9. Glossary of Terms
Astronomical Unit (AU) The semi-major axis of Earth’s orbit
around the Sun, corresponding to a distance of about 150
million km.
Blackbody radiator An idealized object that radiates electromagnetic
energy into space in accordance with Wien’s law and the
Stefan-Boltzmann law.
Black dwarf A near fully cooled-off white dwarf.
Brown dwarf An object with a mass of between ∼0.01 and ∼0.1
M Such objects are not massive enough to initiate hydrogen
fusion reactions within their interiors.
Chandrsekhar limit The limiting mass for a stable white dwarf
star – MWD ≤ 1.4 M The limit is set according to the degenerate
electrons acquiring relativistic speeds.
CNO cycle The catalytic fusion reaction that enables the
conversion of four protons into a helium nucleus with the liberation
of energy. The process begins with the reaction 12C+P ⇒ 13C.
Croll-Milankovitch cycle Climate changes that are driven in
response to small variations in the size, eccentricity, and orientation
of Earth’s orbit.
Degenerate matter Matter that behaves according to the Pauli
exclusion principle.
e-folding time The time required to reduce a quantity by a factor
of e = 2.71828….
Gaia hypothesis An idea developed by James Lovelock which views
Earth’s biosphere as a self-regulating system that operates via a
complex series of interactions between its physical, chemical, and
biotic components.
GRB Gamma-ray burst. This event is believed to be associated
with a hypernova, which is characterized by the release of large
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214 Rejuvenating the Sun and Avoiding Other Global Catastrophes
quantities of very-short wavelength electromagnetic radiation (i.e.,
-rays and X-rays).
Greenhouse effect The process by which a planetary atmosphere
is heated by the absorption of infrared radiation emitted by the
planetary surface (after it has been heated by sunlight).
Hydrostatic equilibrium The achievement of a dynamical balance
between the gravitational force, which is trying to collapse a star,
and the outward (hot gas) pressure forces that are trying to expand
a star.
Hypernova The final disruption stage of a massive, rapidly rotating
star.
Kuiper Belt A disk-like distribution of primarily icy objects
orbiting the Sun out to distances of several thousands of astronomical
units. The existence of these objects was predicted by
Gerald Kuiper in 1951. Pluto is the closest Kuiper Belt object
to the Sun, while Eris is currently the largest known such
object.
Lamor radius The radius of gyration for a charged particle moving
along a helical path along a magnetic field line. For a given
magnetic field strength the gyration radius increases in proportion
to the particle mass.
Malthusian catastrophe The collapse of a population caused by it
outgrowing the capacity to feed itself.
Metals A collective term applied by astronomers to those elements
other than hydrogen and helium.
NEA Near-Earth asteroids. Those asteroids with orbits bringing
them periodically close to Earth.
NEOs Near-Earth objects. The collective name given to those
asteroids and comets that periodically pass close to Earth.
Neutron star The remnant of a star initially more massive
than eight times that of the Sun that has undergone supernova
disruption. Such stars are supported against gravitational collapse
by degenerate neutron pressure.
Oort Cloud An extensive cloud of many trillion (or more) cometary
nuclei that surrounds the Sun and delineates the outer boundary of
the Solar System at distances of more than 100,000 astronomical
units. The existence of this cometary cloud was first suggested by
Jan Oort in 1950.
Glossary of Terms 215
Parsec (pc) The distance at which an object would have a stellar
parallax of one arc second.
Pauli exclusion principle (PEP) A quantum mechanical effect, first
described by Wolfgang Pauli in 1925, which forbids two or more
electrons from occupying the same phase space. A gas obeying the
exclusion principle is said to be degenerate.
Photosphere The outermost region of the Sun where photons can
escape into space without further atomic interactions.
Planetary nebula An ionization nebula produced by a star that
has exhausted helium within its core and is evolving into a white
dwarf.
PP chain The fusion reaction that enables the conversion of four
protons into a helium nucleus with the liberation of energy. The
process begins with the reactionP+P⇒ D + energy.
Quarks Fundamental particles that carry a fractional electrical
charge. Quarks come in six ‘flavors,’ and they are either found in
groups of two (making up so-called mesons) or three (making up
so-called hadrons). The proton is composed, for example, of two
‘up’ quarks and one ‘down’ quark.
Spectral classification A classification scheme based upon
measured spectral lines and which characterizes stars according to
their surface temperature.
Stefan-Boltzmann law One of the fundamental laws pertaining to
blackbody radiators. The law dictates that the energy radiated into
space per second per meter squared (i.e., the flux of electromagnetic
radiation) varies as the temperature of the blackbody radiator raised
to the fourth power.
Stellar parallax The apparent motion in the position of a star, over
a six-month interval, due to Earth’s motion around the Sun
Strange stars Hypothetical stars made of strange quarks held
together by gravity rather than the strong interaction force of
elementary particle physics.
Terraforming The process by which a planet is made habitable.
Torrino scale A scale developed to describe the impact threat
associated with near-Earth asteroids and comets.
Triple- reaction The fusion reaction that enables the conversion
of three helium nuclei into a carbon nucleus, liberating energy in
the process.
216 Rejuvenating the Sun and Avoiding Other Global Catastrophes
White dwarf Post helium-burning phase of stars with initial masses
of less than eight times that of the Sun. Such stars are supported
against gravitational collapse by degenerate electron pressure.
Wien’s law If a blackbody radiator of temperature T emits a
maximum energy flux at wavelength max, then Wien’s law dictates
that the product max T is a constant.
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