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Light-year (ly) A unit of distance
that is the distance traveled by
light in one year, equal to 5,878
million miles (9,460 billion km).
Main sequence See dwarf star.
Messier object One of the nebulae
first cataloged by Charles Messier
in 1781.
Meteorite A lump of rock or
metal that falls from space and
reaches the surface of Earth in
one piece or in many fragments.
Nebula A cloud of gas and dust in
interstellar space. Before the 20th
century, any diffuse object in the
sky was known as a nebula; many of
these are now known to be galaxies.
Neutrino A subatomic particle
with very low mass and zero
electric charge, which travels
at close to the speed of light.
Neutron A subatomic particle
made of three quarks with zero
electric charge.
Neutron star A very dense,
compact star composed almost
entirely of densely packed neutrons.
Neutron stars form when the core
of a high-mass star collapses in
a supernova explosion.
Nova A star that suddenly
becomes thousands of times
brighter before returning to its
original brightness over a period
of weeks or months.
Nuclear fusion A process whereby
atomic nuclei join together to
form heavier nuclei, releasing
energy. Inside stars like the sun,
this process involves the fusion
of hydrogen atoms to make helium.
Oort cloud Also known as the
Oort–Öpik cloud. A spherical region
at the edge of the solar system
containing planetesimals and
comets. It is the origin of long-
period comets.Orbit The path of a body around
another, more massive, body.Parallax The apparent shift in
position of an object due to the
movement of an observer to a
different place.Perihelion The point on its
elliptical orbit around the sun
at which a planet, asteroid,
or comet is closest to the sun.Perturbation A change in the
orbit of a body, caused by the
gravitational influence of other
orbiting bodies. Observed
perturbations in the orbit of
the planet Uranus led to the
discovery of Neptune.Planet A non-luminous body
that orbits a star such as the sun,
is large enough to be spherical
in shape, and has cleared its
neighborhood of smaller objects.Planetesimal A small body of
rock or ice. The planets formed
from planetesimals that joined
together by the process of accretion.Precession A change in the
orientation of a rotating body’s
axis of rotation, caused by
the gravitational influence
of neighboring bodies.Proper motion The rate at which
a star changes its position on the
celestial sphere. This change is
caused by the star’s motion relative
to the motion of other stars.Proton A subatomic particle with
a positive charge, made of three
quarks. The nucleus of the element
hydrogen contains a single proton.Protostar A star in the early
stages of its formation, comprising
a collapsing cloud that is accreting
matter but in which nuclear fusion
has not yet begun.Pulsar A rapidly rotating neutron
star. Pulsars are detected on
Earth by their rapid, regular
pulses of radio waves.Quadrant An instrument for
measuring angles of up to 90°.
Ancient astronomers used
quadrants to measure a star’s
position on the celestial sphere.Quark A fundamental subatomic
particle. Neutrons and protons are
made of three quarks.Quasar Short for “quasi-stellar
radio source,” a compact but
powerful source of radiation
that is believed to be an active
galactic nucleus.Radial velocity The part of the
velocity of a star or other body
that is along the line of sight
directly toward or directly
away from an observer.Radio astronomy The branch of
astronomy that studies radiation
in the long radio wavelength, first
discovered to be coming from
space in the 1930s.Red dwarf A cool, red,
low-luminosity star.Red giant A large, highly luminous
star. A main sequence star becomes
a red giant near the end of its life.GLOSSARY