Glossary items by letter: e — g


The third planet from the Sun and one of four terrestrial planets in the inner solar system. Earth, the only planet where water exists in large quantities, has an atmosphere capable of supporting myriad life forms. The planet is 150 million kilometers (93 million miles) away from the Sun. Earth has one satellite — the Moon.


Traveling around Earth, in the path followed by an object moving in the gravitational field of Earth. For example, the telescope travels around, or orbits, Earth because Earth’s gravitational field keeps the telescope in its path, or orbit.

Electromagnetic Force

A fundamental force that governs all interactions among electrical charges and magnetism. Essentially, all charged particles attract oppositely charged particles and repel identically charged particles. Similarly, opposite poles of magnets attract and like magnetic poles repel.

Electromagnetic Radiation

A form of energy that propagates through space as vibrations of electric and magnetic fields; also called radiation or light. All electromagnetic radiation is a form of light.

Electromagnetic Spectrum

The entire range of wavelengths of electromagnetic radiation, including radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays, and gamma rays.


The science dealing with the physical relationship between electricity and magnetism. The principle of an electromagnet, a magnet generated by electrical current flow, is based on this phenomenon.


A negatively charge elementary particle that typically resides outside the nucleus of an atom but is bound to it by electromagnetic forces. An electron’s mass is tiny: 1,836 electrons equals the mass of one proton.

Electron Volt (eV)

A unit of energy that is equal to the energy that an electron gains as it moves through a potential difference of one volt. This very small amount of energy is equal to 1.602 * 10–19 joules. Because an electron volt is so small, engineers and scientists sometimes use the terms MeV (mega-million) and GeV (giga-billion) electron volts.


A substance composed of a particular kind of atom. All atoms with the same number of protons (atomic numbers) in the nucleus are examples of the same element and have identical chemical properties. For example, gold (with 79 protons) and iron (with 26 protons) are both elements, but table salt is not because it is made from two different elements: sodium and chlorine. The atoms of a particular element have the same number of protons in the nucleus and exhibit a unique set of chemical properties. There are about 90 naturally occurring elements on Earth.

Elementary Particles

Particles smaller than atoms that are the basic building blocks of the universe. The most prominent examples are photons, electrons, and quarks.


A special kind of elongated circle. The orbits of the solar system planets form ellipses.


A special kind of elongated circle. The orbits of the solar system planets are elliptical.

Elliptical Galaxy

A galaxy that appears spherical or football-shaped. Elliptical galaxies are comprised mostly of old stars and contain very little dust and “cool” gas that can form stars.

Emission Line

A bright line in a spectrum caused by emission of light. Each chemical element emits and absorbs radiated energy at specific wavelengths. The collection of emission lines in a spectrum corresponds to the chemical elements contained in a celestial object.


Natural processes that wear or grind away the surface of an object. On Earth, the major agents of erosion are water and wind.

Escape Velocity

The minimum velocity required for an object to escape the gravity of a massive object.

European Space Agency (ESA)

A fifteen-member consortium of European countries for the design, development, and deployment of satellites. The Space Telescope — European Coordinating Facility (ST-ECF) supports the European astronomical community in exploiting the research opportunities provided by the Earth-orbiting Hubble Space Telescope. The ESA members are Austria, Belgium, Denmark, Finland, France, Germany, Ireland, Italy, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland, and the United Kingdom, with Canada as a cooperating state.

Event Horizon

The spherical outer boundary of a black hole. Once matter crosses this threshold, the speed required for it to escape the black hole’s gravitational grip is greater than the speed of light.

Excited State

A greater-than-minimum energy state of any atom that is achieved when at least one of its electrons resides at a greater-than-normal distance from its parent nucleus.


The process of allowing electromagnetic radiation to fall on light-sensitive materials such as photographic films or plates. An exposure is also the image created by the process. A long exposure time is needed in order to obtain an image of dim and distant celestial objects.


An adjective that means “beyond the Earth.” The phrase “extraterrestrial life” refers to possible life on other planets.


The lens or lens group closest to the eye in an optical instrument such as a telescope or microscope.

Fahrenheit Temperature Scale

A temperature scale on which the freezing point of water is 32° F and the boiling point is 212° F.

Faint Object Camera (FOC)

An instrument aboard the Hubble Space Telescope that recorded high-resolution images of faint celestial objects in deep space. Built by the European Space Agency, the camera collected ultraviolet and visible light from celestial objects. The camera served as Hubble’s “telephoto lens” — recording the most detailed images over a small field of view. The FOC’s resolution allowed Hubble to single out individual stars in distant star clusters. The instrument was replaced in March 2002 during Servicing Mission 3B.

Faint Object Spectrograph (FOS)

An instrument aboard the Hubble Space Telescope that acted like a prism to separate light from the cosmos into its component colors, providing a wavelength “fingerprint” of the object being observed. Such information yields clues about an object’s temperature, chemical composition, density, and motion. Spectrographic observations also reveal changes in celestial objects as the universe evolves. The instrument was replaced in February 1997 during the Second Servicing Mission.

Far-Infrared Spectrum

The region of the infrared spectrum that exhibits the longest wavelengths and the lowest frequencies and energies.


A geological term that refers to a fracture or a break in a hard surface like the Earth’s crust. This area is a zone of weakness and may be the site of earthquakes or volcanoes. All planets or moons with a hard crust are candidates for faults or breaks on their surfaces.

Field of View (FOV)

A telescope’s viewing area, measured in degrees, arc minutes, or arc seconds. A telescope that can just fit the full moon into its complete viewing area has a field of view of roughly 30 arc minutes.

Field of view

The field of view is the area of the sky visible through a telescope.


A type of window that absorbs certain colors of light while allowing others to pass through. Astronomers use filters to observe how celestial objects appear in certain colors of light or to reduce the light of exceptionally bright objects. For example, a pair of sunglasses acts as a type of filter, reducing the amount of incoming light while still allowing some light to pass through to the eyes.

Filter Wheels

Rotating wheels in a telescope instrument that allow specific colors of light from a celestial object to pass through and form an image on the detector. The Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope has 12 filter wheels, each of which holds four filters.

Fine Guidance Sensors (FGS)

Targeting devices aboard the Hubble Space Telescope that lock onto “guide stars” and measure their positions relative to the object being viewed. Adjustments based on these precise readings keep Hubble pointed in the right direction. The sensors also are used to perform celestial measurements.

Fine Guidance Sensors (FGS)

The Fine Guidance Sensors are cameras that help keep the Hubble Space Telescope pointed precisely in the right direction.


A nuclear process that releases energy when heavyweight atomic nuclei break down into lighter nuclei. Fission is the basis of the atomic bomb.

Fixed Head Star Trackers (FHST)

Small telescopes with wide fields of view that are aboard the Hubble Space Telescope and used in conjunction with the Fine Guidance Sensors. The star trackers locate the bright stars that are used to orient the telescope for scientific observations.


A sudden and violent outburst of solar energy that is often observed in the vicinity of a sunspot or solar prominence; also known as a solar flare.

Flat Universe

A geometric model of the universe in which the laws of geometry are like those that would apply on a flat surface such as a table top.

Flint glass

The lead glass that was produced in the United States and the United Kingdom prior to the 1860s. This glass is used to make telescope lenses and prisms. Flint glass bends and disperses, or spreads out, light more than crown glass.


The flow of fluid, particles, or energy through a given area within a certain time. In astronomy, this term is often used to describe the rate at which light flows. For example, the amount of light (photons) striking a single square centimeter of a detector in one second is its flux.

Flyby Spacecraft

A spacecraft that travels past a celestial object. Frequently, such a spacecraft is unmanned and takes images of the object.

Focal length

Focal length (shown in red) is the distance between the center of a convex lens or a concave mirror and the focal point of the lens or mirror — the point where parallel rays of light meet, or converge.

Focal point

The focal point of a lens or mirror is the point in space where parallel light rays meet after passing through the lens or bouncing off the mirror. A “perfect” lens or mirror would send all light rays through one focal point, which would result in the clearest image.


Describes the number of wave crests passing by a fixed point in a given time period (usually one second). Frequency is measured in Hertz (Hz).


A nuclear process that releases energy when light atomic nuclei combine to form heavier nuclei. Fusion is the energy source for stars like our Sun.


One of the most energetic gamma-ray bursts (GRBs) ever detected, occurring at 4:47 a.m. EST, January 23, 1999. The “burst” equaled the power of nearly 10 million billion suns. It became the first GRB to be viewed simultaneously in both gamma-ray and optical wavelengths.

Galactic Center

The central hub or nucleus of a galaxy. The Milky Way’s galactic center is about 28,000 light-years from Earth.

Galactic Disk

A flattened disk of gas and young stars in a galaxy. Some galactic disks have material concentrated in spiral arms (as in a spiral galaxy) or bars (as in barred spirals).

Galactic Halo

Spherical regions around spiral galaxies that contain dim stars and globular clusters. The radius of the halo surrounding the Milky Way extends some 50,000 light-years from the galactic center.

Galactic Nucleus

The central concentration of matter (stars, gas, dust, and perhaps a black hole) in a galaxy, typically spanning no more than a few light-years in diameter.

Galactic Plane

The imaginary projection of the Milky Way’s disk on the sky. Most of the galaxy’s stars and interstellar matter reside in this disk. Objects in the galaxy are often referred to as being above, below, or in the galactic plane.


A collection of stars, gas, and dust bound together by gravity. The smallest galaxies may contain only a few hundred thousand stars, while the largest galaxies have thousands of billions of stars. The Milky Way galaxy contains our solar system. Galaxies are classified or grouped by their shape. Round or oval galaxies are elliptical galaxies and those showing a pinwheel structure are spiral galaxies. All others are called irregular because they do not resemble elliptical or spiral galaxies.

Galaxy Cluster

A collection of dozens to thousands of galaxies bound together by gravity.

Galaxy Evolution

The study of the birth of galaxies and how they change and develop over time.

Galaxy Supercluster

A vast collection of galaxy clusters that may contain tens of thousands of galaxies spanning over a hundred million light-years of space. Galaxy superclusters are the largest structures in the universe.

Gamma-Ray Burst (GRB)

A brief, intense, and powerful burst of gamma rays, the highest-energy, shortest-wavelength radiation in the electromagnetic spectrum. These bursts emanate from distant sources outside our galaxy and last only a few seconds. They are the brightest and most energetic explosions known.

Gamma Rays

The part of the electromagnetic spectrum with the highest energy; also called gamma radiation. Gamma rays can cause serious damage when absorbed by living cells.


One of Jupiter’s largest moons. Ganymede, the largest satellite in our solar system, is about 5300 kilometers (3300 miles) wide and larger than the planet Mercury.

Gas Giant

A large planet with a small, rocky core and a deep atmosphere composed mostly of hydrogen and helium. Our solar system contains four gas giants: Jupiter, Saturn, Uranus, and Neptune. This group is also known as Jovian planets.

Gaseous Nebula

A glowing cloud of gas in interstellar space. The cloud of gas may be either an emission nebula, which absorbs ultraviolet light from nearby stars and re-radiates visible light, or a reflection nebula, which reflects light off of its dust particles.

General Theory of Relativity

A theory Einstein developed to explain how gravity influences space and time.


An adjective meaning “centered on the Earth.” Most early civilizations had a geocentric view of the universe.

Geosynchronous Orbit

Also known as geostationary. An orbit in which an object circles the Earth once every 24 hours, moving at the same speed and direction as the planet’s rotation. The object remains nearly stationary above a particular point, as observed from Earth. The International Ultraviolet Explorer (IUE) and some weather satellites are examples of satellites in geosynchronous orbit.

Giant Star

A dying star that has used up the hydrogen fuel in its core and has begun to expand. Giant stars are generally larger than our Sun.


A measure of computer data storage capacity equal to approximately a billion bytes. In computer language, a byte of information represents a letter or digit. So, a billion bytes is equal to a billion letters.

Globular Cluster

A collection of hundreds of thousands of old stars held together by gravity. Globular clusters are usually spherically shaped and are often found in the halos of galaxies. Each star belonging to a cluster revolves around the cluster’s common center of mass.

Goddard High Resolution Spectrograph (GHRS)

A science instrument aboard the Hubble Space Telescope that made finely detailed spectroscopic observations of ultraviolet sources. The GHRS was removed from Hubble in February 1997 and replaced with the Space Telescope Imaging Spectrograph.

Goddard Space Flight Center (GSFC)

NASA’s flight control center in Greenbelt, Maryland, which receives data from orbiting observatories such as the Hubble Space Telescope (HST). HST digital data are then relayed to the Space Telescope Science Institute in Baltimore, Maryland, where they are interpreted into pictures. Goddard also conducts scientific investigations, develops and operates space systems, and works toward the advancement of space science technologies.

Grand Unified Theory (GUT)

A theory stating that that strong and weak nuclear forces and electromagnetic forces are varying aspects of the same fundamental force.

Gravitational Clustering

The process by which a large-scale structure grows as its gravity attracts smaller building blocks. Astronomers believe that all the large-scale structures (such as galaxies, galaxy clusters, and galaxy superclusters) that we see in the universe today formed through gravitational clustering.

Gravitational Constant (G)

A value used in the calculation of the gravitational force between objects. In the equation describing the force of gravity, “G” represents the gravitational constant and is equal to 6.672 * 10–11 Nm2/kg2.

Gravitational Instability

A condition that occurs when an object’s inward-pulling gravitational forces exceed the outward-pushing pressure forces, thus causing the object to collapse on itself. For example, when the pressure forces within an interstellar gas cloud cannot resist the gravitational forces that act to compress the cloud, then the cloud collapses upon itself to form a star.

Gravitational Lens

A massive object that magnifies or distorts the light of objects lying behind it. For example, the powerful gravitational field of a massive cluster of galaxies can bend the light rays from more distant galaxies, just as a camera lens bends light to form a picture.

Gravitational Redshift

The reddening of light from a very massive object caused by photons escaping and traveling away from the object’s strong gravitational field. An example of gravitational redshift is light escaping from the surface of a neutron star.

Gravity Assist

An effect through which an orbiting object, such as a spacecraft or a comet, gains or loses speed by virtue of the gravitational might of a planet or other celestial object that it passes. For example, the Cassini spacecraft in its journey to Saturn used a gravity assist from Earth to increase its velocity by about 36,000 kilometers per hour (22,300 miles per hour).

Gravity (Gravitational Force)

The attractive force between all masses in the universe. All objects that have mass possess a gravitational force that attracts all other masses. The more massive the object, the stronger the gravitational force. The closer objects are to each other, the stronger the gravitational attraction.

Great Red Spot

A circulating storm located in Jupiter’s upper atmosphere. The storm, which rotates around the planet in six days, is the width of two to three Earths. Galileo first observed the spot in the 17th century.

Greenhouse Effect

The result of a planet’s atmosphere trapping infrared heat, rather than allowing it to escape into space. This effect increases the planet’s surface temperature, a phenomenon known as global warming.

Ground State

The minimum energy state of an atom that is achieved when all of its electrons have the lowest possible energy and therefore are as close to the nucleus as possible.

Group of Galaxies

A small collection of galaxies bound together by gravity. The number of galaxies in a group can range from a few to dozens. The Milky Way is a member of the Local Group, a collection of more than 30 galaxies.

Guide Star

A star that a telescope’s guidance system locks onto to ensure that a celestial object is followed and observed as the telescope moves, owing either to the Earth’s rotation or the telescope’s orbital trajectory. The Hubble Space Telescope uses two of its three Fine Guidance Sensors to detect and lock onto guide stars. The telescope’s science operations center has more than 15 million guide stars in its database — the Guide Star Catalogue.


A gyroscope is a spinning wheel mounted on a movable frame that assists in stabilizing and pointing a space-based observatory. Gyroscopes are important because they measure the rate of motion as the observatory moves and help ensure the telescope retains correct pointing during observations. The gyroscopes provide the general pointing of the telescope while the fine guidance sensors provide the “fine tuning.” Gyroscopes are used in navigational instruments for aircraft, satellites, and ships. The Hubble Space Telescope has six gyroscopes for navigation and sighting purposes.

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