Glossary

acceleration

The rate of change in velocity over time.

astrophysics

The branch of astronomy that deals with the physics of astronomical objects and phenomena.

big bang

The fiery birth of the observable universe in an explosion of space itself, which occurred at some time between 12 and 15 billion years ago. According to the prevailing theory, the Big Bang launched the observed expansion of the universe that continues to this day.

binary star system

Two stars orbiting their common center of gravity.

black hole

A region in space where gravity is so strong that space closes back on itself, allowing nothing, not even light, to escape.

constructive interference

The amplification of one wave by another wave that is completely in phase with the first.

cosmic

Of or relating to the universe as a whole.

dark matter

Unseen matter that is detected only by its gravitational pull on visible matter. Most of the universe is evidently made of dark matter. Its nature is yet to be determined.

destructive interference

The cancellation of one wave by another wave that is completely out of phase with the first.

electromagnetic spectrum

The complete array of electromagnetic radiation (light). In order of increasing wavelength (decreasing frequency and energy), the spectrum ranges from gamma rays through X-rays, ultraviolet light, visible light, infrared radiation, microwaves to radio waves.

frequency

The number of repetitions per unit time of a complete waveform.

general relativity

The modern theory of gravity, introduced by Albert Einstein in 1916; it describes gravity as the curvature or warping of space due to the presence of matter.

gravitational waves

Disturbances or ripples in the fabric of space produced by violent events in the cosmos.

gravity

The force of attraction acting between any two masses.

interference

When two waves simultaneously travel through the same medium at one time, they combine and interfere. The shape of the resulting wave is determined by the sum of the separate amplitudes of each wave.

interferometer

An instrument that uses the interference of wave forms to measure very small distances.

mass

A measure of the total amount of matter on a body. It can be defined either by the body's inertia (resistance to altering its motion) or by its gravitational influence on other bodies.

matter

Anything that takes up space.

neutron star

An extremely dense collapsed star consisting mainly of neutrons. A neutron star is what often remains after the supernova explosion of a massive star.

orbit

The path of one celestial body moving around another under the force of gravity.

phase

Two waves are in phase if their crests and troughs line up exactly, and they are out of phase when the troughs and crests do not align. If the crests of one wave line up exactly with the troughs of the other, the waves are said to be completely out of phase.

photodetector

A device to detect and measure the intensity of light.

pulsar

A neutron star that emits radio energy at regular intervals as it spins. It is thereby observed on Earth as a pulsating radio source.

radio waves

Low energy electromagnetic radiation, with long wavelengths and low frequencies.

satellite

A body that orbits around a larger body.

space

Where matter is not.

space-time

The three dimensions of space plus the dimension of time, which measures the procession of events in space. Einstein first proposed this notion of an integrated “space-time” in his 1905 Theory of Special Relativity.

special relativity

The physical theory of space and time developed by Einstein in 1905. It states that all the laws of physics are equally valid in all frames of reference moving at a uniform velocity, and that the speed of light is constant regardless of what frame of reference the observer is in. It also proposed that matter can be converted to energy and vice versa. In 1916 Einstein published his General Theory of Relativity, which added the effects of gravity to Special Relativity.

star

A self-luminous body held together by its own gravity and with a central temperature and pressure sufficient to liberate energy by nuclear fusion.

supermassive black hole

Black holes that have the mass of 10 to 100 billion Suns. They are found in the centers of galaxies called active galactic nuclei, which are incredibly energetic.

supernova

The catastrophic explosion of a star, which blows off most of its mass, increasing in brightness by as much as a billion times. A Type I supernova is due to the thermonuclear detonation of a compact white dwarf star which becomes unstable by accreting mass from an orbiting companion star. A Type II supernova results from the gravitational collapse of a massive star that has exhausted its nuclear fuel.

telescope

An instrument designed to gather and focus electromagnetic radiation (light) to study celestial objects and events.

universe

The physical system that encompasses all matter, energy, and space that exists (see observable universe).

velocity

The speed and direction of an object’s motion.

visible light

The portion of the electromagnetic spectrum corresponding to the visible colors, with wavelengths longer than ultraviolet light and shorter than infrared radiation.

wavelength

The distance between successive wave crests, or troughs. Light of different wavelengths has different properties, such as color.