DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

When calculations are displayed, there can
be points for stars of particular masses, compositions,
and evolutionary phases, or, more often,
continuous trajectories representing the temporal
evolution of a particular model (evolutionary
track) or the loci of a number of different
star masses at the same time (isochrones).
Comparisons between theory and observation
are made by plotting both real stars and tracks
or isochrones. The problem of converting calibration
from observers’ quantities (like spectral
type and absolute visual magnitude) to theorists’
quantities (bolometric luminosity and effective
temperature) remains a serious one.
Major phases of stellar evolution are frequently
named for the loci of the stars in an
HR diagram, including the main sequence, subgiant
branch, Hertzsprung gap, red giant branch,
horizontal branch, and asymptotic giant branch.
Each of these corresponds to a fairly welldefined
nuclear source of stellar energy and to a
fairly definite interior structure in temperature,
density, and chemical composition vs. radius.
Both observations and models show that the primary
determinant of what a star will do throughout
its life is its initial mass. See absolute magnitude,
apparent magnitude, asymptotic giant
branch star, bolometric magnitude luminosity,
color index, effective temperature, Hertzsprung
Gap, horizontal branch star, main sequence star,
metallicity, red giant.
Hubble deep field A high galactic latitude
sky field (of width ≈ 2 arcmin), intensively observed
in four colors with the Wide Field Planetary
Camera mounted on board the Hubble
Space Telescope in December 1995. Three sets
of observations for a total of 35-h exposure time
were obtained with broad band filter centered at
450, 606, and 814 nm, and a set of 50 h exposure
time with a near UV filter centered at 300 nm.
The Hubble deep field was chosen in an area of
low Hi column density, small far-IR flux, with
no radio source brighter than 1 mJy, no bright
stars, and no nearby galaxy clusters. These selection
criteria were aimed at making possible
the identification and morphological study of a
large number of faint, field galaxies. The observations
allowed counting the number of galaxies
in the field down to a magnitude < ∼ 29, an unprecedented
achievement. See Jansky(Jy).
© 2001 by CRC Press LLC
Hubble parameter (Hubble constant)
Hubble diagram A diagram plotting observed
redshift of a galaxy on one axis vs. some
measure of the distance of the galaxy on the
other. Used to verify or to detect deviations
from Hubble’s law: v = H0d, where d is the
distance to the galaxy.
Hubble parameter (Hubble constant) The
proportionality factor H0 in the Hubble law
v = H0l, where v is the velocity with which
a given galaxy G recedes from our galaxy, a
measure of the present expansion rate of the universe,
and l is the distance between G and the
observer in our galaxy. H0 is traditionally measured
in km/(sec · Mpc), so that v comes out
in kilometers per second when l is measured in
megaparsecs (Mpc). The actual numerical value
of H is subject to many random and systematic
errors. In the 1930s H0 was believed to be of the
order of 500 km/(sec · Mpc). Currently it is believed
to be between 50 and 100 km/(sec · Mpc).
In determining the Hubble constant, H0, itis
the distance determination that introduces uncertainty
since the recession is directly measured
by the redshift. This uncertainty in the measured
value of H is usually taken into account by incorporating
the parameter h (called the dimensionless
Hubble parameter) into all formulae that
depend on H ; by definition h = H/(100 km/s
· Mpc) so that h = 1 when H = 100 km/s ·
Mpc. The value of H determines the age of the
universe (i.e., the time since the Big Bang up
to now) and the distances between galaxy clusters,
so knowing it is of fundamental importance
for cosmology. The Hubble parameter is colloquially
called “Hubble constant”, but in the
currently accepted cosmological models it is a
decreasing function of time and “the value of
H ” actually means the current value H0. In inhomogeneous
models the Hubble law does not
apply globally; in them, v is a nonlinear function
of l, a different one for every observer. If
our universe is inhomogeneous, then the Hubble
law applies only as a first approximation to the
actual function v(l) in a sufficiently small neighborhood
of every observer, and the local value
of the Hubble parameter depends on the position
of the observer and on the direction of observation.
Reference: E.P. Hubble, The Realm of
the Nebulae, [republication by] Yale University
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