What Is That?

aberration		in optics, an imperfect focus caused when a mirror or lens fails to bring light to a sharp focus

absolute magnitude		the apparent brightness an object would have if it were 10 parsecs (32.6 light-years) from Earth

absolute zero		the coldest theoretical temperature, equal to 0 kelvin (-459.67° F or -273.15° C)

absorption lines		dark lines in a spectrum caused by the absorption of light by atoms or molecules in a star or planets atmosphere

accretion disk		a disk surrounding a black hole or star in which matter gravitationally falls onto the central object

achromatic lens		a two-element lens, or doublet, that significantly reduces chromatic aberration

active galactic nuclei		the exceptionally bright cores of some galaxies, thought to be fueled by matter falling into supermassive black holes

adaptive optics		a system of telescopes, computers, and deformable mirrors used to compensate for atmospheric blurring

Airy disk		the bright disk-like image of a point source of light, such as a star, as seen in an optical system with a circular aperture

albedo		the percentage of light that an object reflects

altazimuth mount		a mount that enables a telescope to move freely both vertically (in altitude) and horizontally (in azimuth)

altitude		1. the height above sea level 2. the angle between an objects position on the celestial sphere and the horizon

anaglyph		a stereoscopic, composite image in which the right component (usually red in color) is superimposed on the left component (usually blue) to produce a three dimensional effect when viewed through correspondingly colored filters

Andromeda Galaxy		the largest member of the Local Group of galaxies; roughly twice the size of the Milky Way; also known as M31

angular size		the apparent width of an object as seen by an observer, usually expressed in degrees, arcminutes, or arcseconds

anisotropy		the variation of a physical property depending on direction

annular eclipse		a solar eclipse in which the moon does not fully cover the suns disk, allowing observers to see a thin ring of sunlight

antimatter		matter consisting of particles that have the same mass and properties as their matter counterparts, only with the opposite electrical charges

aperture		the diameter of a telescopes primary lens or mirror; the larger the aperture, the greater the telescopes light-gathering power

aphelion		the point farthest from the sun in an objects orbit

apochromatic lens		a lens with three or more elements that reduces chromatic aberration even more than an achromatic lens

apogee		the point in a satellites orbit when it is farthest from Earth

Apollo		1. the U.S. space program that sent astronauts to the moon in the 1960s and '70s 2. an asteroid with a perihelion less than 1.017 AU (and thus comes within the orbit of Earth)

apparent field of view		the angular diameter of the circle of light that the eye sees through an eyepiece

apparent magnitude		the measure of the brightness of an object as seen from Earth

Barlow lens		a lens attached behind the eyepiece of a telescope that increases magnification

barred spiral galaxy		a spiral galaxy with a central bar consisting of stars and gas

baryonic		made up of baryons (elementary particles such as protons and electrons)

baryonic matter		"normal" matter composed of elementary particles called baryons

baryons		elementary particles such as protons and neutrons composed of three quarks

baseline		the line between two observational points or two telescopes of an interferometer

Big Bang		the giant explosion that is theorized to have created the universe 10 billion to 20 billion years ago

billion		1,000,000,000 (in American usage)

binary star		a system of two stars that orbit a common center of gravity; also known as a double star

binoculars		a small, usually hand-held instrument with two tubes that is used to magnify the view of astronomical objects; the two numbers used to describe the binoculars refer to its magnification and its aperture in millimeters, respectively

black hole		a region of space where gravity is so powerful that not even light can escape; black holes can form either from the death of high-mass stars or in the cores of galaxies

blazar		a high-energy, variable type of quasar which astronmers believe has a jet of material aimed in our direction that causes it to appear more energetic than other quasars

blueshift		a decrease in the wavelength of light coming from an object due to its motion toward Earth.

Bok globule		a small, dark nebula thought to be a region of star formation

bolide		a brilliant meteor or fireball that explodes in mid-air

bolides		brilliant meteors or fireballs that explode in mid-air

Bose-Einstein condensate		atoms crowded close together in ultra-low temperatures that behave as if they were one fluid-like superatom

brown dwarf		a gaseous object that forms like a star but lacks the necessary mass to sustain nuclear fusion in its core; a body intermediate in mass between a star and planet

buckyball		a naturally occurring form of carbon known as C-60, its molecular structure resembles the geodesic domes once designed by Buckminster Fuller

bulge		the generally spherical, central region of a spiral galaxy

carbon star		a red giant star with much more carbon than oxygen in its surface layers

Cassegrain telescope		a reflecting telescope in which a secondary mirror reflects light back through a hole in the center of the primary mirror

cataclysmic variable		a close binary system which includes a white dwarf accreting matter from a less massive companion

catadioptric telescope		a telescope that combines the primary mirror of a reflector with a lens placed in front of the mirror that corrects for aberrations; most catadioptric telescopes for amateurs are Schmidt-Cassegrain telescopes

CCD		a silicon chip used to detect light; charge-coupled devices (CCDs) are far more efficient at collecting light than conventional film

celestial pole		the imaginary projection of Earths rotational axis onto the celestial sphere

celestial sphere		the apparent sphere of the sky; an imaginary sphere of immense radius centered on Earth often used to plot the coordinates of objects in the sky

Cepheid variable		a class of luminous stars that vary in brightness; used to calibrate distances to galaxies

Chandrasekhar limit		the maximum mass of a white dwarf star, equivalent to 1.4 solar masses

charge-coupled device		a silicon chip used to detect light; charge-coupled devices (CCDs) are far more efficient at collecting light than conventional film

Charles Messier		A French astronomer and comet hunter who discovered 13 comets independently and codiscovered a half-dozen others. While hunting for comets, Messier compiled a list of fuzzy objects that were not comets in order to avoid them. These catalog entries were later identified as star clusters, nebulae, and galaxies and became the Messier Catalog. Published in various versions beginning in 1771, the catalog grew to 103 objects by 1781. Charles Messier lived from June 26, 1730 to April 12, 1817.

chondrite		a stony meteorite containing small, round, silicate granules called chondrules

chromosphere		a layer in a stars atmosphere lying below the corona and above the photosphere

circumpolar		Circumpolar stars are permanently above the horizon from a given observing point on Earth; that is to say, they never set. At Earth's Geographical North Pole (90° north latitude), all stars in the sky are cirumpolar. On Earth's equator, no stars are circumpolar.

clock drive		a motor attached to an equatorial mount that compensates for Earths rotation and thus keeps the telescope pointing at the same area of sky

coated optics		optics treated with a thin, uniform coating that greatly reduces scattered light and thus makes the image brighter

collimation		the act of putting a telescopes optics into perfect alignment

coma		the bright shroud of gas that surrounds a comets nucleus

comet		a small piece of ice and rock that orbits a star usually in a highly elongated orbit; long-period comets have orbital periods longer than 200 years, short-period comets have orbital periods less than 200 years

comet nucleus		a solid, compact mass of rock and ice that heats up when exposed to sunlight and releases gas and dust

conjunction		a time when two or more bodies appear close together in the sky

constellation		one of the 88 patterns of stars in the sky, often named for a mythological god, hero, or animal

convection		the transfer of heat energy by moving currents of material

core		the central region of a planet, brown dwarf, star, or galaxy

corona		the outer atmosphere of the sun or a star

Damocloid		a rare type of asteroid with an elliptical, comet-like orbit; named for the first one discovered, asteroid 5335 Damocles

dark adaptation		the process by which the human eye becomes well adjusted to seeing dim objects in the dark.

dark energy		a type of "negative gravity" that seems to play a role in the acceeleration of universal expansion

dark matter		matter that exerts gravitational force but does not emit any detectable light or radiation; dark matter comprises most of the mass of the universe but its exact nature remains unknown

dark nebula		a cloud of dust grains that is thick enough to obscure the light from background stars

declination		the angular distance of a celestial object above or below the celestial equator; the celestial sphere equivalent of latitude

deep-sky objects		objects located beyond the solar system; consist of stars, nebulae, star clusters, and galaxies.

degree		1. a unit of angular size equal to 1/360 the circumference of the celestial sphere; the sun and full moon both appear about half a degree wide 2. a unit of measure for temperature along a graded scale

density		the amount of mass per unit volume of an object or region of space

deuterium		an isotope of hydrogen; its nucleus, consisting of one proton and one neutron, has double the mass of the nucleus of ordinary hydrogen

diffraction		the spreading out of light as it passes the edge of an obstacle

Dobsonian telescope		a telescope with a simple but stable altazimuth mount that rotates easily

Doppler effect		the change in wavelength of radiation coming from a source thats moving toward or away from an observer; produces either a blueshift or redshift

dust		tiny particles floating in space

dwarf galaxy		a small galaxy containing a few million stars; the most common type of galaxy in the universe

dwarf star		a main-sequence or smaller star

eccentric		deviating from a circle (used to describe the shape of an orbit)

eccentricity		the extent to which a bodys elliptical orbit deviates from a circle

eclipse		an event in which one body passes in front of another, blocking it partially or completely from view; a specific type of occultation

ecliptic		the plane of Earths orbit around the sun; all the planets except Mercury and Pluto have orbits in nearly the same plane

ejecta		material thrown about by an impact or volcano

electromagnetic radiation		the various forms of light; includes radio waves, infrared light, visible light, ultraviolet light, x rays, and gamma rays

electromagnetic spectrum		the spectrum encompassing the entire range of electromagnetic radiation (light)

electron		a subatomic particle with a negative electric charge; electrons surround the atomic nucleus and are much less massive than protons or neutrons

element		a fundamental unit of matter; consists of a fixed number of protons, although the number of neutrons and electrons can vary

elliptical galaxy		a gravitationally bound system of stars in a spherical or elliptical shape with no spiral structure

elongation		the apparent angular separation of an object from the sun

emission		the discharge of electromagnetic radiation from an object

emission nebula		a cloud of very hot gas that is being illuminated from within by the radiation of energetic, young stars

ephemeris		a table that gives the positions of astronomical objects at certain intervals of time.

equatorial mount		a telescope mount in which one axis lies parallel to Earths rotational axis; the motion of the telescope about this axis can compensate for Earths rotation

equinox		the two times of year when the sun crosses the celestial equator, giving day and night an equal 12-hour length everywhere on Earth

escape velocity		the velocity an object or rocket needs to escape the gravitational clutch of a more massive object

evening star		the planet Venus when it appears in the evening sky

event horizon		the boundary of a black hole from inside which light cannot escape

exit pupil		the image of the objective lens or primary mirror of a telescope formed on the eye side of the eyepiece

exobiologists		a person who studies the origin, development, and distribution of 'living' systems that may exist outside of Earth

extragalactic		beyond the Milky Way Galaxy

extrasolar		beyond the sun

extraterrestrial		beyond Earth

eye relief		the distance between the eyeball and the lens nearest the eye of an eyepiece at which an observer can clearly see the entire field of view

far ultraviolet		ultraviolet radiation with the shortest wavelengths ("farthest" from visible light in the electromagnetic spectrum)

field of view		the area of sky visible in a telescope or binoculars

filter		a device that transmits light of only certain wavelengths; used by astronomers to observe specific wavelengths or to reduce the light of exceptionally bright objects

finder scope		a small, low-powered telescope attached to a larger telescope that helps the observer locate objects in the sky

fireball		an extremely bright meteor; generally brighter than magnitude 4

fireballs		extremely bright meteors; generally brighter than magnitude 4

first quarter		the phase of the moon a quarter of the way around its orbit from new moon; the eastern half is illuminated during this phase

flare		a sudden, violent outburst of energy from the surface of a star

focal length		the distance from a lens or mirror to the point where it brings light to a focus

focal ratio (f/ratio):		the ratio of the focal length of a lens or mirror to its diameter

focus		the point at which rays of light passing through a lens (or reflecting off a mirror) converge

focuser		the device on a telescope that holds an eyepiece and moves to allow an observer to bring light to a sharp focus.

fork mount		an equatorial mount in which the telescope swings in declination between the two prongs of a fork

frequency		the number of wave crests or troughs that pass a particular point in a given interval of time (usually one second); usually expressed in hertz (cycles per second)

full moon		the phase of the moon when it is halfway around its orbit from new moon and opposite the sun in the sky; the full disk is illuminated

galactic disk		the disk of a spiral galaxy

galactic nucleus		the central region of a galaxy; often contains a high density of stars and gas, and a supermassive black hole

galactic plane		the projection of the Milky Ways disk on the sky.

galaxy		an enormous gravitationally bound assemblage of millions or billions of stars

galaxy cluster		a gravitationally bound assemblage of dozens to thousands of galaxies

Galilean moons		Jupiter's four largest moons: Io, Europa, Ganymede, and Callisto; discovered by Galileo Galilei in 1610

Galilean satellites		Jupiter's four largest moons: Io, Europa, Ganymede, and Callisto; discovered by Galileo Galilei in 1610

gamma rays		the form of light (electromagnetic radiation) with the shortest wavelength and the most energy

gamma-ray burst		a short, intense burst of high-energy radiation emanating from the distant universe

gas giant		a large planet made primarily of gas; such as Jupiter, Saturn, Uranus, and Neptune

general relativity		the theory of relativity governing accelerated motion that describes gravity as a curvature of space-time

german equatorial mount		a mount in which the declination axis sits on top of the polar axis, with the telescope on one end of the declination axis and a counterweight on the other

giant molecular cloud		interstellar clouds of cold gas and dust that contain tens or hundreds of thousands of solar masses

gibbous		the phase of the moon between first quarter and last quarter, when the moon appears more than half illuminated

globular cluster		a roughly spherical congregation of hundreds of thousands of stars; most globular clusters consist of old stars and exist in a galaxys halo

gravitational lensing		the distortion or amplification of an objects light due to the presence of a massive object in the light path

gravity		the attractive force that all objects exert on one another; the greater an objects mass, the stronger its gravitational pull

habitable zone (or ecosphere)		the zone around a star in which a planet can maintain liquid water on its surface.

halo		the outer region of a galaxy; contains globular clusters, a few stray stars, and dark matter

heliacal rising		the period of time when an object, such as a star, is briefly seen in the eastern sky before dawn and is no longer hidden from the glare of the sun

heliosphere		a vast region around the sun dominated by the solar wind

helium		the second lightest element; consists of two protons, and usually two neutrons and two electrons; about 8 percent of the atoms in the universe are helium

Hertz		a unit of frequency equal to one cycle per second

Hertzsprung-Russell diagram		a diagram that plots luminosity against temperature for a group of stars

Hubble law		the principle that a distant galaxys recessional velocity is proportional to its distance from Earth

hydrazine		a colorless liquid which burns rapidly and is used as a common rocket and missile fuel

hydrogen		the simplest and lightest element; usually consists of just a single proton and electron; about 90 percent of the atoms in the universe are hydrogen

hypered film		film that has been treated, usually with gas, to enhance its response to low light levels

inclination		the angle between a planets orbit and the ecliptic plane; or the angle between a satellites orbit and its host planets rotational plane

inferior conjunction		the configuration of an inferior planet when it lies between the sun and Earth

inferior planet		a planet that orbits the sun inside of Earths orbit; includes Mercury and Venus

inflation		the theory that the universe experienced a brief and extraordinarily rapid period of expansion a fraction of a second after the Big Bang

infrared		a form of light with slightly lower energy than visible light but with greater energy than radio waves

interacting galaxies		galaxies caught in each others gravitational embrace, often results in galactic mergers or extreme star formation

interferometer		a system of two or more widely separated telescopes that achieves the resolving power of a much larger telescope

interferometry		the technique of using two or more widely separated telescopes to achieve the resolving power of a much larger telescope

intergalactic		the space between the galaxies

International Space Station		a global cooperative program between the United States, Russia, Canada, Japan, and Europe, for the joint development, operation, and utilization of a permanently habitated space station in low-Earth orbit

interplanetary		the space between the planets

interstellar		the space between the stars of a galaxy

interstellar medium		the gas and dust located between the stars

ion		an atom that is electrically charged due to the loss or gain of one or more electrons

ionization		the process by which an atom gains or loses electrons

ionized gas		a gas that has been heated to a state where it contains ions and free-floating electrons; also known as a plasma

ionosphere		an atmospheric layer with a high concentration of ions and free electrons

irregular galaxy		a galaxy without a clearly defined spiral or elliptical shape

isotope		forms of an element in which the atoms all have the same number of protons but different numbers of neutrons

Lagrange point		one of five locations in space relative to two bodies where a third, less massive body can maintain a stable orbit around a common center of mass

Lagrange points		five locations in space relative to two bodies where a third, less massive body can maintain a stable orbit around a common center of mass

Large Magellanic Cloud		an irregular galaxy that orbits the Milky Way Galaxy

last quarter		the phase of the moon three-quarters of the way around its orbit from new moon; the western half is illuminated

latitude		the angular distance north or south from the equator to a point on Earth's surface, measured on the meridian of the point

lens		a curved piece of glass that brings light to a focus

libration		the small oscillations in the moons motion that allow Earth-based observers to see slightly more than half the moons surface

light pollution		light, typically from artificial sources, that reaches the night sky, obscuring the view of faint astronomical objects

light-gathering power		the ability of a telescope to collect light; the larger a telescopes aperture, the greater its light-gathering power

light-year		the distance light travels in one year, equivalent to approximately 5.9 trillion miles (9.5 trillion km)

limb		the apparent edge of a celestial object

limiting magnitude		the apparent magnitude of the faintest objects that can be seen given the local observing conditions and any telescope, film, or other detector you may be using

LINER galaxy		A low-ionization nuclear emission-line region galaxy belongs to a common class of otherwise normal galaxies that display low-ionization line emissions near their central regions

Local Group		the galaxy cluster containing roughly 35 galaxies to which the Milky Way Galaxy belongs

local supercluster		the galaxy supercluster to which the Local Group belongs; it spreads over 100 million light-years and boasts the Virgo Cluster as its dominant member

longitude		the angular distance of a particular place on Earth as measured east or west from the prime meridian running through Greenwich, England

luminosity		the total amount of light that an object radiates

lunar eclipse		a phenomenon caused by the Earth passing between the sun and moon

lunar month		the period of one complete revolution of the moon around Earth, 29.5 days

lunation		the time between two successive new moons; approximately 29.5 days

magnetometer		an instrument used to measure the strength and direction of a magnetic field

magnetosphere		the dynamic region around a planet where the magnetic field traps and controls the movement of charged particles from the solar wind

magnitude		the measurement of an object's brightness; the lower the number, the brighter the object

main sequence		the band of stars on a Hertzsprung-Russell diagram stretching from the upper left to the lower right; stars spend most of their lives in the main sequence phase, in which they are fusing hydrogen into helium in their cores

Maksutov telescope		a catadioptric telescope that uses a deeply curved meniscus lens as the correcting plate

mantle		the portion of a planets interior above the core but below the crust

mare		a dark and relatively smooth area on the surface of the moon or a planet.

mass		a measure of the total amount of matter within an object

mass loss		the loss of mass by a star during its evolution; some of the causes of mass loss include stellar winds, bipolar outflows, and the ejection of material in a planetary nebula or supernova

megaparsec		one million parsecs; equal to 3.26 million light-years

meridian		an imaginary circle on the celestial sphere that connects the zenith to the north (or south) celestial pole

Messier Catalog		A catalog of 107 bright deep-sky objects that belong to a catalog compiled by French astronomer Charles Messier in the 1700s

Messier objects		A catalog of 107 bright deep-sky objects that belong to a catalog compiled by French astronomer Charles Messier in the 1700s

meteor		a flash of light that occurs when a meteoroid burns up in Earths atmosphere; also popularly known as a shooting star

meteor shower		a period of enhanced meteor activity that occurs when Earth collides with a swarm of meteoroids; an individual shower happens at the same time each year and has all its meteors appearing to radiate from a common point

meteorite		a rock from space that survives passage through Earths atmosphere and falls to the ground

meteoroid		a small rock that orbits the sun

microgravity		describes a condition where the apparent weight of an object is much less than its true weight (on Earth). Apparent weight is the weight measured when an object is put on a scale in a given environment, where true weight is the force of Earth's gravity on the object.

microlensing		the effect of gravity from a small astronomical body or bodies focusing light rays, similar in manner to lenses

micron		one-millionth of a meter

microwaves		the most energetic form of radio waves

Milky Way		the band of light that encircles the entire sky and results from the combined light of billions of stars in our galaxys disk

Milky Way Galaxy		the spiral galaxy to which Earth belongs

million		1,000,000 (in American usage)

millisecond pulsar		a neutron star that rotates hundreds of times per second, which typically accretes matter from a stellar companion

naked-eye		something visible or accomplished without the aid of binoculars or a telescope (e.g. a naked-eye object or naked-eye observing)

nebula		a cloud of interstellar gas and dust; some nebulae represent stellar nurseries, others represent stellar graveyards

neutrino		a subatomic particle with little or no mass that is produced in nuclear reactions and in supernovae and very rarely interacts with matter; neutrinos have no electrical charge and travel at or very close to the speed of light

neutron		a subatomic particle with no electric charge that resides in an atomic nucleus; it has about the same mass as a proton

neutron star		the collapsed, extraordinarily dense, city-sized remnant of a high-mass star

new moon		the phase in which the moon is in the same direction as the sun in Earths sky, so it is unilluminated and invisible

Newtonian telescope		a reflecting telescope in which a flat secondary mirror (called the diagonal) in the center of the tube reflects light to a focus outside the tube

NGC		New General Catalogue, a 19th-century compendium of deep-sky objects such as galaxies, globular clusters, and nebulae

NGC objects		deep-sky objects such as galaxies, globular clusters, and nebulae included in the New General Catalogue

North Celestial Pole		the point in the sky to which Earth's Geographical North Pole points

nova		a violent explosion on the surface of a white dwarf, which causes the star to temporarily brighten by a factor of several hundred to several thousand

nuclear fusion		the process by which two atomic nuclei combine to form a heavier atomic nucleus; this is the energy source that causes most stars to shine

nucleosynthesis		the creation of heavy elements from lighter ones by nuclear fusion

Nucleus		the central region of an atom, comet, or galaxy

OB association		a loose grouping of O and B stars, which are the most luminous, most massive, and shortest-lived stars

objective		a telescopes primary lens or mirror that gathers light and brings it to a focus

obliquity		the angle between a planets equator and the plane of its orbit

occultation		the passage of one object in front of a smaller one, temporarily obscuring all or part of the background object from view

omega		1. the ratio of the density of the universe to the critical density 2. the 24th letter of the Greek alphabet

Omega Centauri		a massive globular cluster in the southern constellation Centaurus located about 17,000 light-years from Earth; also known as NGC 5139

Omega nebula		One of the Milky Ways numerous stellar nurseries, the Omega Nebula is about 5,000 light-years from Earth and can be seen in the constellation of Sagittarius the Archer. It is also known as the Swan Nebula, M17, NGC 6618, the Horseshoe Nebula, and the Lobster Nebula.

Oort cloud		a cloud of cometary nuclei that surrounds the sun at a distance of many thousands of astronomical units

open cluster		a system containing a few dozen to a few thousand stars that formed from the same stellar nursery.

opposition		the moment when a planet farther from the sun than Earth appears opposite the sun in the sky; it is the best time to observe a planet

optical double		Two stars at different distances that lie along nearly the same line of sight and thus appear close together

optics		the study of light and its properties; or lenses and mirrors

orbit		the path an object follows around a more massive object or common center of mass; usually elliptical in shape

orbital period		the length of time it takes one body to orbit another

outgassing		the release of gas from a rocky body

PAHs		Polycyclic aromatic hydrocarbons (PAHs) are a class of very stable organic molecules. They are flat molecules made only of carbon and hydrogen atoms. PAH molecules are quite common and highly carcinogenic. They are one of the by-products of combustion from automobiles and airplanes, and some are present in charcoal broiled hamburgers.

parallax		the apparent shift of a relatively nearby object against a fixed background due to the motion of the observer; astronomers observe the parallax of stars to measure their distances

parsec		the distance an object would have to be from Earth so that its parallax when viewed from two points separated by 1 AU would be one arcsecond; equal to 3.26 light-years.

penumbra		the region of a shadow from which part of the light source remains visible

penumbral eclipse		an event that occurs when the moon passes into the outer ring of Earth's shadow (penumbra), causing a slight shading in the moon's appearance

perigee		the point in a satellites orbit when it is closest to Earth

perihelion		the point in an objects orbit when its closest to the sun

period		the time interval for a regular event to take place

pH		a measurement of the alkalinity or acidity of a solution

phase		the regular cycle of changes in the appearance of a moon or planet

photometer		a detector that measures the amount of light coming from an object

Photometry		the measurement of light intensities

photons		individual particles/waves of light

photosphere		the visible surface of the sun

photovoltaic		conversion of light energy into electricity

pixel		short for picture element, the individual light detectors on a CCD chip

planet		a large rocky or gaseous body that orbits a star

planetary nebula		a glowing shell of gas ejected by a dying, low-mass star

planetary nebulae		glowing shells of gas ejected by dying, low-mass stars

planetesimals		asteroid-size bodies in a young planetary system that collide to form larger bodies

planisphere		a two-dimensional map of the sky with an adjustable overlay to show the part of the sky visible at any time of the night or year

plasma		a gas that has been heated to a state where it contains ions and free-floating electrons; also known as ionized gas

plasmasphere		a region of cold, high-density plasma above the ionosphere

plate tectonics		a theory that describes how Earths crust is broken into plates and how those plates move across Earths surface

polar cap		an icy region at the north or south pole of a planet

polarization		a state in which the directions of the electric or magnetic field in an electromagnetic wave changes in a regular pattern; light from celestial objects is often polarized

polycyclic aromatic hydrocarbons		Polycyclic aromatic hydrocarbons (PAHs) are a class of very stable organic molecules. They are flat molecules made only of carbon and hydrogen atoms. PAH molecules are quite common and highly carcinogenic. They are one of the by-products of combustion from automobiles and airplanes, and some are present in charcoal broiled hamburgers.

position angle		the direction in the sky of one celestial object from another, measured eastward from due north

power (or magnification)		the ability of a telescope or binoculars to increase the apparent size of a distant object.

Poynting-Robertson effect		a drag on interplanetary particles caused by their interaction with solar radiation, which causes the particles to lose orbital momentum and spiral into the sun

precession		the slow, periodic change in the direction an objects rotational axis caused by the gravitational influence of another body

primary mirror (or lens)		a telescopes main mirror or lens which gathers light and brings it to a focus

prime meridian		the line of longitude that runs through Greenwich, England

prism		a wedge-shaped piece of glass that breaks white light into its constituent colors

prograde		objects that move or appear to move in the same direction of most solar system bodies, or for moons, the same direction as the planet rotates

prominence		a large eruption of gas streaming off the surface of the sun into the corona

proper motion		the apparent yearly motion of a star across the sky

proton		a subatomic particle that resides in an atoms nucleus and possesses a positive electric charge

protoplanet		a body that is accreting gas, dust, and rocks en route to becoming a full-fledged planet

protoplanetary disk		a disk of gas and dust that surrounds a newborn star; planets form from collisions of particles inside the disk

protostar		a cloud of hot, dense gas and dust that is gravitationally collapsing to form a star

pulsar		a rapidly rotating neutron star that bathes Earth in regular pulses of electromagnetic radiation

radial velocity		the velocity of an object toward or away from an observer

radiant		1. the point in the sky from which the meteors belonging to a meteor shower appear to originate 2. vividly bright and shining

radiation		electromagnetic waves (in astronomical usage)

radio galaxy		a galaxy that emits an unusually large amount of radio waves

radio telescope		a telescope designed to detect radio waves coming from space

radio waves		the form of light with the longest wavelength and the least energy

radiometer		a device that measures the total energy or power from an object in the form of radiation, especially infrared radiation

red dwarf		a low-mass, main-sequence star much smaller, cooler, and less luminous than the sun

red giant		a cool star near the end of its life cycle that has expanded to a size of a few dozen to a hundred times the diameter of the sun

red supergiant		a cool, massive star near the end of its life that has expanded to a size from a hundred to a thousand times the diameter of the sun

redshift		an increase in the wavelength of light coming from an object due to its motion away from Earth, the expansion of the universe, or a strong gravitational field

reflection nebula		a cloud of gas and dust that is visible because the dust reflects a nearby stars light

reflector		a telescope that uses a curved mirror to gather light

refractor		a telescope that uses a glass lens to gather light

regolith		the powdery soil of the moon produced by meteorite impacts

relativity		the theories of physics developed by Albert Einstein that describe measurements made by two observers who are in relative motion

resolution		the ability of a telescope or camera to pick out fine detail

resolving power		the ability of a telescope or camera to pick out fine detail

reticule		a grid or pattern of two or more fine wires set inthe focal plane of a telescope eyepiece and used in determining the position and/or size of a celestial object

retrograde		objects that move or appear to move in the opposite direction of most solar system bodies; for example planets that appear to move east-to-west in the sky or objects that revolve or rotate clockwise as seen from north of the solar system

reusable launch vehicle (RLV)		a single-stage-to-orbit spacecraft that may be reused on successive missions

revolution		the orbital motion of one body around another body or a common center of mass

ribonucleic acid		a nucleic acid that transmits genetic information

rich-field telescope		a telescope designed to show a large field of view at low magnification

right ascension		the angular distance of a celestial object east of the vernal equinox; the celestial sphere equivalent of longitude

satellite		a small body that orbits a planet or asteroid

Schmidt camera		a catadioptric telescope used as a camera to take wide-angle photos of the sky

Schmidt-Cassegrain telescope		a compact telescope in which light passes through a correcting lens at the front of the telescope, then reflects off a primary mirror back up to a secondary mirror, which directs the light through a hole in the primary and out the back of the scope; a popular telescope for backyard observers.

secondary mirror		a relatively small mirror used in a telescope to redirect the light gathered by the primary mirror

seeing		the quality of observing conditions induced by turbulence in Earths atmosphere, which blurs the images of astronomical objects

semimajor axis		the average distance of an orbiting body from its parent body

SETI		the search for extraterrestrial intelligence

setting circles		circular scales on the two axes of an equatorial mount that help an observer point a telescope to a specific right ascension and declination

shock wave		a powerful wave caused by a sudden change in density, pressure, or temperature that travels though a medium faster than sound travels through that same medium

sidereal		relating to or measured with respect to the stars

sidereal year		the amount of time it takes one body to revolve about another with respect to the stars

singularity		the central point of a black hole where matter is concentrated into an area of zero volume and infinite density

Small Magellanic Cloud		a small, irregular dwarf galaxy that orbits the Milky Way Galaxy

solar eclipse		an eclipse of the sun caused by the moon passing between Earth and the sun

solar filter		a filter used to block almost all of the suns light so our star can be viewed safely and comfortably

solar mass		the amount of mass contained in the sun, about 330,000 times that of Earths mass

solar system		the system containing the sun and all the smaller bodies in orbit around it

solar wind		the stream of charged subatomic particles emanating from the sun

solstice		either of the two points on the celestial sphere where the sun is farthest north or south of the celestial equator; when the sun is at a solstice, the amount of daylight hours is greatest for summer and least for winter

South Celestial Pole		the point in the sky to which Earth's Geographical South Pole points

space-time		the intertwining of the three dimensions of space with one dimension of time within which events can be specified exactly

special relativity		the theory of relativity governing uniform motion; it states the equivalence of mass and energy and differs from Newtonian physics only when speeds approach that of light

spectral class		the designation of a star based on its spectrum, which is determined by its surface temperature

spectral line		a particular wavelength of light corresponding to the energy transition of a specific atom or molecule

spectral type		the designation of a star based on its spectrum, which is determined by its surface temperature

spectrograph		an instrument attached to a telescope to record the spectrum of an astronomical object

spectrometer		an instrument attached to a telescope to record the spectrum of an astronomical object

spectroscope		an instrument for examining spectra

spectroscopy		the study of spectra from astronomical objects

spectrum		1. the energy emitted by a radiant source 2. the entire range of electromagnetic radiation (light)

speed of light		the fastest possible speed in a vacuum, equivalent to 186,000 miles per second (300,000 km per second)

spiral arm		a concentration of gas, dust, and young stars that winds its way out from the nuclear region of a spiral galaxy

spiral galaxy		a spiral-shaped system of billions of stars, gas clouds, and dust

standard candle		an astronomical object of known luminosity; can be used to determine distances

star		a self-luminous sphere of hot gas held together by gravity; ordinary stars generate energy by nuclear fusion in their cores

star atlas		an collection of maps that marks the positions of stars, nebulae, galaxies, and other astronomical objects on a coordinate system

star hopping		the technique of using recognizable patterns of stars to hop from one part of the sky to another; useful in observing both with the naked eye and a telescope

star party		a gathering of people to observe the night sky

starburst galaxy		a galaxy undergoing an extremely high rate of star formation

stellar evolution		the life cycle of stars

stellar wind		a stream of electrically charged subatomic particles given off by stars

sublimate		the transition of a solid substance evaporating into a gas without passing through a liquid phase

sublimated		the transition of a solid substance evaporating into a gas without passing through a liquid phase

sublimation		the process by which a solid substance evaporates into a gas without passing through a liquid phase

summer		a season that begins around June 21 in the Northern Hemisphere

sunspot		a dark, temporary, relatively cool spot on the surface of the sun

sunspot cycle		a cycle averaging 11 years in which the number of sunspots increases and decreases.

supercluster		an enormous congregation of galaxy clusters that stretches across hundreds of millions of light-years

superfluid		an unusual state of matter characterized by apparently frictionless flow, found only in liquid helium cooled to near absolute zero

superior conjunction		the configuration of an inferior planet when it lies on the far side of the sun

superior planet		a planet farther from the sun than Earth; includes Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto

superluminal motion		motion that appears to be faster than the speed of light

supermassive black hole		a black hole at the core of a galaxy that contains millions or billions of solar masses

supernova		the cataclysmic explosion of a star

supernova remnant		an expanding cloud of gas that represents the outer layers of an exploded star

synchronous rotation		when a satellite rotates at the same rate at which it revolves around a more massive object; a body with synchronous rotation shows only one hemisphere to the object it orbits

telescope		a tubed instrument used to brighten and magnify the view of astronomical objects (telescopes gather more light than the eye)

terminator		the boundary on a planet or moon separating the illuminated side from the unilluminated.

terrestrial		of or relating to Earth

terrestrial planet		a small, rocky planet such as Mercury, Venus, Earth, and Mars

thermal radiation		electromagnetic radiation emanating from any object not at absolute zero

tidal force		the difference in gravitational force between two points on an object caused by the gravity of another object; the tidal force often leads to a deformation of an object

tides		the distortion of a body caused by the gravitational influence of another body

Trans-Neptunion Object		an object in our solar system lying beyond the orbit of Neptune; abbreviated TNO

transit		the passage of a smaller body in front of a larger body; also, the passage of a celestial body across an observers meridian

transparency		the clarity of the sky

tremolite		a common mineral in some metamorphic rocks, composed mainly of calcium and magnesium; it occurs from the conversion of dolomite (a sedimentary rock), silica, and water

trillion		1,000,000,000,000 (in American usage)

Trojan		an asteroid that lies in or near one of the Lagrange points 60 degrees ahead or behind Jupiter along the planet's orbit

tropical year		the time it takes Earth to revolve around the sun with respect to the vernal equinox

true field of view		the angle of sky seen through an eyepiece when it is attached to a telescope; the true field equals the apparent field divided by the magnification

type Ia supernova		the explosion of a white dwarf that occurs when it accretes enough mass from a companion star to go above the Chandrasekhar limit

type II quasars		a quasar enshrouded in gas and dust that emits very little visibile light, however, is easily seen in the infrared and x-ray region of the electromagnetic spectrum

type II supernova		the explosion of a massive star that occurs when its core runs out of nuclear fuel; these explosions leave behind a neutron star or a black hole

ultraviolet light		radiation with higher energy than visible light, but without as much energy as x rays

ultraviolet radiation		radiation with higher energy than visible light, but without as much energy as x rays

umbra		the dark, central region of a shadow from which none of the light source can be seen

unidentified infrared bands (UIBs)		mysterious objects in space that give off as yet unidentified infrared emission patterns

Universal Time		the local time of day on a line of longitude centered on Greenwich, England (also known as Greenwich Mean Time); it forms the basis for all civil timekeeping

Universe		everything that exists