DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

star spots
observable Stark effect, with greater fields being
required to produce the quadratic Stark effect.
star spots The analog of sunspots on other
stars. They are usually darker (cooler) than their
surroundings (though bright spots also occur)
and are associated with concentrations of magnetic
field in their vicinity, as are sunspots. The
fieldapparentlyinhibitsenergytransportbyconvection
in the atmospheres of the relatively cool
stars that have spots, leading to cooler, darker
regions. Star spots are particularly conspicuous
(and sometimes found near the poles rather than
near the equator as in the sun) in young, rapidly
rotating stars, and in close binary pairs, where
the rotation period is synchronized to the short
orbital period. They show up as changes in the
brightness and spectrum of the star that approximately
repeats at the rotation period.
statcoulomb Unit of electric charge, 3.3356
×10 −10 coulomb. Previously defined as equal
to the charge that exerts a force of 1 dyne on
an equal charge at a distance of 1 centimeter in
vacuum.
state parameter See electric regime (cosmic
string).
steady-state model The cosmological model
of Bondi, Gold, Hoyle and Narlikar in which
matter is continuously created to fill the voids
left as the universe expands. Consequently, such
a universe has no beginning and no end and always
maintains the same density. This model
of the universe is thus derived from a stronger
version of the cosmological principle, that the
universe is not only homogeneous and isotropic
(see homogeneity, isotropy), but also unchanging
in time. This stronger version, the “perfect
cosmological principle,” was postulated as
a universal law of nature. The rate at which
new matter would have to appear to offset the
dilution caused by expansion is one hydrogen
atom in one liter of volume once in 10 9 years.
There is no experimental evidence against this
assumption since the rate is undetectably small.
The steady state models were motivated by the
discrepancy between the age of the universe and
the age of the Earth — the latter was apparently
longer than the former; the steady-state model
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removed this discrepancy by making the universe
eternally-lived. The discrepancy was later
resolved as an error in estimating the value of
the Hubble parameter that led to a drastic underestimation
of the age of the universe. The
steady-state model was definitively proven false
when the microwave background radiation was
discovered in 1965. This radiation finds a simple
explanation in models with a Big Bang, and
no natural explanation in the steady-state model.
steepness (wave) A term used in the study of
water waves, defined as the ratio of wave height,
H , to wavelength, L, orH/L. A common rule
of thumb is that the wave steepness cannot exceed
1/7 in deep water before wave breaking will
occur.
Stefan–Boltzmann constant (σ ) Constant in
the equation for the radiant emittance M (radiant
energy flux per unit area) from a black body at
thermodynamic temperature T : M = σT 4 .
σ = 2π 5k4 15c2h3 = 5.66956 × 10−5
erg cm −2 deg −4 sec −1
where k is Boltzmann’s constant, h is Planck’s
constant, and c is the speed of light.
stellar activity The complex of X-ray, visible
light, and radio wave phenomena associated
with rapid rotation, strong magnetic fields, and
the presence of a chromosphere and corona on
a star (generally one whose photospheric temperature
is less than about 6,500 K). These phenomena
include emission lines of hydrogen, calcium,
and sodium emitted by the chromosphere,
X-rays and occasionally radio waves emitted by
the corona, starspots, with magnetic fields up
to several thousand gauss (generally detectable
only on the sun), and flares.
stellar classification A classification of stars
according to the observed surface temperature;
see below.
There is a further subdivision within each
range with the numbers from 0 to 9, eg G5, A0.
The stellar classification is often called the
Henry Draper system. It was first published
(1918–1924) by Annie Jump Cannon of Harvard