Astrobiology Glossary S – Space

Term Definition
Saturn The sixth planet in the solar system, noted for its obvious ring structure. Saturn is almost ten times the Earth’s distance from the Sun. The planet completes a circuit around the Sun in about 30 Earth years. Saturn is the second largest and the least dense planet in our solar system. The planet has more than 21 moons, including Titan, the second largest known moon in our solar system.
Schwarzchild Radius The distance from the ‘center’ of a black hole to its ‘edge’ (called an event horizon). If the Earth became a black hole, all of its mass would be squeezed into a sphere with a Schwarzschild radius of 0.03 cm, about the size of a bacterium.
Secondary Atmosphere A gas or gases, such as helium, that a planet discharges from its interior after having lost its primary or primordial atmosphere.
Secondary Mirror A small mirror in a reflecting telescope that redirects light from the larger primary mirror toward the light-sensitive scientific instruments. In a Cassegrain-type telescope like the Hubble Space Telescope, the secondary mirror is slightly convex and directs light from the primary mirror back through a hole in the center of the primary mirror. Hubble’s secondary mirror measures 12.2 inches (0.3 meters) in diameter.
Seismic Wave The transfer of energy throughout a celestial object, such as a planet, resulting from an external impact or an internal event. On Earth, seismic waves are generated primarily by earthquakes.
SETI The Search for Extraterrestrial Intelligence.SETI projects aim to detect radio or other signals from advanced civilizations elsewhere in the galaxy.
Seyfert Galaxy A galaxy characterized by a moderately bright, compact active galactic nucleus, presumably powered by a black hole.
Shock Wave A high-pressure wave that travels at supersonic speeds. Shock waves are usually produced by an explosion.
Short-Period Comet Comets that orbit mainly in the inner solar system. Usually these objects orbit the Sun in less than 200 years. Halley’s comet is an example of a short-period comet.
Singularity A black hole’s center, where the matter is thought to be infinitely dense, the volume is infinitely small, and the force of gravity is infinitely large.
SNC meteorites Another term for martian meteorites. The name derives from the initials of three examples, Shergotty, Nakhla and Chassigny.
Snowball Earth Name given to events in which the Earth was glaciated and the oceans frozen from pole to pole. Snowball Earth events have been suggested as occuring at about 2.3 billion years ago and between 750 and 580 million years ago.
Solar Arrays Two rigid, wing-like arrays of solar panels that convert sunlight directly into electricity to operate the Hubble Space Telescope’s scientific instruments, computers, and radio transmitters. Some of the energy generated is stored in onboard batteries so the telescope can operate while in Earth’s shadow (which is about 36 minutes out of each 97-minute orbit). The solar arrays are designed for replacement by visiting astronauts during servicing missions.
Solar Constant The average amount of solar radiation reaching a planet; usually expressed in watts (energy per unit time) per square meter. For Earth, the solar constant equals 1,372 W/m2. Each planet has a unique solar constant depending on its distance from the Sun.
Solar Cycle The periodic changing of the Sun’s magnetic field, which determines the number of sunspots and the amount of particles emitted in the solar wind. The period of the cycle is about 11 years.
Solar Eclipse A phenomenon in which the Moon’s disk passes in front of the Sun, blocking sunlight. A total eclipse occurs when the Moon completely obscures the Sun’s disk, leaving only the solar corona visible. A solar eclipse can only occur during a new phase of the Moon.
Solar Maximum The midpoint in the solar cycle where the amount of sunspot activity and the output of cosmic particles and solar radiation is highest.
Solar Minimum The beginning and the end of a sunspot cycle when only a few sunspots are usually observed, and the output of particles and radiation is normal.
Solar System The Sun and its surrounding matter, including asteroids, comets, planets and moons, held together by the Sun’s gravitational influence.
Solar Wind Streams of charged particles flowing from the Sun at millions of kilometers per hour. This high-speed solar wind varies in composition, always streams away from the Sun, and interacts with other regions of matter in the solar system.
South Celestial Pole (SCP) A direction determined by the projection of the Earth’s South Pole onto the celestial sphere. The SCP is exactly 180 degrees from the North Celestial Pole and corresponds to a declination of -90 degrees.
Space Infrared Telescope Facility (SIRTF) A space-borne infrared telescope that will study planets, comets, stars, galaxies, and other celestial objects. NASA plans to launch SIRTF in December 2002 on a Delta rocket. SIRTF represents the fourth and final satellite in NASA’s Great Observatories program, which includes the Hubble Space Telescope and the Chandra X-Ray Observatory.
Space Shuttle A reusable U.S. spacecraft operated by astronauts and used to transport cargo, such as satellites, into space. The spacecraft uses rockets to launch into space, but it lands like an airplane. A space shuttle carried the Hubble Space Telescope into space in 1990. Astronauts aboard subsequent space shuttles have visited the telescope to service it.
Space Telescope Imaging Spectrograph (STIS) An instrument that acts like a prism to separate light from the cosmos into its component colors, providing a wavelength ‘fingerprint’ of the object being observed. The 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. Astronauts installed STIS aboard the Hubble Space Telescope in February 1997 during the Second Servicing Mission. STIS spans ultraviolet, visible, and near-infrared wavelengths. The spectrograph can sample 500 points along a celestial object simultaneously.
Space Telescope Science Institute (STScI) The astronomical research center responsible for operating the Hubble Space Telescope as an international scientific observatory. Located in Baltimore, Maryland, STScI is managed by AURA (Association of Universities for Research in Astronomy) under contract to the National Aeronautics and Space Administration (NASA) and the National Science Foundation (NSF).
Spacetime The four-dimensional coordinate system (three dimensions of space and one of time) in which physical events are located.
Spectral Class (Spectral Type) A classification scheme that groups stars according to their surface temperatures and spectral features.
Spectral Line In a spectrum, an emission (bright) or absorption (dark) at a specific frequency or wavelength.
Spectrograph (Spectrometer) An instrument that spreads electromagnetic radiation into its component frequencies and wavelengths for detailed study. A spectrograph is similar to a prism, which spreads white light into a continuous rainbow.
Spectroscopy The study and interpretation of a celestial object’s electromagnetic spectrum. A spectrograph or spectrometer is used to analyze an object’s electromagnetic spectrum.
Speed Of Light (c) The speed at which light (photons) travels through empty space is roughly 3 * 108 meters per second or 300 million meters per second.
Spiral Arms A pinwheel structure, composed of dust, gas, and young stars, that winds its way out from the core of a normal spiral galaxy and from the ends of the bar in a barred spiral galaxy.
Spiral Galaxy A spiral-shaped system of stars, dust, and gas clouds. A typical spiral galaxy has a spherical central bulge of older stars surrounded by a flattened galactic disk that contains a spiral pattern of young, hot stars, as well as interstellar matter.
Sprites Gamma-ray flashes produced in Earth’s atmosphere by severe lightning storms and upper atmospheric events.
Standard Candle An object whose properties allow us to measure large distances through space. The absolute brightness of a standard candle can be determined without a measurement of its apparent brightness. Comparing the absolute brightness of a standard candle to its apparent brightness therefore allows us to measure its distance. For example, the distinct variations of Cepheid variable stars in other galaxies tell us their absolute brightness. By accurately measuring the apparent brightness of these stars, astronomers can precisely determine the distance to the galaxy in which they reside.
Star A gaseous, self-luminous object held together by its own gravity. The core of a star is extremely hot and releases energy by fusing lighter atomic nuclei into heavier nuclei. Our Sun, the center of our solar system, is a star of average temperature and size.
Star Cluster A group of stars born at almost the same time and place, capable of remaining together for billions of years because of their mutual gravitational attraction.
Starburst Galaxy A galaxy undergoing an extremely high rate of star formation. Starburst galaxies contain massive, deeply embedded stars that are among the youngest stars observed.
Stellar Black Hole A black hole formed from the death of a massive star during a supernova explosion. A stellar black hole, much like a supermassive black hole, feeds off of nearby material – in this case, the dead star. As it gains mass, its gravitational field increases.
Stellar Evolution The process of change that occurs during a star’s lifetime from its birth to its death.
Stellar Parallax The apparent change in the position of a nearby star when observed from Earth due to our planet’s yearly orbit around the Sun. This method allows astronomers to calculate distances to stars that are less than 100 parsecs from Earth.
Stromatolite Layered structures built by colonies of microorganisms which are commonly found in the Archean and Proterozoic fossil records. Modern examples can be found in sites such as Shark Bay in Western Australia.
Strong Force The force that binds protons and neutrons within atomic nuclei and is effective only at distances less than 10-13 centimeters.
Sun The star at the center of our solar system. An average star in terms of size and mass, the Sun is a yellow dwarf of spectral type G2. It is about 5 billion years old, contains 2 * 1030 kilograms of material, and has a diameter more than 100 times that of Earth.
Sunspot A region on the Sun’s photosphere that is cooler and darker than the surrounding material. Sunspots often appear in pairs or groups with specific magnetic polarities that indicate electromagnetic origins.
Sunspot Cycle The change in strength of the Sun’s magnetic field, which determines the number of sunspots and the amount of particles emitted in the solar wind. The period of the cycle is about 11 years.
Supermassive Black Hole A black hole possessing as much mass as a million or a billion stars. Supermassive black holes reside in the centers of galaxies and are the engines that power active galactic nuclei and quasars.
Supernova The explosive death of a massive star whose energy output causes its expanding gases to glow brightly for weeks or months.
Supernova Remnant The glowing, expanding gaseous remains of a supernova explosion.

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