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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.
- a way of viewing scientific theories and models that says they
truly characterize the way the universe operates; they represent reality (contrast
- process of electrons becoming bound to
protons to make neutral atoms.
- 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.
- 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.
- 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
- 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 solar system object (e.g., a planet) moves ``backward'' (westward)
with respect to its normal eastward drift against the stars. It happens when
the Earth is closest to the object.
- 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
0h at the vernal
equinox point to 24h 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
- 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
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.
Glossary links (select a letter for definitions of astronomy terms
beginning with that letter):
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
last updated: 27 May 2001
Is this page a copy of Strobel's
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