Glossary – G


G2 Star
our Sun is a type G main sequence star, but more specifically, it is a type G2V. The “G” means that it’s a yellow star. The “2″ means that it is about 2/10ths of the way between a yellow “G” and an orange “K.” These arbitrary letter designations are historical in origin and are based on atomic absorption lines in the star’s spectrum. The “V” is a Roman numeral 5. It means the star is a main sequence star, often known as a dwarf star.
a collection of gas, dust, and billions of stars held together by its gravity.
Galactic Coordinate System
A coordinate system used for measuring the angular positions of celestial objects. The system’s angular coordinates are galactic latitude and galactic longitude.
Galactic Latitude (b)
Used in the galactic coordinate system to measure the angular position of a celestial object above or below the galactic plane of the Milky Way. Galactic latitude is typically measured in degrees, with positive angles to the North and negative angles to the South.
Galactic Longitude (l)
Used in the galactic coordinate system to measure the angular position of a celestial object around the Milky Way’s galactic plane. Galactic longitude is typically measured in degrees, with positive angles Eastward about the plane.
Gamma-ray Burst
A huge explosion in space that emits electromagnetic radiation primarily in gamma-rays.
Galaxy Cluster
thousands of nearby galaxies that are gravitationally bound together into a large group. Clusters contain at least as much mass in hot x-ray emitting gas as they do in galaxies. However, they are dominated by dark matter, which comprises 80% to 90% of their total mass.
Gamma Ray
the very highest energy end of the electromagnetic spectrum, with the shortest wavelengths. Gamma-rays typically have wavelengths a few hundred times shorter than low-energy x-rays, and are usually shorter than a few hundred picometers (pm, `10^-12m`).
Gamma-Ray Burst
a huge explosion in space marking the birth of a black hole.
General Theory of Relativity
commonly called General Relativity, this theory postulates that mass and energy curve and distort spacetime. The severity of the curvature and distortion caused by the mass/energy is the strength of the gravitational attraction. General Relativity expands upon Einstein’s earlier Special Theory of Relativity.
Generation Ship
a theoretical space craft that could be constructed with the intent of sustaining several generations of people through the duration of its voyage.
the Gamma-ray Large Area Space Telescope (GLAST) is an Earth-orbiting satellite launched on June 11, 2008 to explore the gamma- ray sky. Its called Gamma-Ray because it measures the highest energy light, known as gamma rays; Large Area because the telescope can see almost one-fifth of the entire sky at once; and space telescope because, well, it’s in space! Update: GLAST was renamed to Fermi Gamma-ray Space Telescope 60 days after it launched.
the subatomic particle which carries the strong nuclear force. “Gluon” is a fitting name for the particle that glues atomic nuclei together, overcoming the repulsion from the positively charged protons in nuclei.
the Global Positioning System consists of a network of Earth-orbiting satellites that are used to triangulate the position of a receiver (generally a radio or microwave receiver). The receiver uses signals emitted by the satellites to compute its position on Earth, typically to accuracies of meters or even less.
Gravitational Lensing
the gravitational bending of light due to the gravity of a foreground object.
Gravitational Wave
objects that undergo a change of mass, and/or have asymmetrical rotations are hypothesized to affect spacetime in such a way as to generate ripples in spacetime. The ripples, or gravitational waves, are produced when the objects change their physical configuration, i.e. their size, shape or extent.
the force exerted by one massive body on another. The force of gravity from an object depends on its mass and inversely on how far you are from it.
Grays (unit)
a standard international derived unit of ionizing radiation dose; the absorption of one joule of radiation energy by one kilogram of matter.
Great Red Spot
is a storm that has existed on Jupiter for at least 300 years. in Jupiter’s southern hemisphere with crimson-colored clouds that spin counterclockwise at wind speeds that exceed those in any storm on Earth. The Great Red Spot has slowly changed over the years, and is currently about 1.3 times as wide as our planet. It is not known whether it will ever disappear.
Greenhouse Effect
the process by which a planet’s atmosphere prevents radiant energy from its surface from escaping into space, thereby increasing the surface temperature of the planet. The effect is caused by the trapping of infrared radiation by gases such as carbon dioxide, methane and water vapor in the planet’s atmosphere. On Earth, sunlight warms the planet surface, and the surface then re-emits the solar energy in the form of infrared light. Greenhouse gases prevent a portion of the infrared from escaping to space.