Basic Vocabulary

A mixture of two or more metals. Brass (a mixture of copper and zinc) and bronze (a mixture of copper and tin) are common alloys

A type of telescope mounting that supports the weight of the telescope and allows it to move in two directions to locate a specific target. One axis of support is vertical (called the altitude) and allows the telescope to move up and down. The other axis is horizontal (called the azimuth) and allows the telescope to swing in a circle parallel to the ground. This makes it easy to position the telescope: swing it around in a circle and then lift it to the target. However, tracking an object as the Earth turns is more complicated. The telescope needs to be adjusted in both directions while tracking, which requires a computer to control the telescope.

To make larger or more powerful; increase. Radio signals are amplified because they are very weak.

An electrical device used to send or receive electromagnetic waves. The aerial (a long piece of metal attached to the front or rear fender) on a car is the antenna for the radio.

An orderly arrangement or impressive display. For radio telescopes, an array is a group of individual radio dishes that work together. The VLA (Very Large Array) has 27 telescope dishes arranged in a "Y" pattern.

A scientist who studies the universe and the celestial bodies residing in it, including their composition, history, location, and motion. A scientist who studies celestial objects using visible light is called an optical astronomer while one who studies celestial objects in the radio wavelengths is called a radio astronomer.

Astronomy is the study of the universe and the celestial bodies that reside in it, including their composition, history, location, and motion.

The blurring of an image due to the layer of gases surrounding the surface of Earth. As starlight travels through the atmosphere, pockets of air act like little lenses and bend the light in unpredictable ways. This distortion causes stars to appear to twinkle.

A broadly accepted theory for the origin and evolution of our universe. The theory states that the observable universe started roughly 15 billion years ago from an extremely dense and extraordinarily hot "bang."

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 also be associated with the black hole.

A type of reflecting telescope whose eyepiece is located behind the primary mirror. The primary mirror is cast with a hole in the center. When light enters the telescope, it reflects from the primary mirror to the secondary mirror. The secondary mirror reflects the light back through the hole in the primary mirror to the eyepiece.

An electronic detector that records visible light from stars and galaxies to make photographs. These detectors are very sensitive to the extremely faint light of distant galaxies. They can see objects that are 1,000 million times fainter than the eye can see. CCDs are electronic circuits composed of light-sensitive picture elements (pixels), tiny cells that, placed together, resemble mesh on a screen door. The same CCD technology is used in digital cameras.

Visible light is made of different colors. When visible light passes through a glass lens or a prism, it gets dispersed, or split, into its many colors. A lens focuses each color at a different point, causing a fringe of color to appear around bright objects

Looking at only red and blue light:

A moveable mirror system used in solar telescopes. The mirror follows the Sun and keeps its image in the same location as Earth rotates.

A galactic "car wreck" in which two galaxies pass close enough to gravitationally disrupt each other's shape. The collision rips streamers of stars from the galaxies, fuels an explosion of star birth, and can ultimately result in both galaxies merging into one. Note: Stars do not collide when galaxies merge, due to the great distances between stars.

The outermost layer of the atmosphere of a star, including the Sun. The corona is visible during a solar eclipse or when special adapters or filters are attached to a telescope to block the light from the star's central region. The gaseous corona extends millions of kilometers from the star's surface and has a temperature in the millions of degrees.

Electromagnetic energy filling the universe that is believed to be the radiation remaining from the Big Bang. It is sometimes called the "primal glow." This radiation is strongest in the microwave part of the spectrum but has also been detected at radio and infrared wavelengths. The intensity of the cosmic microwave background from every part of the sky is almost exactly the same.

Originally the main material used to make flat planes of glass for windows, it is composed of soda-lime glass. It can be used to make lenses and prisms. Crown glass bends and disperses, or spreads out, light less than flint glass.

Matter that is too dim to be detected by telescopes. Astronomers infer its existence by measuring its gravitational influence. Dark matter makes up most of the total mass of the universe.

A device used to measure the amount of electromagnetic radiation emitted by celestial objects. Frequently, detectors are used to sense light that is not visible.

The distance from one side of a circle to the other measured through the center. For telescopes, the diameter of a lens or mirror is measured from one side to the opposite side, passing through the center.

Visible light is actually made up of different colors. Each color bends by a different amount when refracted by glass. That's why visible light is split, or dispersed, into different colors when it passes through a lens or prism. Shorter wavelengths, like purple and blue light, bend the most. Longer wavelengths, like red and orange light, bend the least.

A system of two stars that are gravitationally bound to each other. They orbit each other around a common center. They can also be called binary stars.

The entire range of wavelengths of light. Arranged from longest to shortest wavelength, it includes radio waves, microwaves, infrared, visible, ultraviolet, X-rays, and gamma rays. All electromagnetic waves travel at the same speed in space.

A special kind of elongated circle. The orbits of the solar system planets are elliptical.

The process of allowing electromagnetic radiation to fall on light-sensitive materials such as photographic films or plates. An exposure is also the image created by the process. A long exposure time is needed in order to obtain an image of dim and distant celestial objects.

The lens or lens group closest to the eye in an optical instrument such as a telescope or microscope.

The field of view is the area of the sky visible through a telescope.

The lead glass that was produced in the United States and the United Kingdom prior to the 1860s. This glass is used to make telescope lenses and prisms. Flint glass bends and disperses, or spreads out, light more than crown glass.

Focal length (shown in red) is the distance between the center of a convex lens or a concave mirror and the focal point of the lens or mirror — the point where parallel rays of light meet, or converge.

The focal point of a lens or mirror is the point in space where parallel light rays meet after passing through the lens or bouncing off the mirror. A "perfect" lens or mirror would send all light rays through one focal point, which would result in the clearest image.

Describes the number of wave crests passing by a fixed point in a given time period (usually one second). Frequency is measured in Hertz (Hz).

A collection of stars, gas, and dust bound together by gravity. The smallest galaxies may contain only a few hundred thousand stars, while the largest galaxies have thousands of billions of stars. The Milky Way galaxy contains our solar system.

Light with the shortest wavelengths and the highest energies and frequencies in the electromagnetic spectrum; also called gamma radiation. Gamma rays are produced by violent events such as supernova explosions. They are also produced by the decay of radioactive materials. Gamma rays can kill living cells, so it is good that Earth's atmosphere can stop them. Gamma radiation is used in medicine to kill cancer cells.

A spinning wheel mounted on a non-stationary frame that stabilizes and points a space-based observatory. This spinning wheel resists applied external forces and tends to retain its original orientation in space. For example, balancing on a moving bicycle is easier than balancing on a stationary one because of this tendency.

A region of the electromagnetic spectrum that has slightly longer wavelengths and lower frequencies than visible light, but is not visible to the human eye. This region of light is comparable to the range of sounds that are too low for the human ear to hear. Infrared light can be detected as the heat from a fire or a light bulb.

Any device that measures and/or records energy from astronomical objects. Some astronomical instruments include spectrometers, photometers, spectroheliographs, and charge-coupled devices.

The process used to combine the signal from two or more telescopes to produce a sharper image than each telescope could achieve separately.

An instrument that combines the signal from two or more telescopes to produce a sharper image than the telescopes could achieve separately.

A carefully ground or molded piece of glass, plastic, or other transparent material that causes light to bend and either come together or spread apart to form an image.

A set of two lenses, one concave and one convex, made from different types of glass. Together the lenses correct both spherical and chromatic aberrations. A single lens alone cannot correct these aberrations.

A region of space in which magnetic forces may be detected or may affect the motion of an electrically charged particle. As with gravity, magnetism has a long-range effect and magnetic fields are associated with many astronomical objects.

Enlargement in the size of an optical image. For telescopes, magnification is not as important as the ability to gather light, which depends on the diameter of the primary lens or mirror.

The process of enlarging the size of an optical image.

Electromagnetic waves found in the region between infrared and radio wavelengths. Microwave wavelengths fall between one millimeter and one meter. Microwaves can be used to quickly heat and cook food.

The Milky Way, a spiral galaxy, is the home of Earth. The Milky Way contains more than 100 billion stars and has a diameter of 100,000 light-years.

The support structure for a telescope that bears the weight of the telescope and allows it to be pointed at a target. The mountings of today's research telescopes also allow astronomers to track the object as it appears to move across the sky.

A very small fraction of a meter. There are a billion (1,000,000,000) nanometers (nm) in one meter.

A cloud of gas in space, usually one that is glowing. Historically, "nebula" was a general term used to indicate any light or dark patch of the night sky that was "fuzzy," or not sharply defined, as a star or planet would be.

An extremely compact ball of neutrons created from the central core of a star that collapsed under gravity during a supernova explosion. Neutron stars are extremely dense: they are only 10 kilometers or so in size, but have the mass of an average star (usually about 1.5 times more massive than our Sun). A neutron star that regularly emits pulses of radiation is known as a pulsar.

A type of reflecting telescope whose eyepiece is located along the side of the telescope. When light enters the telescope, it reflects from the primary mirror to the secondary mirror. The secondary mirror reflects the light at a right angle through the side of the telescope to the eyepiece.

The act of noticing or perceiving something. In science, observations refer to noting or recording a fact or occurrence. A telescope is a tool astronomers use to make observations of celestial objects.

A building, group of buildings, or spacecraft specifically designed and fitted with equipment to study celestial objects.

A person who grinds lenses and mirrors.

The science that deals with the properties of light; in this case specifically dealing with the way light changes directions when it is either refracted and dispersed by a lens or reflected from a mirror.

The path followed by a body moving in a gravitational field. For example, the planets travel around the Sun because the Sun's gravitational field keeps them in their paths.

If cross-sections of a spherical surface and a parabolic surface were made by slicing each surface in half, these would be the shapes you would see:

The "perfect" lens does not exist. Due to the nature of glass, light is dispersed when passing through glass. In the case of convex lenses, red light bends less than blue light, so the focal points are in different places, making the image blurry. A single lens cannot counter this effect.

Regularly occurring changes in the appearance of the Moon or a planet. Phases of the Moon include new, full, crescent, first quarter, gibbous, and third quarter.

A technique for measuring the brightness of celestial objects. Astronomers measure the brightness of celestial objects with photometers.

A large convex lens in a refracting telescope that captures light from celestial objects and focuses it toward the eyepiece.

A large concave mirror in a reflecting telescope that captures light from celestial objects and focuses it toward a smaller secondary mirror.

A prism is usually a triangular-shaped piece of glass used to refract, or bend, light. The shape of the glass causes the light to disperse, or spread out, as it bends, producing a rainbow of colors from the white light.

An eruption of gas from the chromosphere of a star. Solar prominences are visible as part of the corona during a total solar eclipse. These eruptions occur above the Sun's surface (photosphere), where gases are suspended in a loop, apparently by magnetic forces that arch upward into the solar corona and then return to the surface.

A neutron star that emits rapid and periodic pulses of radiation. A neutron star is an extremely compact ball of neutrons created from the central core of a star that collapsed under gravity during a supernova explosion. Neutron stars are extremely dense: they are only 10 kilometers or so in size, but have the mass of an average star (usually about 1.5 times more massive than our Sun). A neutron star that regularly emits pulses of radiation is known as a pulsar.

A quasar is the bright center of a galaxy, 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.

Radiation is a term used to refer to any and all wavelengths of the electromagnetic spectrum. Arranged from longest to shortest wavelength, electromagnetic radiation includes radio, microwaves, infrared, visible, ultraviolet, X-rays, and gamma rays.

Celestial objects that give off radio waves. These sources can be stars, pulsars, galaxies or even a cloud of gas between the stars.

The region of the electromagnetic spectrum with the longest wavelengths and lowest frequencies. Radio waves are about 100,000 times longer than visible light waves.

The part of the radio telescope that detects long wavelength electromagnetic radiation and converts it to an electrical signal so that we can sense it.

Reflection occurs when light changes direction as a result of "bouncing off" a surface like a mirror.