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Astrobiology Glossary H – Space

Term Definition
Habitable Zone The region around a star in which an orbiting planet could maintain conditions suitable for life.
Habitable Zone the region around a star where liquid water can exist on a planet’s surface
Habitable Zone A region around a star where planets with liquid water may be present. A planet on the near edge of the habitable zone would have a surface temperature slightly lower than the boiling point of water. A planet on the distant edge of the habitable zone would have a surface temperature slightly higher than the freezing point of water.
Hadean The period from the formation of the Earth (about 4550 million years ago) to 4000 million years ago. The first division of the Precambrian. The Hadean period is followed by the Archean.
HDF-N Hubble Deep Field North (HDF-N) is a tiny region of the northern sky near the Big Dipper toward which the Hubble Space Telescope was pointed for ten straight days in 1995. Because this observation was designed to detect very faint light from the most distant galaxies Hubble can observe, the field contains few bright celestial objects. Seemingly devoid of light, this small area provided a ‘keyhole’ view of the universe’s past, reaching across space and time to see infant galaxies. By probing these remote regions of space, astronomers are gaining more information on galaxy development.
HDF-S Hubble Deep Field South (HDF-S) is a tiny region of the southern sky near the Southern Cross toward which the Hubble Space Telescope was pointed for ten straight days in 1998. Because this observation was designed to detect very faint light from the most distant galaxies Hubble can observe, the field contains few bright celestial objects. Seemingly devoid of light, this small area provided a ‘keyhole’ view of the universe’s past, reaching across space and time to see infant galaxies. By probing these remote regions of space, astronomers are gaining more information on galaxy development.
Heavy Bombardment During the first few hundred million years of the solar system the earth and other planets were subject to an intense bombardment by the debris left over from the formation of the solar system. It is during this heavy bombardment phase that most of the craters on the moon were formed. The emergence of life on Earth appears to roughly coincide with the end of the heavy bombardment.
Heliocentric An adjective meaning ‘centered on the Sun.’
Hertzsprung-Russell Diagram A plot showing the relationship between the brightness (luminosity) and the surface temperatures of many stars. Often the spectral class, which is based on the temperature of the star, is used as a label.
High Speed Photometer (HSP) An original science instrument aboard the Hubble Space Telescope that made very rapid photometric observations of celestial objects in near-ultraviolet to visible light. The instrument was removed in December 1993 during the First Servicing Mission.
Homeostasis A state of self-regulated,internal balance in an organism.
Host Galaxy A galaxy in which a cosmic phenomenon, such as a supernova explosion or a gamma-ray burst, has occurred.
Hot Jupiter A massive planet orbiting very close to a star, such as the planet of 51 Pegasi with an orbital period of only four days. Many of the extrasolar planets so far found are of this type.
Hubble Constant (Ho) A number that expresses the rate at which the universe expands with time. Ho appears to be between 60 and 75 kilometers per second per megaparsec.
Hubble Space Telescope (HST) An orbiting telescope that collects light from celestial objects in visible, near-ultraviolet, and near-infrared wavelengths. The telescope was launched April 24, 1990 aboard the NASA Space Shuttle Discovery. The 12.5-ton (11,110-kg), tube-shaped telescope is 13.1 m (43 ft) long and 4.3 m (14 ft) wide. It orbits the Earth every 96 minutes and is mainly powered by the sunlight collected by its two solar arrays. The telescope’s primary mirror is 2.4 m (8 ft) wide. The telescope is operated jointly by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). HST is one of the many NASA Origins Missions, which include current satellites such as the Far Ultraviolet Space Explorer (FUSE) and future space observatories such as the Next Generation Space Telescope (NGST).
Hubble’s Law Mathematically expresses the idea that the recessional velocities of faraway galaxies are directly proportional to their distance from us. Hubble’s Law describes the relationship of velocity and distance by the equation V=Ho * d, where V is the object’s recessional velocity, d is the distance to the object, and Ho is the Hubble constant. Essentially, the more distant two galaxies are from each other, the faster they are traveling away from each other. American astronomer Edwin Hubble discovered this relationship in 1929 when he observed that galaxies and clusters of galaxies were generally moving away from each other.
Hyperthermophile An organism adapted to life at very high temperatures. Hyperthermophiles have optimum growth temperatures above 80 degrees C, and a number can grow at temperatures above 100 degrees C.