DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

spect to normal galaxies of the same morphological
type. Associated features include often high
ultraviolet emission, galactic superwinds, dominant
photoionization lines in the far infrared
spectra, evidence for active periods in the recent
star formation history, etc. Spectral features
of Wolf Rayet stars (which are very bright
and have broad, easily-identified emission lines)
imply a recent burst of massive star formation,
since these last only 3 to 4 Myr before they explode.
Red supergiants, that last ≈ 10 Myr, signify
a somewhat older burst. Starburst galaxies
often show evidence of some sort of interaction,
harassment or merger in their recent past. Prototype
Starburst galaxies are the edge-on spiral
NGC 253 and the dwarf irregular galaxy Messier
82. FIR-strong galaxies, blue galaxies, UV excess
galaxies, or Hii galaxies are often systems
where star formation is enhanced with respect
to normal galaxies, and can be synonyms of
Starburst galaxies. The different names reflect
the different technique and the different spectral
range of observation and discovery.
Stardust A spacecraft launched on February
7, 1999, that is the first space mission that will
fly close to a comet and bring cometary material
back to Earth for analysis. It was developed
under NASA’s Discovery Program of low-cost
solar system exploration missions. Its primary
goal is to collect comet dust and volatile samples
during a planned close encounter with comet
Wild 2 in January of 2004. The spacecraft will
also bring back samples of interstellar dust including
the recently discovered dust streaming
into the solar system from the direction of Sagittarius.
A unique substance called aerogel is the
medium that will be used to catch and preserve
comet samples. When Stardust swings by Earth
in January 2006, the samples encased in a reentry
capsule will be jettisoned and parachuted to
a pre-selected site in the Utah desert. Groundbased
analysis of these samples should yield important
insights into the evolution of the sun and
planets, and possibly into the origin of life itself.
Stardust is a collaborative effort between
NASA, university and industry partners.
star formation The process by which diffuse
gas collapses to form a star or system of stars.
This follows the concentration of the atomic gas
© 2001 by CRC Press LLC
Stark effect
within galaxies into dense, opaque, molecular
clouds, and the further condensation of regions
within these clouds into dense cores that become
unstable to collapse. During collapse, a
star and its surrounding protoplanetary accretion
disk grow from infalling matter.
Stars are born with vastly less magnetic flux
and angular momentum than their parent clouds.
The loss of magnetic flux is attributed to ambipolar
diffusion, the motion of neutral material
across magnetic field lines, that operates in
molecular clouds. Magnetic flux may be also
lost by reconnection. Angular momentum is lost
by magnetic braking before collapse, and then
by the emission of a powerful, collimated wind
powered by accretion through the disk. Such
winds strongly affect the star-forming cloud by
adding turbulent motion and by casting away
material.
star formation rate (SFR) The rate at which
new stars are being formed in a galaxy, or in any
star forming region, often measured in units of
solar masses per year. The star formation rate
can be estimated from measurement of the luminosity
of the Hydrogen Hα spectral line, or from
the luminosity emitted in the far infrared, if the
initial mass function of stars is assumed. Galaxies
of late morphological types, on the contrary,
have yet to exhaust their molecular gas, and
are still forming stars. The star formation rate
in galaxies varies widely along the Hubble sequence:
For galaxies in the local universe, typical
values are close to zero for elliptical galaxies
and ∼ 10 solar masses per year for Sc spirals.
Stark effect The sub-division of energy levels
within an atom caused by the external application
of an electric field. For hydrogenic
atoms, the energy shift is directly proportional
to the applied field strength; this is known as the
linear Stark effect. For other atoms and ions,
the displacement of the energy levels is proportional
to the square of the applied electric field
strength, giving rise to the quadratic Stark effect.
The latter results from the additional condition
that the induced dipole moment is also
proportional to the intensity of the applied electric
field. For hydrogen atoms, electric fields of
intensity > 10 V/cm are required to produce an
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