| B
| Spectral type for blue stars, such as Rigel, Spica, and
Regulus. B-type stars are hot, but even hotter blue stars are
designated spectral type O. [C95]
|
| Baade's Window
| A clearing in the dust
clouds of the constellation Sagittarius where astronomers can view
stars in the Galactic bulge. Baade's window lies four degrees south of
the Galactic center, so an observer's line of sight passes within 1800
light-years of the Milky Way's center. [C95]
|
| Bahcall.Soneira Model
| A model for the Galaxy first published by John
Bahcall and Raymond Soneira in 1980. In its original form, it sought
to reproduce star counts in different parts of the sky by employing
only a (thin) disk and a halo; it had no thick disk. [C95]
|
| Barnard's Star
| Discovered in 1916 by Edward Emerson Barnard, this red
dwarf lies 5.96 light-years away and is the second nearest star system
to the Sun. Barnard's Star has the largest proper motion of any star,
10.3 arcseconds per year, which means that the star moves the
equivalent of a lunar diameter every 180 years. [C95]
|
| Barred Spiral Galaxy
| A spiral galaxy whose bulge is oval instead of
round. The Milky Way may be a barred spiral. [C95]
|
| Baryonic
| Consisting of baryons-protons and neutrons-baryonic matter
is "normal" matter. The Sun and the Earth are made of baryonic matter. [C95]
|
| Beryllium
| Element with atomic number four. Beryllium is rare and
fragile, and nuclear reactions in stars destroy it. Most and possibly
all beryllium originated when cosmic rays smashed into heavier atoms
in space and split them into lighter ones, such as beryllium. [C95]
|
| Betelgeuse
| A red supergiant star in the constellation Orion and the
brightest red supergiant in Earth's sky. [C95]
|
| B2FH
| An epic paper, published in 1957, by Margaret Burbidge, Geoffrey
Burbidge, William Fowler, and Fred Hoyle, who described in detail how
the stars had created nearly every element in the universe. [C95]
|
| Big Bang
| According to standard cosmology, the explosion that started
the universe expanding 10 to 15 billion years ago. [C95]
|
| Binary Star
| A star system having two stars that revolve around each
other. [C95]
|
| Black Hole
| An object with such strong gravity that nothing-not even
light, the fastest thing in the universe-an escape it. The most famous
black hole candidates are Cygnus X-1, discovered in 1971, and
A0620-00, discovered in 1986. A massive black hole, containing a
million solar masses, probably lies at the Milky Way's center. [C95]
|
| Blue Giant
| A giant star with spectral type O or B. [C95]
|
| Blueshift
| The shift to the blue of an object's spectrum. A blueshift
arises when an object moves toward us: its light waves get compressed
and reduced in wavelength, "so that the entire spectrum is shifted to
shorter, or bluer, wavelengths. The greater a star's blueshift, the
faster the star is moving toward us. A few galaxies also show
blueshifts, the most famous being Andromeda, but most show redshifts,
due to the expansion of the universe. [C95]
|
| Blue Supergiant
| A supergiant star with spectral type O or B. All blue
supergiants are hot and young. Rigel, in the constellation Orion, is
the best example. [C95]
|
| Boron
| Element with atomic number five. It is rare and fragile, and
nuclear reactions in stars destroy it. Most boron is created in space,
by cosmic rays that smash into heavier atoms and split them. [C95]
|
| Brightness
| Refers to the amount of light coming from an
object. Apparent brightness is the light we Sec; intrinsic brightness,
which has more importance but is more difficult to measure, is the
light an object actually gives off also known as luminosity. By
itself, the word "bright" can mean either apparent or intrinsic
brightness, depending on the context. [C95]
|
| Brown Dwarf
| A star with too little mass to ignite its hydrogen-I
fuel. If brown dwarfs exist, they shine faint red for a time, as they
convert gravitational energy into heat, and then fade and cool. Their
masses range from 1 to 8 percent of the Sun's mass. [C95]
|
| B-Type
| Having a spectral type of B, that is, hot and blue, such as
Rigel and Regulus. [C95]
|
| Bulge
| The stellar population that lies within several thousand
light-years of the Galactic center. The bulge is old, dense, and
metal-rich. [C95]
|
| Canopus
| The brightest star in the constellation Carina and the second
brightest star in the night sky. It is spectral type F and shines
yellow-white. [C95]
|
| Capella
| The brightest star in the constellation Auriga and the sixth
brightest star in the night sky, Capella lies 42 light-years away and
consists of two yellow giants. [C95]
|
| Carbon
| Element with atomic number six and the basis of all
terrestrial life. Carbon is produced during helium burning in red
giants and is ejected into the Galaxy when these stars form planetary
nebulae. Some carbon also comes from high-mass stars that explode as
supernovae. [C95]
|
| Carbon Burning
| The stage when a star fuses carbon into heavier
elements, making neon and magnesium. Carbon burning eventually occurs
in all stars born with more than eight solar masses. [C95]
|
| Carbon Dioxide
| A molecule consisting of one carbon atom and two
oxygen atoms (CO,). It is a gas in Earth's atmosphere that helps to
keep the planet warm by trapping solar heat. [C95]
|
| Carbon Monoxide
| A molecule consisting of one carbon atom and one
oxygen atom (CO). It is the most abundant interstellar molecule after
molecular hydrogen and is especially useful because it radiates at
radio wavelengths, so astronomers can use it to map the distribution
of molecular hydrogen. [C95]
|
| Carbon Star
| A cool star that has a large amount of carbon on its
surface. [C95]
|
| Carina
| 1. A constellation in the southern sky and home of the bright
star Canopus. 2. A dwarf galaxy that orbits the Milky Way. Discovered in
1977, Carina lies 350,000 light-years from the Galactic center. [C95]
|
| Cepheid
| A yellow supergiant that pulsates, alternately brightening
and dimming. Cepheids allow astronomers to measure distances, because
the longer a Cepheid's period of variation, the greater the Cepheid's
mean intrinsic brightness. To determine a Cepheid's distance, all an
astronomer has to do is measure the Cepheid's period; comparing the
star's mean intrinsic brightness with the star's mean apparent
brightness then yields the distance. Cepheids are so bright that we
can see them in other galaxies,
allowing us to establish distances to entire galaxies beyond the Milky
Way. [C95]
|
| Chandrasekhar Limit
| The most mass a white dwarf can have, about 1.4
solar masses. If a white dwarf receives material from a companion star
and exceeds the Chandrasekhar limit, the white dwarf explodes as a
type Ia supernova. [C95]
|
| Cluster
| 1. A gathering of hundreds, thousands, or even a million
stars. Star clusters come in two varieties: open clusters and
globular clusters. 2. A gathering of hundreds or thousands of
galaxies. The nearest large galaxy cluster is the Virgo cluster. [C95]
|
| CNO Cycle
| One way that a star converts hydrogen into helium. During
the CNO cycle, carbon, nitrogen, and oxygen catalyze the nuclear
reaction, so the total number of carbon, nitrogen, and oxygen nuclei
remains the same. However, carbon and oxygen gradually get converted
into nitrogen. The CNO cycle powers the hydrogen burning that occurs
in main-sequence stars with more than 1.5 solar masses and in giants
and supergiants of all masses. [C95]
|
| CO
| See Carbon Monoxide. [C95]
|
| C02
| See Carbon Dioxide. [C95]
|
| Cold Dark Matter
| Hypothetical subatomic particles that move slowly
compared with the speed of light. [C95]
|
| Color
| Color indicates temperature: blue stars are hot, yellow stars
are warm, and red stars are cool. [C95]
|
| Comet
| A small icy body that orbits a star. The best-known comet is
Halley's. Most comets in our solar system spend most of their time far
beyond Neptune and Pluto. [C95]
|
| Constellation
| 1. As used by astronomers, constellation refers to a
particular region of the sky. There are 88 official constellations
that blanket the entire sky, so every star, known or unknown, is in
one constellation or another. Constellations are like states, and stars
are like cities within those
states. 2. As used by many people, constellation means a pattern of
stars. By analogy, if you draw lines between the cities of a
particular state, you would have the equivalent of this type of
constellation. [C95]
|
| Cosmic Rays
| High-energy particles that travel through space. They can
smash into atoms and split them apart, creating lighter elements, such
as lithium, beryllium, and boron. [C95]
|
| Cosmological Constant
| A possible third parameter in cosmology, in
addition to the Hubble constant and omega (fl). Most cosmologists
believe the cosmological constant is zero, but if it is not, it would
make the universe older than astronomers calculate from the Hubble
constant and fl. The size of the cosmological constant is designated
by the Greek letter lambda (h). [C95]
|
| Cosmological Redshift
| The redshift produced by the expansion of the
universe and the reason most galaxies in the universe have
redshifts. Contrary to popular belief, this is not a Doppler shift. A
Doppler redshift arises when an object moves away from us. Most
galaxies move away from us, but this is not the cause of their
redshifts. Instead, as a light wave travels through the fabric of
space, the universe expands and the light wave gets stretched and
therefore redshifted. It's a subtle difference, but a difference it
is. The farther a galaxy, the longer its light waves have traveled
through space and the more redshifted they have become. [C95]
|
| Cosmology
| The branch of astronomy that deals with the universe as a
whole, especially its origin, structure, and evolution. [C95]
|
| Crab Nebula
| A supernova remnant in the constellation Taurus. The star
that produced it exploded in A.D. 1054. [C95]
|
| Cygnus X-1
| A black hole candidate in the constellation Cygnus and a
source of x-rays. [C95]
|
| Dark Halo
| The massive outer region of the Milky Way that surrounds
the disk and stellar halo. The dark halo consists mostly of dark
matter, whose form is unknown. Though it emits almost no light, the
dark halo outweighs the rest of the Galaxy. [C95]
|
| Dark Matter
| Material astronomers cannot see but whose presence they
believe in either because they detect its gravitational influence or
because certain theories predict its existence. For example,
astronomers believe that the outer part of the Galaxy harbors dark
matter, because they notice its gravitational influence on the stars
they can see; and inflationary cosmologists believe that the universe
is full of dark matter, because inflation predicts that the universe
has a-large density. [C95]
|
| Delta Cephei
| A pulsating star in the constellation Cepheus. It was
the second Cepheid discovered and lent its name to the entire class of
stars. [C95]
|
| Deneb
| The brightest star in the constellation Cygnus, Deneb is a
white A-type supergiant that generates more light in a single night
than the Sun produces during an entire century. Deneb lies 1500
light-years away and is the most distant first-magnitude star. [C95]
|
| Density
| An object's mass divided by its volume. Cotton has a low
density; lead has a high density. Red giants have a low density; white
dwarfs have a high density. [C95]
|
| Density Wave Theory
| One possible explanation for spiral
arms. According to this theory, the spiral arms represent regions of
somewhat enhanced density (density waves) that rotate more slowly than
the galaxy's stars and gas. As gas enters a density wave, it gets
squeezed and makes new stars, some of which are short-lived blue stars
that light the arms. [C95]
|
| Deuterium
| Hydrogen-2, the rare heavy isotope of hydrogen. Deuterium
has one proton and one neutron, whereas normal hydrogen has one proton
and no neutrons. [C95]
|
| Differential Rotation
| As an object rotates, different parts of it may
move at different rates. The Galaxy rotates differentially. [C95]
|
| Dissipation
| When a galaxy forms and gas clouds start crashing into
and impeding one another. The Galaxy's disk probably formed through
dissipation. [C95]
|
| Doppler Shift
| The blueshift or redshift produced by an object's
motion toward or away from us. If a star moves toward us, its light
waves get compressed and its spectrum is blueshifted; if a star moves
away from us, its light waves get stretched and its spectrum is
redshifted. The Doppler shift allows astronomers to measure the radial
velocities of stars. The Doppler shift is not responsible for the
redshifts that most galaxies exhibit; that is a cosmological redshift. [C95]
|
| Double Star
| A star system having two stars that revolve around each
other. [C95]
|
| Draco
| 1. A constellation in the northern sky. 2. A dwarf galaxy that
orbits the Milky Way and lies about 250,000 light-years from the
Galactic center. Draco is the least luminous galaxy known, with an
absolute magnitude of -8.6. [C95]
|
| Dwarf
| 1. A star that is on the main sequence-that is, a star fusing
hydrogen into helium at its core. 2. A small, faint galaxy,
exemplified by those that orbit the Milky Way: Ursa Minor, Draco,
Sculptor, Sextans, Carina, Fornax, Leo II, and Leo I. [C95]
|
| Dynamical Friction
| The process by which a large mass gets slowed down
as it moves through a sea of smaller objects and feels their
gravitational pull. For example, a satellite galaxy that moves through
the Milky Way's dark halo gets slowed down by the dynamical friction
with the dark matter and will spiral into the Milky Way as a result. [C95]
|
| Eccentric
| An orbit that has a high eccentricity that is, highly
elliptical. [C95]
|
| Eccentricity
| A measure of how round or elliptical an orbit is. A
perfect circle has an eccentricity of 0 percent, and an extremely
elliptical orbit has an eccentricity of just under 100 percent. The
Sun has an orbital eccentricity of 6 percent, which means that at
perigalacticon the Sun is 6 percent closer to the Galactic center than
its mean distance and at apogalacticon the Sun is 6 percent farther
from the Galactic center than its mean distance. [C95]
|
| Eclipsing Binary
| A double star in which at least one of the two stars
passes in front of and/or behind the other so that the system's total
light periodically fades. The most famous eclipsing binary is Algol. [C95]
|
| Electromagnetic Radiation
| Visible light, radio waves, infrared
radiation, ultraviolet radiation, x-rays, and gamma rays. In a vacuum,
all electromagnetic radiation travels at the speed of light. The
shorter its wavelength, the more energetic it is. [C95]
|
| Electron
| A small, negatively charged particle that appears in every
neutral atom, surrounding the positively charged nucleus like bees
around honey. [C95]
|
| Element
| Different elements are distinguished by the number of protons
in their nuclei. All hydrogen atoms have one proton; all helium atoms
have two protons; all oxygen atoms have eight protons. [C95]
|
| Elliptical Galaxy
| A galaxy that looks round or elliptical. One
example is M87, in the constellation Virgo. [C95]
|
| ELS
| A classic paper published in 1962 by Olin Eggen, Donald
Lynden-Bell, and Allan Sandage, who argued that the Galaxy formed from
a single huge cloud of gas that rapidly collapsed. [C95]
|
| Epsilon Eridani
| A young orange dwarf star in the constellation
Eridanus that is visible to the naked eye and lies just 10.7
light-years away. [C95]
|
| Epsilon Indi
| An old orange dwarf star in the southern constellation
Indus that lies 11.2 light-years away. [C95]
|
| Equipartition of Energy
| If all stars have the same kinetic energy,
equipartition of energy prevails. Because kinetic energy depends on
both a star's mass and its velocity, high-mass stars must move more
slowly than low-mass stars, if equipartition of energy prevails. [C95]
|
| Eta Aquilae
| A pulsating star in the constellation Aquila. It was the
first Cepheid discovered, in 1784. [C95]
|
| Extragalactic Astronomy
| The field that deals with objects beyond the
Milky Way, especially galaxies and quasars. [C95]
|
| F
| Spectral type for yellow-white stars, which are slightly hotter
than the Sun. The brightest F-type stars in Earth's sky are Canopus
and Procyon. Field Star. A star that is not part of any star
cluster. Most stars, including the Sun, are field stars. [C95]
|
| Flare Star
| A star that emits flares, which can outshine the entire
star. Many red dwarfs are flare stars. [C95]
|
| Fomalhaut
| The brightest star in the constellation Piscis Austrinus,
Fomalhaut is a white A-type main-sequence star 21 light-years away. [C95]
|
| Fornax
| 1. A faint constellation in the southern sky. 2. A dwarf
galaxy that orbits-the Milky Way and lies 440,000 light-years from the
Galactic center. It was discovered in 1938. [C95]
|
| 47 Tucanae
| A globular cluster in the southern constellation Tucana
and a member of the thick-disk population. [C95]
|
| Free-Fall
| A collapse in which gas clouds do not hit or impede one
another. According to ELS, the Galactic halo formed in a free-fall
collapse. [C95]
|
| F-Type
| Having a spectral type of F, that is, yellow-white, like
Canopus and Procyon. [C95]
|
| G
| Spectral type for yellow stars, such as the Sun, Alpha Centauri A,
and Capella. [C95]
|
| Galactic
| 1. When capitalized, the word refers to our Galaxy. 2. When
not capitalized, it refers to a galaxy. [C95]
|
| Galactic Anticenter
| The point in the Galactic plane that lies
directly opposite the Galactic center. Here we gaze toward the edge of
the Galactic disk. The nearest bright star to the anticenter is El
Nath, in the constellation Taurus. [C95]
|
| Galactic Astronomy
| The study of the Milky Way. [C95]
|
| Galactic Latitude
| The angle between the line of sight to a star and
the Galactic plane. Galactic latitude ranges from +90 degrees to -90
degrees; the Galactic plane has a Galactic latitude of 0
degrees. Regions north of
the Galactic plane have positive Galactic latitude; regions south have
negative Galactic latitude. The point with a Galactic latitude of #90
degrees is called the north Galactic pole, and the point with a
Galactic latitude of -90 degrees
is called the south Galactic pole. [C95]
|
| Galactic Longitude
| A measure of a star's position with respect to the
Sun and Galactic center. Galactic longitude ranges from 0 degrees to
360 degrees. Imagine the Sun at the center of a giant clock, with the
Galactic center located in the direction of six o'clock. A Galactic
longitude of 0 degrees would correspond to the direction of six
o'clock, a Galactic longitude of 90 degrees to the direction of three
o'clock, a Galactic longitude of 180 degrees to the direction of
twelve o'clock, and a Galactic longitude of 270 degrees to the
direction of nine o'clock. [C95]
|
| Galactic Plane
| The plane that contains the disk of the Milky Way. By
definition, one direction perpendicular to this plane is called
"above" or "north," and the opposite direction, also perpendicular to
the Galactic plane, is called "below" or "south." From Earth, due
Galactic north is marked by the north Galactic pole, which lies near
the bright star Arcturus, and due Galactic south is marked by the
south Galactic pole, which lies in the faint constellation Sculptor. [C95]
|
| Galactic Pole
| Either of the two points in the sky where we look
perpendicular to the disk of the Milky Way. The north Galactic pole is
the Galactic pole located above the disk; the south Galactic pole is
the Galactic pole located below the disk. [C95]
|
| Galactic Tide
| See Tide. [C95]
|
| Galactocentric Distance
| A star's distance from the Galactic
center. The Sun's Galactocentric distance is about 27,000 light-years. [C95]
|
| Galaxy
| A huge collection of millions, billions, or trillions of
stars. When referring to the Milky Way, "galaxy" is capitalized,
otherwise not; thus: "Andromeda is the nearest giant galaxy to the
Galaxy." [C95]
|
| Galaxy Cluster
| A conglomeration of hundreds or thousands of galaxies.
The nearest large galaxy cluster is the Virgo cluster. [C95]
|
| Giant
| 1. A star that has evolved off the main sequence and is roughly
a hundred times as luminous as the Sun. Giants can be of any color,
but yellow, orange, and red giants are the most common. 2. A planet
much more massive than Earth. The solar system has four giant planets,
all far from the Sun: Jupiter, Saturn, Uranus, and Neptune. [C95]
|
| Giant Molecular Cloud
| A huge complex of interstellar gas and dust,
consisting mostly of molecular hydrogen, that typically stretches over
150 light-years and contains 200,000 solar masses. Giant molecular
clouds give birth to new stars. [C95]
|
| Globular Cluster
| A star cluster that packs hundreds of thousands of
stars into a region only about a hundred light-years across. Bright
globular clusters include Omega Centauri, 47 Tucanae, and M13. In the
Milky Way, all globular clusters are old. Most are members of the
Galactic halo, and the rest are members of the thick disk. [C95]
|
| Gold
| Element with atomic number 79. It is produced entirely by the
r-process, in supernovae. [C95]
|
| Gravitational Lensing
| The bending of light caused by the gravity of
an object lying between us and the light source. This may cause the
light source to look brighter than it normally does. [C95]
|
| Groombridge 1830
| A famous halo star that lies 28 light-years away in
the constellation Ursa Major. Its proper motion, discovered in 1841,
was then the largest known, displacing that of 61 Cygni. [C95]
|
| Group
| A small gathering of galaxies, smaller than a cluster. The
Milky Way is part of the Local Group, which contains about thirty
galaxies. [C95]
|
| G-Type
| Having a spectral type of G, that is, yellow, like the Sun. [C95]
|
| HI
| Neutral hydrogen gas. It emits radio waves that are 21 centimeters
long. [C95]
|
| HII
| Ionized hydrogen-that is, hydrogen with its electron missing. [C95]
|
| HII Region
| An area of ionized hydrogen. Most HII regions are red and
arise from hot blue O and B stars, whose ultraviolet light can ionize
all the hydrogen for dozens or even hundreds of light-years in every
direction. The most famous HII region is the Orion Nebula. [C95]
|
| Half-Life
| The length of time it takes for half the number of
radioactive atoms to decay. [C95]
|
| Halo
| The somewhat round population of old, metal-poor stars in the
Milky Way. Also, the huge entity that surrounds the disk and contains
most of the Galaxy's dark matter. To distinguish between the two,
astronomers call the former the stellar halo and the latter the dark
halo. Most of the stellar halo lies closer to the Galactic center than
the Sun, while most of the dark halo lies farther from the Galactic
center than the Sun. [C95]
|
| HD
| Henry Draper Catalogue, which lists over 200,000 stars. It was
published in nine volumes between 1918 and 1924. [C95]
|
| HD 19445
| A subdwarf in the constellation Aries that was used, along
with HD 140283, to establish that some stars have much lower
metallicities than the Sun. [C95]
|
| HD 122563
| A yellow giant star in the constellation Bootes whose
metallicity is only Oz percent of the Sun's. [C95]
|
| HD 140283
| A subdwarf or subgiant in the constellation Libra that was
used, along with HIS 19445, to establish that some stars have much
lower metallicities than the Sun. [C95]
|
| Helium
| The second lightest (atomic number two) and second most common
element in the universe. Most of it was produced by the big bang, with
main-sequence stars making an additional contribution. It has two
stable isotopes, helium-3 (two protons and one neutron) and helium-4
(two protons and two neutrons). The latter isotope is by far the more
common; it is also the most stable and tightly bound of the light
nuclei. [C95]
|
| Helium Burning
| The stage when a star fuses helium into carbon and
oxygen. All stars born with more than half a solar mass eventually
burn helium. [C95]
|
| Hertzsprung-Russell Diagram
| A plot of stellar color, temperature, or
spectral type versus stellar luminosity. The H-R diagram segregates
three principal types of stars: the main sequence, which forms a
diagonal band from bright blue stars to faint red ones; red giants and
supergiants, which appear in the upper right of the diagram; and white
dwarfs, which lie below the main sequence. [C95]
|
| High-Velocity Star
| A star whose U and/or V and/or W velocities are
much greater or much less than zero. Such stars usually have eccentric
orbits around the Galaxy. [C95]
|
| Hipparcos
| A European satellite that from 1989 to 1993 measured the
parallaxes of stars. [C95]
|
| Horizontal Branch Star
| A metal-poor star, similar in mass to the Sun,
that fuses helium into carbon and oxygen at its core. Such stars range
in color from blue to yellow. RR Lyrae stars are horizontal-branch
stars. Stars bluer than RR Lyraes are called blue horizontal-branch
stars; stars redder are called red horizontal-branch stars, even
though they are actually yellow. All other things being equal, the
more metal-poor a globular cluster, the bluer its horizontal branch;
the older a globular cluster, the bluer its horizontal branch, too. [C95]
|
| Hot Dark Matter
| Subatomic particles that moved almost as fast as
light when the universe was young. [C95]
|
| H-R Diagram
| See Hertzsprung-Russell Diagram. [C95]
|
| Hubble Constant
| The present expansion rate of the universe, in units
of kilometers per second per megaparsec. The larger the Hubble
constant, the younger the universe. [C95]
|
| Hubble Time
| The inverse of the Hubble constant and a crude measure of
the universe's age. For a Hubble constant of 50, one can calculate
that the Hubble time is 19.6 billion years; for a Hubble constant of 80, the
Hubble time is 12.2 billion years. If there is no cosmological
constant, the universe is younger than the Hubble time. In particular,
if the mass density of the universe (designated fl) is 0.1, the
universe's age is 90 percent of the Hubble time; if D is 1.0, the
universe's age is 67 percent of the Hubble time. [C95]
|
| Hyades
| The nearest star cluster, 150 light-years from Earth. The
cluster lies in the constellation Taurus and is 600 million years
old. It is an open star cluster. [C95]
|
| Hydrogen
| The lightest and most common element in the universe. It has
atomic number one and was produced by the big bang. Hydrogen-1 (one
proton and no neutrons) is the most common isotope; hydrogen-2 (one
proton and one neutron), or deuterium, is rarer; and hydrogen-3 (one
proton and two neutrons), or tritium, is radioactive. [C95]
|
| Hydrogen Burning
| The fusion of hydrogen into helium and the process
by which all main-sequence stars generate energy. Every star born with
more than 0.08 solar masses burns hydrogen. [C95]
|
| IC 4182
| A nearby galaxy in which a type Ia supernova exploded in
1937. Inflation. The idea that, when it was a fraction of a second
old, the universe expanded dramatically. If inflation is correct, then
the mass density of the universe (fl) should be 1.0, if there is no
cosmological constant; if there is a cosmological constant and
inflation is correct, the sum of fl and the cosmological constant (X)
should be 1.0. [C95]
|
| Infrared
| Radiation that has a somewhat longer wavelength than visible
light. It penetrates dust much more readily than visible light does. [C95]
|
| Inhomogeneous Early Universe
| The idea that during the first few
minutes after the big bang, the universe had regions of different
density. An inhomogeneous early universe can produce elements
different from those of the standard homogeneous early universe. [C95]
|
| Interarm Region
| The area between a spiral galaxy's spiral arms. These
areas look dark, not because they lack stars, but because they contain
none of the young, luminous stars that light the arms. [C95]
|
| Interstellar Cloud
| A collection of gas and dust that lies between the
stars. [C95]
|
| Interstellar Medium
| The space between the stars. [C95]
|
| Intrinsic Brightness
| The amount of light an object actually emits, as
opposed to how bright the object looks from Earth. An apparently
bright star can be intrinsically bright and far away or intrinsically
faint and nearby. [C95]
|
| Ionized Hydrogen
| A hydrogen atom that has lost its electron. Hydrogen
gets ionized by hot O and B stars in HII regions. The most famous HII
region is the Orion Nebula. [C95]
|
| Iron
| Element with atomic number 26, created mostly by type Ia
supernovae, with an additional contribution from type Ib, Ic, and II
supernovae. It is the most stable element. [C95]
|
| IRS
| Infrared source. [C95]
|
| IRS 7
| A red supergiant that lies less than one light-year from the
Galactic center. It resembles a comet, because a wind hits its
atmosphere and creates a tail that points away from the Galactic
center. [C95]
|
| IRS 16
| A star cluster near the Galactic center. Isotope. An element
with different numbers of neutrons. For example, hydrogen-I and
hydrogen-2 are both isotopes of hydrogen; each has one proton in its
nucleus, but the former has no neutrons and the latter has one. [C95]
|
| Jupiter
| The largest planet in the solar system, having about 0.001
solar masses. The planet has more mass than all of the other planets
put together. [C95]
|
| K
| Spectral type for orange stars, such as Arcturus, Aldebaran, and
Alpha Centauri B. K-type stars are somewhat cooler than the Sun. [C95]
|
| Kapteyn's Star
| A nearby star discovered in 1897 by Jacobus
Kapteyn. It is a red subdwarf that lies 13 lightyears away and is both
the nearest halo star to the Sun and the nearest star that orbits the
Galaxy backward. [C95]
|
| Kapteyn Universe
| An incorrect model for the Galaxy proposed by
Jacobus Kapteyn in which the Milky Way was small and the Sun located
at or near the Galaxy's center. [C95]
|
| Kelvin
| The temperature scale that astronomers usually use. On the
Kelvin scale, the coldest possible temperature is 0 degrees. This
corresponds to -273° Celsius or -460° Fahrenheit. Room temperature is
about 295 degrees Kelvin. [C95]
|
| Kilometer Per Second
| The unit of speed in astronomy. One kilometer
per second is 2237 miles per hour-five times the speed of an airplane. [C95]
|
| Kinematics
| The motions of stars, especially as these motions pertain
to the stars' orbits around the Galaxy. [C95]
|
| K-Type
| Having spectral type K, that is, orange, like Arcturus and
Aldebaran. [C95]
|
| Lambda (h).
| See Cosmological Constant. [C95]
|
| Large Magellanic Cloud
| The nearest and largest of the many galaxies
that orbit the Milky Way. It is 160,000 light-years away. [C95]
|
| Lensing
| See Gravitational Lensing. [C95]
|
| Leo I
| The most distant galaxy that orbits the Milky Way. A dwarf
galaxy, Leo I was discovered in 1950 and lies 890,000 light-years from the
Galactic center. It is close to the bright star Regulus, whose glare
interferes with the study of the galaxy. [C95]
|
| Leo II
| The second most distant galaxy that orbits the Milky Way,
lying 720,000 light-years from the Galactic center. Like Leo I, Leo II
is a dwarf galaxy that was discovered in 1950.[C95]
|
| Life Zone
| The region around a star where a planet can have liquid
water and so may support life. [C95]
|
| Light Elements
| Usually, hydrogen, helium, and lithium, which have
atomic numbers of one, two, and three; sometimes also beryllium and
boron, which have atomic numbers of four and five. [C95]
|
| Light-Year
| The distance light travels in one year: 5.88 trillion
miles, or 9.46 trillion kilometers. The nearest star system to the Sun
is 4.3 light-years away. [C95]
|
| Lithium
| Element with atomic number three. Some lithium formed in the
big bang, along with huge amounts of hydrogen and helium. Lithium has
two stable isotopes: the rarer, lithium-6 (three protons and three
neutrons); and the more common, lithium-7 (three protons and four
neutrons). [C95]
|
| Local Bubble
| The region of the Galaxy near the Sun which has little
neutral hydrogen gas. It extends about a hundred light-years in most
directions but up to a thousand in some. The local bubble may have
been produced by supernovae. [C95]
|
| Local Group
| The gravitationally bound collection of nearby galaxies
ruled by the Andromeda Galaxy and the Milky Way, which are its largest
members. The Local Group has about 30 known galaxies. [C95]
|
| Local Standard of Rest
| An imaginary point, located at the Sun's
distance from the Galactic center, that revolves clockwise around the
Galaxy on a circular orbit. Astronomers measure a star's U, V, and \V
velocities with respect to the local standard of rest rather than with
respect to the Sun, because the Sun has a slightly noncircular
orbit. The orbital velocity of the local standard of rest around the
Galaxy is about 220 kilometers per second. [C95]
|
| Local Supercluster
| The supercluster to which the Local Group
belongs. It is shaped like a cigar, with the Virgo cluster of galaxies
at its center and the Local Group near one end. [C95]
|
| Low-Velocity Star
| A star whose U, V, and \V velocities are all near
zero. Such stars have nearly circular orbits around the Galaxy. [C95]
|
| Luminosity
| The total amount of energy radiated by a star-that is, its
true, or intrinsic, brightness. [C95]
|
| Luminosity Class
| A measure of a star's intrinsic brightness, as
determined from the star's spectrum. Supergiants have luminosity class
I, bright giants have luminosity class II, giants have luminosity
class III, subgiants have luminosity class IV, and main-sequence stars
have luminosity class V. [C95]
|
| Luminosity Function
| 1. The number of stars in the Galaxy with a
particular absolute magnitude. The luminosity function reveals that
luminous stars are rare and intrinsically faint stars common. 2. The
distribution of galaxies by absolute magnitude. Luminous galaxies are
rare and intrinsically faint ones are common. [C95]
|
| Luminous
| Intrinsically bright, as opposed to being just apparently
bright. [C95]
|
| M
| Spectral type for red stars, such as Betelgeuse, Antares, and
Proxima Centauri. [C95]
|
| M13
| A great globular cluster in the constellation Hercules. [C95]
|
| M31
| The Andromeda Galaxy, the largest member of the Local Group. It
is a giant spiral galaxy that lies 2.4 million light-years away. [C95]
|
| M32
| An elliptical galaxy that orbits the Andromeda Galaxy. [C95]
|
| M33
| The Pinwheel Galaxy, the third largest member of the Local Group,
after Andromeda and the Milky Way. It is a spiral galaxy that
lies 2.6 million light-years away. [C95]
|
| M42
| The Orion Nebula, a star-forming region in the constellation
Orion. [C95]
|
| M45
| The Pleiades, a beautiful open star cluster in the constellation
Taurus. It is 410 light-years away. [C95]
|
| M51
| The Whirlpool Galaxy, a stunning spiral in the constellation
Canes Venatici. [C95]
|
| M81
| A giant spiral galaxy 11 million light-years away in the
constellation Ursa Major. It rules the M81 group, the second nearest
galaxy group to the Local Group. [C95]
|
| M87
| A giant elliptical galaxy in the Virgo cluster. [C95]
|
| M104
| The Sombrero Galaxy, in which galactic rotation was first
detected. It lies in the constellation Virgo. [C95]
|
| MACHO
| Massive compact halo object. MACHOs are dark stars or planets
that may make up the Milky Way's dark halo. [C95]
|
| Magellanic Clouds
| The Large and Small Magellanic Clouds, the two
nearest and largest of the galaxies that orbit the Milky Way. The
Magellanic Clouds lie in the southern sky and cannot be seen from the
United States. [C95]
|
| Magellanic Stream
| A strand of gas spanning 300,000
light-years that the Milky Way has ripped out of the Magellanic Clouds. [C95]
|
| Magnesium
| Element with atomic number twelve. It is the fifth most
common metal in the universe and was produced by high-mass stars that
exploded. [C95]
|
| Magnitude
| A measure of a star's brightness. Apparent magnitude
measures a star's apparent brightness that is, how bright a star looks
from Earth. Absolute magnitude measures a star's intrinsic
brightness, that is, how much light the star actually emits. [C95]
|
| Main-Sequence Star
| A star, like the Sun, that fuses hydrogen into
helium at its core. Ninety percent of all stars are main-sequence
stars; examples are Sirius, Vega, Altair, and Alpha Centauri A, B, and
C. [C95]
|
| Main-Sequence Turnoff
| The point on the H-R diagram of a star cluster
where main-sequence stars are beginning to leave the main
sequence. The main-sequence turnoff measures age: all other things
being equal, the older a star cluster, the fainter the main-sequence
turnoff. [C95]
|
| Mass Number
| See Atomic Mass Number. [C95]
|
| Mass-to-Light Ratio
| The amount of mass in an object divided by its
luminosity, both measured in solar units. The Sun has a mass-to-light
ratio of one, because it has one solar mass and one solar luminosity.
Stars brighter than the Sun, such as upper main·sequence stars,
giants, and supergiants, have low mass-to-light ratios, because most
have somewhat more mass than the Sun but much more luminosity. Stars
fainter than the Sun, such as red, orange, and white dwarfs, have high
mass-to-light ratios, because most have smaller masses than the Sun
but much smaller luminosities. Dark matter, by definition, has a high
mass-to-light ratio: it has much mass but radiates little or no light. [C95]
|
| Megaparsec
| A unit of distance equal to a million parsecs, or 3.2616
million light-years. [C95]
|
| Merger
| The formation of a galaxy from the collision of two or more
separate galaxies. [C95]
|
| Metal
| To an astronomer, a metal is any element heavier than hydrogen
and helium; thus, not only are iron and copper metals, but so are
elements like oxygen and neon. [C95]
|
| Metallicity
| An object's abundance of metals. In practice, this
usually means the abundance of iron, which is easy to measure. [C95]
|
| Metallicity Gradient
| The progressive change in metallicity from the
center of a galaxy to its edge. A galaxy exhibiting a metallicity
gradient is more metal-rich at its center than at its edges. [C95]
|
| Metal-Poor
| Having a low metallicity. [C95]
|
| Metal-Rich
| Having a high metallicity. [C95]
|
| Microwave Background
| The 2.7 degree Kelvin radiation that pervades
the universe and is believed to be the afterglow of the Big Bang. [C95]
|
| Milky Way
| I. Our Galaxy. 2. The band of light that stretches across
the sky during summer and winter, produced by innumerable stars in the
plane of the Galaxy. [C95]
|
| Mintaka
| One of the three stars in Orion's belt, and the star along
whose line of sight interstellar gas was first spectroscopically
detected. [C95]
|
| Mira
| A red giant in the constellation Cetus that varies in brightness
as it pulsates. When brightest, Mira is visible to the naked eye; when
dimmest, Mira can be viewed only with optical aid. Mira is the prototype of all
pulsating red giants, which are called Miras in its honor. [C95]
|
| Molecular Clous
| A cloud of interstellar gas and dust that consists
mostly of molecular hydrogen. [C95]
|
| Molecular Hydrogen
| A molecule consisting of two hydrogen atoms (H2)
and the most common molecule in space. [C95]
|
| M-Type
| Having a spectral type of M, that is, red like Betelgeuse and
Antares. [C95]
|
| Nebula
| A region of gas and dust in space, like the Orion Nebula, the
Ring Nebula, and the Lagoon Nebula. Galaxies were once called
"nebulae" as well, before astronomers knew what these objects really
were. [C95]
|
| Neon
| Element with atomic number ten and the third most common metal
in the universe. It is produced by carbon burning in high-mass stars
and released into the Galaxy when they explode. [C95]
|
| Neon Burning
| The stage in which a star burns neon into oxygen and
magnesium. [C95]
|
| Neutral Hydrogen
| A hydrogen atom that has a proton and an electron
and so is electrically neutral. Neutral hydrogen produces radio waves
that are 21 centimeters long. [C95]
|
| Neutrino
| A neutral subatomic particle with little or no mass that
travels at or near the speed of light. Neutrinos hardly ever interact
with matter. [C95]
|
| Neutrino Process
| Nucleosynthesis induced by neutrinos. It may create
fluorine and boron. [C95]
|
| Neutron
| A neutral, massive subatomic particle that occurs in all
nuclei except hydrogen-I. For example, helium-4 has two neutrons, and
oxygen-16 has eight. [C95]
|
| Neutron Star
| A dead, collapsed star that consists mostly of neutrons
and is only about 20 kilometers across. Neutron stars are much denser
than white dwarfs. [C95]
|
| NGC
| New General Catalogue, a catalogue of 7840 nebulae, star
clusters, and galaxies that was published in 1888 by John Dreyer. [C95]
|
| NGC 253
| A large edge-on spiral galaxy and the largest member of the
Sculptor Group, the group of galaxies nearest to the Local Group. [C95]
|
| NGC 288
| A globular cluster that played a key role in proving that not
all globulars are the same age. ln the late 1980s, NGC 288 was shown
to be 3 billion years older than NGC 362. [C95]
|
| NGC 362
| A globular cluster. See NGC 288. [C95]
|
| NGC 4565
| An edge-on spiral galaxy in the constellation Coma Berenices
that resembles the Milky Way. [C95]
|
| Nitrogen
| Element with atomic number seven and the fourth most common
metal in the universe. It formed during hydrogen burning in
main-se-quence stars and red giants, via the CNO cycle. [C95]
|
| Non-baryonic Matter
| Material that consists of exotic subatomic
particles. These subatomic particles can move slowly (cold dark
matter) or fast (hot dark matter). Cosmologists who believe in
inflation-or anyone else who thinks the mass density of the universe
(Omega) is 1.0-believe that most of the universe consists of
nonbaryonic matter. [C95]
|
| North Galactic Pole
| A point in the constellation Coma Berenices where
we look perpendicular to and above the Galactic plane. The nearest
bright star to the north Galactic pole is Arcturus, in the neighboring
constellation Bootes. [C95]
|
| North Star
| See Polaris. [C95]
|
| Nova
| An exploding star, but one that never attains the enormous
luminosity of a supernova. A nova usually arises from a double-star
system in which one member is a white dwarf. The other star dumps
material onto the white dwarf, and the nova occurs when this material
catches nuclear fire and explodes. The explosion does not destroy
either star. [C95]
|
| Nucleon
| A proton or neutron. For example, oxygen-16, with eight
protons and eight neutrons, has sixteen nucleons. [C95]
|
| Nucleosynthesis
| The transformation of one element or isotope into
another. Nucleosynthesis occurred just after the big bang, but today
most nucleosynthesis takes place in stars-for example, the Sun
presently converts hydrogen into helium. [C95]
|
| Nucleus
| The central part of an atom, which contains the atom's
protons and neutrons. [C95]
|
| O
| Spectral type for the hottest blue stars, even hotter than B-type
stars. O-type stars are rare and short-lived. [C95]
|
| OB
| Spectral type O or B, that is, hot and blue. [C95]
|
| OB Association
| A loose gathering of O and B stars that typically
stretches over hundreds of light-years and contains a few dozen OB
stars. [C95]
|
| Old Thin Disk
| The older part of the thin-disk population, ranging in
age from about 1 to 10 billion years. The Sun and most other nearby
stars belong to the old thin disk. The scale height of the old thin
disk is about 1000 light-years. [C95]
|
| Omega (fl)
| The mass density of the universe. If fl is greater than
1.0, the universe is so dense that the gravitational attraction of all
the mass will halt the expansion and cause the universe to collapse;
if fl is less than 1.0, the universe does not have enough mass to
reverse the expansion and will expand forever. The larger fl, the
younger the universe. [C95]
|
| Omega Centauri
| A bright globular cluster in the constellation
Centaurus. [C95]
|
| Omicron2 Eridani
| A triple star that lies 16 light-years away and has
the first white dwarf ever discovered. [C95]
|
| Open Cluster
| A small, loose cluster of stars that typically contains
several hundred members. The best examples are the Hyades and the
Pleiades, both in the constellation Taurus. Open clusters line the
Galactic plane, in contrast with globular clusters, which are members
of the Galaxy's halo or thick disk. [C95]
|
| Orange Dwarf
| A main-sequence star with spectral type K. These stars
are somewhat fainter, cooler, and smaller than the Sun and account for
15 percent of the Galaxy's stars. [C95]
|
| Orange Giant
| A giant star with spectral type K. These stars are about
a hundred times more luminous than the Sun but are cooler. The two
brightest in the sky are Arcturus and Aldebaran. [C95]
|
| Orion Arm
| The spiral arm containing the Sun. It lies between the
Sagittarius arm and the Perseus arm. [C95]
|
| Orion Nebula
| A large cloud of gas and dust giving birth to young
stars in the constellation Orion and visible to the naked eye. It is
an HII region 1500 light-years away. [C95]
|
| O-Type
| Having a spectral type of 0-that is, hot and blue. [C95]
|
| Oxygen
| The most abundant metal in the universe, and the third most
abundant element overall, after hydrogen and helium. Oxygen has atomic
number eight and is produced by massive stars-those born with over
eight solar masses-which eject the element into the Galaxy when they
explode. [C95]
|
| Oxygen Burning
| The stage when a star fuses oxygen into silicon and
sulfur. It occurs only in stars born with over eight solar masses. [C95]
|
| Parallax
| The tiny shift in a star's apparent position that occurs
when the star is viewed from slightly different perspectives as the
Earth revolves around the Sun. The larger a star's parallax, the
closer the star is to Earth. [C95]
|
| Parsec
| A unit of distance equal to 3.261633 light-years. A star that
is one parsec from the Sun has a parallax of one arcsecond. [C95]
|
| Perigalacticon
| The point in a star's orbit around the Galaxy when the
star lies closest to the Galactic center. The Sun is near
perigalacticon now. [C95]
|
| Period-Luminosiry Relation
| Cepheids obey this relation: the longer
the Cepheid's pulsation period, the more luminous the star. Since
measuring a Cepheid's period is easy, the period-luminosity relation
allows astronomers to determine the Cepheid's intrinsic brightness and
hence distance. If the Cepheid is part of another galaxy, the
Cepheid's distance gives the distance to the entire galaxy. [C95]
|
| Perseus Arm
| The spiral arm that lies next out from the arm containing
the Sun. The most famous members of the Perseus arm are the young star
clusters h and Chi Persei. [C95]
|
| Photon
| A particle of light. [C95]
|
| Pinwheel Galaxy
| M33, a spiral galaxy that lies 2.6 million
light-years away and is the third largest member of the Local Group,
after Andromeda and the Milky Way. [C95]
|
| Planet
| An object that formed in the disk surrounding a star. To be
called a planets an object must be more massive than Pluto (1/500 the
Earth's mass) and less massive than ten times Jupiter's mass. Unlike
stars, planets do not produce light of their own hut merely reflect
that of the star(s) they orbit. [C95]
|
| Planetary Nebula
| A bubble of gas surrounding a hot, dying star. The
star is so hot that it makes the planetary nebula glow, which allows
astronomers to see it. The star was once the core of a red giant,
which ejected its outer atmosphere and created the planetary. A
planetary nebula has nothing to do with a planet, but through a small
telescope, it looks like a planet's disk, hence the misleading name. [C95]
|
| Platinum
| Element with atomic number 78. It is produced almost
entirely by the r process~ in supernovae. [C95]
|
| Pleiades
| A nearby star cluster in the constellation Taurus that lies
410 light-years away. It is young, containing blue stars, and has an
age of 70 million years. [C95]
|
| Polaris
| The North Star, a second-magnitude star in the constellation Ursa
Minor. The star is a yellow-white F-type supergiant that lies 330
light-years away. Until recently, Polaris was also a Cepheid; in fact,
it was the nearest Cepheid to Earth-but as this book goes to press
there are signs that the star is ceasing to pulsate. [C95]
|
| Population
| See Stellar Population. [C95]
|
| Primordial Nucleosynthesis
| The creation of elements that occurred
just minutes after the big bang. According to standard theory,
primordial nucleosynthesis gave the universe only five nuclei, all
lightweight: hydrogen-1b, hydrogen-2 (or deuterium), helium-3,
helium-4, and lithium-7. [C95]
|
| Procyon
| The brightest star in the constellation Canis Minor and one
of the nearest stars, lying just 11.4 light-years from Earth. Procyon
is the eighth brightest star in the night sky. It consists of two
stars: Procyon A, a bright yellow-white F-type star that has just
started to evolve off the main sequence; and Procyon B, a dim white
dwarf. [C95]
|
| Proper Motion
| The apparent movement of a star, year after year,
caused by the star's velocity across the line of sight. If the star's
distance is known, this velocity-called the tangential velocity, can
be computed. The star with the largest proper motion is Barnard's
Star, whose proper motion is 10.3 arc-seconds per year. [C95]
|
| Protogalaxy
| An object that becomes a galaxy. [C95]
|
| Proton
| Acle with positive electric charge. Every atom subatomic
particle has at least one proton in its nucleus; the number of protons
determineS the element. For example, all atoms with one proton are
hydrogen, all atoms with two protons are helium, and so on. [C95]
|
| Proton-Proton Reaction
| The nuclear sequence by which the Sun and all
other main-sequence stars with less than 1.5 solar masses fuse
hydrogen into helium. [C95]
|
| Proxima Centauri
| The faintest of the three stars that make up the
Alpha Centauri star system. See Alpha Centauri. [C95]
|
| PSR B 1257+12
| A pulsar in the constellation Virgo and the site of the
first solar system to be discovered outside our own. The planets were
detected in 1991. [C95]
|
| Pulsar
| A fast-spinning neutron star that emits radiation toward Earth
every-time it rotates. [C95]
|
| Quasar
| The brightest objects in the universe, quasars can generate
over a trillion times as much light as the Sun from a region little
larger than the solar system. Most are extremely distant, which means
that they existed long ago. [C95]
|
| Radial Velocity
| The speed at which an object moves toward or away
from us. It can be measured from a star's spectrum: a star moving
toward us has a blueshifted spectrum, and a star moving away from us
has a redshifted spectrum. The larger the blueshift or redshift, the
larger the radial velocity. The present radial-velocity champion is a
star in the constellation Lacerta named Giclas 233-27, which moves
toward us at 583 kilometers per second. [C95]
|
| Radio
| Electromagnetic radiation with the lowest energy and longest
wavelength. Unlike visible light, radio waves penetrate dust and can
be detected from throughout the Galaxy. [C95]
|
| Reddening
| The scattering away of blue light that occurs when light
passes through gas and dust, thereby leaving red (and infrared) light
more dominant. This phenomenon occurs at sunset, when the Sun looks
orange or red because its light passes through the thick air on the
Earth's horizon. Reddening is unrelated to redshift. [C95]
|
| Red Dwarf
| A main-sequence star with spectral type M. Red dwarfs are
much fainter, cooler, and smaller than the Sun but are the most common
type of star in the Galaxy, accounting for 70 percent of all
stars. The nearest red dwarf, Proxima Centauri, lies just 4.25
light-years away, but neither it nor any other is visible to the naked
eye. [C95]
|
| Red Giant
| A giant star with spectral type M. Such stars are in a more
advanced state of evolution than the Sun, for they do not burn
hydrogen into helium at their cores. Instead, they may fuse hydrogen
into helium in a layer surrounding their cores, or they may fuse
helium into carbon and oxygen, or they may do both. Often, astronomers
use "red giant" loosely, to include not only M giants but G and K
giants, too. [C95]
|
| Redshift
| The shift to the red of a star's spectrum caused by the
star's movement away from us. This movement stretches the star's light
waves and increases their wavelength. Since red has a longer
wavelength than blue, this shift is called a redshift. The larger a
star's redshift, the faster the star is moving away from us. Most
galaxies also show redshifts, not because of the galaxy's movement
away from us (although the galaxy is moving away from us) but because
of the expansion of the universe. As a galaxy emits a light wave
toward us, the light wave travels through the fabric of space; en
route to Earth, it is stretched by the expansion of space and exhibits
a redshift. The farther the galaxy, the larger the redshift. To
distinguish this type of redshift from one caused by movement,
astronomers call it the "cosmological redshift." [C95]
|
| Red Supergiant
| A supergiant with spectral type M. Red supergiants are
the largest stars in the universe: if put in place of the Sun, some
would touch Saturn. The two brightest red supergiants in Earth's
sky are
Betelgeuse and Antares. [C95]
|
| Resolution
| The ability to see detail in an object. [C95]
|
| Rigel
| A blue supergiant some 900 light-years away in the constellation
Orion. Rigel is the brightest star in Orion and the seventh brightest
star in the night sky. [C95]
|
| Ring Nebula
| A famous planetary nebula in the constellation Lyra. [C95]
|
| R-Process
| The creation of elements heavier than zinc through the
rapid bombardment of other elements by neutrons. The r process occurs
in supernovae. Examples of reprocess elements are gold, iodine, and
europium. [C95]
|
| RR Lyrae Star
| An old metal-poor white or yellow-white giant star that
pulsates like a Cepheid and therefore varies in brightness. Most RR
Lyrae stars have periods of under one day, which is shorter than
periods for Cepheids. RR Lyrae stars are also fainter than Cepheids,
with absolute magnitudes around +0.7, corresponding to a luminosity
about 45 times the Sun's. RR Lyrae stars are excellent distance
indicators because they all have nearly the same intrinsic
brightness. They take their name from the star RR Lyrae, in the
constellation Lyra. [C95]
|
| Sagittarius A* (pronounced "ay star")
| The very center of the Milky
Way, Sagittarius A* is a strong source of radio waves and probably a
massive black hole. [C95]
|
| Sagittarius Arm
| The spiral arm that lies next in from the arm
containing the Sun. [C95]
|
| Satellite Galaxy
| A galaxy that orbits a larger one. The Milky Way has
at least ten satellite galaxies: the Large Magellanic Cloud, the Small
Magellanic Cloud, Ursa Minor, Draco, Sculptor, Sextans, Carina,
Fornax, Leo II, and Leo I. [C95]
|
| Scale Height
| The mean distance of a group of stars from the Galactic
plane. In general, old stars have larger scale heights than young
ones. [C95]
|
| Sculptor
| 1. A faint constellation in the southern sky. 2. A dwarf
galaxy that orbits the Milky Way in the constellation Sculptor. It is
255,000 light-years from the Galactic center. [C95]
|
| Sculptor Group
| The nearest group of galaxies to the Local Group, 4 to
10 million lightyears away. Its brightest member is the beautiful
edge-on spiral NGC 253. [C95]
|
| Searle and Zinn
| The idea, published in 1978 by Leonard Searle and
Robert Zinn, that the outer part of the Galaxy's halo formed from the
accretion of smaller galaxies. [C95]
|
| Second Parameter
| The color of a globular cluster's horizontal branch
is determined largely by its metallicity: all other things being
equal, the more metal-poor a cluster, the bluer its horizontal
branch. However, all other things are not always equal, because
globulars with similar metallicities sometimes have different
horizontal-branch colors, so a second parameter must be
responsible. Searle and Zinn speculated that the second parameter was
age and said that all globulars had not formed at the same time. [C95]
|
| Sextans
| 1. A faint constellation south of Leo. 2. A dwarf galaxy that
orbits the Milky Way. Discovered by computer in 1990, Sextans lies
295,000 light-years from the Galactic center. [C95]
|
| Silicon
| Element with atomic number fourteen and the sixth most common
metal in the universe. It is produced by high-mass stars that explode. [C95]
|
| Silicon Burning
| The end of the line for a high-mass star, silicon
burning creates iron and other elements of similar mass and presages a
supernova. [C95]
|
| Silver
| Element with atomic number 47. It is produced by both the
r-process and the s-process, but more by the former. [C95]
|
| Sirius
| The brightest star in the night sky. It is a white, A-type
star that lies just 8.6 light-years from Earth in the constellation
Canis Major. Orbiting the main star (officially called Sirius A) is a
faint white dwarf, Sirius B. Sirius A is the nearest A-type
main-sequence star to Earth; Sirius B is the nearest white dwarf to
Earth. [C95]
|
| 61 Cygni
| The first star other than the Sun to have its parallax, and
hence distance, measured. The star is a double orange dwarf that lies
in the constellation Cygnus 11.4 light-years away. [C95]
|
| Small Magellanic Cloud
| The second largest, and the second nearest, of
the galaxies that orbit the Milky Way. It lies in the southern sky,
190,000 light-years away. [C95]
|
| Snickers
| A possible satellite galaxy of the Milky Way, reported in
1975. Its existence is in dispute. [C95]
|
| Solar Mass
| The amount of mass in the Sun, and the unit in which
stellar and galactic masses are expressed. [C95]
|
| Solar Motion
| The velocity of the Sun through space, relative to the
local standard of rest. The solar motion is U = -9 kilometers per
second, V = +12 kilometers per second, and W = +7 kilometers per
second. [C95]
|
| Solar System
| Objects that orbit a star-planets, asteroids, comets. [C95]
|
| Sombrero Galaxy
| A spiral galaxy in the constellation Virgo. It was
the first galaxy whose rotation was detected. [C95]
|
| South Galactic Pole
| A point in the constellation Sculptor toward
which our line of sight is perpendicular to and below the Galactic
disk. [C95]
|
| Space Velocity
| A star's total velocity with respect to the local
standard of rest. This is the combination of the star's U, V, and W
velocities: space velocity = ill> + V, + W2. For example, the Sun (U =
-9, V = +12, W = +7) has a space velocity of
17 kilometers per second. [C95]
|
| Spectral Type
| Classification of a star's spectrum, which correlates
with the star's temperature and color. There are seven main spectral
types. From hot and blue to cool and red, they are O, B, A, F, G, K,
and M. For further precision, astronomers divide each spectral
type. For example, from warmest to coolest, spectral type G is GO, Gl,
G2, G3, and so on to G9. The Sun is spectra] type G2. [C95]
|
| Spectroscopy
| Measuring the spectrum of an object. [C95]
|
| Spectrum
| The breakdown of light into a rainbow of colors. A good
spectrum reveals a star's spectral type, radial velocity (from the
spectrum's Doppler shift), and metallicity. [C95]
|
| Spica
| The brightest star in the constellation Virgo, Spica consists
of two blue B-type stars about 220 light-years from Earth. [C95]
|
| Spiral Galaxy
| A galaxy that looks like a pinwheel. The Milky Way, the
Andromeda Galaxy, M33, and M51 are all spiral galaxies.
Spitzer-Schwarzschild Scattering Mechanism. The process by which stars
in the Milky Way's disk encounter interstellar clouds and are
accelerated by them. Over time, this perturbs the stars, so that older
disk stars have more elliptical orbits, larger velocity dispersions,
and greater scale heights than younger disk stars. This mechanism
cannot, however, explain the motions of halo stars. [C95]
|
| S-Process
| The process by which elements heavier than copper are
formed through a slow flux of neutrons. The s-process operates in red
giant stars; prominent s-process elements include barium, zirconium,
yttrium, and lanthanum. [C95]
|
| Standard Candle
| An object-usually a star or a galaxy of known
intrinsic brightness. Measuring the apparent brightness of a standard
candle yields its distance. [C95]
|
| Star Cluster
| A gathering of hundreds, thousands, or even a million
stars. Star clusters are of two types: the less massive open clusters
and the more massive globular clusters. [C95]
|
| Star' Count
| Determination of the number of stars in a region of the
sky as a function of apparent magnitude and sometimes color. [C95]
|
| Star Stream
| Discovered by Kapteyn in 1902, a star stream is a group
of stars traveling in more or less the same direction. Kapteyn found
what he thought were two oppositely directed star streams, but
astronomers now recognize that these simply reflect the tendency of
stars to have their largest velocities in the U direction. [C95]
|
| Star System
| A few stars that orbit each other. For example, a double
star system consists of two stars; a triple star system consists of
three stars; and so on. [C95]
|
| Steady State
| A cosmological model that proposes that the universe is
eternal, with no beginning or end. [C95]
|
| Stellar Evolution
| How a star changes with time. [C95]
|
| Stellar Halo
| See Halo. [C95]
|
| Stellar Parallax
| See Parallax. [C95]
|
| Stellar Population
| A Galaxy-wide group of stars of all types that
have similar ages, locations, kinematics, and metallicities. As
astronomers presently know the Milky Way, they recognize four stellar
populations: the thin disk; the thick disk; the stellar halo; and the
bulge. [C95]
|
| Subdwarf
| A metal-poor main-sequence star. On the H-R diagram,
subdwarfs lie slightly below the metal-rich main sequence, because
they are fainter than metal-rich main-sequence stars of the same
color. [C95]
|
| Subgiant
| A star making the transition from the main sequence to the
giant stage. [C95]
|
| Sulfur
| Element with atomic number sixteen and the eighth most common
metal in the universe. It was produced by oxygen burning in high-mass
stars that exploded. [C95]
|
| Sun
| The star that Earth orbits. The Sun is a yellow main-sequence
star that is spectral type G2, shines with apparent magnitude -26.74,
and has an absolute magnitude of +4.83. The Sun is 4.6 billion years
old. It lies 27,000 light-years from the Galactic center, or about 40
percent of the way from the center to the edge of the Galactic disk. [C95]
|
| Supercluster
| A huge assemblage of galaxies. The Milky Way and the
entire Local Group are part of the Local Supercluster, which is
centered on the Virgo cluster. [C95]
|
| Supernova
| A titanic explosion that destroys a star. Type Ia
supernovae are explosions of white dwarfs that receive material from a
companion star and exceed the Chandrasekhar limit, whereas type Ib,
Ic, and II supernovae are explosions of high-mass stars-those born
with more than eight solar masses that run out of fuel. Type Ia, Ibs,
and Ic supernovae have no hydrogen in their spectra, whereas type II
supernovae do have hydrogen in their spectra. [C95]
|
| Surface Brightness
| The measure of the amount of light that an object,
especially a galaxy, emits per area of the sky. Even a luminous galaxy
can be hard to see if it has a low surface brightness. [C95]
|
| Tangential Velocity
| A star's velocity across an observer's line of
sight. To calculate a star's tangential velocity, one must know the
star's distance and proper motion. [C95]
|
| Tau Ceti
| A G-type main-sequence star that lies in the constellation
Cetus, 11.4 light-years away. It is a single star like the Sun and
could support life. [C95]
|
| Technetium
| Radioactive element with atomic number 43. It was seen in
red giants in 1952; because it is unstable, its presence indicated
that the stars themselves had made it. [C95]
|
| Temperature
| The hotter a star, the bluer it is: blue stars are hot,
yellow stars warm, and red stars cool. [C95]
|
| Thick Disk
| The stellar population that contains Arcturus and about 4
percent of the other stars near the Sun. It has a scale height of
about 3500 light-years and consists of old stars. [C95]
|
| Thin Disk
| The stellar population that contains the Sun and most other
nearby stars. Most of its stars have a scale height of 1000
light-years and orbit the Galaxy on fairly circular orbits. The stars
of the thin disk range in age from 0 to about 10 billion years. The
thin disk breaks into two subpopulations, the young thin disk (ranging
in age from 0 to 1 billion years) and the old thin disk (ranging in
age from 1 to about 10 billion years). The young thin disk has a
smaller scale height than the old thin disk, and the former's stars
have more circular orbits. [C95]
|
| Tide
| A differential gravitational force. In the Galaxy, a tide
results because the Milky Way's gravity pulls more strongly on the
side of an object facing the Galactic center than on the object's
other side, so the object may get torn apart. [C95]
|
| Triple Star
| A star system having three stars that revolve around one
another. Hydrogen-3 the heaviest isotope of hydrogen, which contains
one proton and two neutrons. It is radioactive. Turnoff. See
Main-Sequence Turnoff. [C95]
|
| 21-Centimeter Radiation
| Emission of radio waves from neutral hydrogen gas. [C95]
|
| U
| See U Velocity. [C95]
|
| Ultraviolet
| Electromagnetic radiation that has a somewhat shorter
wavelength than visible light. In generals the hotter and bluer a
star, the more ultraviolet radiation it produces. [C95]
|
| Ultraviolet Excess
| Property of a star that emits more ultraviolet
radiation than one would have expected, based on its visual color. In
general, the greater the ultraviolet excess, the lower the star's
metallicity, because metals in a star's atmosphere absorb ultraviolet
radiation. [C95]
|
| Ursa Minor
| 1. The Little Bear (or Little Dipper), a constellation in
the northern sky that contains Polaris, the North Star. 2. A dwarf
galaxy in the constellation Ursa Minor that orbits the Milky Way and
lies 215,000 light~years from its center. [C95]
|
| U Velocity
| The component of a star's motion away from the Galactic
center. If a star moves away from the Galactic center, the star's U
velocity is positive; if a star moves toward the Galactic center, the
U velocity is negative; and if the star moves neither toward nor away
from the Galactic center, the U velocity is zero. The Sun has a U
velocity of -9 kilometers per second, so the Sun is moving toward the
Galactic center at 9 kilometers per second. [C95]
|
| V
| See V Velocity. [C95]
|
| Variable Star
| A star whose light varies. Some variables vary simply
because they consist of two stars, one of which eclipses the other;
Algol is the most famous example. Other variables, however, vary
because the stars themselves actually change in brightness; the most
famous are the Cepheids, RR Lyraes, and Miras, all of which pulsate. [C95]
|
| Vega
| The brightest star in the constellation Lyra and the fifth
brightest star in the night sky. Vega is a white A-type main sequence
star 25 light-years away. [C95]
|
| Velocity Dispersion
| The spread of a velocity distribution-that is,
how stars move relative to one another. Technically, the velocity
dispersion is the standard deviation of the velocity
distribution. Stars with similar velocities have a small velocity
dispersion, whereas stars with wildly different velocities have a
large velocity dispersion. [C95]
|
| V Velocity
| A star's velocity in the direction of Galactic rotation,
as measured relative to a nearby star that has a circular orbit. If a
star revolves faster than such a star, the V velocity is positive; if
it revolves more slowly, the V velocity is negative; and if both
revolve at the same rate, the V velocity is zero. The Sun has a V
velocity of +12 kilometers per second, so it revolves 12 kilometers
per second faster than it would if it had a circular orbit. Since a
star on a circular orbit revolves around the Galaxy at 220 kilometers
per second, a star with a V velocity of 0 is not stationary; rather,
it revolves at 220 kilometers per second. The Sun therefore revolves
around the Galaxy at 220 + 12 = 232 kilometers per second. [C95]
|
| W
| See W Velocity. [C95]
|
| Warp
| The deviation from flatness in the outer Galactic disk. Some parts
of the outer disk lie above the Galactic plane; others lie below it. [C95]
|
| Whirlpool Galaxy
| MS 1, the most beautiful spiral galaxy in the sky. It
lies in the constellation Canes Venatici. [C95]
|
| White Dwarf
| A small, faint, dense, dying star that has used up its
nuclear fuel and is slowly fading from view. A typical white dwarf has
60 percent of the Sun's mass but is little larger than the
Earth. White dwarfs are common, accounting for 10 percent of all stars
in the Galaxy; the nearest is Sirius B, just 8.6 light-years away. But
no white dwarf is visible to the naked eye. [C95]
|
| White Giant
| A giant star of spectral type A. Some RR Lyrae stars are
white giants. [C95]
|
| White Supergiant
| A supergiant star with a spectral type of A. White
supergiants are rare; the nearest is Deneb, which lies 1500
light-years away. [C95]
|
| WIMP
| Weakly interacting massive particle. Some astronomers believe
that these exotic subatomic particles make up most of the mass of the
universe. [C95]
|
| Wolf-Rayet Star
| A hot blue star with a peculiar spectrum. Wolf-Rayet
stars have ejected their outer layers, exposing some of the elements
the stars have created. [C95]
|
| W-Velocity
| A star's velocity perpendicular to the Galactic plane. Ifa
star is moving up, its W velocity is positive; if a star is moving
down, its W velocity is negative; and if a star does neither, its W
velocity is zero. The Sun has a W velocity of +7 kilometers per
second, so it is moving up at 7 kilometers per second. In general, the
greater a star's W velocity when it crosses the Galactic plane, the
farther above and below the plane the star will travel. [C95]
|
| W Virginis Star
| Also known as a population II Cepheid, a W Virginis
star is a bright yellow star that pulsates like a Cepheid but is older
and fainter. W Virginis stars appear in globular clusters. [C95]
|
| X-Process
| The unknown nucleosynthetic process that Burbidge,
Burbidge, Fowler, and Hoyle said had formed the light nuclei deuterium,
lithium, beryllium, and boron. [C95]
|
| X-Ray
| An energetic form of electromagnetic radiation that is more
powerful than ultraviolet radiation but less powerful than gamma rays. [C95]
|
| Yellow Giant
| A giant star with a spectral type of G. The nearest and
brightest yellow giants are the two composing the double star Capella. [C95]
|
| Yellow Supergiant
| A supergiant star with a spectral type of G. [C95]
|
| Young Thin Disk
| A subpopulation in the thin disk whose stars range in
age from 0 to 1 billion years old. The stars of the young thin disk
have a scale height of 350 light-years and have very circular orbits
around the Galaxy. [C95]
|
| Zirconium
| Element with atomic number 40. It arises almost entirely
from the s-proccss, in red giant stars. [C95]
|