Glossary – R

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Radian
1/(2π) of the circumference of a circle. One radian = 57.3 degrees.
Radiation Belt
regions around planets that contain high-energy charged particles which are trapped within the planet’s magnetic field.
Radio
the name given to the lowest energy region of the electromagnetic spectrum. Radio waves have wavelengths of meters (m), or even kilometers (km, 103m).
Radiation
the tendency of unstable atomic nuclei to break apart into stable ones by emitting particles and (electromagnetic) radiation.
Radioactivity
the natural or artificial process that causes the nucleus of a stable atom to become unstable thereby breaking apart to become a new element.
Radio Galaxy
a galaxy that is a powerful source of radio waves.
Radio Lobe
A large radio-wave emitting cloud of matter located at the ends of the jets in some active galaxies, formed when the matter from the jet is slowed by intergalactic material.
Radiography
an imaging technique that uses X-rays to view the internal structure of objects like the human body.
Red Giant
at the end of its life, a dwarf star will turn into a red giant, growing to several times its normal size and with a relatively low surface temperature. The low surface temperature makes it appear red, thus the name, red giant.
Red Shift
the name given to the apparent change in the wavelength of light due to the Doppler Effect. Scientists know what the regular spectrum of a galaxy should look like (based on the spectrum of light emitted from known elements). If the light waves from a galaxy appears to have shifted towards higher frequency (blue), it is moving towards us, and if it has shifted towards a lower frequency (red), that means the object is moving away.
Relativistic Speed
a speed near to the speed of light. At such high speeds objects show effects predicted by Einstein’s Theory of Special Relativity.
Resolution
the degree of detail visible in an image.
Return Value
this is the final result of a function that is returned to the function that initiated it.
Right Ascension (RA)
Used in the equatorial coordinate system to measure the angular position of a celestial object around the celestial equator. Right ascension is typically measured in sidereal hours, minutes, and seconds, with positive angles Eastward about the celestial equator from the vernal equinox.