Glossary

 

Big Bang

A broadly accepted theory for the origin and evolution of our universe. The theory says that the observable universe started roughly 13.7 billion years ago from an extremely dense and incredibly hot initial state.

Black Hole

A region of space containing a huge amount of mass compacted into an extremely small volume. A black hole's gravitational influence is so strong that nothing, not even light, can escape its grasp. Swirling disks of material — called accretion disks — may surround black holes, and jets of matter may arise from their vicinity.

Brown Dwarf

An object too small to be an ordinary star because it cannot produce enough energy by fusion in its core to compensate for the radiative energy it loses from its surface. A brown dwarf has a mass less than 0.08 times that of the Sun.

Centripetal force

The inward, center-directed force on a body moving in a curved line around another body. Centripetal force is necessary for an object to move in a circle.

Circumstellar disk

A ring of dusty material around a star. Such disks are expected to be the birthplace of planets.

Intergalactic medium

The sparse gas and dust located between galaxies .

Interstellar gas and dust

Gas, mostly hydrogen, and small particles of solid matter, similar to smoke, in the space between stars.

Interstellar medium

The sparse gas and dust located between the stars of a galaxy.

Kuiper belt

A region in our outer solar system where many short-period comets (possessing orbits of less than 200 years) originate. This region begins beyond Neptune's orbit and encompasses an estimated distance of between 30 and 100 astronomical units. There may be as many as 100 million Kuiper belt comets.

Mass function

A calculation of how many stars in a region and how massive the stars are, which shows how the cloud of gas that created the stars collapsed and broke up into smaller pieces.

Micrometers (microns)

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

Quasar

The brightest type of active galactic nucleus, believed to be powered by a supermassive black hole. The word “quasar” is derived from quasi-stellar radio source, because this type of object was first identified as a kind of radio source. Quasars also are called quasi-stellar objects (QSOs). Thousands of quasars have been observed, all at extreme distances from our galaxy.

Second Lagrange Point (L2)

The mathematician Joseph-Louis Lagrange determined that there are five points around two orbiting masses where a third, smaller mass can orbit at a constant distance from the larger masses. At those points, the gravitational pull of the large masses are balanced by the centripetal force necessary to rotate with the objects. JWST will be placed at the Second Lagrange Point, or L2. Normally, an object circling the Sun beyond the Earth would take more than a year to finish an orbit. But the balance of forces at the Second Lagrange Point means JWST will be able to keep up with the Earth as it orbits the Sun. JWST will remain at a fairly constant distance from the Earth throughout the year with only small maneuvers necessary.

Supernovae

The explosive death of a massive star whose energy output causes its expanding gases to glow brightly for weeks or months.