HALO:
the outer region of a galaxy containing globular
culusters a few stray stars, and dark matter
HUBBLE CONSTANT:
the value that expresses the expansion rate
of the universe and is equal to the recessional
velocity of a galaxy divided by the distance
to the galaxy; usually expresssed in kilometers
per second per megaparsec(3.26 million
light-years
INFRARED LIGHT:
a form of light with slightly lower energy
than visible light but with greater energy
than radio waves
KELVIN:
a unit of temperature equal to one degree
on the Celsius scale and 1.8 degrees on the
fahrenheit scale; also the absolute temperature
scale difined so that 0 kelvin is absolute zero
KUIPER BELT:
a region in the outer solar system beyond
Neptune's orbit that contains billions of
small, icy bodies; Pluto is the largest known
Kuiper belt member
LIGHT-YEAR:
the distance light travels in one year;
equivalent to approximately 5.9 trillion
miles(9.5 trillin km).
LOCAL GROUP:
the galaxy cluster containing roughly 35 galaxies
to which the Milky Way Galaxy belongs
LUMINOSITY:
the total amount of light that an object radiates
MAGNITUDE:
the measurement of the brightness of an object;
the lower the munber, the brighter the object.
Absolute:
the apparent brightness an object would have if
it were 10 parsec(32.6 light-years) from Earth
Apparent:
the measure of the brightness of an object
as seen from Earh
MASS:
a measure of the total amount of matter
within an object
MERIDIAN:
an imaginary circle on the celestial sphere
that connects the zenith to the north (or south)
celestial pole
METEOR:
a flash of light that occurs when a meteoriod
METEORITE:
a rock from space that survives passage through
Earth's atmosphere and falls to the ground
MILKY WAY:
the band of light that encircles the entire sky
and resuls from the combined light of billions of
stars in our galaxy's disk
NUBULA:
a cloud of interstellar gas and dust' some
nebulae represent stellar nurseries, others
represent stellar graveyards
NEUTRON STAR:
the collapsed, extraordinarily dense,
city-sized remnant of a high-mass star
NOVA:
a violent explosion on the surface of a
white dwarf, which causes the star to
temporarily brighten by a factor of several
hundred to several thousand
OBJECTIVE:
a telescope's primary lens or mirror that
gathers light and brings it to a focus
OCCULTATION:
the passage of one object in front if a
smaller one, temporarily obscuring all or
part of the background object from view
OORT CLOUD:
a cloud of cometary nuclei that surrounds
the sun at a distance of many thousands of
astronomical units
OPEN CLUSTER:
a system containing a few dozen to a few
thousand stars that formed from the same
stellar nursery
OPPOSITION:
the moment when a planet farther from the
sun than earth appears opposite the sun in
the sky; it is the best time to observe a planet
PERIHELION:
the point closest to the sun in an
object's orbit
PHASE:
the regular cycle of changes in the
appearance of a moon or panet
PHOTONS:
individual "particles" of light
PLANETARY NEBULA:
glowing shell of gas ejected by a dying
low-mass star
PULSAR:
a rapidly rotating neutron star that bathes
Earth in regular pulses of electromagnetic
radiation
QUASAR:
the highly energetic core of a young galaxy
thought to be powered by a supermassive black
hole; short for quasi-stellar object
RADIO WAVES:
the form of light with the longest wavelength
and the least energy
RED GIANT:
a cool star near the end of its life cycle
that has expaded to a diameter a few dozen to
hundred times that of the sun
REDSHIFT:
an increase in the wavelength of light coming
from an object due to its motion away from Earth,
the expansion of the universe, or a strong
gravitational field
REFLECTOR:
a telescope that uses a curved mirrior
to gather light
REFRACTOR:
a telescope that uses a glass lens
to gather light
RESOLUTION(or resolving power):
the ability of a telescope or camera to pick
out fine detail
REVOLUTION:
the orbital motion of one body around another
body or a common center of mass
RIGHT ASCENSION:
the angular distance of a celestial object
east of the vernal equinox; the celestial sphere
equivalent of longitude
SEEING:
the quality of observing conditions induced by
turbulence in Earth's atmosphere, which blurs
the images of astronomical objects
SINGULARITY:
the central point of a black hole where matter
is concentrated into an area of zero volume
and infinite density
SOLAR WIND:
the stream of charged subatomic particles
emanating from the sun
SUPERNOVA:
the cataclysmic explosion of a high-mass star
TERMINATOR:
the boundary on a planet or moon separating the
illuminated side from the unilluminated
ULTRAVIOLET RADIATION:
a form of light with higher energy than visible
light, but without as much energy as x rays
UNIVERSAL TIME:
the local time of day on a line of longitude
centered on Greenwich, England; it forms the
basis for all civel time keeping
VISIBLE LIGHT:
the form of electromagnetic radiation that
humans can see with their eyes
WHITE DWARF:
the dense, collapsed, Earth-sized remnant of an
intermediate-mass star like the sun
X RAYS:
electromagetic radiation more energetic
than ultraviole light but less energetic
than gamma rays
ZENITH:
the point on the celestial sphere directly
over the head of an observer
ZODIACAL LIGHT:
a faint, coneshaped glow of light seen in
the west after nightfall or in the east
before dawn caused by sunlight reflecting and
scattering off interplanetary dust particles
lying along the eclipic plane
---->A-G
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