Glossary

Accretion: Gradual addition of matter to an object. Proto-stars accrete matter from their natal cloud. Planets form through the accretion of planetesimals and gas in the proto-planetary disks, according to the leading theory.

Adaptive optics: A technique that provides sharp images of astronomical objects with the help of a deformable mirror to correct in real time for the distortions caused by the Earth’s atmosphere. (The shape of the mirror is adjusted with many small electromechanical devices on its backside.)

Angular momentum: Roughly speaking, the spin energy of an object. It is a conserved physical quantity. It causes a figure skater to spin faster as he folds in his arms and to slow down when he stretches them out again.

Angular resolution: The ability to see details, defined as the minimum angle at which two objects in the sky can be seen as separate (rather than being blended into one).

Arcminute: A unit for measuring angles, equal to 1/60 of a degree.

Arcsecond: A unit for measuring small angles, equal to 1/3600 of a degree, or 1/60 of an arcminute.

Asteroid: A small rocky body. Most asteroids in our solar system reside in the “asteroid belt” between the orbits of Mars and Jupiter, but some have orbits that bring them close to the Earth.

Astrometry: Measurement of the precise positions (thus motions) of stars in the sky. A planet tugging on its star gravitationally would produce a periodic (albeit extremely small) wobble in the star’s position, so astrometry can be used to find extrasolar planets.

Astronomical unit (AU): The average distance between the Sun and the Earth, roughly about 150 million kilometers, or 93 million miles. It is a useful unit for discussing distances within a planetary system. Mercury is roughly 0.4 AU from the Sun, Jupiter is at 5 AU, and Neptune at 30 AU.

Binary star: Two stars that orbit each other. Binary stars are common, and in most cases, the pair is believed to have been born together out of the same natal cloud, which fragmented as it contracted.

Biosignature: A substance, often a molecule, indicating the existence of living organisms. Also known as a biomarker.

Brown dwarf: An object not massive enough to become a star, because its core temperature never gets high enough to fuse hydrogen through nuclear reactions. The upper mass limit of brown dwarfs is at about 8 percent of the mass of the Sun (or 80 Jupiter masses); the lower limit is arbitrary but is sometimes taken to be 13 Jupiter masses, above which deuterium fusion can occur. Most brown dwarfs are found as free-floating objects, but some are in orbit around stars or other brown dwarfs.

Carbon cycle: The long-term cycle by which carbon is exchanged among the Earth’s atmosphere, living organisms, oceans, soil, and interior. It is important for maintaining a stable climate and for life.

Charge-coupled device (CCD): A device for digital imaging that is widely used in astronomy because of its sensitivity and reliability.

Chlorophyll: A green pigment found in most plants that is vital for photosynthesis.

Chondrite: The most common type of stony meteorite, which remains little changed since the solar system formed 4.5 billion years ago. Chondrites are so named because they contain chondrules.

Chondrules: Round, millimeter-size pebbles found in chondrites. They formed as molten droplets before being incorporated into parent bodies.

Comet: A small icy body. As a comet approaches the Sun, sublimation (turning material directly from solid to gas) can produce a tenuous envelope called the coma as well as an extended tail.

Constellation: A grouping of stars in the night sky that forms a pattern, usually derived from mythology. The stars of a given constellation do not necessarily have any physical relation among them; they are located at vastly different distances from the Earth and from one another. By agreement of the International Astronomical Union, the sky is divided into eighty-eight constellations, or unequal regions.

Coronagraph: An instrument that uses an opaque (or nearly opaque) mask to block the light from a bright object, like the Sun or a star, in order to reveal much fainter objects nearby.

Doppler shift: The change in frequency and wavelength of electromagnetic radiation that occurs when the source and the observer are moving toward or away from each other.

Doppler technique: The use of Doppler shifts to measure the wobble of a star due to the gravitational tug of an unseen companion in orbit around it. It is a highly successful technique for finding extrasolar planets.

Earthshine: Light reflected off the Earth and its atmosphere onto the dark part of the Moon. It is the reason the entire disk of the Moon is dimly lit even when only a thin crescent is directly illuminated by sunlight.

Eccentricity: A measure of how circular or elongated an orbit is. Solar system planets are in nearly circular orbits, thus they have low eccentricity, while some comets and extrasolar planets are in elliptical or highly eccentric orbits.

Electron: A negatively charged elementary particle, which is usually in orbit around the nucleus of an atom.

Fusion: The joining together of light atomic nuclei to form a heavier nucleus, accompanied by the release of energy (and possibly other particles). Fusion powers most stars.

Galaxy: A large collection of stars, typically hundreds of millions to hundreds of billions, as well as dust and gas held together by gravity. Also see Milky Way.

Gas giant planet: A massive planet, such as Jupiter, composed primarily of gaseous material, thus lacking a solid surface (though it may have a solid core).

Greenhouse effect: The warming of a planet by certain gases in the atmosphere that permit visible light from the parent star to reach the surface but prevent the planet surface from re-radiating some of the heat back into space at longer, infrared wavelengths. Carbon dioxide, methane, and water vapor are among the important greenhouse gases. Without the greenhouse effect, the Earth would be too cold for liquid water, but a runaway greenhouse effect has turned Venus into an inferno.

Habitable zone: Usually defined as the region around a star where the temperatures are in the correct range for water to exist in liquid form. The assumption is that life as we know it requires liquid water.

Heavy elements: See Metals.

Helium: The second lightest and second most common element in the universe, with two protons in its nucleus. Stars fuse hydrogen into helium; helium itself fuses into carbon and oxygen.

Hot Jupiter: A gas giant planet located very close to its star, taking only a few days to circle it, thus is heated to high temperatures.

Hydrogen: The lightest and most abundant element in the universe. The most common form of hydrogen contains only a single proton in its nucleus. A rare form called heavy hydrogen or deuterium contains a neutron in addition to the proton.

Infrared: Radiation whose wavelengths are longer than those of visible light, but shorter than those of microwaves.

Infrared excess: If a star is brighter in the infrared than it should be, given its temperature, it is said to exhibit an infrared excess. Such emission indicates the presence of dust (or a cool companion) around the star.

Interferometer: The combination of two or more telescopes to achieve the angular resolution of a much larger single telescope.

Interferometry: The technique of using interferometers for high angular-resolution observations.

Isotopes: Different types of atoms of the same chemical element, each harboring a different number of neutrons. Some isotopes are radioactive and thus decay into other types of atoms by spontaneously emitting particles and radiation.

Kuiper Belt: The region beyond the orbit of Neptune (i.e., 30–55 AU from the Sun) that contains tens of thousands of small bodies, as well as a handful of known dwarf planets like Pluto, left over from the era of solar system formation.

Light-year: The distance that light travels in a year, just under 10 trillion kilometers (or about 6 trillion miles).

Magma: Molten rock.

Main-sequence star: A star that fuses hydrogen into helium in its core. Stars spend the bulk of their lifetime in this phase, the length of which depends on the star’s mass. The Sun’s main-sequence lifetime is about 10 billion years.

Mass: The amount of material in an object. The more massive an object is, the stronger its gravitational pull.

Metals (or heavy elements): In the astronomers’ parlance, all elements heavier than hydrogen and helium.

Meteorite: A solid object, of rocky and/or metallic composition, that has reached the ground from space. Meteorites are usually fragments of asteroids or comets, but some originate from the surface of Mars or the Moon.

Microlensing (or gravitational microlensing): The bending of light rays by the gravitational field of an intervening object that magnifies the apparent brightness of a more distant light source. It has been used to detect extrasolar planets.

Micron: A unit of distance (or size) equal to one-millionth of a meter. A human hair is about 100 microns thick.

Migration (of planets): A large change in the orbit of a planet, due to interactions with other bodies or with the dusty disk in which the planet is embedded. It is the favored explanation for the origin of “hot Jupiters.”

Milky way: Our Galaxy. It consists of a fattened disk with spiral arms, a central bulge and a large spherical halo. The Sun is located in the outskirts of the Galactic disk. There is strong evidence for a supermassive black hole at the center of the Galaxy.

Milliarcsecond: A unit for measuring extremely small angles, equal to 1/3,600,000 of a degree, or one-thousandth of an arcsecond.

Millimeter waves: Radiation with wavelengths longer than infrared but shorter than radio. Millimeter waves are particularly useful for studies of cool astronomical objects, such as molecular clouds and protoplanetary disks.

Millisecond pulsar: A pulsar with a spin period of mere milliseconds.

Molecular cloud: One of the clouds of gas and dust in interstellar space whose densities and temperatures permit molecules like molecular hydrogen (H₂) to form. They are the birth sites of stars. Giant molecular clouds span hundreds of light-years and spawn thousands of stars, while small clouds, called Bok globules, may form just a few stars.

Molecule: A stable grouping of two or more atoms.

Nebula: A cloud of gas and dust in space. Some nebulae, like the Orion Nebula, are birth sites of stars and shine from the reflected light of newborn stars; others, like the Crab Nebula, are made of material ejected by dying stars.

Neutron: A subatomic particle with no electric charge and a mass slightly larger than that of a proton, usually found in atomic nuclei.

Neutron star: Dense, compact remnant of a massive star that exploded as a supernova. It is made almost entirely of neutrons.

Orbit: The path that one body follows around another; for example, the path of a planet around a star.

Orbital period: The time it takes for a body to complete one revolution around another. The orbital period of the Earth around the Sun is one year.

Orbital resonance: Two bodies are said to be in resonance when their orbital periods have a simple ratio. For example, when Neptune completes two orbits around the Sun, Pluto completes three, so they are in a 2:3 resonance.

Organic: Carbon-based molecules; often used to imply compounds related to life.

Ozone: A molecule that consists of three oxygen atoms (O₃). The ozone layer in the Earth’s upper atmosphere prevents harmful ultraviolet radiation from reaching the surface. Ozone is also a greenhouse gas and a possible biosignature in an extrasolar planet.

Period: See Orbital period.

Photosynthesis: A process that converts carbon dioxide and water into organic compounds and oxygen using the energy of sunlight. The emergence of photosynthetic organisms led to a rise in the oxygen level in the Earth’s atmosphere. It is vital for present life on Earth.

Planetesimal: One of the small solid bodies that build up in a protoplanetary disk, which in turn accumulate to build up planets.

Plate tectonics: Slow, large-scale motions of plates of the Earth’s (or another rocky planet’s) crust.

Proton: A positively charged subatomic particle found in the nucleus of every atom. The number of protons determines each element; e.g., hydrogen atoms have one proton, helium atoms have two protons, carbon atoms have six protons.

Protoplanetary disk: The rotating disk of dust and gas that surrounds a newborn star, out of which planets form.

Protostar: An early phase in the formation of a star, after it has fragmented out of a gas cloud but before it has contracted enough for nuclear fusion to begin. A dusty cocoon, which blocks visible light but allows infrared and microwave radiation to escape, surrounds a protostar.

Pulsar: A rapidly rotating neutron star that emits beams of radiation (usually radio waves), similar to the beams of a lighthouse, which are detected as pulses as the beams sweep past the Earth.

Radial velocity: The velocity of an object toward or away from the observer. Also see Doppler shift.

Radiometric dating: A technique for dating materials, based on a comparison of radioactive isotopes and their decay products, given known decay rates.

Red dwarf: A star with a much lower mass than the Sun (about 10-50 percent of a solar mass) but a higher mass than a brown dwarf. It is the most common type of star in the Galaxy.

Seti: The Search for Extraterrestrial Intelligence.

Solar mass: The amount of mass in the Sun. It is a common unit for expressing masses of stars. The Sun is about a thousand times more massive than Jupiter and about 330,000 times more massive than the Earth.

Spectral Lines: Bright (emission) or dark (absorption) lines superimposed on the spectrum of an object. Each element or molecule has a characteristic set of spectral lines, kind of like a fingerprint.

Spectral resolution: A measure of how well a spectrograph can resolve spectral features, defined as the smallest difference in wavelengths that can be distinguished.

Spectrograph: An instrument for separating light into its constituent wavelengths and making measurements.

Spectroscopy: The detection and analysis of spectra, including spectral lines, to derive properties of astronomical objects.

Spectrum: The spread of light into its component wavelengths or colors. The simplest example is a rainbow.

Subgiant Star: A star that has ceased fusing hydrogen in its core, which contracts while the outer layers expand and cool.

Super-Earth: A planet with a mass between one and ten times that of the Earth.

Supernova: A huge explosion of a massive star at the end of its life or of a stellar cinder that accumulates material from a companion. Elements heavier than iron (e.g., gold) are produced only in supernovae.

Terrestrial planet: A rocky planet, or planet with a solid surface; e.g., the Earth, Venus, Mars, and Mercury.

Tidal locking: Tides between two objects that orbit each other closely tend to eventually synchronize the orbital and rotational period of the lower mass object. As a result, the same side of it will always face the other, just as one side of the Moon always faces the Earth. Close-in extrasolar planets are expected to be tidally locked to their star, thus they would have a permanent dayside and an eternal nightside.

Transit: The passing of one object in front of another, partially covering the latter temporarily. Searching for small periodic dips in the brightness of star as planets transit is a successful technique for detecting and characterizing extra-solar planets.

T Tauri star: A young low-mass star, less than about 10 million years in age, that is still contracting under its own gravity. Many stars of this type harbor protoplanetary disks.

Ultraviolet (UV): Radiation with wavelengths shorter than those of visible light but longer than those of X-rays.

Wavelength: The distance between two successive crests or troughs of a wave.

White dwarf: The compact, dense core of a star that has shed its outer layers. It is the end state of stars like the Sun that are not massive enough to explode as supernovae.