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.

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.

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

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.

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.

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.

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

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.

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.

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

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.

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.

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.

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

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.

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.

A natural satellite orbiting around a planet.

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.

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

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.

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

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.

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.

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

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

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.