accretion disk: A superheated disk of dust and gas spiraling into a high-gravity object such as a star, white dwarf, or black hole. The intense energies of the disk cause it to emit electromagnetic radiation.
active galaxy: A galaxy that is emitting intense electromagnetic radiation out of its nucleus, generally due to matter falling into a supermassive black hole.
alt-azimuth mount: A type of telescope mount in which the axes are aligned to the horizon and the zenith. The altitude axis points the telescope up and down, to any point above the horizon. The azimuth axis rotates the telescope to any compass angle. Alt-azimuth mounts must move on both axes to compensate for the Earth’s rotation.
annular eclipse: An eclipse of the sun in which the moon fits inside the disk of the sun, leaving a bright, ring of sunlight.
aperture: The diameter of a telescope’s primary mirror or lens. Larger diameters are able to gather more light and have better resolving power.
arcminute: A measure of angular distance between two points in the sky. There are 180 degrees of sky from the eastern horizon, through the zenith, to the western horizon. There are 60 arcminutes in a degree. The full moon is approximately 30 arcminutes in diameter.
arcsecond: A measure of angular distance equal to 1/60 of an arcminute.
averted vision: An observing technique in which you look at a faint object out of the corner of your eye, instead of directly at it. Peripheral vision is more sensitive to light than direct vision.
Big Bang: The rapid expansion of space at the beginning of the universe, which explains why most galaxies seem to be moving away from each other.
Caldwell Catalog: A catalog of beautiful, non-Messier, deep-sky objects for amateur astronomers to observe. Compiled by Sir Patrick Caldwell Moore in 1995.
Cepheid variable: A type of variable star used to determine true distances to galaxies and other objects.
circumpolar: An object that never sets because it is close to a celestial pole (as seen from the observer). The Little Dipper, for example, is circumpolar.
comet: A small, icy body in the solar system that heats up and emits gases when its orbit brings it close to the sun.
conjunction: An event in which two or more objects appear close to each other in the sky; for example, a conjunction between Jupiter and Mars.
constellation: A region of the celestial sphere, defined by the International Astronomical Union, and based on traditional patterns of stars.
cosmos: All that is or ever was or ever will be. A synonym for the universe.
degree: A measure of angular distance in the sky. There are 180 degrees between the eastern horizon and the western horizon. There are 60 arcminutes in a degree and 60 arcseconds in an arcminute.
eclipse: An event in which one object passes in front of the sun, casting a shadow on another object. When the moon passes in front of the sun, it is a solar eclipse. When the Earth passes in front of the sun, as seen from the moon, it is a lunar eclipse.
ecliptic: The apparent path of the sun on the celestial sphere, due to the plane of Earth’s orbit. All planets orbit around the sun close to the same plane, and thus the planets all appear near the ecliptic.
electromagnetic radiation: A type of energy, consisting of photons, and emitted by various processes at various wavelengths. Almost everything we know about objects in space comes from detecting and focusing electromagnetic radiation in all its forms: radio waves, microwaves, infrared light, visible light, ultraviolet light, x-rays, and gamma rays.
equatorial mount: A type of telescope mount that is aligned with the Earth’s axis of rotation. Equatorial mounts can compensate for the Earth’s rotation by counter-rotating on a single axis, making them ideal for astrophotography.
eyepiece: A removable lens that you look through on a telescope. Different eyepieces provide different magnification levels.
filter: An optical component, added to a telescope, to exclude certain frequencies of light. Some filters can exclude light pollution (to enhance the view of nebulae); others can exclude all but certain colors, to enhance the view of planets.
focal length: The length of a telescope’s focal point. Longer focal lengths have greater magnification.
focal ratio: The ratio of a telescope’s focal length to its aperture. For example, an f/4 telescope has a focal length four times longer than its aperture. Shorter focal ratios make extended objects (like galaxies and nebulae) appear brighter.
galaxy: An enormous assemblage of gravitationally bound stars, gas, and dust. The Milky Way is a galaxy. There are billions of galaxies in the universe, each containing from billions to trillions of stars.
globular cluster: A spherical cluster of stars, generally found away from the galactic disk.
ionization: A process in which an atom gains or loses an electron, giving it a negative or positive charge. Nebulae, which are often composed of ionized gas, emit visible light.
Kuiper Belt: A disk of asteroids and comets orbiting beyond Neptune. Pluto is the largest member of the Kuiper Belt.
light pollution: The brightening of the night sky caused by artificial lights. Light pollution decreases the contrast of faint objects like galaxies and nebulae, making it harder to observe them. Light pollution can sometimes be mitigated by filters, but the best solution is to observe in National Parks and other areas far from city lights.
magnification: The scaling up of a visual image in a telescope. Higher magnification can reveal more detail in small objects (such as planetary nebulae), but it also makes objects dimmer, which is undesirable for faint objects such as galaxies. The amount of magnification depends on the telescope’s focal length and the eyepiece being used.
magnitude: A measure of the brightness of a star or other object. The star Vega is magnitude 0. Objects fainter than Vega have higher magnitude numbers; objects brighter than Vega have negative magnitude numbers. In dark skies, the naked eye can see stars down to magnitude 6.
mare: A large, dark plain on Earth’s moon. Early astronomers thought they might be seas, and named them maria, which is Latin for sea.
Messier Catalog: A catalog of beautiful deep-sky objects compiled by comet-hunter Charles Messier as a means to avoid objects that could be confused for comets.
NGC: New General Catalog. A comprehensive catalog of deep-sky objects compiled by John Dreyer in 1888. Most of the brightest deep-sky objects have an NGC designation.
nebula: A vast cloud of interstellar gas and dust. Diffuse nebulae are extended and have ill-defined boundaries; some diffuse nebulae give birth to entire clusters of stars. Planetary nebulae are the remains of a red giant star after it has collapsed into a white dwarf.
occultation: An event in which an object passes in front of another, temporarily obscuring it. For example, the moon can occlude Mars when it passes in front of it (as seen from Earth).
open cluster: A loose association of stars, generally born in the same nebula, and still moving together in the galaxy. Open clusters are only loosely bound by gravity, and eventually disperse.
opposition: The point at which a planet is opposite the sun as seen from Earth. This is one of the best times to see the planet, since it is fully illuminated, and closer to Earth than at other points in its orbit. Inner planets (Mercury and Venus) are never at opposition.
parallax: The difference in the apparent position of an object when seen from two slightly different points, which can be used to determine true distance. The nearest stars exhibit parallax when seen from opposite ends of Earth’s orbit (e.g., July vs. December), which can be used to estimate the star’s distance.
pareidolia: A phenomenon in which the brain sees a familiar object from a random arrangement of shapes. For example, seeing objects in an ink blot or faces on a rock cliff.
penumbra: In an eclipse, the region in which an object is only partially in shadow. See also, umbra.
photon: A fundamental particle which makes up all electromagnetic radiation, including visible light.
prominence: A solar phenomenon in which bright gaseous features extend outward from the sun’s surface. Prominences may be visible during a solar eclipse or with suitable filters.
quasar: A quasi-stellar radio source; the most distant and energetic kind of active galaxy.
reflecting telescope: A kind of telescope using a curved mirror as the primary means to collect and focus light.
refracting telescope: A kind of telescope using a lens as the primary means to collect and focus light.
retrograde: The apparent backwards motion of certain planets as seen from Earth. Mars, for example, exhibits retrograde motion when Earth passes it in its orbit.
rich-field telescope: A kind of telescope with short focal ratios, general f/5 or better. Rich-field telescopes are ideal for observing nebulae and galaxies.
spectrum: The range of light at various wavelengths. By measuring the intensity of an object’s light at various wavelengths, we can learn about its physical composition. For example, nebulae emit light at the same wavelength as ionized hydrogen, suggesting that they are mostly composed of hydrogen.
sunspot: A cool spot on the sun’s surface caused by concentrations of magnetic field lines.
supernova: A cataclysmic release of energy when a star runs out of fuel and collapses due to its own gravity.
supernova remnant: The scattered remains of a supernova explosion, generally consisting of dust and gas, expanding away from the star.
terminator: The line on the moon’s surface dividing the sunlit side from the shadowed side. Lunar features such as craters and mountains are easily visible at the terminator because their shadows are long.
turbulence: The random motion of the atmosphere causing stars to twinkle. Turbulence limits the amount of telescopic magnification that can be used.
umbra: In an eclipse, the region in which an object is completely in shadow. See also, penumbra.
wide-field telescope: See rich-field telescope.
zenith: The point in the sky directly overhead from the observer’s point of view.