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All About Astronomy
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Astronomy Dictionary

Click on an underlined word for more information on that subject.



Carl Sagan (1934-1996) was an American astronomer who discovered that the surface of Venus was extraordinarily hot and noxious (contrary to previous models of a mild Venusian surface). Sagan also showed that the universe has many organic (carbon-based) chemicals and that life is likely to exist throughout the cosmos. He was a great popularizer of astronomy, was involved in many NASA flights and SETI, and also pioneered the field of exobiology.


[Abbreviation: Sgr] Sagittarius is the ninth constellation of the zodiac. To the ancients, it represented a centaur (half-man, half-horse) archer who was aiming at the Scorpion (the next constellation) which bit Orion. Its central section (the archer's chest) also resembles a teapot. The center of the Milky Way Galaxy is in the direction of Sagittarius. Many meteor showers, including the Sagittariids, seem to radiate from Sagittarius. The brightest star in Sagittarius (Alpha Sgr) is Rukbat (which means "knee" in Arabic).
The first space station, Salyut 1 (also called DOS 1 and Zarya), was launched on April 19, 1971 from Baikonur. This small cylindrical-shaped Soviet space station had room for a crew of three; it was 13.1 meters long and about 4.2 meters across. The crew (ladimir Shatalov, Alexei Yeliseyev and Nikolai Rukavishnikov) unsuccessfully docked with the space station on June 7, 1971 (the docking mechanism was damaged during the docking process). They couldn't enter the space station, but spent 21 days orbiting the Earth. The next crew (Georgi Dobrovolski, Vladislav Volkov and Viktor Patsayev) spent 24 days on the station; they died during the return trip to Earth (on June 30). The space station orbited for 179.93 days but was never used again.


Allan Rex Sandage (1926-) is an astronomer who, in the 1950's, measured the rate of the expansion of the Universe, Hubble's constant (H), which he calculated to be 50 km/sec/mpc. From this, Sandage estimated of the age of the Universe (T) to be 19.2 billion years [T = 2/3 x (1/H) ]. These calculations have changed through the years and now, H=~ 75 (T=12.9 billion years) is more generally accepted. Sandage also discovered quasars in 1964.
The SAO number is one of many ways of identifying or referring to a particular star. The SAO (Smithsonian Astrophysical Observatory) cataloged 258,996 stars and assigned them numbers; these numbers are the stars' SAO numbers, and are in the form: SAO ####. For example, the star Vega is SAO 67174.
The saros is the roughly 18-year periodic cycle of the Earth-Moon-Sun system. Every 6,585 days, the Earth, Moon and Sun are in exactly the same position. When there is a lunar eclipse, there will also be one exactly 6,585 days later.
Sarsen stones (also known as Druid stones and greywether; Saracen means Druid) are huge, vertical, linteled, sandstone blocks found in South-central England (such as Stonehenge). These stones were carefully set in a circular pattern during the Bronze Age in England. They were perhaps intended to predict the summer and winter solstices, forming a simple calendar.


Satellites are objects that orbit a planet or a moon. Many man-made satellites and one natural satellite (the Moon) orbit the Earth.


Saturn is the sixth planet from the Sun. It is the second-largest planet in our solar system (Jupiter is the largest). It has beautiful rings that are made of ice chunks that range in size from the size of a fingernail to the size of a car; it also has many moons. Saturn is made mostly of gas: hydrogen and helium.
A scalar is a number (a magnitude) without a direction (compare with vector). For example, speed is a scalar; it tells you how fast something is traveling but not the direction.


Giovanni Schiaparelli (1835-1910) was an Italian astronomer (and director of the Milan Observatory) who first mapped Mars (in 1877) and brought attention to the network of "canali" (Italian for canals or channels) on Mars. These "canals" were later found to be dry and not to be canal at all. A Martian impact crater (Crater Schiaparelli, 461 km = 277 mi in diameter) and a hemisphere of Mars have been named after Schiaparelli. His late niece Elsa was an influential fashion designer.


A Schmidt-Cassegrain telescope (SCT) is a wide-angle reflecting telescope with a correcting lens that minimizes spherical aberration and a concave mirror that receives light and focuses an image. A second mirror reflects the light through a gap in the primary mirror, allowing the eyepiece or camera to be mounted at the back end of the tube. The Cassegrain telescope (named for the French sculptor Sieur Guillaume Cassegrain) was developed in 1672; the correcting plate (a lens) was added in 1930 by the Estonian astronomer and lens-maker Bernard Schmidt (1879-1935).


Heinrich Schwabe was an an amateur German astronomer discovered that sunspots appeared in an 11-year cycle. Schwabe was a pharmacist who observed the sun daily and published his observations, "Solar Observations During 1843," in 1843.
The Schwarzschild radius is the radius of the event horizon of a black hole. This is the distance from a black hole at which nothing can escape, bot even light. Within the Schwarzschild radius, the escape velocity from the black hole is greater than the speed of light. The size of the Schwarzschild radius may be proportional to the mass of the black hole. For a typical black hole with a mass 10 times that of the Sun, the Schwarzschild radius would be roughly 18.6 miles (30 km). The Schwarzschild radius is named for the German astronomer Karl Schwarzschild, who predicted the existence of collapsed stellar bodies that cannot emit radiation, in 1916.


People study science to learn about the physical world.
Scientific notation is a mathematical format used to write very large and very small numbers; this system avoids using a lot of zeros. In scientific notation, there is a base number (a number between 1 and 10) multiplied by a power of ten. For example, the number 250 written in scientific notation is 2.5 x 102. For another example, the number 0.000052 written in scientific notation is 5.2 x 10-5.
Scintillation is the twinkling of stars (fluctuation of intensity) seen through a planet's atmosphere. Scintillation in caused when the star's light is distorted by the Earth's atmosphere. Scintillation is greater for bright stars that are low on the horizon. It is also known as astronomical scintillation.


[Abbreviation: Sco] Scorpius (the scorpion) is a constellation of the zodiac. This constellation is seen along the ecliptic between Libra and Sagittarius. The brightest star in Scorpius is Antares, a red supergiant star that is about 500 light-years away from Earth and is about 230 times as big as the Sun. The second-brightest (Beta 1 Sco) is Graffias.


Seafloor spreading is the movement of two oceanic plates away from each other, which results in the formation of new oceanic crust and a mid-ocean ridge.


The Sea of Tranquility (Mare Tranquillitatis) is a large plain on the moon. The first moon landing, Apollo 11, in 1969, was to this sea.


The seasons of the year are: spring, summer, fall (autumn), and winter. The seasons are caused by the tilt of the Earth's axis.
Seawinds is a weather radar system (a radar scatterometer) designed to measure oceanic winds. It has also mapped Earth's polar regions, since radar can work through clouds and darkness. Seawinds (on the satellite QuikSCAT) was launched by NASA in June, 1999.
Second generation stars do not just burn hydrogen, they also burn heavier elements, like helium and metals, and were formed from supernova explosions (the debris of exploded population II stars). Our Sun is a seond or third generation star.
In astronomy, secular mean that a phenomenon takes a tremendous amount of time to unfold, and occurs gradually. Secular change, for example, is a long-standing, continuous (and nonperiodic) change to a system.


Sedimentary rock is rock that has formed from sediment. Most fossils are found in sedimentary rock.
Sedna is a reddish-colored, planet-like body located in the Oort Cloud, approximately 13 billion kilometers (8 billion miles) away (three times farther from Earth than Pluto). Sedna is abour 800-1100 miles in diameter (this is about three-fourths the size of Pluto). Sedna turns on its axis once every 40 days (its day is 40 Earth days long); this is a very slow rotation. Sedna was discovered by Mike Brown, Chad Trujillo, and David Rabinowitz on Nov. 14, 2003. The name Sedna is from an Inuit (a North American Indian tribe) goddess of the ocean.


A semidiurnal tide tide is a tide having a period of about 12 hours; it has two high waters and two low waters during a tidal day.


The semi-major axis of an ellipse (a flattened circle) is half the length of the line segment across the longest part of the ellipse.


The semi-minor axis of an ellipse (a flattened circle) is half the length of the line segment across the shortest part of the ellipse.


SETI is an acronym for "Search for ExtraTerrestrial Intelligence." It is an ongoing project in which astronomers are searching the sky for signs of intelligent life elsewhere in our galaxy. Radio telescopes are used to "listen" for electromagnetic signals containing complex patterns of the sort that would likely be generated by intelligent beings.
The sextant is an astronomical instrument that is used to determine latitude for navigation. It does this by measuring angular distances, like the altitude of the sun, moon and stars. The sextant was invented independently in both England and America in 1731. The sextant replaced the astrolabe. The word sextant comes from the Latin word meaning "one sixth."
A Seyfert galaxy is an active spiral galaxy. Its nucleus (center) has bright emission lines, including visible wavelengths. The brightness varies over relatively short time periods (less than a year). They may have massive black holes at their centers. Seyfert galaxies were first described by Carl Seyfert in 1943.


A Soft Gamma Repeater (SGR) is probably a magnetar (a highly-magnetic neutron star) that emits bursts of soft (low-energy) gamma rays at irregular intervals. The gamma ray bursts may be caused by starquakes on the surface crust of the neutron star. (The starquakes are caused by aberrations in the star's strong magnetic field.) These SGR bursts are different from gamma ray bursts, which are one-time events. SGRs were discovered in 1979.
Harlow Shapley (1885-1972) was an American astronomer who was the first person to accurately estimate the size of the Milky Way Galaxy and our position in it.
Alan B. Shepard Jr. (1923-1998) piloted America's first manned space mission. This astronaut briefly flew into space on May 5, 1961, in Freedom 7, a Mercury space capsule. The capsule splashed down at sea and was retrieved by helicopter. Shepard also piloted Apollo XIV to the moon, accompanied by Edgar D. Mitchell and Stuart A. Roosa. They took off on January 31, 1971. Shepard and Mitchell landed on the moon in the lunar module (landing near the Fra Mauro Crater) on February 5, 1971, while Roosa orbited the moon in the command module. Shepard hit golf balls on the moon during this historic trip


A shepherd satellite is a moon of a planet that orbits along side a ring of that planet; the gravitational forces of the moon confines the ring and giving it a sharp edge. Examples of shepherd satellites include Saturn's moons Prometheus and Pandora (pictured above) which shepherd its narrow, outer F ring. Peter Goldreich and Scott Tremaine proposed the idea of shepherding moons in 1979 to explain why Uranus' rings were so narrow.
Shocked quartz is quartz that has undergone deformation due to extreme pressure and heat. It has been found in the layer that marks the K-T boundary, lending credence to the Alvarez impact theory, which explains the huge K-T extinction that killed off the dinosaurs and many other groups of organisms.


A shock wave is a very strong pressure wave in any elastic medium (such as air, water, or a solid), produced by supersonic craft, lightning, explosions, or other extreme phenomena that create sudden, huge changes in pressure.


Eugene Merle Shoemaker (1928-1997) and Carolyn Spellman Shoemaker (1929 - ) are scientists who have made many important discoveries in astronomy, finding many asteroids and comets. In 1994, Eugene and Carolyn Shoemaker and David H. Levy discovered the short-period comet, Shoemaker-Levy 9 (SL-9). Some of Eugene Shoemaker's ashes (1 ounce) were sent to the moon on Lunar Prospector in 1999 - his are the first human remains resting on another celestial body (Eugene had always wanted to go into space).


Shoemaker-Levy 9 (SL-9) was a short-period comet that was discovered by Eugene and Carolyn Shoemaker and David H. Levy. As the comet passed close by Jupiter, Jupiter's gravitational forces broke the comet apart . Fragments of the comet collided with Jupiter for six days during July, 1994, causing huge fireballs in Jupiter's atmosphere that were visible from Earth.


A shooting star is not a star; it is a meteor (made of rock and/or iron) which is burning up in the Earth's atmosphere.


A short period comet has an orbital period under 200 years. This type of comet may originate from the Kuiper belt. Halley's comet is a short period comet; its period is about 76 years.
Planet Sidereal Period (in Earth Years)
Mercury 0.24
Venus 0.62
Earth 1.0
Mars 1.9
Jupiter 11.862
Saturn 29.456
Uranus 84.07
Neptune 164.81
Pluto 247.7


The sidereal period is the time it takes a planet (or other body) to complete one orbit around the sun (with reference to the stars). It is usually expressed in Earth years.


Sidereal time is time measured relative to the stars (the period between successive conjunctions with any star) instead of relative to the motion of the Sun). One sidereal day, equal to 23 hours and 56 minutes, is the period during which the earth completes one rotation on its axis (this is the same as the time it takes to come into alignment with a particular star). A sidereal month is 27.322 days long.


A sidereal year is measured relative to the stars (the period between successive conjunctions with any star). One sidereal year, equal to 365 days, 6 hours, 9 minutes, and 9.5 seconds, is the period during which the earth completes one revolution around the Sun (this is the same as the time it takes to come into alignment with a particular star).
Silicates are minerals composed of silicon and oxygen with one or more other elements. Silicates make up about 95% of the Earth's crust.


Simple impact craters have bowl-shaped depressions, usually with smooth walls. On the moon, this type of crater usually has a diameter less than 9 miles (15 km). Their depth is usually about 20% of the diameter.
A singularity is a point in space-time at which the density of matter and the gravitational field are infinite (forming a black hole). Singularities are points at which the mathematical solution to the space-time equations are undefined.
Sinope is Jupiter's sixteenth and outermost moon. Sinope is 17.5 miles (28 km) in diameter and orbits 14,700,000 miles (23,700,000 km) from Jupiter. Sinope has a mass of 8 x 1016kg. It orbits Jupiter in 758 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). Very little is known about Sinope. Sinope was discovered by S. Nicholson in 1914.
A sinus is a large plain on a planet or a moon. The Sinus Iridium (bay of rainbows) is a semi-circular, dark plain off the NW corner of the Moon's Mare Imbrium.


Sirius (meaning "scorching" in Greek), also known as the dog star, is the brightest star in the sky (except for the sun). It is in the constellation Canis Major (The Great Dog). Sirius is a main sequence star that is about 70 times more luminous than the sun. It is about 8.6 light-years from Earth. It has an apparent magnitude of -1.46 and an absolute magnitude of +1.4. Sirius has a companion star (called the Pup), which is a white dwarf.


The sky on Earth appears blue because our atmosphere scatters the blue colors from the sunlight (which consists of all colors of light). The sky on planets with a different atmospheric composition would appear to be a different color.

Skylab was America's first orbiting space station. It was made from Saturn and Apollo equipment. Skylab was launched May 14, 1973 and orbited at an altitude of 270 miles (435 km). It orbited the Earth every 93 minutes. Three three-man crews (SL-2: Charles Conrad, Paul J. Weitz, Joseph Kerwin; SL-3: Alan L. Bean, Jack R. Lousma, Owen Garriott; SL-4: Gerald P. Carr, William R. Pogue, Edward Gibson) lived in Skylab for a total of 171 days and 13 hours; astronauts traveled to and from Skylab via Apollo spacecraft. These astronauts carried out astronomical observations and experiments on how humans react to a microgravity environment in space. Skylab re-entered the Earth's atmosphere and fell to Earth on July 11, 1979 scattering debris over the Indian Ocean and parts of Western Australia.
Small Magellanic Cloud


The Small Magellanic Cloud (SMC) is an irregular-shaped galaxy in the Local Group. The irregular shape may be the result of a disturbance, perhaps a collision of two galaxies. The Small Magellanic Cloud (SMC) is near the constellation Tucana, and is under 200,000 light-years away.


A Soft Gamma Repeater (SGR) is probably a magnetar (a highly-magnetic neutron star) that emits bursts of soft (low-energy) gamma rays at irregular intervals. The gamma ray bursts may be caused by starquakes on the surface crust of the neutron star. (The starquakes are caused by aberrations in the star's strong magnetic field.) These SGR bursts are different from gamma ray bursts, which are one-time events. SGRs were discovered in 1979.


SOHO (an acronym for "SOlar and Heliospheric Observatory") is a space-based telescope that takes images of the Sun. It was launched on December 2, 1995, and may operate until 2003.
Sojuner rover


Sojuner rover was the first robotic explorer to explore Mars. Weighing only 25.4 pounds (11.5 kilograms), Sojuner was launched aboard NASA's Pathfinder mission on Dec. 4 1996. The 25-pound, six-wheeled microrover explored an ancient Martian flood plain (Ares Vallis) in 1997. Each wheel has a diameter of 5 inches (13 cm). The rover has a maximum speed of 1.3 feet per minute (0.4 meters per min.). It was powered by a solar panel.


When the location (by latitude on the Sun) of sunspots is graphed year by year, a pattern that looks like a series of butterflies emerges.


Solar conjunction is when a planet is the closest it will be to the Sun as viewed from the Earth. Generally, this means the planet can't be seen, because the brightness of the Sun drowns the planet out. An exception is when the planet transits across the Sun.


The solar constant is amount of solar power flux (energy flow) that passes through the Earth's orbit. The currently-accepted estimate of the solar constant is 1,367 Watts/m2. Earth-based instruments record lower values of the solar constant because energy is absorbed and deflected by the Earth's atmosphere.


The corona is the top layer of the sun's atmosphere; it extends for millions of miles beyond the Sun's surface. The corona has very high temperatures (over a million ;K) and a very low density. During a complete solar eclipse (pictured above), only the ghostly corona is visible.


The solar cycle is the roughly 11-year, quasi-periodic variation in the frequency or number of sunspots, solar flares, and other solar activity.


A solar day is the amount of time that passes between two subsequent times when the Sun reaches its highest position above the horizon (passing through the meridian). The solar day varies greatly throughout the year, so the mean solar day is used instead. This is calculated as the average of all of the solar days in one year. A solar day is exactly 24 hours (since this is how we define hour, as 1/24 of a day); a solar day is slightly longer than a sidereal day. A solar day is not the time it takes for the Earth to rotate on its axis (that is the sidereal day).


A solar eclipse happens when the moon blocks our view of the sun.


A solar flare is a magnetic storm on the sun, which appears to be a very bright spot, and a gaseous surface eruption. Solar flares are classified based upon their x-ray energy output at peak burst intensity.


A solar halo is a luminous ring that is sometimes seen surrounding the Sun. Some parts of the halo are very bright, others are not very bright. Sometimes, only a part of the ring is visible. Within the solar halo, on opposite sides of the Sun, there can be two very bright spots called "sun dogs" or "mock suns." The halo is produced as sunlight is reflected and refracted through tiny, flat ice crystals in the atmosphere. Sun dogs and halos always are at an angle of 22° away from the Sun, due to the hexagonal structure of the ice crystals. The diameter of the halo is about an eighth of the sky.


A solar mass is the amount of mass in our Sun; it is also the unit in which the masses of other stars, galaxies, and other large celestial bodies are expressed. The solar mass is 1.99 x 1030. A black hole, for example has anywhere from a million to a billion solar masses.


A solar maximum is the highest level in solar activity (like flares, prominences, sunspots, soronal holes, etc.), and occurs between consecutive solar minima.


A solar minimum is a low level in solar activity (like flares, prominences, sunspots, soronal holes, etc.), and occurs between consecutive solar maxima.


A solar plume is a long, feathery jet of high-speed electrified gas that is expelled from the Sun's corona. Solar plumes emanate near the Sun's poles and travel over 13 million miles (21 million km) into space.


A solar prominence is an arc of gas that erupts from the surface of the Sun. Prominences can loop hundreds of thousands of miles into space and can last for many months.


A solar sail is a large, flat surface that harnesses the force of photons (light) from the Sun to propel a spacecraft around the Solar Sysyem. The solar wind does not exert enough pressure to push a solar sail. After observing the tail of comets over 400 years ago, the astronomer Johannes Kepler theorized that space vessels might someday use the solar breeze for power, much as sailing ships do.


A solar system is a group of planets, moons, asteroids, and comets that orbit around a sun. In our solar system, eight planets, over 61 moons, and many other objects orbit around our Sun.


The plane of our solar system is also called the ecliptic. It is the plane in which most of our Solar System lies (Mercury and Pluto orbit are inclined from this plane). The plane of our solar system is tilted about 5.5 degrees from the plane of the Milky Way galaxy. The Earth's axis is tilted at a 23.5° from the ecliptic (this causes the seasons).


The solar wind is a continuous stream of ions (electrically charged particles) that are given off by the sun. Since the particles are emitted from the Sun as the Sun rotates, the solar wind blows in a pinwheel pattern through the solar system. It takes the solar wind about 5 days to reach Earth; it has a velocity of about 500 miles/hour (800 km/sec). The solar wind travels along vibrating magnetic waves which are propagated by the Sun


The solar wind (heliospheric) termination shock is the shock that occurs as the solar wind hits the heliopause and its speed slows greatly (down to about 20 km/s).


The solar year (also called the tropical year) is the amount of time that passes between two consecutive returns of the sun to the vernal equinox. In 1900, the solar year lasted 365 days, 5 hours, 48 minutes, and 46 seconds (shorter than the sidereal year); the length of the solar year is decreasing 0.53 second per century ad the Earth slows down in its orbit.


A solid is a phase of matter in which the molecules are very close together and cannot move around.


The solstices are days when the Sun reaches its farthest northern and southern declinations. The winter solstice occurs on December 21 and marks the beginning of winter (this is the shortest day of the year). The summer solstice occurs on June 21 and marks the beginning of summer (this is the longest day of the year).


Solunar means pertaining to the rising and setting times of the sun and the moon, the phases of the moon, and of solar and lunar eclipses. Solunar tables list when these various events occur.


A sonic boom is a very loud sound that is caused by a shock wave (pressure disturbance in the air) coming from an object which is traveling faster than the speed of sound. Fireballs and some fast aircraft can cause sonic booms.


Crux (or Crux Australis) is the scientific name of the Southern Cross constellation. This well-known, cross-shaped Southern Hemisphere constellation is on the Australian flag. The brightest star in Crux is Acrux (alpha Cru), a double-star system at the base of the cross. The second-brightest star is Becrux or Mimosa (Beta Cru); the third-brightest is Gacrux (Gamma Cru). The Jewel Box (also known as Kappa Crucis) is an open cluster of about 100 stars in the Southern Cross. Crux lies on the Milky Way and is surrounded by the constellation Centaurus on three sides.


Soyuz are Russian spacecraft that have been used often since 1967 - this small workhorse carries only one or two people. In Russian, soyuz means "union." Soyuz 1 was launched on April 23, 1967 - it crashed on reentry, killing cosmonaut Vladimir Komarov (Soyuz's parachutes did not open). Russian cosmonauts used Soyuz spacecrafts to shuttle them back and forth from the MIR space station (one was kept with MIR for use as an eacape pod).


Spacecraft are vehicles that travel beyond the Earth's atmosphere.


A space elevator is a device that would carry people, cargo, and power from the Earth into space. The space elevator would be constructed by extending a long cable from the Earth's surface into space; the cable would be held up by a satellite in geostationary Earth orbit. Fat-moving electromagnetic elevator cars would travel along the cable from Earth into space and back.


A space probe is an unmanned research spacecraft.


People fly in NASA's space shuttle to go into Earth orbit and return to Earth again. There have been over 100 Space Shuttle flights into orbit. The first space shuttle, STS-1 Columbia (OV-102), launched on April 12, 1981 (from Florida), and landed April 14, 1981 (at Edwards Air Force Base, CA). NASA's orbiter vehicles include Atlantis, Challenger, Columbia, Discovery, and Endeavor.


The Space Shuttle Challenger was the second NASA Space Shuttle to be used (Columbia was the first). Challenger's first launch was the sixth Space Shuttle launch, called STS-6 Challenger; it took off on April 4, 1983 (from Florida), and landed on April 16, 1983 (at Edwards Air Force Base, CA). The first crew consisted of Paul J. Weitz, Karol J. Bobko, Donald H. Peterson, and F. Story Musgrave. It orbited the Earth 81 times, flying for 5 days, 0 hours, 23 min, 42 seconds; the first Shuttle space walk occured on this mission (it lasted four hours, 17 minutes). After nine flights into space, STS-51L (the mission number) Challenger, exploded 73 seconds into its launch on January 28, 1986, killing the crew (Gregory Jarvis, S. Christa McAuliffe, Ronald McNair, Ellison Onizuka, Judy Resnik, Francis Scobee and Michael Smith).


The first Space Shuttle, STS-1 Columbia (OV-102), launched on April 12, 1981 (from Florida), and landed April 14, 1981 (at Edwards Air Force Base, CA). The first crew consisted of Commander John W. Young and Pilot Robert L. Crippen. It orbited the Earth 37 times, flying 1,074,567 miles in 2 days, 6 hours, 20 min, 53 seconds. After dozens of flights into space, STS-107 (the mission number) Columbia, disintegrated during atmospheric re-entry on February 1, 2003, killing the seven crew members (Rick D. Husband, Kalpana Chawla, William C. McCool, David M. Brown, Laurel B. Clark, Michael P. Anderson, Ilan Ramon).


A space station is an orbiting structure where people can live in space. The first space station, Salyut 1, was launched on April 19, 1971. It led to Mir, which was launched in 1986. Construction (in orbit) on the new International Space station began in 1998. The first crew was launched on a Russian Soyuz (in Russian, soyuz means "union") spacecraft for a three-month stay beginning January 2000.


Astronauts wear space suits when they walk in space.
The curvature of space-time is a distortion of space-time that is caused by the gravitational field of matter. The degree of curvature depends on the strength of the gravitational field (which depends on the massiveness of the objects in that part of space). An object traveling in space moves along the curves in space-time.


Space trash (also called space junk) is debris that orbits the Earth. It is mostly debris from man-made satellites and other space missions (including old satellites and bits of exploded rocket stages). Space trash is a considerable danger to spacecraft. The first known collision between a satellite (a French microsatellite) and space trash happened on July 24, 1997.


Space weather is a new field of science that studies the interactions between the Sun and the Earth. It attempts to predict solar flares, coronal mass ejections, geomagnetic storms and other space phenomena.


Special relativity supplanted Newtonian mechanics, yielding different results for very fast-moving objects. The Theory of Special Relativity is based on the idea that speed has an upper bound; nothing can pass the speed of light. The theory also states that time and distance measurements are not absolute but are instead relative to the observer's frame of reference. Space and time are viewed as aspects of a single phenomenon, called space-time. Energy and momentum are similarly linked. As a result, mass can be converted into huge amounts of energy, and vice versa, according to the formula E=mc2. Albert Einstein devised the Theory of Special Relativity.


Spectral classes are groups of stars that have similar characteristic emission lines in their spectra (indicating that they have similar compositions).


A spectral line is a bright or dark line found in the spectrum of some radiant source. Bright lines indicate emission, dark lines indicate absorption. A bright spectral line represents light emitted at a specific frequency by an atom or molecule. Each different element and molecule gives off light at a unique set of frequencies. Astronomers can determine the composition of gases in stars by looking for characteristic frequencies (this is called spectroscopy). For example, carbon monoxide (CO) has a spectral line at 115 Gigahertz (equal to a wavelength of 2.7 mm).


The spectral type of stars is a system of classification of stars based on the stars' spectra, emission lines that correlate with each star's surface temperature (and color). There are seven major spectral types. Stars range from blue and hot to red and cool. The spectral types are: O, B, A, F, G, K, and M (from hottest to coolest). Each of these letters is divided into 10 numerical classes, from hotter to cooler: 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. For example, our Sun has the spectral type G2.


A spectrograph is an image of the electromagnetic spectrum of a light source. Spectrographs identify which elements are present in that star.


A spectroscope is a scientific instrument that breaks up the light from a star into its component colors in order to identify which elements are present in that star.


Spectroscopy is a scientific technique in which the visible light coming from objects (like stars and nebulae) is examined to determine the object's composition, temperature, density, and velocity.


The spectrum is the band of colors that white light is composed of, in the order: red, orange, yellow, green, blue, indigo, violet (from long to short wavelength). Newton first discovered that sunlight could be divided into the visible spectrum.


Speed is a measure of how fast something is moving. Speed is a scalar.


The speed of light is the speed at which electromagnetic waves can move in a vacuum: 299,792,458 meters/sec (186,000 miles/second). According to Einstein's Theory of Relativity, nothing can go faster than the speed of light.


The speed of sound is the speed at which sound moves through air. At sea level and standard atmospheric pressure, the speed of sound is 1116.45 feet per second (340.294 meters per second).


Spicules are very bright spikes that extend from the Sun into the chromosphere.


Spiral galaxies are galaxies with a central, dense area and spiraling arms. There are two types of spiral galaxies, "S" (normal spiral) and "SB" (barred spiral, with an elongated center). The Milky Way and M31 (commonly known as Andromeda Galaxy) are two of a multitude of known spiral galaxies.


Spring tides are especially strong tides. They occur when the Earth, the sun, and the Moon are in a line. The gravitational forces of the Moon and the sun both contribute to the tides. Spring tides occur during the full moon and the new moon.


The Soviet Sputnik space missions were the first to orbit the Earth. The word Sputnik means "Traveling Companion" in Russian. The first Sputnik mission was launched on on October 4, 1957; this was Earth's first artificial satellite. Sputnik 1 was about the size of a basketball, weighing roughly 183 pounds. It was sent into an elliptical orbit around the Earth, revolving in about 98 minutes. Sputnik 2 was launched on November 3, 1957 with the dog Laika aboard. Sputnik 3 failed on February 3, 1958. The last Sputnik mission, Sputnik 3 was successfully re-launched on May 15, 1958 and remained in orbit for 2 years.


Each star in the sky is a glowing ball of gas. Our sun is a medium-sized star. The first stars in the Universe appeared about 200 million years after the Big Bang (which occurred about 13.7 billon years).


A starburst galaxy is a bright, blue-glowing galaxy that has a high rate of star formation (sometimes, the star-formation area is only at the core of the galaxy, and these galaxies are called nuclear starbursts). This type of galaxy is filled with glowing hydrogen gas clouds, dense interstellar dust, and many, hot, newly-formed stars. These galaxies have strong radio emissions that are the remnants of recent supernovas. Starburst galaxies were formed by violent gravitational encounters, like near-collisions of nearby galaxies. Examples of starburst galaxies include NGC 253 and the barred spiral galaxy NGC 1808. Starburst galaxies may be related to active galactic nuclei.


A star catalog is a listing of stars, their positions, and catalog numbers.


Stars are classified by their spectra (the elements that they absorb) and their temperature. For example, the Sun is a G1 star. There are seven main types of stars. In order of decreasing temperature, they are: O - He II absorption; B - He I absorption; A - H absorption; F - Ca II absorption; G - strong metallic lines; K -bands developing; M - very red. O and B stars are uncommon but very bright; M stars are common but dim. An easy mnemonic for remembering these is: "Oh be a fine guy/girl, kiss me."


A star cluster is a group of stars that are close together in space.


A star diagonal is a simple mirrored device on some refracting telescope that bends the light 90 degrees. This lets the observer look down through the eyepiece instead of kneeling to look directly through the telescope. The resulting image has left and right reversed.


NASA's Stardust Mission is designed to learn about comets. It took a sample of comet particles and returned them to Earth. The small spacecraft (about 770 pounds = 350 kg) was launched February 7, 1999, and rendezvoused with comet Wild 2 in January, 2004. It returned to Earth on January 15, 2006, and landed in western Utah, USA.
Starquakes are fractures in the crust of neutron stars and pulsars caused by the star's strong magnetic field. As the crust cracks in the starquake, energy is released and x-rays are emitted. Another type of starquake occurs on ordinary stars (like our Sun). In these quakes, violent seismic events on stars are caused by solar flares (jets of hot gases ejected thousands of miles from the surface of a star). When a starquake occurs, energy is released in seismic waves on the relatively fluid surface of the star. These waves radiate in concentric circles from the epicenter of the starquake.


Project Starshine (Student Tracked Atmospheric Research Satellite for Heuristic International Networking Equipment) is a 19 inch hollow, spherical satellite covered with 900 tiny aluminum mirrors. It was launched on May 1999 from NASA's Space Shuttle. Students around the world helped polish the quarter-sized mirrors and will track this satellite visually for several months, during morning and evening twilight. The students then calculated its orbit from shared data and then deduce the atmospheric density from drag-induced changes in its orbit over time. Starshine orbited Earth for 8 months and then disintegrated in February, 2000.


When a time exposure photogaph of the night sky is taken, the spinning movement of the Earth causes stars to appear not as individual points, but as curved star trails, concentric circles around the North Celestial Pole.


Matter can exist in four states or phases (solid, liquid, gas, and plasma) and a few other extreme phases, like critical fluids and degenerate gases. The phase diagram of water (above) shows its phase at various temperatures and pressures.


Static electricity is a stationary electric charge that is built up on a material.


The Steady-State Theory is a cosmological theory that postulates that the Universe has always been and will always be the same way it is today. It assumes that the universe is uniform, infinite, and not expanding. It was proposed in 1948 by Hermann Bondi, Thomas Gold and Sir Fred Hoyle. The Steady-State Theory is an alternate to the Big Bang Theory.


Stars can be classified by their surface temperature and their absorption spectra. There are seven main types of stars. In order of decreasing temperature, they are: O - He II absorption; B - He I absorption; A - H absorption; F - Ca II absorption; G - strong metallic lines; K -bands developing; M - very red. O and B stars are uncommon but very bright; M stars are common but dim. The Sun is a G star, about average. The standard mnemonic for remembering the classes is: Oh Be A Fine Guy Kiss Me. It is supplemented by the giants and supergiants: R- and N-type stars (also known as carbon stars or C-type stars) and S-type stars.


A stellar nursery is a nebula ( a large cloud of hydrogen gas in space) in which star formation is occurring (stars are formed from gas). These nebulae are frequently illuminated by ultraviolet light which is emitted from the newborn stars. One example of a stellar nursery is the Eagle nebula pictured above.


Stellar parallax is the apparent change in the position of a star that is caused only by the motion of the Earth as it orbits the Sun.
Stellar scintillation is the twinkling of stars (fluctuation of intensity) seen through a planet's atmosphere. Scintillation in caused when the star's light is distorted by the Earth's atmosphere. Scintillation is greater for bright stars that are low on the horizon. It is also known as astronomical scintillation.
Stellar wind is gas that is ejected from the surface of a star (including the Sun). Older (evolved) stars give off stronger stellar winds.
Stishovite is a very dense form of quartz that has only been found in meteorite impact craters, in which quartz has undergone high-pressure shock.


The stratosphere is the atmospheric layer between the troposphere and the mesosphere. The stratosphere is characterized by a slight temperature increase with altitude and the absence of clouds. The stratosphere extends between 11 and 31 miles (17 to 50 kilometers) above the earth's surface. The earth's ozone layer is located in the stratosphere.
Bengt Georg Daniel Strömgren (January 21, 1908 - July 4, 1987) was a Danish astronomer who studied the structure of stars, including stellar atmospheres, their composition (what elements they contain), and ionization in stars. He theorized that ionized hydrogen gas clouds surround hot stars (the "Strömgren sphere"). Stromgren also determined the relationship between the gas density of a star, its luminosity, and the size of the "Strömgren sphere" of ionized gas that surrounds it. He also did work on photoelectric photometry (measuring the intensity of the photoelectric effect from an astronomical object).
Friedrich Georg Wilhelm von Struve (April 15, 1793 - Nov. 23, 1864) was a German-born astronomer who studied binary stars. He catalogued 120,000 stars (from the north celestial pole to 15° S declination), noting 3,112 binary stars, most of which were previously unknown. He published his catalog, "Stellarum Duplicium Mensurae Micrometricae" [meaning "Micrometric Measurement of Double Stars"] in 1837. In 1838, Struve measured the parallax of the star Vega; he was one of the first people to measure parallax.


Sub-atomic particles are particles that are smaller than atoms. There are two main groups of subatomic particles: leptons (elementary particles that are not made up of other smaller particles) and hadrons (which are made up of smaller subatomic particles called quarks). Leptons are light-weight particles, like the electron, positron, tau, muon and so on. Hadrons are subdivided into two groups, mesons (like the pion, psi, upsilon and other) and baryons (heavy particles like the proton, neutron, and others). There are six types of quarks: up, down, charmed, strange, top, and bottom.


A subduction is a phenomenon in which one part of the Earth's crust (a plate) is pushed underneath another plate as two plates collide. The descending crust melts as it is pushed deep into the Earth's mantle. Subduction destroys crust and recycles it back into the mantle.
A subduction zone is an area on a planet's crust in which the edge of an oceanic continental plate is being pushed beneath another plate.


Subliming is when a material goes directly from being a solid to being a gas (it skips the liquid phase altogether).


The solstices are days when the Sun reaches its farthest northern and southern declinations. The winter solstice occurs on December 21 and marks the beginning of winter (this is the shortest day of the year). The summer solstice occurs on June 21 and marks the beginning of summer (this is the longest day of the year).


The Summer Triangle is a group of three bright stars (Deneb, Vega, and Altair) which are visible during the summer in the Northern Hemisphere. The Summer Triangle is not a constellation, but an asterism.


The Sun is a star at the center of our solar system. Our Sun is a medium-sized yellow star that is 93,026,724 miles (149,680,000 km) from Earth. Its diameter is 865,121 miles (1,391,980 km). At its core, nuclear reactions produce enormous amounts of energy, through the process of converting hydrogen atoms into helium atoms (nuclear fusion). Its absolute magnitude is +4.83. The solar mass is 1.99 x 1030 kg.


A sun dog is one of two very bright spots in the sky that are sometimes seen in a solar halo (a luminous ring that is sometimes seen surrounding the Sun). The two sun dogs are located on opposite sides of the sun. They are also called "mock suns." The halo and the two sun dogs are produced as sunlight is reflected and refracted through tiny, flat ice crystals in the atmosphere. Sun dogs and halos are always at an angle of 22° away from the sun, due to the hexagonal structure of the ice crystals. Sun dogs are also called mock suns, false suns, or parhelia (meaning "beside the sun" in Greek).


A sungrazer is a comet that either crashes into the Sun or gets so close that it burns up.
Sunquakes are violent seismic events on the Sun that are caused by solar flares (jets of hot gases ejected thousands of miles from the surface of the Sun). When a sunquake occurs, energy is released in seismic waves on the relatively fluid surface of the sun. These waves radiate in concentric circles from the epicenter of the sunquake. These seismic waves seem to be compression waves (perhaps like "P" waves generated by earthquakes). Sunquakes would rate about 11.3 on the Richter scale. These huge quakes release about 40,000 times more energy than the 1906 San Francisco earthquake. Sunquakes were first observed by Alexander G. Kosovichev (Stanford University) and Valentina V. Zharkova (Glasgow University, UK).


Sunspots are cool, dark patches on the Sun's surface. They are caused by disturbances in the sun's magnetic field which make the sunspot about 2700°F (1500°C) cooler than the surrounding area. Sunspots occur where the sun's magnetic field loops up out of the solar surface. The number of sunspots follows an 11-year cycle; the current cycle will peak in the middle of 2000. Sunspots are visible from Earth. (WARNING: do NOT look at the sun; it can damage your eyes permanently!) The sunspot cycle was discovered by H. Schwabe in 1843.


A supercluster is a huge group of up to a thousand galaxies. These tend to be shaped as flattened disks, sheets, or filaments. These superclusters then form surfaces like the surfaces of bubbles, with virtual voids in between. These may be the largest structures in the universe.
A supercritical (or critical) fluid is a liquid/gas under extreme pressure. These supercritical fluids have unique characteristics, the density of a liquid and the mobility of a gas. Supercritical fluids exist deep inside some planets; for example, there is supercritical water deep inside the Earth.


A supergiant is the largest known type of star; some are almost as large as our entire solar system. Betelgeuse and Rigel are supergiants. These stars are rare. When supergiants die they supernova and become black holes.


Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto are called superior planets because they are farther from the Sun than Earth. [Planets that are closer to the Sun than Earth are called inferior planets.]


A supernova is a huge explosion that occurs at the end of a mid- to heavy-weight star's life. A supernova releases a tremendous amount of energy, expelling the outer layers of the star and becoming extremely bright. What remains is a neutron star (from a middle-weight star, mass between 1.5 to 3 times the mass of the Sun) or a black hole (from a supergiant star, mass over 3 times the mass of the Sun).


A supersynchronous orbit is a type of transfer orbit (a temporary orbit that a satellite is put into before being injected into its final, desired orbit). A satellite is in a supersynchronous orbit when its orbital apogee (farthest point of the orbit) is much greater than the eventual geosynchronous altitude. Later, the satellite will be maneuvered into its final geosynchronous or geostationary orbit.


Surface gravity is the strength of the gravitational field (the acceleration due to gravity) at the surface of the planet.


Swift-Tuttle is a comet that is responsible for the Perseid meteor shower; every July and August the Earth crosses this comet's path causing a meteor shower. Comet Swift-Tuttle is the biggest celestial object that makes repeated passes near the Earth. This comet orbits the Sun with a period of about 130 years. Comet Swift-Tuttle was discovered on July 16, 1862, by Lewis Swift (from Marathon, New York), and independently discovered a few days later (July 19, 1862) by Horace Parnell Tuttle (from Harvard University) and many other astronomers. Comet Swift-Tuttle is the same as Comet Kegler, which was first seen in 1737. Comet Swift-Tuttle was last seen in 1992; its next visit will be in 2126.


Each of the planets in our Solar System have been given symbols, pictured above.


A satellite is in synchronous orbit (also called synchronous rotation) when its orbital period is the same as its period of rotation about its axis. The Moon is in a synchronous orbit, so the same side of the moon always faces Earth. Observing from the moon, the Earth is always in the same spot. Sometimes the term synchronous orbit is used to mean geosynchronous or geostationary (causing confusion).


A synchrotron is a device in which particles are acceletated to very high speeds (usually in circular paths) within a magnetic field. Physicists used synchrotrons to study particles, subatomic interactions, and other basic phenomena.


Synchrotron radiation is the electromagnetic radiation emitted by charged particles that are moving (in circular orbits) at extremely high speeds (close to the speed of light) in a magnetic field. This happens in a synchrotron and also, perhaps, in space, as extra-galactic celestial radio sources generate microwave radiation as electrons move in curved paths in magnetic fields.


Synodic means pertaining to two successive conjunctions of celestial bodies. For example, a synodic month is the time between successive new moons (29.531 days). The synodic month is is slightly longer than a sidereal month.


A synodic month (also called a lunar month) is the time between successive new or full moons. It lasts 29 days, 12 hours, and 44 minutes.


Syzygy occurs when the moon (or a planet) is in opposition or conjuction with the Earth and Sun; the three bodies are positioned in a straight line. At syzygy, the moon (or planet) is seen as new or full (from Earth).
Astronomy Dictionary

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