Glossary -- R

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radial velocity

the velocity along the line of sight.

radial velocity curve

a plot of how an object's velocity along the line of sight changes over time.

radiative zone

the region of a star's interior where energy is transported outward with photons. For the Sun, it is the region above the core.

radio galaxy

usually an elliptical galaxy emitting very large amounts of radio energy from the core (up to millions of times a typical galaxy's radio emission) and having strong radio emission from regions extending out several million light years from the galaxy nucleus.

radioactive dating

a technique that gives absolute ages of a material (rather than merely relative ages) from the number of radioactive active atoms remaining in the material.

realism

a way of viewing scientific theories and models that says they truly characterize the way the universe operates; they represent reality (contrast with instrumentalism).

recombination

process of electrons becoming bound to protons to make neutral atoms.

reddening

the preferential scattering of the shorter wavelengths of light as it passes through a dust cloud, so that a large fraction of the bluer wavelengths of light are scattered away from your line of sight while a large fraction of the redder wavelengths of light make it through the dust cloud unaffected. Dust clouds in space make stars behind the dust clouds appear redder than they would be if the dust was not there.

red giant

a dying star that has become large in diameter and cool on the surface while the core has shrunk and increased in temperature. Nuclear fusion takes place in a shell around the compressing core. They are more luminous than when the star was in the main sequence stage, even though their surface is cool, because they have a HUGE surface area. Therefore, they are plotted in the upper right part of the Hertzsprung-Russell diagram.

redshift

the shift of spectral lines from an object to longer wavelengths because the object is moving away from the observer. The greater the speed of the object, the greater the redshift will be.

reflector telescope

telescope that uses a large mirror at the back of the telescope to gather and focus the light. It has no size limit and is the type preferred for large research telescopes.

refraction

the bending of waves when they pass from one transparent medium (or vacuum) to another (e.g., sunlight bending as it passes through the Earth's atmosphere).

refractor telescope

telescope that uses a large glass lens at the front end of the telescope to gather and focus the light. The glass lens has a maximum size limit and suffers to some degree from chromatic aberration.

representative sample

a collection of objects that includes all parts of the population of the objects in their proper proportions; an unbiased sample that gives accurate results.

resolving power

the ability of a telescope to detect very small details and produce sharp images. It depends on the diameter of the telescope's objective or the interferometer AND the wavelength of light used to observe, such that the more wavelengths that can be fit across the objective or interferometer, the sharper the image will be.

retrograde motion

when a planet moves ``backward'' (westward) with respect to its normal eastward drift against the stars. It happens when the Earth is closest to the planet.

rich cluster

a cluster of hundreds to thousands of galaxies.

right ascension

(RA): position on the celestial sphere measured with respect to the vernal equinox position on the celestial equator. It is a projection of longitude lines onto the sky and converted to time units. An object's right ascension is fixed with respect to the stars. Varies from 0^h at the vernal equinox point to 24^h in a full circle.

rotation curve

how the orbital velocities of objects in the disk of a spiral galaxy vary with increasing distance from the center of the galaxy. The rotation curve is used to study the distribution of mass in a galaxy.

RR Lyrae

(variable star): a type of low-mass variable stars that all have the same average luminosity. RR Lyrae are valuable for determining distances to star clusters.

runaway greenhouse

a process in which the heating of a planet increases the greenhouse effect in a feedback loop resulting in a dramatic change in the atmospheric composition and dramatic rise in the surface temperature. Venus' atmosphere is an example of this process.

runaway refrigerator

a process in which the cooling of a planet's surface decreases the greenhouse effect in a feedback loop resulting in a dramatic change in the atmospheric composition and dramatic cooling in the surface temperature. Mars' atmosphere is an example of this process.

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last update: 06 August 1999