AstroGlossary - S
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satellite
A natural or artificial object in orbit around another body. For examples, the Moon is the largest natural satellite of the Earth, and the planets are satellites of the Sun. In addition, there are satellite galaxies that orbit the Milky Way. Saturn
The sixth planet from the Sun in the aolar system. Saturn orbits the Sun at an average distance of 9.5 AU. The planet has a mass 95.2 times the mass of Earth and a radius 9.45 times the radius of Earth. Saturn is a gas planet and has no solid surface. It is made of hydrogen, helium, ammonia, and methane. Saturn is most famous for its beautiful ring system. In addition to the rings, the planet has 47 known moons, the largest of which is Titan. Schwarzschild Radius
The distance between the central singularity and the event horizon of a black hole. The length of the Schwarzschild Radius depends on the mass of the black hole. Anything inside this radius can not escape the black hole. semimajor axis
The distance from the center of an ellipse, through a focus to the edge, or perimeter. This is the longest length of an ellipse. For example, the semimajor axis of the Earth's orbit is 149,597,870.7 km, or 1.0 AU. sidereal time
Measurement of time based on the position of the stars, rather than the Sun. It takes Earth 23 hours, 56 minutes, 4.091 seconds to complete one full turn relative to the stars, versus exactly 24 hours relative to the Sun. Therefore, a solar day, which is measured with respect to the Sun, is slightly longer than a sidereal day, which is measured with respect to the stars. The difference between solar and sidereal time is due to Earth's motion; as we move along our orbit around the Sun, the relative position of the Sun compared to the background stars changes. As a result, the stars rise about four minutes earlier each night. singularity
A theoretical point at the core of a black hole where all of the black hole's mass is concentrated. The singularity is compressed so tightly that it is almost infinitely dense. The curvature of space-time is infinite, and the normal laws of physics break down. solar eclipse
A dramatic celestial phenomenon in which light from the Sun is blocked from the Earth by the Moon. In order for this to occur, the Earth, Moon, and Sun must be in a line in that order, which means that the phase of the Moon must be new. The Moon's shadow sweeps across the Earth's surface as the Moon moves over the face of the Sun. Totality occurs when the Sun is completely blocked by the Moon. The darkest part of the shadow is called the umbra, and it is surrounded by an area of partial shadow called the penumbra. Because the Moon's shadow stretches from the Moon to the Earth in a cone shape, only a small fraction of the Earth's surface experiences the eclipse. The solar eclipse alignment of Earth, Moon and Sun does not occur every month, because the Moon's orbit is tilted five degrees from a plane containing the Earth and Sun, so the Moon's shadow usually passes above or below the Earth. solar flare
A violent eruption of plasma from the chromosphere of the Sun that is whipped up by intense magnetic activity. During the eruption, flares rise thousands of kilometers above the chromosphere, and the plasma temperatures quickly soar to 20 million degrees. Large flares release 10^25 Joules, or about the energy of a few million volcanic eruptions on the Earth. Sunspot and solar flare frequency are strongly related. In addition, flares often disturb the Earth's atmosphere electrically, thus interfering with radio transmissions. The aurora borealis and aurora australis are results of flare activity that injects energetic particles into Earth's magnetic field. solar system
The system of our Sun and its planets, including moons, asteroids, and comets. solar wind
A flow of atomic nuclei and electrons from the Sun that travels at an average speed of about two million miles per hour (400 km/s). solstice
An event in the Earth's orbit during which the tilt of the Earth's axis is pointed most directly towards or away from the Sun. The summer solstice for the northern hemisphere occurs within a few days of June 21 every year. It is on this day that the position of the Sun in the sky at noon is at its highest altitude of the year, and the position of the Sun at Sunrise and Sunset is farthest north for the year. The winter solstice is around December 21, marking the date on which the Sun is lowest in the sky at noon and rises and sets farthest south. The day of the summer solstice is the longest day of the year, and the day of the winter solstice is the shortest day of the year. space-time
A four-dimensional coordinate system, or reference frame, that physicists and astronomers use to describe the universe. Space-time has three spatial axes (x, y, z) and one time axis (t). A point in this reference frame is called an event, because it is something that happens in space and time. Einstein's General Theory of Relativity explains that the speed of light is constant between all reference frames, regardless of their motion relative to each other. spectroscopy
A technique used by astronomers that allows them to determine the properties, such as composition, temperature, and motion through space, of celestial objects by analyzing the spectra of celestial objects. For example, because each atomic element absorbs and emits light in a unique set of wavelengths, the astronomer can sift through the spectrum of a star and determine what elements are present in the star's atmosphere. From the shapes and depths of spectral lines, the astronomer can calculate fundamental qualities of a star, such as how fast the gases churn through the stellar atmosphere or the star's effective temperature. An astronomer may also be interested in correlations between the abundances of certain elements and the physical behavior of the star, the age of the star, or the abundances of other elements. For instance, compared with the Sun, stars with low amounts of iron are also low in almost every other element with respect to hydrogen. Spectroscopy is performed by astronomers with instruments called spectrographs. spectrum
A wavelength or frequency band of electromagnetic radiation that contains information about the radiating source of energy. Optical astronomers rely on the spectrum of light from red to blue (visible spectrum) for detailed information carried by absorption or emission lines, color appearance and brightness. An optical instrument called a spectrograph refracts the light into a spectrum of color with a grating or prism. speed of light
The maximum velocity for everything in the universe; 186,282.397 miles (299,792.458 km) per second, or fast enough to go to the Moon and back in less than three seconds. See also: spicule
A structural component of the chromosphere that erupts like a plasma geyser driven by dense magnetic fields. Spicules rise between 3,000 and 10,000 kilometers above the magnetic mesh beneath. At the boundary between the photosphere and chromosphere, spicules look like peach fuzz. Spicules live short, dynamic lives of a few minutes but spew plasma at speeds of more than ten kilometers a second. star
A dense, glowing ball of hydrogen, helium, and trace quantities of heavier elements that shines with energy released from a hydrogen thermonuclear fusion reaction in the center, or core. Our Sun is an example of a star. Stars can appear red, orange, yellow, blue, or white, according to their masses and effective temperatures. Stars are born from clouds of gas and dust, live for millions or billions of years, depending on their masses, and die sometimes spectacular deaths. Stars usually exist in very large collections called galaxies. subatomic
Having a size smaller than the size of an atom, which is about one angstrom, or 10^-10 meters. Sun
The closest star to the Earth and the center of the solar system. sunspot
A cooler, and therefore darker, region of the Sun's photosphere caused by a solar magnetic disturbance. Strong, dense magnetic fields generated by circulating plasma sometimes become entangled and surge through the photosphere, creating the sunspot. The knot causes the temperature to fall to around 1000 K in the sunspot region, which darkens the photosphere. The dark center of the sunspot is called the umbra, and it is surrounded by a dim filamentary area called the penumbra. Sunspots range from Earth-size "pimples" to swollen scars halfway across the surface. Sunspot activity generally follows an 11-year cycle, called the "sunspot cycle." supercluster
An enormous chain of galaxies linked by their mutual gravity. Superclusters look as if they ride on the surfaces of bubbles. Our galaxy is a peripheral member of the Virgo Supercluster, centered around a giant elliptical galaxy M87. Luminous matter does not bind this cluster: Dark matter outweighs luminous matter 10 to 1. Our galaxy seems to be drifting toward the Virgo Supercluster at about 250 km/s. supergiant
A star of at least eight solar masses that has evolved off the main sequence and depleted a significant portion of its hydrogen fuel supply. Helium becomes the new fuel, and "burns" at a much higher temperature in the core while hydrogen continues to "burn" in a surrounding shell. This shell expands the outer atmosphere of the star to over 10 up to 1000 solar radii. The entire life of the star from main sequence to supergiant is only a few million years. supernova
A violent stellar explosion that can shine as brightly as an entire galaxy of billions of normal stars. Astronomers divide supernovae into two groups: Type I and Type II. Type I supernovae most likely form as a white dwarf "steals" hot gas from a companion star. If enough gas piles up on the surface of the white dwarf, a runaway thermonuclear explosion blasts the star to bits, leaving nothing behind. These are the brightest supernovae, and can be used to measure the distances to other galaxies. Type II supernovae are the final stage in the evolution of stars that are at least eight times as massive as the Sun. Such a star reaches a point where it can no longer produce nuclear energy in its core. Without the outward pressure created by this energy, gravity wins out and causes the star's core to collapse to form a neutron star or black hole. The star's outer layers "rebound" violently, blasting into space at several percent of the speed of light. supernova remnant
The remainder of the stellar atmosphere that the supernova blows away into interstellar space. Astronomers see spectral emission of elements processed in the late stages of stellar evolution, like nitrogen, oxygen, and neon. The material is still hot and rushes through space at hundreds of kilometers per second. synodic month
The period of the Moon's orbit measured relative to the its appearance in Earth's sky, instead of relative to the background stars. The synodic month (29.53 days) is longer that of a sidereal month (27.3 days), because the Earth travels 70.6 million kilometers, or 1/12 of its orbit, around the Sun as the Moon orbits the Earth once. The sidereal month is the true measure of the Moon's orbital period, but the synodic period, such as from one full moon to the next, is what we on Earth measure as the lunar month.
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