Meaning, Glossary and Acronym > Space Glossary

The Milky Way, the galaxy containing our solar system, is about 100,000 light-years in diameter and about 10,000 light-years thick.

Galaxies are immense systems containing billions of stars. Astronomers have estimated that the universe could contain 40 to 50 billion galaxies. Galaxies have different shapes: some are spiral, others are elliptical, or oval-shaped, and some are irregular.

The Milky Way is our own galaxy. Just about all that you can see in the sky belongs to our galaxy—a system of roughly 200 billion stars. The Milky Way is a spiral-shaped galaxy about 100,000 light-years in diameter and about 10,000 light-years in thickness.

The solar system is made up of the Sun (solar means sun) at its center, the nine planets that orbit it, and the various satellites, asteroids, comets, and meteorites that are also controlled by the Sun's gravitational pull.

The Sun is the closest star to Earth and the center of our solar system. Every second, it converts 49 million tons (45 million metric tons) of matter into pure energy, which reaches us in the form of light. The Sun weighs more than 300,000 times as much as Earth and is 109 times larger.

Sunspots appear as dark spots on the Sun, and are believed to be cooler than the rest of the Sun. They appear in 11-year cycles.

Planet is the term used for a body in orbit around the Sun. The word comes from the Greek planetes, and means “wanderers.” Our solar system has nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. An easy way to remember their names in the correct order is to keep in mind the following sentence: My Very Educated Mother Just Served Us Nine Pickles. The first letter of each word is the first letter of each planet. Since 1994, evidence has been found that planets also exist beyond our solar system. At least 10 planets existing in other solar systems have been discovered.

Satellite (or moon) is the term for a body in orbit around a planet. As long as our own Moon was the only moon known, there was no need for a general term for the moons of planets. But when Galileo Galilei discovered the four main moons of the planet Jupiter, Johannes Kepler wrote Galileo a letter suggesting he call them “satellites” (from the Latin satelles, which means attendant). The word means the same thing as “moon.”

Orbit is the term for the path traveled by a body in space. It comes from the Latin orbis, which means circle. Some orbits are nearly circular, but the orbits of most planets are ellipses—shaped like ovals.

Asteroids, also known as the minor planets, are small bodies orbiting the Sun that resemble planets. More than 5,000 asteroids have been discovered, and most are found between Mars and Jupiter. Usually having an irregular shape, asteroids—at least those discovered thus far—can range in size from 580 miles (940 km) in diameter, which is the size of the asteroid Ceres, to just 33 ft. (10 m) in diameter.

Comets are made up of frozen dust and gases, and have been described as large, dirty snowballs with icy centers. They often travel on extremely elongated orbits around the Sun. Some comets have orbits that take just 10 years to circumnavigate while other comets have orbits that take hundreds of thousands of years to circulate. The tail of a comet, called a coma, forms when the comet comes within 100 million miles of the Sun. It is then affected by the solar wind (hydrogen and helium that travel away from the Sun at high speeds), which causes a tail of dust and gases to form behind the comet.

Meteors are fragments of comets, planets, moons, or asteroids that have broken off. It is estimated that a billion meteors enter our atmosphere every day. Contact with our atmosphere causes most to disintegrate before reaching Earth. Those that do not disintegrate completely but fall to Earth are called meteorites.

Stars are composed of intensely hot gasses, deriving their energy from nuclear reactions going on in their interiors. Our Sun is the nearest star. Stars are very large — some are even bigger than planets. Our Sun has a diameter of 865,400 miles—making it a comparatively small star.

White dwarfs occur when a star runs out of energy and shuts down. The force of gravity at its center pulls the mass of the star in on itself, forcing it to collapse. It resembles the glowing cinders of a fire that has died down. It is called a white dwarf because it emits a white glow.

Brown Dwarfs are also called failed stars. They lack enough energy to be true stars but are also too massive and hot to be planets.

A supernova is an extremely large exploding star. Just before the star dies, it releases huge amounts of energy, briefly becoming millions of times brighter than it was. Then it immediately shrinks.

Neutron stars are formed after a supernova explodes and shrinks. The shrunken form of the star becomes incredibly dense and compact as gravity pulls all of its matter inward. It becomes so compressed that a million tons of its matter would hardly fill a thimble. This density crushes together the electrons and protons that make up its atoms, turning them into neutrons.

Pulsars are believed to be rapidly spinning neutron stars that give off bursts of radio waves at regular intervals. Pulsar is a shortened version of Puls[ating st]ar.

Quasars (quas[istell]ar objects) are believed to be the most remote objects in the universe. Despite their small size they produce tremendous amounts of light and microwave radiation: not much bigger than Earth's solar system, they pour out 100 to 1,000 times as much light as an entire galaxy containing a hundred billion stars.

A black hole is created by the total gravitational collapse of a massive star or group of stars. It is the final phase of some stars, in which gravity sucks the star in on itself—it implodes rather than explodes. This makes it so dense that not even light can escape its gravitational field.

A nebula is a giant glowing cloud thought to be made up of dust and gas. Nebulae were thought to have been galaxies that appeared as a blur because they were so far away, but as more powerful telescopes were created, they showed that nebulae were not clumps of stars but in fact a hazy cloud of gasses. A nebula is illuminated by bright stars nearby. More than 300 nebulae have been named.