Glossary
Astronomy glossary
514 definitions · page 7 of 9.
- prograde
- objects that move or appear to move in the same direction of most solar system bodies, or for moons, the same direction as the planet rotates
- prominence
- a large eruption of gas streaming off the surface of the sun into the corona
- proper motion
- the apparent yearly motion of a star across the sky
- proton
- a subatomic particle that resides in an atom’s nucleus and possesses a positive electric charge
- protoplanet
- a body that is accreting gas, dust, and rocks en route to becoming a full-fledged planet
- protoplanetary disk
- a disk of gas and dust that surrounds a newborn star; planets form from collisions of particles inside the disk
- protostar
- a cloud of hot, dense gas and dust that is gravitationally collapsing to form a star
- Proxima Centauri
- the nearest star to the sun at a distance of 4.2 light-years
- pulsar
- a rapidly rotating neutron star that bathes Earth in regular pulses of electromagnetic radiation
- quadrillion
- 1,000,000,000,000,000 (in American usage)
- quantum mechanics
- the physical laws that describe the behavior of matter at the atomic and subatomic level
- quasar
- the highly energetic core of a young galaxy thought to be powered by a supermassive black hole; short for quasi-stellar object
- radial velocity
- the velocity of an object toward or away from an observer
- radiant
- 1. the point in the sky from which the meteors belonging to a meteor shower appear to originate2. vividly bright and shining
- radiation
- electromagnetic waves (in astronomical usage)
- radiation pressure
- a very small amount of pressure exerted on a surface by light or other electromagnetic radiation
- radio galaxy
- a galaxy that emits an unusually large amount of radio waves
- radio telescope
- a telescope designed to detect radio waves coming from space
- radio waves
- the form of light with the longest wavelength and the least energy
- radiometer
- a device that measures the total energy or power from an object in the form of radiation, especially infrared radiation
- red dwarf
- a low-mass, main-sequence star much smaller, cooler, and less luminous than the sun
- red giant
- a cool star near the end of its life cycle that has expanded to a size of a few dozen to a hundred times the diameter of the sun
- red supergiant
- a cool, massive star near the end of its life that has expanded to a size from a hundred to a thousand times the diameter of the sun
- redshift
- an increase in the wavelength of light coming from an object due to its motion away from Earth, the expansion of the universe, or a strong gravitational field
- reflection nebula
- a cloud of gas and dust that is visible because the dust reflects a nearby star’s light
- reflector
- a telescope that uses a curved mirror to gather light
- refractor
- a telescope that uses a glass lens to gather light
- regolith
- the unconsolidated residual or transported rock and soil that overlies solid bedrock on Earth, the moon, or another planetthe powdery soil of the moon produced by meteorite impacts
- relativity
- the theories of physics developed by Albert Einstein that describe measurements made by two observers who are in relative motion
- resolution
- the ability of a telescope or camera to pick out fine detail
- resolving power
- the ability of a telescope or camera to pick out fine detail
- reticule
- a grid or pattern of two or more fine wires set inthe focal plane of a telescope eyepiece and used in determining the position and/or size of a celestial object
- retrograde
- objects that move or appear to move in the opposite direction of most solar system bodies; for example planets that appear to move east-to-west in the sky or objects that revolve or rotate clockwise as seen from north of the solar system
- reusable launch vehicle (RLV)
- a single-stage-to-orbit spacecraft that may be reused on successive missions
- revolution
- the orbital motion of one body around another body or a common center of mass
- ribonucleic acid
- a nucleic acid that transmits genetic information
- rich clusters
- large galaxy clusters with unusually high population densities
- rich-field telescope
- a telescope designed to show a large field of view at low magnification
- RNA
- a nucleic acid that transmits genetic information
- rotation
- the spin of a galaxy, star, planet, moon, or asteroid about a central axis
- rotation period
- the length of time it takes a body to complete one rotation
- satellite
- a small body that orbits a planet or asteroid
- Schmidt camera
- a catadioptric telescope used as a camera to take wide-angle photos of the sky
- Schmidt-Cassegrain telescope
- a compact telescope in which light passes through a correcting lens at the front of the telescope, then reflects off a primary mirror back up to a secondary mirror, which directs the light through a hole in the primary and out the back of the scope; a…
- secondary mirror
- a relatively small mirror used in a telescope to redirect the light gathered by the primary mirror
- seeing
- the quality of observing conditions induced by turbulence in Earth’s atmosphere, which blurs the images of astronomical objects
- semimajor axis
- the average distance of an orbiting body from its parent body
- SETI
- the search for extraterrestrial intelligence
- setting circles
- circular scales on the two axes of an equatorial mount that help an observer point a telescope to a specific right ascension and declination
- Seyfert galaxy
- a galaxy (usually a spiral) with a very bright nucleus and strong spectral emission lines; the first was discovered in 1943 by Carl Seyfert
- shock wave
- a powerful wave caused by a sudden change in density, pressure, or temperature that travels though a medium faster than sound travels through that same medium
- short-period comet
- Comets that have orbital periods of less than 200 years.
- sidereal
- relating to or measured with respect to the stars
- sidereal year
- the amount of time it takes one body to revolve about another with respect to the stars
- siderostat
- a flat mirror that can be moved to reflect light from a celestial object to a specific spot
- singularity
- a point at which space and time are infinitely distorted, such as the central point of a black hole where matter is concentrated into an area of zero volume and infinite density
- small Magellanic Cloud
- a small, irregular dwarf galaxy that orbits the Milky Way Galaxy
- solar eclipse
- an eclipse of the sun caused by the moon passing between Earth and the sun
- solar filter
- a filter used to block almost all of the sun’s light so our star can be viewed safely and comfortably
- solar irradiance
- the radiant energy emitted by the sun over all wavelengths that falls each second on one square meter of Earth's atmosphere