Astronomy Background • 2.8 Glossary of Basic Astronomical Terms by Andrew Fraknoi (Foothill College & ASP) A Black Hole The collapsed remnant of a star whose gravity is so great that nothing, not even light, can es- Andromeda Galaxy The closest major galaxy to our cape it. More technically, a region where gravity has own, located about 2.5 million light years away in the warped space-time so much that straight lines have constellation of Andromeda. Also known by its cata- become circles. log number M31, it is the only other large galaxy in Brown Dwarf A failed star; a star that cannot (for any the Local Group of galaxies. significant length of time) sustain itself by producing Aperture The size (diameter) of the light-collecting energy in its core through nuclear reactions (fusion). lens or mirror in a telescope. C Apparent Magnitude How bright a star or other ob- ject in the sky looks from Earth, expressed in the old- CCD (charge-coupled device) An electronic detec- fashioned magnitude system. The brighter the star tor of electromagnetic radiation (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, Sirius, has a magnitude of –1.42, the cord the amount of light coming in and read them out star Vega 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 eye. (Bear in mind that how bright a star looks to us depends on both how bright it really Comet A relatively small body made of ice and dust is — its luminosity — and how far away it is.) that orbits the Sun. Most comets remain far away and frozen solid, but when a comet approaches the Sun, its Asteroid A relatively small rocky object orbiting the ice can evaporate and its dust can be freed, producing Sun. Most asteroids are found between Mars and Jupi- a large cloud of material around the frozen core, and ter in a region called the asteroid belt, 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. Astronomical Unit The average distance 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 stars in the sky. To- miles. Astronomers use this unit to express distances day, astronomers have given the word a more precise within the solar system. 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, Orion (the Hunter) is a pattern of bright stars easily recognized in the Big Bang The high-energy, very dense beginning of winter sky. In the new system, the “box” in the sky the universe; the start of its expansion. The Big Bang that includes the hunter pattern is called Orion, and Theory 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. The Universe at Your Fingertips • Astronomical Society of the Pacific Page 1 Astronomy Background 2.8 • Glossary of Basic Astronomical Terms 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 orbit with a number of other objects. system. Topics that cosmologists think about include Ceres, Pluto, and Eris are examples of dwarf planets. the organization, origin, and ultimate fate of the uni- verse. E D Eclipse When one body gets into the shadow of an- other in space. For example, we get an eclipse of the Dark Energy The energy that is causing the expansion Moon 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- Electric charge The property of some particles 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 nature of this energy is un- are the electron and the proton. known at present. Electromagnetic radiation Waves of energy gener- Dark Matter 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 speed of light. Examples include istence we know about from seeing its gravity affect radio waves, infrared waves, visible light, ultraviolet the material in the universe that we can see. It appears waves, x-rays, and gamma rays. that there is more of this mysterious dark matter 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 observation 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 planet takes to spin (turn since the big bang. Recent observations indicate that on its axis) once. this expansion is speeding up (accelerating). Declination 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 latitude system the Sun. we use on Earth. Declination is measured in degrees north and south of the celestial equator (the circle in Eyepiece 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 Degree A measure of angle equal to one 360th of a full circle. The dome of the sky from a point on the hori- Frequency The number of cycles (repetitions) per sec- zon to the opposite point measures 180 degrees. ond in a wave, such as light. The higher the frequency, Density The amount of mass 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 human eye and a solar panel are both When fusion occurs, some mass turns into energy. examples of detectors; telescopes by themselves are not. G Doppler shift The change in the wavelength (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 gas by motion of its source toward us or away from us. and dust), separated from other galaxies by large gulfs Dwarf planet An object orbiting the Sun which is of space. We live in one such group called the Milky Way Galaxy. (Astronomers generally write “Galaxy” The Universe at Your Fingertips • Astronomical Society of the Pacific Page 2 Astronomy Background 2.8 • Glossary of Basic Astronomical Terms 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. Galaxy Cluster a group of galaxies containing dozens to thousands of members I Gamma Ray 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 William Herschel, 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 Theory of Relativity (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 cosmology. Light Year 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.