Glossary

Glossary items by letter: q — s

Quark

A basic building block of protons, neutrons, and other elementary particles.

Quasar

The brightest type of active galactic nucleus, believed to be powered by a supermassive black hole. The word “quasar” is derived from quasi-stellar radio source, because this type of object was first identified as a kind of radio source. Quasars also are called quasi-stellar objects (QSOs). Thousands of quasars have been observed, all at extreme distances from our galaxy.

RADAR (Radio Detection and Ranging)

A method of detecting, locating, or tracking an object by using beamed, reflected, and timed radio waves. RADAR also refers to the electronic equipment that uses radio waves to detect, locate, and track objects.

Radial Motion

The component of an object’s velocity (speed and direction) as measured along an observer’s line of sight.

Radiation

The process by which electromagnetic energy moves through space as vibrations in electric and magnetic fields. This term also refers to radiant energy and other forms of electromagnetic radiation, such as gamma rays and X-rays.

Radiative Process

An event involving the emission or absorption of radiation. For example, a hydrogen atom that absorbs a photon of light converts the energy of that radiation into electrical potential energy.

Radio Waves

The part of the electromagnetic spectrum with the lowest energy. Radio waves are the easiest way to communicate information through the atmosphere or outer space.

Radioactivity

The spontaneous decay of certain rare, unstable, atomic nuclei into more stable atomic nuclei. A natural by-product of this process is the release of energy.

Rate Sensor Units (RSUs)

Boxes that house Hubble’s gyroscopes. Each rate sensor unit contains two gyroscopes. Astronauts remove the rate sensor units when they replace gyroscopes, so gyroscopes are always replaced two at a time.

Reaction Wheel

One of four spinning flywheels aboard the Hubble Space Telescope. The flywheels work together to make the observatory rotate either more rapidly or less rapidly toward a new target.

Receiver

The part of the radio telescope that detects long wavelength electromagnetic radiation and converts it to an electrical signal so that we can sense it.

Recessional Velocity

The velocity at which an object moves away from an observer. The recessional velocity of a distant galaxy is proportional to its distance from Earth. Therefore, the greater the recessional velocity, the more distant the object.

Red Giant Star

An old, bright star, much larger and cooler than the Sun. Betelgeuse (alpha Orionis) is an example of a red giant.

Redshift

The lengthening of a light wave from an object that is moving away from an observer. For example, when a galaxy is traveling away from Earth, its light shifts to the red end of the electromagnetic spectrum.

Reflection

Reflection occurs when light changes direction as a result of "bouncing off" a surface like a mirror.

Reflector

A type of telescope, also known as a reflecting telescope, that uses one or more polished, curved mirrors to gather light and reflect it to a focal point.

Reflector (Reflecting telescope)

A type of telescope, also known as a reflecting telescope, that uses one or more polished, curved mirrors to gather light and reflect it to a focal point.

Refraction

Refraction is the bending of light as it passes from one substance to another. Here, the light ray passes from air to glass and back to air. The bending is caused by the differences in density between the two substances.

Refractor

A telescope, also known as a refracting telescope, that uses a transparent lens to gather light and bend it to a focus.

Refractor (Refracting telescopes)

A type of telescope, also known as a refracting telescope, that uses a transparent convex lens to gather the light and bend it to a focal point.

Regolith

The layer of loose rock resting on bedrock (sometimes called mantle rock), found on the Earth, the Moon, or a planet. Regolith is made up of soils, sediments, weathered rock, and hard, near-surface crusts. On the surface of the Moon, regolith is a fine rocky layer of fragmentary debris (or dust) produced mainly by meteoroid collisions.

Relativity

A theory of physics that describes the dynamical behavior of matter and energy. The consequences of relativity can be quite strange at very high velocities and very high densities. A direct result of the theory of relativity is the equation E = mc2, which expresses a relationship between mass (m), energy (E), and the speed of light (c).

Resolution (Resolving power)

A measure of the smallest separation at which a telescope can observe two neighboring objects as two separate objects.

Resolve

The ability of a telescope to distinguish objects that are very close to each other as two separate objects.

Revolution

The orbital motion of one object around another. The Earth revolves around the Sun in one year. The moon revolves around the Earth in approximately 28 days.

Right Ascension (RA)

A coordinate used by astronomers to locate stars and other celestial objects in the sky. Right ascension is comparable to longitude, but it is measured in hours, minutes, and seconds because the entire sky appears to pass overhead over a period of 24 hours. The zero hour corresponds to the apparent location of the Sun with respect to the stars on the day of the vernal (spring) equinox (approximately March 21).

Rille

A long, narrow depression on the Moon's surface. A rille can be straight, have a sweeping arc, or meander, with many curves going in random directions.

Robotic Optical Transient Search Experiment (ROTSE)

A terrestrial telescope that searches for the optical counterparts of gamma-ray bursts. When orbiting satellites detect a gamma-ray burst, ROTSE begins searching for its visible-light afterglow. ROTSE-I (an array of four electronic telephoto cameras) and ROTSE-II (a set of identical telescopes) are located in Los Alamos, New Mexico.

Roche Limit

The smallest distance at which two celestial bodies can remain in a stable orbit around each other without one of them being torn apart by tidal forces. The distance depends on the densities of the two bodies and their orbit around each other.

Rocky Planet

A planet located in the inner solar system and made up mostly of rock. The rocky planets are Mercury, Venus, Earth, and Mars. This group is also known as terrestrial planets.

Rotation

The spin of an object around its central axis. Earth rotates about its axis every 24 hours. A spinning top rotates about its center shaft.

Satellite

A man-made object that orbits Earth, the Moon, or another celestial object.

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.

Scintillation

A flash of light produced when gamma rays strike a certain material. The high energy of gamma rays makes them hard to capture but they can be detected using scintillation.

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.

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.

Servicing Missions

Hubble was the first space telescope designed to be serviced in space. Scientists believed that periodic servicing missions would extend Hubble’s operating life and keep the observatory up-to-date. Astronauts have visited Hubble five times. The first servicing mission was in December 1993 and the second in February 1997. The third mission was split into two visits. Part A took place in December 1999 and part B in March 2002. The final servicing mission visit occurred in May 2009.

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.

Shock Wave

A high-pressure wave that travels at supersonic speeds. Shock waves are usually produced by an explosion.

Short-Period Comet

A comet that orbits mainly in the inner solar system. Short-period comets usually 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.

Soft Capture Mechanism

When Hubble reaches the end of its mission, NASA must be able to safely return the telescope to Earth. When that time comes, the space shuttle will no longer be operating, so another means of capturing the telescope must be available. The soft capture mechanism is a compact device that, when attached to the Hubble Space Telescope, will assist in its safe de-orbit. This device has structures and targets that will allow a next generation space vehicle to more easily capture and guide the telescope into a safe, controlled re-entry.

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 an hour. The composition of this high-speed solar wind may vary, but it always streams away from the Sun. The solar wind is responsible for the Northern and Southern Lights on Earth and causes the tails of comets to point away from the Sun.

Solar panels

Two rigid, wing-like structures that convert sunlight directly into electricity to operate a 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.

Solar telescope

A special reflecting telescope designed to study our closest star, the Sun. Solar telescopes differ from normal telescopes in that they are stationary and use small tracking mirrors to direct sunlight into the primary mirror. This is necessary because the Sun appears to move across the sky due to Earth’s rotation.

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.

Southern Hemisphere

Half of a spherical or roughly spherical body; for example, the Southern Hemisphere of Earth is the half below the equator.

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)

The Space Telescope Imaging Spectrograph (STIS) is a general-purpose spectrograph that spans ultraviolet, visible, and near-infrared wavelengths. It was installed in February 1997 during the Second Servicing Mission. A spectrograph works by breaking up light from an object into its individual wavelengths so that its composition, temperature, motion, and other chemical and physical properties can be analyzed. STIS stopped functioning in 2004 due to a power supply failure, but astronauts replaced a low-voltage power supply board during Servicing Mission 4.

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.

Spectrograph (Spectrometer/spectroscope)

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.

Spectroheliograph

An instrument used in solar telescopes to photograph the Sun in a single wavelength of light. Different wavelengths reveal different features of the Sun’s surface.

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.

Spectrum

The entire range of electromagnetic rays from the longest radio waves to the shortest gamma rays. Arranged from longest to shortest wavelengths, the spectrum of electromagnetic radiation includes radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays and gamma rays.

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.

Spherical aberration

Spherical aberration is an optical defect of a lens or mirror caused by its rounded shape. Spherical lenses and mirrors produce a distorted (blurry) image.

Spherical aberration in lenses

The shape of a spherical lens causes a problem called spherical aberration.

In spherical aberration, parallel light rays that pass through the central region of the lens focus farther away than light rays that pass through the edges of the lens. The result is many focal points, which produce a blurry image. To get a clear image, all rays need to focus at the same point.

Spherical aberration in mirrors

The shape of a spherical telescope mirror causes a problem called spherical aberration.

In spherical aberration, parallel light rays that bounce off the central region of a spherical mirror focus farther away than light rays that bounce off the edges. The result is many focal points, which produce a blurry image. To get a clear image, all rays need to focus at the same point.

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 huge ball of gas held together by gravity. The central core of a star is extremely hot and produces energy. Some of this energy is released as visible light, which makes the star glow. Stars come in different sizes, colors, and temperatures. Our Sun, the center of our solar system, is a yellow 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.

Static

Random noise in a radio receiver. It can also be heard in telephone lines and cell phones.

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.

Stellar nursery

A region in space where stars are forming from a cloud of gas and dust.

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.

Sunspots

A sunspot is 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.

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. A supernova remnant is the glowing, expanding gaseous remains of a supernova explosion.

Supernova Remnant

The glowing, expanding gaseous remains of a supernova explosion.

 
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