamental unit of matter; consists of a fixed number of protons, although the number of neutrons and electrons can vary 

elliptical galaxy

  a gravitationally bound system of stars in a spherical or elliptical shape with no spiral structure 

elongation

  the apparent angular separation of an object from the sun 

emission

  the discharge of electromagnetic radiation from an object 

emission nebula

  a cloud of very hot gas that is being illuminated from within by the radiation of energetic, young stars 

ephemeris

  a table that gives the positions of astronomical objects at certain intervals of time. 

equatorial mount

  a telescope mount in which one axis lies parallel to Earths rotational axis; the motion of the telescope about this axis can compensate for Earths rotation 

equinox

  the two times of year when the sun crosses the celestial equator, giving day and night an equal 12-hour length everywhere on Earth 

escape velocity

  the velocity an object or rocket needs to escape the gravitational clutch of a more massive object 

evening star

  the planet Venus when it appears in the evening sky 

event horizon

  the boundary of a black hole from inside which light cannot escape 

exit pupil

  the image of the objective lens or primary mirror of a telescope formed on the eye side of the eyepiece 

exobiologists

  a person who studies the origin, development, and distribution of 'living' systems that may exist  outside of Earth 

extragalactic

  beyond the Milky Way Galaxy 

extrasolar

  beyond the sun 

extraterrestrial

  beyond Earth 

eye relief

  the distance between the eyeball and the lens nearest the eye of an eyepiece at which an observer can clearly see the entire field of view 

 

eyepiece

 

 

                          a magnifying lens used to view the image produced by a telescopes primary lens or mirror. 

F

far ultraviolet

  ultraviolet radiation with the shortest wavelengths ("farthest" from visible light in the electromagnetic spectrum)
 

field of view

  the area of sky visible in a telescope or binoculars 

filter

  a device that transmits light of only certain wavelengths; used by astronomers to observe specific wavelengths or to reduce the light of exceptionally bright objects 

finder scope

  a small, low-powered telescope attached to a larger telescope that helps the observer locate objects in the sky 

fireball

  an extremely bright meteor; generally brighter than magnitude 4 

fireballs

  extremely bright meteors; generally brighter than magnitude 4 

first quarter

  the phase of the moon a quarter of the way around its orbit from new moon; the eastern half is illuminated during this phase 

flare

  a sudden, violent outburst of energy from the surface of a star 

focal length

  the distance from a lens or mirror to the point where it brings light to a focus 

focal ratio (f/ratio):

  the ratio of the focal length of a lens or mirror to its diameter 

focus

  the point at which rays of light passing through a lens (or reflecting off a mirror) converge 

focuser

  the device on a telescope that holds an eyepiece and moves to allow an observer to bring light to a sharp focus. 

fork mount

  an equatorial mount in which the telescope swings in declination between the two prongs of a fork 

frequency

  the number of wave crests or troughs that pass a particular point in a given interval of time (usually one second); usually expressed in hertz (cycles per second) 

full moon

  the phase of the moon when it is halfway around its orbit from new moon and opposite the sun in the sky; the full disk is illuminated 

G

galactic disk

  the disk of a spiral galaxy 

galactic nucleus

  the central region of a galaxy; often contains a high density of stars and gas, and a supermassive black hole 

galactic plane

  the projection of the Milky Ways disk on the sky. 

galaxy

  an enormous gravitationally bound assemblage of millions or billions of stars 

galaxy cluster

  a gravitationally bound assemblage of dozens to thousands of galaxies 

Galilean moons

  Jupiter's four largest moons: Io, Europa, Ganymede, and Callisto; discovered by Galileo Galilei in 1610 

Galilean satellites

  Jupiter's four largest moons: Io, Europa, Ganymede, and Callisto; discovered by Galileo Galilei in 1610 

gamma rays

  the form of light (electromagnetic radiation) with the shortest wavelength and the most energy 

gamma-ray burst

  a short, intense burst of high-energy radiation emanating from the distant universe 

gas giant

  a large planet made primarily of gas; such as Jupiter, Saturn, Uranus, and Neptune 

general relativity

  the theory of relativity governing accelerated motion that describes gravity as a curvature of space-time 

german equatorial mount

  a mount in which the declination axis sits on top of the polar axis, with the telescope on one end of the declination axis and a counterweight on the other 

giant molecular cloud

  interstellar clouds of cold gas and dust that contain tens or hundreds of thousands of solar masses 

gibbous

  the phase of the moon between first quarter and last quarter, when the moon appears more than half illuminated 

globular cluster

  a roughly spherical congregation of hundreds of thousands of stars; most globular clusters consist of old stars and exist in a galaxys halo 

gravitational lensing

  the distortion or amplification of an objects light due to the presence of a massive object in the light path 

gravity

  the attractive force that all objects exert on one another; the greater an objects mass, the stronger its gravitational pull 

H

habitable zone (or ecosphere)

  the zone around a star in which a planet can maintain liquid water on its surface. 

halo

  the outer region of a galaxy; contains globular clusters, a few stray stars, and dark matter 

heliacal rising

  the period of time when an object, such as a star, is briefly seen in the eastern sky before dawn and is no longer hidden from the glare of the sun 

heliosphere

  a vast region around the sun dominated by the solar wind 

helium

  the second lightest element; consists of two protons, and usually two neutrons and two electrons; about 8 percent of the atoms in the universe are helium 

Hertz

  a unit of frequency equal to one cycle per second 

Hertzsprung-Russell diagram

  a diagram that plots luminosity against temperature for a group of stars 

Hubble law

  the principle that a distant galaxys recessional velocity is proportional to its distance from Earth 

hydrazine

  a colorless liquid which burns rapidly and is used as a common rocket and missile fuel 

hydrogen

  the simplest and lightest element; usually consists of just a single proton and electron; about 90 percent of the atoms in the universe are hydrogen 

hypered film

  film that has been treated, usually with gas, to enhance its response to low light levels 

I

inclination

  the angle between a planets orbit and the ecliptic plane; or the angle between a satellites orbit and its host planets rotational plane 

inferior conjunction

  the configuration of an inferior planet when it lies between the sun and Earth 

inferior planet

  a planet that orbits the sun inside of Earths orbit; includes Mercury and Venus 

inflation

  the theory that the universe experienced a brief and extraordinarily rapid period of expansion a fraction of a second after the Big Bang 

infrared

  a form of light with slightly lower energy than visible light but with greater energy than radio waves 

interacting galaxies

  galaxies caught in each others gravitational embrace, often results in galactic mergers or extreme star formation 

interferometer

  a system of two or more widely separated telescopes that achieves the resolving power of a much larger telescope 

interferometry

  the technique of using two or more widely separated telescopes to achieve the resolving power of a much larger telescope 

intergalactic

  the space between the galaxies 

International Space Station

  a global cooperative program between the United States, Russia, Canada, Japan, and Europe, for the joint development, operation, and utilization of a permanently habitated space station in low-Earth orbit
 

interplanetary

  the space between the planets 

interstellar

  the space between the stars of a galaxy 

interstellar medium

  the gas and dust located between the stars 

ion

  an atom that is electrically charged due to the loss or gain of one or more electrons 

ionization

  the process by which an atom gains or loses electrons 

ionized gas

  a gas that has been heated to a state where it contains ions and free-floating electrons; also known as a plasma 

ionosphere

  an atmospheric layer with a high concentration of ions and free electrons 

irregular galaxy

  a galaxy without a clearly defined spiral or elliptical shape 

isotope

  forms of an element in which the atoms all have the same number of protons but different numbers of neutrons 

J

jet

  a narrow stream of gas or particles ejected from an accretion disk surrounding a star or black hole 

Jet Propulsion Laboratory

  the lead U.S. center for robotic exploration of the solar system, located in Pasadena, California; JPL spacecraft have visited all known planets except Pluto 

jet stream

  a high-speed, wandering wind current in the upper troposphere that blows from west to east and affects weather 

Jovian planet

  a planet with characteristics similar to Jupiter (see gas giant). 

K

Kelvin

  a unit of temperature equal to one degree on the Celsius scale and 1.8 degrees on the Fahrenheit scale; also the absolute temperature scale defined so that 0 kelvin is absolute zero 

Kuiper belt

  a region in the outer solar system beyond Neptunes orbit that contains billions of small, icy bodies; Pluto is the largest known Kuiper belt object 

L

Lagrange point

  one of five locations in space relative to two bodies where a third, less massive body can maintain a stable orbit around a common center of mass 

Lagrange points

  five locations in space relative to two bodies where a third, less massive body can maintain a stable orbit around a common center of mass 

Large Magellanic Cloud

  an irregular galaxy that orbits the Milky Way Galaxy 

last quarter

  the phase of the moon three-quarters of the way around its orbit from new moon; the western half is illuminated 

latitude

  the angular distance north or south from the equator to a point on Earth's surface, measured on the meridian of the point 

lens

  a curved piece of glass that brings light to a focus 

libration

  the small oscillations in the moons motion that allow Earth-based observers to see slightly more than half the moons surface 

light pollution

  light, typically from artificial sources, that reaches the night sky, obscuring the view of faint astronomical objects 

light-gathering power

  the ability of a telescope to collect light; the larger a telescopes aperture, the greater its light-gathering power 

light-year

  the distance light travels in one year, equivalent to approximately 5.9 trillion miles (9.5 trillion km) 

limb

  the apparent edge of a celestial object 

limiting magnitude

  the apparent magnitude of the faintest objects that can be seen given the local observing conditions and any telescope, film, or other detector you may be using 

LINER galaxy

  A low-ionization nuclear emission-line region galaxy belongs to a common class of otherwise normal galaxies that display low-ionization line emissions near their central regions 

Local Group

  the galaxy cluster containing roughly 35 galaxies to which the Milky Way Galaxy belongs 

local supercluster

  the galaxy supercluster to which the Local Group belongs; it spreads over 100 million light-years and boasts the Virgo Cluster as its dominant member 

longitude

  the angular distance of a particular place on Earth as measured east or west from the prime meridian running through Greenwich, England 

luminosity

  the total amount of light that an object radiates 

lunar eclipse

  a phenomenon caused by the Earth passing between the sun and moon 

lunar month

  the period of one complete revolution of the moon around Earth, 29.5 days 

lunation

  the time between two successive new moons; approximately 29.5 days 

M

magnetometer

  an instrument used to measure the strength and direction of a magnetic field 

magnetosphere

  the dynamic region around a planet where the magnetic field traps and controls the movement of charged particles from the solar wind 

magnitude

  the measurement of an object's  brightness; the lower the number, the brighter the object 

main sequence

  the band of stars on a Hertzsprung-Russell diagram stretching from the upper left to the lower right; stars spend most of their lives in the main sequence phase, in which they are fusing hydrogen into helium in their cores 

Maksutov telescope

  a catadioptric telescope that uses a deeply curved meniscus lens as the correcting plate 

mantle

  the portion of a planets interior above the core but below the crust 

mare

  a dark and relatively smooth area on the surface of the moon or a planet. 

mass

  a measure of the total amount of matter within an object 

mass loss

  the loss of mass by a star during its evolution; some of the causes of mass loss include stellar winds, bipolar outflows, and the ejection of material in a planetary nebula or supernova 

megaparsec

  one million parsecs; equal to 3.26 million light-years 

meridian

  an imaginary circle on the celestial sphere that connects the zenith to the north (or south) celestial pole 

Messier Catalog

  A catalog of 107 bright deep-sky objects that belong to a catalog compiled by French astronomer Charles Messier in the 1700s 

Messier objects

  A catalog of 107 bright deep-sky objects that belong to a catalog compiled by French astronomer Charles Messier in the 1700s 

meteor

  a flash of light that occurs when a meteoroid burns up in Earths atmosphere; also popularly known as a shooting star 

meteor shower

  a period of enhanced meteor activity that occurs when Earth collides with a swarm of meteoroids; an individual shower happens at the same time each year and has all its meteors appearing to radiate from a common point 

meteorite

  a rock from space that survives passage through Earths atmosphere and falls to the ground 

meteoroid

  a small rock that orbits the sun 

microgravity

  describes a condition where the apparent weight of an object is much less than its true weight (on Earth). Apparent weight is the weight measured when an object is put on a
scale in a given environment, where true weight is the force of Earth's gravity on the object. 

microlensing

  the effect of gravity from a small astronomical body or bodies focusing light rays, similar in manner to lenses 

micron

  one-millionth of a meter 

microwaves

  the most energetic form of radio waves 

Milky Way

  the band of light that encircles the entire sky and results from the combined light of billions of stars in our galaxys disk 

Milky Way Galaxy

  the spiral galaxy to which Earth belongs 

million

  1,000,000 (in American usage) 

millisecond pulsar

  a neutron star that rotates hundreds of times per second, which typically accretes matter from a stellar companion 

 

minor planet

 

 

a rocky body that orbits the sun; also known as an asteroid 


mirror

  a piece of glass coated with a highly reflective material 

molecule

  a combination of two or more atoms that represents the smallest part of a compound that has the chemical properties of that compound 

moon

  a smaller body orbiting a larger body; often refers to Earths moon 

morning star

  the planet Venus when it appears in the morning sky 

motor drive

  see clock drive 

multicultural astronomy

  the variety of ways in which cultures of the past and present have observed, recorded, interperted, and made use of astronomy to structure their lives and/or satisfy their curiosity about the universe 

multiple star system

  a gravitationally bound system in which two or more stars orbit a common center of mass. 

N

naked-eye

  something visible or accomplished without the aid of binoculars or a telescope (e.g. a naked-eye object or naked-eye observing) 

nebula

  a cloud of interstellar gas and dust; some nebulae represent stellar nurseries, others represent stellar graveyards 

neutrino

  a subatomic particle with little or no mass that is produced in nuclear reactions and in supernovae and very rarely interacts with matter; neutrinos have no electrical charge and travel at or very close to the speed of light 

neutron

  a subatomic particle with no electric charge that resides in an atomic nucleus; it has about the same mass as a proton 

neutron star

  the collapsed, extraordinarily dense, city-sized remnant of a high-mass star 

new moon

  the phase in which the moon is in the same direction as the sun in Earths sky, so it is unilluminated and invisible 

Newtonian telescope

  a reflecting telescope in which a flat secondary mirror (called the diagonal) in the center of the tube reflects light to a focus outside the tube 

NGC

  New General Catalogue, a 19th-century compendium of deep-sky objects such as galaxies, globular clusters, and nebulae 

NGC objects

  deep-sky objects such as galaxies, globular clusters, and nebulae included in the New General Catalogue 

North Celestial Pole

  the point in the sky to which Earth's Geographical North Pole points 

nova

  a violent explosion on the surface of a white dwarf, which causes the star to temporarily brighten by a factor of several hundred to several thousand 

nuclear fusion

  the process by which two atomic nuclei combine to form a heavier atomic nucleus; this is the energy source that causes most stars to shine 

nucleosynthesis

  the creation of heavy elements from lighter ones by nuclear fusion 

Nucleus

  the central region of an atom, comet, or galaxy 

O

OB association

  a loose grouping of O and B stars, which are the most luminous, most massive, and shortest-lived stars 

objective

  a telescopes primary lens or mirror that gathers light and brings it to a focus 

obliquity

  the angle between a planets equator and the plane of its orbit 

occultation

  the passage of one object in front of a smaller one, temporarily obscuring all or part of the background object from view 

omega

  1. the ratio of the density of the universe to the critical density
2. the 24th letter of the Greek alphabet 

Omega Centauri

  a massive globular cluster in the southern constellation Centaurus located about 17,000 light-years from Earth; also known as NGC 5139 

Omega nebula

  One of the Milky Ways numerous stellar nurseries, the Omega Nebula is about 5,000 light-years from Earth and can be seen in the constellation of Sagittarius the Archer. It is also known as the Swan Nebula, M17, NGC 6618, the Horseshoe Nebula, and the Lobster Nebula. 

Oort cloud

  a cloud of cometary nuclei that surrounds the sun at a distance of many thousands of astronomical units 

open cluster

  a system containing a few dozen to a few thousand stars that formed from the same stellar nursery. 

opposition

  the moment when a planet farther from the sun than Earth appears opposite the sun in the sky; it is the best time to observe a planet 

optical double

  Two stars at different distances that lie along nearly the same line of sight and thus appear close together 

optics

  the study of light and its properties; or lenses and mirrors 

orbit

  the path an object follows around a more massive object or common center of mass; usually elliptical in shape 

orbital period

  the length of time it takes one body to orbit another 

outgassing

  the release of gas from a rocky body 

P

PAHs

  Polycyclic aromatic hydrocarbons (PAHs) are a class of very stable organic molecules. They are flat molecules made only of carbon and hydrogen atoms. PAH molecules are quite common and highly carcinogenic. They are one of the by-products of combustion from automobiles and airplanes, and some are present in charcoal broiled hamburgers.

 

parallax

  the apparent shift of a relatively nearby object against a fixed background due to the motion of the observer; astronomers observe the parallax of stars to measure their distances 

parsec

  the distance an object would have to be from Earth so that its parallax when viewed from two points separated by 1 AU would be one arcsecond; equal to 3.26 light-years. 

penumbra

  the region of a shadow from which part of the light source remains visible 

penumbral eclipse

  an event that occurs when the moon passes into the outer ring of Earth's shadow (penumbra), causing a slight shading in the moon's appearance 

perigee

  the point in a satellites orbit when it is closest to Earth 

perihelion

  the point in an objects orbit when its closest to the sun 

period

  the time interval for a regular event to take place 

pH

  a measurement of the alkalinity or acidity of a solution 

phase

  the regular cycle of changes in the appearance of a moon or planet 

photometer

  a detector that measures the amount of light coming from an object 

Photometry

  the measurement of light intensities 

photons

  individual particles/waves of light 

photosphere

  the visible surface of the sun 

photovoltaic

  conversion of light energy into electricity 

pixel

  short for picture element, the individual light detectors on a CCD chip 

planet

  a large rocky or gaseous body that orbits a star 

planetary nebula

  a glowing shell of gas ejected by a dying, low-mass star 

planetary nebulae

  glowing shells of gas ejected by dying, low-mass stars 

planetesimals

  asteroid-size bodies in a young planetary system that collide to form larger bodies 

planisphere

  a two-dimensional map of the sky with an adjustable overlay to show the part of the sky visible at any time of the night or year 

plasma

  a gas that has been heated to a state where it contains ions and free-floating electrons; also known as ionized gas 

plasmasphere

  a region of cold, high-density plasma above the ionosphere 

plate tectonics

  a theory that describes how Earths crust is broken into plates and how those plates move across Earths surface 

polar cap

  an icy region at the north or south pole of a planet 

polarization

  a state in which the directions of the electric or magnetic field in an electromagnetic wave changes in a regular pattern; light from celestial objects is often polarized 

polycyclic aromatic hydrocarbons

  Polycyclic aromatic hydrocarbons (PAHs) are a class of very stable organic molecules. They are flat molecules made only of carbon and hydrogen atoms. PAH molecules are quite common and highly carcinogenic. They are one of the by-products of combustion from automobiles and airplanes, and some are present in charcoal broiled hamburgers.

 

position angle

  the direction in the sky of one celestial object from another, measured eastward from due north 

power (or magnification)

  the ability of a telescope or binoculars to increase the apparent size of a distant object. 

Poynting-Robertson effect

  a drag on interplanetary particles caused by their interaction with solar radiation, which causes the particles to lose orbital momentum and spiral into the sun 

precession

  the slow, periodic change in the direction an objects rotational axis caused by the gravitational influence of another body 

primary mirror (or lens)

  a telescopes main mirror or lens which gathers light and brings it to a focus 

prime meridian

  the line of longitude that runs through Greenwich, England 

prism

  a wedge-shaped piece of glass that breaks white light into its constituent colors 

prograde

  objects that move or appear to move in the same direction of most solar system bodies, or for moons, the same direction as the planet rotates 

prominence

  a large eruption of gas streaming off the surface of the sun into the corona 

proper motion

  the apparent yearly motion of a star across the sky 

proton

  a subatomic particle that resides in an atoms nucleus and possesses a positive electric charge 

protoplanet

  a body that is accreting gas, dust, and rocks en route to becoming a full-fledged planet 

protoplanetary disk

  a disk of gas and dust that surrounds a newborn star; planets form from collisions of particles inside the disk 

protostar

  a cloud of hot, dense gas and dust that is gravitationally collapsing to form a star 

pulsar

  a rapidly rotating neutron star that bathes Earth in regular pulses of electromagnetic radiation 

Q

quantum mechanics

  the physical laws that describe the behavior of matter at the atomic and subatomic level 

quasar

  the highly energetic core of a young galaxy thought to be powered by a supermassive black hole; short for quasi-stellar object 

R

radial velocity

  the velocity of an object toward or away from an observer 

radiant

  1. the point in the sky from which the meteors belonging to a meteor shower appear to originate

2. vividly bright and shining 

radiation

  electromagnetic waves (in astronomical usage) 

radio galaxy

  a galaxy that emits an unusually large amount of radio waves 

radio telescope

  a telescope designed to detect radio waves coming from space 

radio waves

  the form of light with the longest wavelength and the least energy 

radiometer

  a device that measures the total energy or power from an object in the form of radiation, especially infrared radiation 

red dwarf

  a low-mass, main-sequence star much smaller, cooler, and less luminous than the sun 

red giant

  a cool star near the end of its life cycle that has expanded to a size of a few dozen to a hundred times the diameter of the sun 

red supergiant

  a cool, massive star near the end of its life that has expanded to a size from a hundred to a thousand times the diameter of the sun 

redshift

  an increase in the wavelength of light coming from an object due to its motion away from Earth, the expansion of the universe, or a strong gravitational field

 

reflection nebula

  a cloud of gas and dust that is visible because the dust reflects a nearby stars light 

reflector

  a telescope that uses a curved mirror to gather light 

refractor

  a telescope that uses a glass lens to gather light 

regolith

  the powdery soil of the moon produced by meteorite impacts 

relativity

  the theories of physics developed by Albert Einstein that describe measurements made by two observers who are in relative motion 

resolution

  the ability of a telescope or camera to pick out fine detail 

resolving power

  the ability of a telescope or camera to pick out fine detail 

reticule

  a grid or pattern of two or more fine wires set inthe focal plane of a telescope eyepiece and used in determining the position and/or size of a  celestial object 

retrograde

  objects that move or appear to move in the opposite direction of most solar system bodies; for example planets that appear to move east-to-west in the sky or objects that revolve or rotate clockwise as seen from north of the solar system 

reusable launch vehicle (RLV)

  a single-stage-to-orbit spacecraft that may be reused on successive missions 

revolution

  the orbital motion of one body around another body or a common center of mass 

ribonucleic acid

  a nucleic acid that transmits genetic information 

rich-field telescope

  a telescope designed to show a large field of view at low magnification 

right ascension

  the angular distance of a celestial object east of the vernal equinox; the celestial sphere equivalent of longitude 

RNA

  a nucleic acid that transmits genetic information 

rotation

  the spin of a galaxy, star, planet, moon, or asteroid about a central axis 

rotation period

  the length of time it takes a body to complete one rotation 

S

satellite

  a small body that orbits a planet or asteroid 

Schmidt camera

  a catadioptric telescope used as a camera to take wide-angle photos of the sky 

Schmidt-Cassegrain telescope

  a compact telescope in which light passes through a correcting lens at the front of the telescope, then reflects off a primary mirror back up to a secondary mirror, which directs the light through a hole in the primary and out the back of the scope; a popular telescope for backyard observers. 

secondary mirror

  a relatively small mirror used in a telescope to redirect the light gathered by the primary mirror 

seeing

  the quality of observing conditions induced by turbulence in Earths atmosphere, which blurs the images of astronomical objects 

semimajor axis

  the average distance of an orbiting body from its parent body 

SETI

  the search for extraterrestrial intelligence 

setting circles

  circular scales on the two axes of an equatorial mount that help an observer point a telescope to a specific right ascension and declination 

shock wave

  a powerful wave caused by a sudden change in density, pressure, or temperature that travels though a medium faster than sound travels through that same medium 

sidereal

  relating to or measured with respect to the stars 

sidereal year

  the amount of time it takes one body to revolve about another with respect to the stars 

singularity

  the central point of a black hole where matter is concentrated into an area of zero volume and infinite density 

Small Magellanic Cloud

  a small, irregular dwarf galaxy that orbits the Milky Way Galaxy 

solar eclipse

  an eclipse of the sun caused by the moon passing between Earth and the sun 

solar filter

  a filter used to block almost all of the suns light so our star can be viewed safely and comfortably 

solar mass

  the amount of mass contained in the sun, about 330,000 times that of Earths mass 

solar system

  the system containing the sun and all the smaller bodies in orbit around it 

solar wind

  the stream of charged subatomic particles emanating from the sun 

solstice

  either of the two points on the celestial sphere where the sun is farthest north or south of the celestial equator; when the sun is at a solstice, the amount of daylight hours is greatest for summer and least for winter 

South Celestial Pole

  the point in the sky to which Earth's Geographical South Pole points 

space-time

  the intertwining of the three dimensions of space with one dimension of time within which events can be specified exactly 

special relativity

  the theory of relativity governing uniform motion; it states the equivalence of mass and energy and differs from Newtonian physics only when speeds approach that of light 

spectral class

  the designation of a star based on its spectrum, which is determined by its surface temperature 

spectral line

  a particular wavelength of light corresponding to the energy transition of a specific atom or molecule 

spectral type

  the designation of a star based on its spectrum, which is determined by its surface temperature 

spectrograph

  an instrument attached to a telescope to record the spectrum of an astronomical object 

spectrometer

  an instrument attached to a telescope to record the spectrum of an astronomical object 

spectroscope

  an instrument for examining spectra 

spectroscopy

  the study of spectra from astronomical objects 

spectrum

  1. the energy emitted by a radiant source 2. the entire range of electromagnetic radiation (light) 

speed of light

  the fastest possible speed in a vacuum, equivalent to 186,000 miles per second (300,000 km per second) 

spiral arm

  a concentration of gas, dust, and young stars that winds its way out from the nuclear region of a spiral galaxy 

spiral galaxy

  a spiral-shaped system of billions of stars, gas clouds, and dust 

standard candle

  an astronomical object of known luminosity; can be used to determine distances 

star

  a self-luminous sphere of hot gas held together by gravity; ordinary stars generate energy by nuclear fusion in their cores 

star atlas

  an collection of maps that marks the positions of stars, nebulae, galaxies, and other astronomical objects on a coordinate system 

star hopping

  the technique of using recognizable patterns of stars to hop from one part of the sky to another; useful in observing both with the naked eye and a telescope 

star party

  a gathering of people to observe the night sky 

starburst galaxy

  a galaxy undergoing an extremely high rate of star formation 

stellar evolution

  the life cycle of stars 

stellar wind

  a stream of electrically charged subatomic particles given off by stars 

sublimate

  the transition of a solid substance evaporating into a gas without passing through a liquid phase 

sublimated

  the transition of a solid substance evaporating into a gas without passing through a liquid phase 

sublimation

  the process by which a solid substance evaporates into a gas without passing through a liquid phase 

summer

  a season that begins around June 21 in the Northern Hemisphere 

sunspot

  a dark, temporary, relatively cool spot on the surface of the sun 

sunspot cycle

  a cycle averaging 11 years in which the number of sunspots increases and decreases. 

supercluster

  an enormous congregation of galaxy clusters that stretches across hundreds of millions of light-years 

superfluid

  an unusual state of matter characterized by apparently frictionless flow, found only in liquid helium cooled to near absolute zero 

superior conjunction

  the configuration of an inferior planet when it lies on the far side of the sun 

superior planet

  a planet farther from the sun than Earth; includes Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto 

superluminal motion

  motion that appears to be faster than the speed of light 

supermassive black hole

  a black hole at the core of a galaxy that contains millions or billions of solar masses 

supernova

  the cataclysmic explosion of a star 

supernova remnant

  an expanding cloud of gas that represents the outer layers of an exploded star 

synchronous rotation

  when a satellite rotates at the same rate at which it revolves around a more massive object; a body with synchronous rotation shows only one hemisphere to the object it orbits 

T

telescope

  a tubed instrument used to brighten and magnify the view of astronomical objects (telescopes gather more light than the eye) 

terminator

  the boundary on a planet or moon separating the illuminated side from the unilluminated. 

terrestrial

  of or relating to Earth 

terrestrial planet

  a small, rocky planet such as Mercury, Venus, Earth, and Mars 

thermal radiation

  electromagnetic radiation emanating from any object not at absolute zero 

tidal force

  the difference in gravitational force between two points on an object caused by the gravity of another object; the tidal force often leads to a deformation of an object 

tides

  the distortion of a body caused by the gravitational influence of another body 

Trans-Neptunion Object

  an object in our solar system lying beyond the orbit of Neptune; abbreviated TNO 

transit

  the passage of a smaller body in front of a larger body; also, the passage of a celestial body across an observers meridian 

transparency

  the clarity of the sky

 

tremolite

  a common mineral in some metamorphic rocks, composed mainly of calcium and magnesium; it occurs from the conversion of dolomite (a sedimentary rock), silica, and water 

trillion

  1,000,000,000,000 (in American usage) 

Trojan

  an asteroid that lies in or near one of the Lagrange points 60 degrees ahead or behind Jupiter along the planet's orbit
 

tropical year

  the time it takes Earth to revolve around the sun with respect to the vernal equinox 

true field of view

  the angle of sky seen through an eyepiece when it is attached to a telescope; the true field equals the apparent field divided by the magnification 

type Ia supernova

  the explosion of a white dwarf that occurs when it accretes enough mass from a companion star to go above the Chandrasekhar limit 

type II quasars

  a quasar enshrouded in gas and dust that emits very little visibile light, however, is easily seen in the infrared and x-ray region of the electromagnetic spectrum 

type II supernova

  the explosion of a massive star that occurs when its core runs out of nuclear fuel; these explosions leave behind a neutron star or a black hole 

U

ultraviolet light

  radiation with higher energy than visible light, but without as much energy as x rays 

ultraviolet radiation

  radiation with higher energy than visible light, but without as much energy as x rays 

umbra

  the dark, central region of a shadow from which none of the light source can be seen 

unidentified infrared bands (UIBs)

  mysterious objects in space that give off as yet unidentified infrared emission patterns 

Universal Time

  the local time of day on a line of longitude centered on Greenwich, England (also known as Greenwich Mean Time); it forms the basis for all civil timekeeping 

Universe

  everything that exists 

V

Van Allen belts

  the two belts of charged particles from the solar wind that have been trapped by Earths magnetic field above Earths atmosphere.

 

variable star

  a star that varies in luminosity 

vernal equinox

  the time of year around March 21 when the sun crosses the celestial equator heading north 

visible light

  the portion of the electromagnetic spectrum visible to the human eye 

voids

  enormous regions of relatively empty space between galaxy superclusters 

volatiles

  chemical compounds that are gaseous at low temperatures. 

W

waning

  the period between full moon and new moon 

wavelength

  the distance between two successive wave crests or troughs 

waxing

  the period between new moon and full moon 

weight

  the force exerted on an object due to gravity 

white dwarf

  the dense, collapsed, Earth-sized remnant of an intermediate-mass star like the sun 

winter

  a season that begins around December 21 in the Northern Hemisphere 

X

X rays

  electromagnetic radiation more energetic than ultraviolet light but less energetic than gamma rays 

Y
 
no definitions listed at this time

Z

zenith

  the point on the celestial sphere directly over the head of an observer.

 

zenithal hourly rate

  the number of meteorites expected to be seen per hour when a meteor shower's radiant is at an observer's zenith; abbreviated ZHR 

zodiac

  a belt around the sky about 18 wide and centered on the ecliptic that encompasses the traditional 12 constellations through which the sun travels during its yearly motion 

zodiacal light

  a faint, cone-shaped glow of light seen in the west after nightfall or in the east before dawn, caused by sunlight reflecting and scattering off interplanetary dust particles lying along the ecliptic plane