Astrobiology Glossary G – Space

Term Definition
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.
Galaxy 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.
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 Rays Light with the shortest wavelengths and the highest energies and frequencies in the electromagnetic spectrum; also called gamma radiation.
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.
Ganymede 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 massive planet such as Jupiter or Saturn composed mainly of gases with no solid surface.
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.
Genetic Code The set of rules by which three letter words in a DNA or RNA sequence describe an amino acid to be incorporated into a protein.
Genome The complete set of genetic information for a particular organism.
Geocentric 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.
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.
Gradualism The theory that evolution occurs gradually and at a steady pace, as opposed to occurring in leaps and bounds.
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 Lensing The effect on the appearance of an astronomical source due to the bending of light by the gravitational field of an intervening object.
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 (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.
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).
GRB990123 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.
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.
Gyroscope A spinning wheel mounted on a non-stationary frame that stabilizes and points a space-based observatory. This spinning wheel resists applied external forces and tends to retain its original orientation in space. For example, balancing on a moving bicycle is easier than balancing on a stationary one because of this tendency. Gyroscopes are used in navigational instruments for aircraft, satellites, and ships. The Hubble Space Telescope has six gyroscopes on board for navigation and sighting purposes.

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