Word List

This list is always growing.

Absolute Zero
The cold­est pos­si­ble tem­per­a­ture in the uni­verse where all mol­e­c­u­lar activ­ity stops. On the Kelvin scale this tem­per­a­ture is 0 degrees, or about –273 Cel­sius or –460 Fahrenheit

Astro­nom­i­cal Unit
(AU) The aver­age dis­tance between the Earth and the Sun; about 93 mil­lion miles or 150 mil­lion kilometers.

Atom
The small­est unit of mat­ter that pos­sesses chem­i­cal prop­er­ties. All atoms have the same basic struc­ture: a nucleus, con­tain­ing an equal num­ber of pos­i­tively charged pro­tons and neg­a­tively charged elec­trons. Each atom cor­re­sponds to a unique chem­i­cal ele­ment deter­mined by the num­ber of pro­tons in the nucleus.

Big Bang
The cur­rent, accepted the­ory for the ori­gin and evo­lu­tion of the uni­verse. The the­ory says the observ­able uni­verse started almost 14 bil­lion years ago from an extremely dense and hot state.

Binary star sys­tem
A star sys­tem with two stars orbit­ing around a com­mon cen­ter of mass. Most star sys­tems in the galaxy are binary star systems.

Black holes
A region of space con­tain­ing an object of high mass packed into an extremely small vol­ume. The grav­i­ta­tional influ­ence of a black hole is so strong that noth­ing, not even light can escape. Astronomers see black holes only by observ­ing swirling disks of mate­r­ial, called accre­tion disks, around them.

Blue star
Mas­sive, hot stars that appear blue in color. Spica in the con­stel­la­tion of Virgo is an example.

Cel­sius
A tem­per­a­ture scale where the freez­ing point of water is 0 and the boil­ing point is 100.

Con­stel­la­tion
A group of bright stars that appear in the sky. Astronomers rec­og­nize 88 con­stel­la­tions in the north­ern and south­ern hemi­spheres. Ancient observers named many con­stel­la­tions after gods, heroes, ani­mals and other mytho­log­i­cal beings.

Elec­trons
A neg­a­tively charged ele­men­tary par­ti­cle found out­side the nucleus of an atom but bound to it by elec­tro­mag­netic forces.

Ellip­ti­cal galaxy
A galaxy that appear spher­i­cal in shape like an egg or a foot­ball. Ellip­ti­cal galax­ies are usu­ally made up of older stars and con­tain lit­tle gas and dust to cre­ate new stars.

Fahren­heit
A tem­per­a­ture scale pro­posed in 1724 by physi­cist Daniel Gabriel Fahren­heit. The freez­ing point of water is set at 32 and the boil­ing point at 212.

Fusion
A nuclear process that releases energy when atomic nuclei com­bine to form heav­ier nuclei. Fusion is the Suns energy source.

Galaxy
A col­lec­tion of stars, gas and dust bound by grav­ity. The Milky Way Galaxy con­tains our Sun and solar sys­tem. Galax­ies are grouped by their shape. Round or oval galax­ies are ellip­ti­cal galax­ies. Galax­ies show­ing a pin­wheel struc­ture are called spi­ral galax­ies. Galax­ies that do not resem­ble either ellip­ti­cal galax­ies or spi­ral galax­ies are con­sid­ered irreg­u­lar galaxies.

Gaseous neb­ula
A glow­ing cloud of gas in inter­stel­lar space. Emis­sion neb­u­las absorb ultra­vi­o­let light from nearby stars. The ultra­vi­o­let light causes the cloud to glow as vis­i­ble light. Or the cloud of gas could be a reflec­tion neb­ula. Dust within the reflec­tion neb­ula reflects light from nearby stars.

Giant star
A dying star that has used all of its hydro­gen fuel for nuclear fusion and has begun to expand. Giant stars are usu­ally larger than the Sun.

Glob­u­lar clus­ter
A col­lec­tion of hun­dreds of thou­sands of stars of sim­i­lar age held together by grav­ity. The clus­ters are usu­ally shaped as a sphere and are often found in the halos of galaxies.

Infrared
Radi­a­tion that has longer wave­lengths and lower fre­quen­cies than vis­i­ble light. We feel infrared radi­a­tion as heat.

Inter­stel­lar dust
Small par­ti­cles of solid mat­ter, sim­i­lar to smoke, found between the stars.

Inter­stel­lar space
The dark regions of space between stars.

Irreg­u­lar galaxy
A galaxy that appears to be dis­or­ga­nized. Irreg­u­lar galax­ies, usu­ally rich in inter­stel­lar dust and gas, lack dis­tinct spi­ral or ellip­ti­cal shapes. The Large and Small Mag­el­lanic Clouds, satel­lites of the Milky Way, are exam­ples of irreg­u­lar galaxies.

Jets
Nar­row, high-energy streams of gas and dust usu­ally ejected in oppo­site direc­tions from a core star.

Light-year
The dis­tance that a par­ti­cle of light, or a pho­ton, trav­els in a year. Light trav­els about 6 tril­lion miles, or 10 tril­lion kilo­me­ters, in a year. Astronomers mea­sure the dis­tance between stars and galax­ies in light years.

Lumi­nos­ity
The amount of energy radi­ated into space by a celes­tial object, such as a star. Lumi­nos­ity is sim­i­lar to the bright­ness of the celes­tial object.

Mag­ne­tar
A mag­ne­tar is a spin­ning, neu­tron star, or pul­sar, with a very strong mag­netic field. The mag­netic field of a mag­ne­tar is tril­lions of times stronger than Earth’s mag­netic field.

Mass
A mea­sure of the total amount of mat­ter con­tained within an object

Meteor
A small piece of rock or dust that hits Earth’s atmos­phere from space. Another name for a meteor is a shoot­ing star. One can see a meteor as a visual streak of light in the sky. A meteor is mov­ing so fast that it heats up and glows while mov­ing through the atmos­phere. Most mete­ors bun up before hit­ting the ground. If the meteor hits the ground, we call it a meteorite.

Micron
A unit of length equal to one mil­lionth of a meter.

Milky Way
The Milky Way Galaxy is a spi­ral galaxy that is home to Earth, the Sun, and to most of the stars we see in our night sky. The Milky Way con­tains more than 100 bil­lion stars, is 100,000 light-years wide and about 1,000 light years thick.

Moon
A nat­ural satel­lite orbit­ing around a planet.

Neb­ula
A glow­ing cloud of gas in inter­stel­lar space.

Neu­tron star
The left­over cen­tral core of a star that col­lapsed under grav­ity dur­ing a super­nova explo­sion. After the super­nova blows off the outer lay­ers of the star, it col­lapses under its own grav­ity. The star col­lapses so much that the pro­tons and elec­trons spin­ning around the atoms of the star com­bine to form neu­trons. Neu­tron stars are extremely dense. They have the mass of an aver­age star, about 1.5 times the mass of the Sun. Imag­ine our entire sun packed into an area of just 20 kilo­me­ters (12 miles) in diam­e­ter! Grav­ity is very strong on a neu­tron star. On Earth, a spoon­ful of neu­tron star mate­r­ial would weigh bil­lions of tons.

Nova
A binary star sys­tem, made up of a white dwarf and com­pan­ion star, that rapidly bright­ens, then slowly fades to nor­mal brightness.

Pho­ton
A packet of elec­tro­mag­netic energy, such as light.

Plan­e­tary neb­ula
An expand­ing shell of glow­ing gas released by a star late in life. Our Sun will cre­ate a plan­e­tary neb­ula at the end of its life 4 bil­lion years from now.

Pro­tons
A pos­i­tively charged ele­men­tary par­ti­cle inside the nucleus of an atom.

Pul­sar
Pul­sars are pow­er­ful, spin­ning neu­tron stars and give off reg­u­lar pulses of energy, like the tick­ing of a very pre­cise clock.

Red giant
An old, bright star. Red giants are much larger and cooler than the Sun. Antares is an exam­ple of a red giant.

Shoot­ing star
A small piece of rock or dust that hits Earth’s atmos­phere from space. A more sci­en­tific name for a shoot­ing star is meteor. One can see a meteor as a visual streak of light in the sky. A meteor is mov­ing so fast that it heats up and glows while mov­ing through the atmos­phere. Most mete­ors bun up before hit­ting the ground. If the meteor hits the ground, we call it a meteorite.

Solar sys­tem
The Sun and sur­round­ing objects, includ­ing plan­ets, icy dwarf plan­ets, moons, aster­oids, comets and dust. All these objects are bound to the Sun by gravity.

Solar wind
Streams of highly charged par­ti­cles flow­ing out from the Sun or a star at mil­lions of kilo­me­ters per hour. On Earth, the solar wind inter­acts with the mag­netic field to form aurora over the north and south poles. The solar wind also causes comet tails to point away from the Sun.

Speed of light
The speed at which light (pho­tons) travel through empty space. 300 mil­lion meters per sec­ond or about 186,000 miles per second.

Spi­ral galaxy
A pinwheel-shaped col­lec­tion of stars, dust and gas clouds. A typ­i­cal spi­ral galaxy has a spher­i­cal bulge of older stars sur­rounded by a flat­tened disk of younger stars, dust and gas. The Milky Way Galaxy is a spi­ral galaxy.

Star
A huge ball of gas held together by grav­ity. The cen­tral core of the star is very hot and pro­duces energy through fusion. Stars come in many shapes, col­ors and tem­per­a­tures. Our Sun, the star at the cen­ter of our solar sys­tem, is a yel­low star of aver­age size and temperature.

Star clus­ter
A group of stars born at about the same time. Stars in these clus­ters are bound by grav­ity and stay together for bil­lions of years. The Pleiades is an example.

Super­nova
The explo­sive death of a mas­sive star. The energy out­put from a supernova’s rapidly expand­ing gas cloud can glow brighter than an entire galaxy for a few weeks.

Super­nova rem­nant
The glow­ing, expand­ing gaseous remains of a super­nova explosion

Ultra­vi­o­let
Radi­a­tion that has shorter wave­lengths and higher fre­quen­cies than vis­i­ble light.

Uni­verse
All of space and time, along with the energy con­tained within it. The best cur­rent the­ory is that the uni­verse is expand­ing and all energy and mat­ter were cre­ated dur­ing the Big Bang.

Vari­able star
A star whose lumi­nos­ity, or bright­ness, changes over time.

Vis­i­ble light
The part of the elec­tro­mag­netic spec­trum that human eyes can detect. The col­ors of the rain­bow make up vis­i­ble light.

White dwarf
The hot, com­pact remains of a star with mass like our Sun that has exhausted its sources of fuel for ther­monu­clear fusion. White dwarfs are about the size of the Earth.

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