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- 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.
