DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Hubble radius
Press 1982; Mon. Not. Roy. Astr. Soc., 113, 658,
(1953).
Hubble radius Distance at which the recession
velocity of a galaxy will be the speed of
light: cH−1 o = 3000 h−1 Mpc. Galaxies located
at larger distances will not be in casual
contact with us. Hence, it gives the scalelength
of the particle horizon. See Hubble parameter.
Hubble–Reynolds law Empirical law describing
the brightness profile of an elliptical
galaxy, introduced by J.H. Reynolds in 1913.
According to Hubble–Reynolds law, the surface
brightness depends upon the distance from the
galaxy center, r, as (r) = 0/(1 + r/r0),
where the scaling parameter r0 is the radius at
which the surface brightness falls to one quarter
its central value 0. This law, remarkable
because of its simplicity, predicts a deficit of
light close to the center and more light in the
outer envelope of a galaxy with respect to de
Vaucouleurs’ law. See de Vaucouleurs’ law.
Hubble sequence A classification scheme
of galaxies created by E. Hubble. In the Hubble
sequence, galaxies are subdivided into elliptical
galaxies, S0 galaxies (galaxies showing
evidence of an amorphous disk and a bulge, but
no spiral arm, also called lenticular galaxies),
spiral galaxies, either barred or non barred, and
irregular galaxies. An elliptical galaxy is conventionally
indicated with the uppercase letter
E and an integer number ranging from 0 to 7,
increasing with the apparent flattening, and defined
as the integer part of 10 × (1 − (b/a)),
where (b/a) is the axial ratio measured on a
photograph or digital image. Spiral galaxies are
farther subdivided along the sequence in S0, Sa,
Sb, Sc according to three criteria: (1) decreasing
bulge prominence with respect to disk; (2) spiral
arms less tightly wound; (3) appearance of
arms more resolved. At the end of the sequence,
irregular galaxies (subdivided into Magellanic
and amorphous or M82-type) do not show regularly
decreasing surface brightness nor spiral
arms and are of patchy appearance. Elliptical
and S0 galaxies are collectively referred to as
“early morphological types”, and Sc and irregulars
as “late type” galaxies. Hubble attributed
to these terms an evolutionary meaning, i.e., he
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thought that a spiral could be an evolved elliptical
galaxy. This view is not considered appropriate
anymore, since the angular momentum
per unit mass, a constant for an isolated galaxy,
increases along the sequence from the elliptical
to the most flattened galaxies (Sc). Nevertheless,
gravitational interaction between galaxies
can affect the morphology of spiral galaxies to
the point of changing their Hubble type. See
elliptical galaxies, spiral galaxy.
Hubble’s law Relation between the recession
velocity of a galaxy v and our distance to
it d: v =Hod where Ho is called Hubble parameter
to honor Edwin Hubble who discovered
this relation in 1928. In an expanding universe,
this direct proportionality is a consequence of
the homogeneity and isotropy (see cosmological
principle). The law is only exact on the
average. Local irregularities in the matter distribution
create small deviations. See peculiar
motion.
Hubble space telescope (HST) A spacebased
telescope of 2.4 m aperture, launched
in 1990 and orbiting in a low terrestrial orbit.
Although the optical design of HST is similar
to that of a mid-sized ground-based telescope,
the absence of atmosphere allows the telescope
to operate at a resolution close to the diffraction
limit (0.03 arcsec at 3000 Å), and to detect
UV light which is absorbed by the terrestrial
atmosphere. Currently available instruments
on board HST include two imaging cameras, a
long-slit spectrograph, and a camera and spectrometer
operating in the near infrared. The
Wide Field Planetary Camera, which is composed
of three CCD detectors in an L-shape configuration
plus a single, smaller CCD detector at
the center of field, has limiting magnitude of 28
(with 1-h exposure time and S/N ratio 5), and
reaches a resolution of 0.053 sec of arc. The
highest resolution, 0.042 sec of arc, is achieved
with the Faint Object Camera, which has a much
smaller field of view, 7×7 square arcsec. (Traditional
ground-based optical telescopes have resolution
limited by atmospheric seeing to about
1 arcsec, although modern adaptive optics techniques
can improve ground-based resolution in
some wavelengths to well below 1 arcsec.) The
Space Telescope Imaging Spectrometer, which