DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

spiral galaxy
more evolved stars. (Although all masses of
stars are formed in the star forming region, the
massive blue stars are brighter, and they live a
substantially short time, so they remain near the
spiral wave that triggered their formation; they
expire before they have a chance to move far
from their birthplace.) Dust lanes, filamentary
absorption features that appear dark on the emission
background of the galaxy’s disk, are often
associated with spiral arms. The physical origin
of the spiral pattern of galaxies is a subject of
current debate; it is presumably a density wave
in the rotating galactic disk, moving through the
disk at approximately 20 km/sec; the regions of
compression become regions of star formation.
“Grand design” spirals have been linked to the
gravitational perturbation by a nearby companion
galaxy, as in the case of M51. See also M51.
spiral galaxy A galaxy showing a bright
spiral pattern, superimposed to smooth disk
emission. Spiral galaxies are composed of a
spheroidal bulge and a flattened system of gas
and stars (the disk), over which the spiral pattern
is seen. While the bulge of a spiral galaxy
loosely resembles an elliptical galaxy, the disk
shows an exponential decrease in surface brightness
with increasing distance from the nucleus.
Both the prominence of the bulge and the shape
of the spiral pattern vary along the Hubble sequence
from S0, the lenticular galaxies, to Sa
(which show large nuclii and faint spiral structure),
to Sb, to Sc galaxies which have small
nuclei and strong spiral arms. Many galaxies
show a bar across the nucleus, in which case
a B is added to the classification, as in SBc.
In total count, S0 amount to 22%, and Sa,b,c
galaxies to 61% of all galaxies observed. Spiral
galaxies are most common in the “field”, i.e.,
not in clusters (“morphology-density relation”).
See also Hubble sequence, spiral arm.
split-hull barge A barge that is constructed
so that its hull can be opened to drop the contents
of its hold. The hinge runs along the long axis
of the barge.
Spoerer’s law Describes the spatial distribution
of sunspots during the solar cycle: sunspots
always start at relatively high latitudes (about
30 ◦ ) and move towards the equator. In addition,
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during the solar cycle the latitude of emergence
of sunspots moves also toward the equator.
spontaneous symmetry breaking In condensed
matter physics, the zero magnetization
of an (initially) isotropic ferromagnetic system
takes on a nonvanishing value (and thus, the
magnetization points in a particular direction)
as the temperature decreases below the critical
temperature Tc. Thus, the initial symmery (the
isotropy) is spontaneously broken. The case for
particle physics is described by a scalar field φ
called the Higgs field, and now the order parameter
characterizing the transition is the vacuum
expectation value |φ| of this field. The vacuum
expectation value is the expected value of a field
in its lowest possible energy configuration.
Research done in the 1970s in finite-temperature
field theory led to the result that the
temperature-dependent effective potential for
the Higgs field can be written roughly as
VT(|φ|) =− 1
2 m2 (T)|φ| 2 + λ
4! |φ|4
withT 2
c = 24m2 0 /λ,m2 (T)=m2 0 (1−T 2 /T2 c ),
this potential has one minimum energy (the
“false vacuum”) at |φ| =0 whenT is large, but
a different value given by 〈|φ|〉 2 = 6m2 (T)/λ
wherem0 andλ are two positive constants when
T is small. Because this minimum depends only
on the magnitude of |φ|, not on its sign or phase,
there are possibly more than one low temperature
minima, a situation called degenerate true
vacuua.
For energies much larger than the critical
temperature (in appropriate units), the fields are
in the highly symmetric state characterized by
|φ| = 0. But, when energies decrease the symmetry
of the system is spontaneously broken:
the scalar field rolls down the potential and sits
in one of the nonvanishing degenerate new minima.
See cosmic phase transition.
sporadic E Sporadic E (Es) layers are transient
localized thin patches of relatively high
electron density occurring at (95 to 140 km) E
layer altitudes. While the peak sporadic E electron
density can be several times larger than the
normal E region, it is independent of the regular
solar produced E layer. Although the layers