Glossary for 400 Years of the Telescope
Special thanks to Andy Fraknoi (Foothill College) for his contribution of this glossary.
A | B | C | D | E | F | G | I | M | N | O | P | R | S | T | U
Adaptive optics — A relatively new technique for removing the blurring effects of the Earth’s atmosphere from astronomical images. Stars and other celestial objects “twinkle” because the air through which the light travels to us is being moved in complex ways by the winds in it. Also clouds and smog in our air can move across an astronomical object and briefly obscure it. Astronomers first measure the changes in the light coming from a bright star in the field of view. Then they can use a mirror whose reflecting surface can be instantly adjusted to remove these changes before the light from the telescope is recorded. This can produce images from the surface of the Earth that begin to have some of the clarity and detail of pictures taken from space.
All Sky Survey — A program in which astronomers record the appearance of the entire sky as seen from Earth, either once or over and over again. Such a record can then be compared to earlier images or future ones to see what changes in the sky over the years.
Alt-azimuth mount — A way to set up and support a telescope, where it moves up and down and sideways. The vertical movement direction is called altitude and the direction of movement along the horizon is called azimuth. This kind of mount is simpler and cheaper to produce than the kind that lines up with the direction of the Earth’s equator and pole.
Aperture — The size of the opening that collects light in a telescope. So, for example, the aperture of the big telescope on Mt. Palomar in Southern California is 200 inches (5 meters); that’s the size of the mirror that collects the light.
Asteroid — A relatively small rocky object orbiting the Sun. Most asteroids are found between Mars and Jupiter in a region called the asteroid belt, but some asteroids cross the orbits of other planets. Those asteroids that move across the Earth’s orbit are called Earth-crossing asteroids, and they represent a long-term danger to the inhabitants of our planet.
Baryonic matter — Matter consisting of protons and neutrons in the nucleus of the atom. Scientists often use this term to refer to the ordinary or everyday matter that we are familiar with, as opposed to more unusual forms such as dark matter.
Big Bang — The high-energy, very dense beginning of the universe; the start of its expansion. The big bang is now thought of as an explosion of space, time, matter, and energy, which is not an easy thought to wrap one’s mind around.
Cosmological constant — A kind of universal repulsive force, introduced by Einstein into his general theory of relativity, to keep the universe from collapsing. (Back then scientists did not suspect that the universe was actually expanding and thus Einstein worried that gravity would soon make the universe collapse.) Later, Einstein called the introduction of this rather arbitrary force “the biggest blunder of my life.” But today, with the discovery that the expansion of the universe is speeding up, scientists are reconsidering the possibility of the kind of repulsive force Einstein envisioned.
Cosmology — The branch of astronomy that deals with the universe as a whole – as a single system. Topics that cosmologists think about include the large-scaled properties, origin, and ultimate fate of the universe.
Dark energy — The energy that is causing the expansion of the universe to accelerate (speed up). Astronomers have discovered this acceleration by using distant supernovae as distance markers and noting that the space has not been stretching at a constant rate over cosmic time. The nature of this energy is unknown at present.
Dark matter — Material which we cannot detect (by observing light or other radiation from it) but whose existence we know about from seeing its gravity affect the material in the universe that we can see. It appears that there is more of this mysterious dark matter in the universe than the regular matter which we see as stars and galaxies. What dark matter is made of is currently unknown.
Detector — A device that registers (detects) light or other radiation. The human eye and a solar panel are both examples of detectors; telescopes by themselves are not.
Expanding universe — The observation that the groups of galaxies are all moving away from one another – we now know that this is because space itself has been “stretching” since the big bang. Recent observations indicate that this expansion is speeding up (accelerating).
Eyepiece — A magnifying lens astronomers use to view the image produced by a telescope.
Flare — An outburst of light (and/or other radiation) and charged particles from the surface of the Sun or a star.
Focal length — The distance between the main lens and mirror of a telescope and the point where the rays of light come together (come to a focus). In some refracting telescopes, which are enclosed, the long focal length required a long telescope tube.
Galaxy — A great “island” of millions to hundreds of billions of stars, separated from other galaxies by large gulfs of space. We live in one such island called the Milky Way Galaxy. When astronomers use the word in a sentence, the Milky Way is written as the Galaxy, and all other such giant star groups are written as a galaxy (lower case g).
General theory of relativity (or general relativity) — An overarching theory, developed by Albert Einstein, which relates gravity and the nature of space and time in the universe. Astronomers use the general theory of relativity as the basis of all ideas of modern cosmology.
Gravity — The pull of all material in the universe on all other material. Gravity is one of the fundamental forces governing the universe.
Interferometry — The technique of combining light (or other radiation) from two or more telescopes, allowing astronomers to see more detail than from any of the telescopes by themselves.
Milky Way galaxy — The galaxy of stars in which the Sun and the Earth are located; a spiral-shaped collection of hundreds of billions of stars and raw material (gas and dust). The Sun is located about halfway between the center and outer edge of the Galaxy’s disk.
Millimeter astronomy — The study of astronomical objects using not visible light, but another kind of radiation whose waves have wavelengths on the order of a millimeter (one thousandth of a meter.) These kinds of waves fall into the broad range astronomers call radio waves.
Moon — An object that orbits a planet (sometimes a moon is also called a satellite.) When astronomers write about the Earth’s satellite, they call it the Moon, but other planets’ satellites are written as moons (lower case m).
Nebula — A cloud of gas and dust among the stars. A nebula can be observed in regions where new stars have recently been born and around stars that are dying or have died. Before galaxies were understood, they were also classified as nebulae (as in the term “spiral nebula”) but that term is not used today. The plural is nebulae.
Optical telescope — A telescope that is designed to collect visible light (as opposed to other forms of radiation, such as radio waves, that are not visible to the human eye.)
Phases — The different appearances of a planet or moon as it moves around its orbit. For example, we say that the Moon goes through phases as the light we see from it changes from new moon to full moon.
Planet — An object of significant size that has cleared out the region in which it orbits. According to the new definition of a planet adopted by the International Astronomical Union, planets must have enough mass to be spherical and must not share their orbits with objects that are like them. According to this definition, the Sun has only 8 planets.
Radiation — Waves of energy generated by electric and magnetic changes in matter; examples include radio waves, infrared waves, visible light, ultraviolet waves, x-rays, and gamma rays. Such waves move away (radiate) from the source at the speed of light.
Radio waves — The kind of radiation (see above) that has the longest wavelength (lowest energy). Many types of radio waves easily penetrate the atmosphere of the Earth and other planets, and, also being relative cheap to produce, are ideal for communicating through space.
Redshift — The change in the colors of an astronomical object caused by its motion away from us. Christian Doppler discovered in the 19th century that the motion of a source of light (or other radiation) away from us or toward us changes its colors in a subtle but measurable way. The faster an object moves away the greater its redshift is. By spreading out the light of a star or galaxy into a spectrum, astronomers can thus measure its speed. Galaxies beyond our immediategroup of neighbors ALL show a redshift, since they are participating in the expansion of the universe.
Reflecting telescope — A telescope in which the light is collected (and reflected to a focus) by a mirror. Reflecting telescopes can be made much larger than refracting telescopes.
Refracting telescope — A telescope in which the light is collected (and refracted or bent) by a lens. Refracting telescopes (or refractors) were the first type of telescope used; Galileo’s telescopes were refractors.
Resolution — The ability of a telescope to make out fine details in an astronomical object or to separate the image of two objects that are close to each other on the sky. (Resolution in astronomy is measured in units of angle on the sky: degrees, minutes of arc, or seconds of arc.)
SETI — The Search for Extra-Terrestrial Intelligence. A series of scientific projects designed to find evidence of intelligent life on worlds around other stars. The largest part of the effort is directed toward finding radio signals from civilizations around stars in the nearer parts of the Milky Way Galaxy.
Solar cycle — The eleven-year period over which activity (including sunspots) on the Sun’s surface rises and falls. When the Sun’s activity is at maximum, there are more sunspots, flares, and other forms of energetic disturbance on the Sun.
Solar system — The group name we give to the Sun and its family of planets, moons, and assorted smaller chunks of material. The Earth is a part of the solar system.
Spectrum — The array of colors (wavelengths) of light spread out by a prism or other instrument. “Coded” into this array of light are clues to the composition, temperature, motion and other properties of astronomical objects, making the study of spectra (plural of spectrum) a key part of what astronomers do. (More generally, a spectrum is the array of waves spread out by wavelength for any kind of radiation.)
Star — A large ball of hot gas that shines under its own power. The Sun is our closest example of a star. The other stars are basically like the Sun; they only look small and dim to us because they are so far away.
Sunspot — A darker, cooler region on the surface of the Sun.
Supernova — The explosion of a star at the end of its life. Some really massive stars explode because their internal structure becomes unstable. Other stars, which live in a binary system (where two stars orbit each other), explode in a more complicated scenario: One of the two stars dies first and becomes a compressed “star corpse” called a white dwarf. The other star expands as it ages, and its outer materials falls on the white dwarf, which then becomes “overloaded,” heats up tremendously, and explodes. It is this second kind of supernova, which always flares up to be the same built-in brightness, which has allowed astronomers recently to measure that the expansion of the universe is speeding up.
Telescope — An instrument that gathers light (or another type of radiation) and brings it to a focus for viewing or recording. Telescopes allow astronomers to see or photograph objects that are too dim to be seen with the naked eye. Telescope images can be magnified (made to look bigger) and show more detail than our eyes can see.
Universe — Everything that is accessible or can become accessible to our observations; the total of all matter, energy, space, and time of which we can be aware. The term “cosmos” is sometimes used to mean the same thing.
A Few Other Astronomical Glossaries on the Web:
NASA Imagine the Universe Site Dictionary (for younger readers):
Amazing Space Glossary from the Space Telescope Science Institute:
Case Western Reserve University, brief definitions of astronomical terms:
Glossary from the PBS Program Seeing in the Dark:
Catalog of the Cosmos from the PBS Nova Program Death Star:
NASA/IPAC Extragalactic Database Glossary (a bit technical, but extremely thorough):