Glossary
Astronomy glossary
514 definitions · page 4 of 9.
- focal ratio (f ratio)
- the ratio of the focal length of a lens or mirror to its diameter
- 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 Way’s 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 galaxy’s halo
- gravitational lens
- a massive object which magnifies or distorts the light from a more distant object along the same line of sight
- gravitational lensing
- the distortion or amplification of an object’s light due to the presence of a massive object in the light path
- gravitational waves
- weak, wavelike disturbances which represent the radiation related to the gravitational force; produced when massive bodies are accelerated or otherwise disturbed
- gravity
- the attractive force that all objects exert on one another; the greater an object’s mass, the stronger its gravitational pull
- gravity waves
- weak, wavelike disturbances which represent the radiation related to the gravitational force; produced when massive bodies are accelerated or otherwise disturbed
- 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
- HII region
- an area filled with clouds of ionized hydrogen; the ionization is usually caused by radiation from newborn stars
- HST
- The Hubble Space Telescope makes its observations from above Earth's atmosphere. The telescope orbits 600 kilometers (375 miles) above Earth, working around the clock. It was originally designed in the 1970s and launched in 1990. The telescope is named for…
- Hubble law
- the principle that a distant galaxy’s recessional velocity is proportional to its distance from Earth
- Hubble Space Telescope
- The Hubble Space Telescope makes its observations from above Earth's atmosphere. The telescope orbits 600 kilometers (375 miles) above Earth, working around the clock. It was originally designed in the 1970s and launched in 1990. The telescope is named for…
- 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
- ilm that has been treated, usually with gas, to enhance its response to low light levels
- igneous rock
- rock formed by the solidification of magma
- inclination
- the angle between a planet’s orbit and the ecliptic plane; or the angle between a satellite’s orbit and its host planet’s 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 Earth’s orbit; includes Mercury and Venus
- inflation
- 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 other’s gravitational embrace, often results in galactic mergers or extreme star formation
- interference fringes
- a wave-like pattern resulting from the successful combination of two beams of light which amplifies the light
- interferometer
- a system of two or more widely separated telescopes that achieves the resolving power of a much larger telescope
- interferometric fringes
- a wave-like pattern resulting from the successful combination of two beams of light which amplifies the light
- 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