Astronomy • 2.8

Glossary of Basic Astronomical Terms by Andrew Fraknoi (Foothill College & ASP)

A The collapsed remnant of a whose is so great that nothing, not even , can es- Andromeda The closest major galaxy to our cape it. More technically, a region where gravity has own, located about 2.5 million light away in the warped space- so much that straight lines have of Andromeda. Also known by its cata- become . log number M31, it is the only other large galaxy in A failed star; a star that cannot (for any the of . significant of time) sustain itself by producing The () of the light-collecting in its core through nuclear reactions (fusion). or in a . C Apparent How bright a star or other ob- ject in the looks from , expressed in the old- CCD (charge-coupled device) An electronic detec- fashioned magnitude system. The brighter the star tor of electromagnetic (including light). In looks, the smaller the magnitude number. The bright- photography, CCD’s are light-sensitive arrays that re- est star in the sky, , has a magnitude of –1.42, the cord the amount of light coming in and read them out star has a magnitude of zero, while dim Barnard’s as numbers (digits). They made digital photography Star, with a magnitude of 9.5, is too faint to be seen possible, replacing film as the collector of light. with the naked . (Bear in mind that how bright a star looks to us depends on both how bright it really A relatively small body made of and is — its — and how far away it is.) that the . Most remain far away and frozen solid, but when a comet approaches the Sun, its A relatively small rocky object orbiting the ice can evaporate and its dust can be freed, producing Sun. Most are found between and Jupi- a large of material around the frozen core, and ter in a region called the , although some making it much easier to spot. Light and energy from asteroids have orbits that cross the orbits of inner plan- the Sun can push some of this cloud into a direction ets (like Earth). Those are the ones we worry about. away from the Sun, producing the comet’s tail. The average between the Constellation In the old days, a constellation was any Earth and the Sun, about 150 million km or 93 million recognizable pattern of bright in the sky. To- . use this unit to express , astronomers have given the word a more precise within the . meaning: it is one of 88 sectors into which astrono- Axis An imaginary line drawn through the center of a mers divide the sky (much like the territory of the U.S. body, around which it spins. is divided into sectors called states.) Each constella- tion “box” is named after a pattern of bright stars in B it. For example, in the old system, (the Hunter) is a pattern of bright stars easily recognized in the The high-energy, very dense beginning of winter sky. In the new system, the “box” in the sky the ; the start of its expansion. The Big Bang that includes the hunter pattern is called Orion, and is the popular name for a whole set of ideas for it includes all the stars and galaxies in the sky that lie the origin and early evolution of the universe. in the box.

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Cosmology The branch of astronomy that deals with large enough for gravity to make it round, but one that the properties of the universe as a whole — as a single shares its general with a number of other objects. system. Topics that cosmologists think about include , , and are examples of dwarf . the organization, origin, and ultimate fate of the uni- verse. E D When one body gets into the shadow of an- other in space. For example, we get an eclipse of the The energy that is causing the expansion when the Earth gets right between the Sun and of the universe to accelerate (speed up). Astronomers the Moon and casts its shadow on the Moon. have discovered this acceleration by using distant su- The property of some of mat- pernovae (exploding stars) as distance markers and ter that causes them to attract or repel other charged noting that space has not been stretching at a constant particles. Examples of particles with electric charge rate over cosmic time. The of this energy is un- are the and the . known at present. Electromagnetic radiation of energy gener- Dark Material which we cannot detect (by ob- ated by electric and magnetic changes in matter; these serving light or other radiation from it) but whose ex- waves travel at the . Examples include istence we know about from seeing its gravity affect waves, waves, visible light, the material in the universe that we can see. It appears waves, x-rays, and gamma rays. that there is more of this mysterious in the universe than the regular matter which we see as Expansion of the Universe (Expanding Uni- stars and galaxies. What dark matter is made of is cur- verse) The that the groups of galaxies are rently unknown. all moving away from one another — we now know that this is because space itself has been “stretching” Day In astronomy, the time a takes to spin (turn since the big bang. Recent indicate that on its axis) once. this expansion is speeding up (accelerating). A system of measuring the position of ob- Extra-solar planet A planet orbiting a star other than jects in the sky that is similar to the system the Sun. we use on Earth. Declination is measured in degrees north and south of the celestial (the in A magnifying lens used to view the image the sky that you would see if you extended the Earth’s produced by a telescope. equator out into the sky.) F A measure of equal to one 360th of a full circle. The dome of the sky from a point on the hori- The number of cycles (repetitions) per sec- zon to the opposite point measures 180 degrees. ond in a , such as light. The higher the frequency, The amount of per unit volume in a the more energy a wave carries. body. A cubic centimeter of lead is a lot more dense Fusion A process by which light atomic nuclei come than a cubic centimeter of Jell-O. together under tremendously hot conditions to pro- Detector A device that registers (detects) light or other duce energy; fusion is what allows the stars to shine. radiation. The eye and a solar panel are both When fusion occurs, some mass turns into energy. examples of detectors; by themselves are not. G Doppler shift The change in the (color) of Galaxy A great island of millions to hundreds of bil- light — or other electromagnetic radiation — caused lions of stars (plus “raw material” in the form of by of its source toward us or away from us. and dust), separated from other galaxies by large gulfs An object orbiting the Sun which is of space. We live in one such group called the Galaxy. (Astronomers generally write “Galaxy”

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when referring to ours and “galaxy” — lower case — ergize the wreckage around it. Such hypernovae are when referring to others.) thought to be the explanation for one kind of gamma- ray burst. a group of galaxies containing dozens to thousands of members I A form of electro-magnetic radiation, consisting of waves with the highest energy. Infrared light A type of electro-magnetic radiation with a longer wavelength than visible light. Infrared Gamma-ray Burst Brief event which generates a huge light, discovered by , is the type of amount of gamma-ray energy, lasting from a fraction wave that human beings and chairs give off. (They also of a second to a few minutes. Such bursts are now reflect visible light when it is shining on them, but this known to come from rare, extremely violent events in is not their radiation; it comes from another source.) other galaxies. Infrared is a way for astronomers to detect cooler ob- General (or General Relativ- jects, such as stars that are in the process of forming. ity) An overarching theory, developed by Albert Einstein, which relates gravity and the nature of space L and time in the universe. Astronomers use the general theory of relativity as the basis of modern . Light The distance that light travels in one year; Its ideas also help to explain the strange properties of roughly 9.5 trillion kilometers or 6 trillion miles. The black holes. nearest star is a little over 4 light years away. A crowded group of 100,000 or more Local Group The group of several dozen galaxies to stars all traveling together, as part of a galaxy. These which our Milky Way Galaxy belongs. Most of the clusters are spherical in shape, hence their name. The members of the group are galaxies smaller than the Milky Way Galaxy has more than 150 such globular Milky Way. clusters. Luminosity The total energy output of a star or other Gravity The pull of all matter on all other matter in the celestial object; a measure of the total of all the visible universe. Gravity is one of the fundamental forces in light and other electromagnetic radiation an object the cosmos, and is responsible for pulling structures gives off. like stars together. (A more “sophisticated” or over- An eclipse of the Moon; it happens arching theory of gravity is given by the General The- when the Moon moves directly between the Earth and ory of Relativity.) the Sun, and thus into the Earth’s shadow. H M

Hubble’s Law In an expanding universe, the farther Magnitude see away a galaxy is from an observer, the faster its speed of moving away. This principle was first noted in a Mass The total amount of material in a body. systematic way by the American Edwin Messier Catalog A catalog of “fuzzy” celestial objects Hubble (in the 1920s). In the general theory of relativ- compiled by in the 18th century. He ity, this behavior is expected from the fact that space- compiled the catalog so he would not mistake the ob- time itself is stretching. The more space there is be- jects in it for comets (which were his passion). But the tween galaxies (or groups of galaxies, to be precise), catalog turned out to be a pioneering list of the most the more space there is to stretch, and the faster the noticeable nebulae, star clusters, and galaxies, and galaxies get “carried away” from each other by that is still used today. Astronomers often call objects by stretching. their number in this catalog: the The explosion, at the end of its , of a is M31, the Crab is M1. very massive star, whose core collapses to be a super- Meteor A bit of solid debris from space, which vapor- compressed, spinning black hole, stirring and en- izes during its passage through the Earth’s .

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Its high-speed interaction the air with air causes a brief A telescope that is designed to col- flash of light (which is sometimes called a “shooting lect visible light (as opposed to other forms of electro- star” — although it has nothing to do with stars.) magnetic radiation, such as radio waves, that are not visible to the human eye.) A rock from space which has survived pas- sage through the Earth’s atmosphere. Orbit The path that an object takes as it revolves around another object due to their mutual gravity. For Milky Way Galaxy The galaxy of stars in which the example, the Earth has an orbit around the Sun, and Sun and the Earth are located; one of the two large the Moon and the Hubble both have spiral galaxies in the Local Group. orbits around the Earth. A combination of two or more chemi- Ozone Layer A region of the Earth’s atmosphere cally bound together. A molecule, for example, (roughly 10 to 20 miles above the surface) which has consists of two atoms of and one of a higher concentration of ozone (a molecule of oxy- . gen with three oxygen atoms in it). This ozone absorbs Moon Any object that orbits a planet (sometimes also harmful ultraviolet radiation coming from the Sun, called a .) When astronomers write about the contributing to the conditions that make life on the Earth’s satellite, they call it the Moon, but other plan- Earth’s surface possible. ets’ are written as (lower case m). P N A unit of distance equal to 3.26 light years (or Nebula A cloud of glowing gas and dust among the 206,265 astronomical units.) This unit is still used by stars. A nebula can often be observed in regions where astronomers, but is not so often heard when astrono- new stars have recently been born and around stars mers talk to the public. that are dying or have died. Before galaxies were un- Phases of the Moon The changing appearance of the derstood, they were also classified as nebulae (as in Moon as it revolves around the Earth, caused by the the term “spiral nebula”) but we don’t use the term differing amounts of we can see reflected that way any more. The plural isnebulae. from the Moon from our vantage point. (More gener- The remains of a massive star that has ex- ally, phases are the different appearances of a planet or ploded at the end of its life as a . These rem- moon as it moves around its orbit.) nants are very dense, because the violence of the star’s Planet An object of significant size that orbits a star, death has compressed them until and but is not itself a star. According to the new definition have merged to become neutrons. Typically, neutron of a planet adopted by the International Astronomical stars can contain more than twice the mass of our Sun Union, planets must have enough mass to be spherical in a ball about 10–20 km across. Magnetic, spinning and must have their own independent orbits around neutron stars can sometimes be seen as . the Sun — as opposed to sharing orbits with a number O of other bodies. The Sun has eight planets according to this definition. (Pluto is part of the and A place where telescopes and other as- Ceres is part of the Asteroid Belt, making them dwarf tronomical instruments are housed and used. Obser- planets.) vatories that have visible-light detecting telescopes A shell or shells of gas ejected by a are now built on remote mountain tops, to escape the relatively low-mass star that is in the process of dying, of civilization. and becoming a dwarf. The name is very mis- A group of stars within the main disk leading — this “last gasp” material from a star at the of a galaxy, containing a few dozen to a few thousand end of its life has nothing to do with planets. member stars, all of whom were born in roughly the Prominence An eruption of hot gas from the surface same place at roughly the same time. Another name of the Sun. Seen against the darkness of space, it can used for such a group is galactic cluster. look like a giant arcing flame.

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Pulsar Originally, short for “pulsating ”. greater its is. By spreading out the light of a These objects, now known to be compact, rotating, star or galaxy into a spectrum, astronomers can thus magnetic neutron stars, signal their presence to us by measure the shift of its colors and thus its speed to- giving off extremely regular pulses of electromagnetic ward or away from us (objects moving toward us show radiation. a ). Galaxies beyond our immediate group of neighbors ALL show a redshift, since they are partici- Q pating in the expansion of the universe. Originally, short for “quasi-stellar object” — A telescope in which the light is things that first looked like a dim blue stars, but turned collected (and reflected) by a mirror. out to be the bright centers of a distant galaxies. These A telescope in which the light is central regions shine with so much energy that they collected (and refracted or bent) by a lens. are much more easily detected than the full galaxy that Resolution The ability of a telescope to make out fine surrounds them. Today, we know that their brilliance details in an or to separate the is connected with the energy produced by enormous image of two objects that are close to each other on black holes in the crowded center of each galaxy. the sky. (Resolution in astronomy is measured in units R of angle on the sky: degrees, minutes of arc, or sec- onds of arc.) Radial Outward from the center, like the radius of a A system of measuring the position circle, or the spokes of a bicycle wheel. Astrono- of objects in the sky that is similar to the mers often talk about “” — the speed system we use on Earth. Right Ascension is measured of something in the cosmos in the direction outward in and minutes around the (a from us. circle in the sky that would result if we pushed the Radiation Waves of energy that move away (radiate) Earth’s equator out into the sky.) from their source at the speed of light. By far the most common form is electro-magnetic radiation, of which S light is the best-known example. Second of Arc A very small measure of angle, often Radio Waves A type of electro-magnetic radiation used in astronomy; equal to 1/3600th of a degree. A consisting of waves of long wavelength and low ener- U.S. dime, seen from about two miles away, takes up gy. Radio waves on Earth travel from the transmitting one second of arc. tower of a radio station to the in your car, for SETI The Search for Extra-Terrestrial Intelligence. The example. Objects in the universe give off natural radio branch of astronomy devoted to finding evidence of waves (which would sound like “static” if we were to intelligent life among the stars. translate them into sound.) An eclipse of the Sun by the Moon; A stage in the life of every star, when it when the Moon moves in front of the Sun as seen runs out of its initial fuel for fusion, and, as part of its from Earth. readjustment, expands to become much larger than its original size. As the hot gas of the star expands, Solar System The Sun and its family of planets, moons, it cools down and becomes reddish in color. A well- and assorted smaller chunks of material. Strictly known red giant is , in the constellation fig- speaking, this term refers only to the Sun and not to ure of Orion, the hunter. the other stars; but astronomers often talk about “oth- er solar systems”, when they refer to families of planets Redshift The change in the colors of an astronomi- around other stars. cal object caused by its motion away from us. Chris- tian Doppler discovered in the 19th century that the Spectrum The array of wavelength or colors in a beam motion of a source of light (or other radiation) away of light (which can be studied when the white light is from us or toward us changes its colors in a subtle but spread out in a spectroscope), or a photograph of this measurable way. The faster an object moves away, the array of colors. More generally, the array of

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found in any beam of electro-magnetic radiation. ers at random intervals. Many amateur astronomers contribute to astronomy by monitoring such stars Spectroscope A device that allows to ob- carefully over long periods of time. (One class of vari- serve the spectrum of radiation from some source. Us- able stars, called Cepheids, has special properties that ing a spectroscope, for example, astronomers spread allow them to be used to determine distances.) the light of the Sun out into its component colors and can learn about the Sun’s , composition, The collapsed, hot remnant of a low- and motion. mass star at the end of its life. When the Sun becomes a white dwarf, it is expected to be about as small as Star A sphere of hot gas that shines under its own pow- two across. White dwarf are extremely dense er. The energy that allows a star to shine comes from objects, shining only by radiating away their heat en- the process of . The Sun is our closest ergy. Ultimately, white dwarfs fade away into black example of a star. dwarfs — too cool to be seen with visible light. A cooler region of the Sun’s surface, which Year The time it takes a planet to revolve once around looks dark in comparison to the hotter material its star. The Earth’s year is 365 ¼ days long, but the around it. years of other planets are different. (of galaxies) A grouping of galaxy clus- ters. Such may contain as much mate- A Few Astronomical Glossaries on the Web rial as 10,000 or more Milky Way Galaxies and stretch over hundreds of millions of light years. NASA Imagine the Universe Site Dictionary (for younger readers): Supernova An enormous explosion that causes the http://imagine.gsfc.nasa.gov/docs/dictionary.html “death” of a massive star. Typically, such an explosion leaves behind a neutron star, or a black hole. Another Amazing Space Glossary from the Space Telescope type of supernova is an explosion that occurs because Institute: a white dwarf in a system has re-ignited http://amazing-space.stsci.edu/glossary/def.php. after a companion star has dumped a lot of fresh ma- s=topic_astronomy terial on it. (It is this second kind of supernova, which always flares up to be roughly the same luminosity, Case Western Reserve , brief definitions of that has allowed astronomers to measure that the ex- astronomical terms: pansion of the universe is speeding up.) http://burro.astr.cwru.edu/stu/extras_glossary.html Glossary from the PBS Program Seeing in the Dark: T http://www.pbs.org/seeinginthedark/resources- links/glossary.html Telescope An instrument that gathers light (or an- other type of electromagnetic radiation) and brings Catalog of the Cosmos from the PBS Program it to a . Telescopes allow astronomers to see or : photograph objects that are too dim to be seen with http://www.pbs.org/wgbh/nova/gamma/cosm_ the . nf.html When a smaller object passes in front of a larger NASA/IPAC Extragalactic Database Glossary (a bit one in space; for example, a planet may transit a star. technical, but extremely thorough): U-Z http://nedwww.ipac.caltech.edu/level5/Glossary/ frames.html Universe The sum total of all matter, radiation, and space; everything that is accessible or can become ac- cessible to our observations.

Variable Star A star whose luminosity (output of ener- © 2010 Astronomical Society of the Pacific gy) changes with time — some change regularly, oth- www.astrosociety.org

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