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21-cm Radiation
Radio Emission Produced by cold, low-density
hydrogen in interstellar space.
A
Absolute Bolometeric Magnitude
The Absolute Magnitude we would observe if
we could detect all wavelengths
Absolute Visual Magnitude
Intrinsic brightness of a star; the apparent
visual magnitude the star would have if it were 10 Parsecs away from earth.
Absolute Zero
The lowest possible temperature. The temperature
at which particles in a material, atoms or molecules, contain no energy
of motion that can be extracted from a body.
Absorption Line
A dark line in a spectrum. Produced by the
absence of photons absorbed by atoms or molecules.
Absorption Spectrum
A Spectrum that contains absorption lines.
Acceleration
A change in velocity
Acceleration of Gravity
Falling objects fall with an increasing acceleration
of 9.8 meters per second per second.
Accretion
The sticking together of solid particles
to form a larger particle.
Accretion Disk
The whirling disk of gas that forms around
a compact object such as a white dwarf, neutron star or black hole as
matter is drawn in
Achromatic Lens
A telescope lens composed of two lenses ground
from different types of glass and designed to bring two selected colors
to the same focus and correct for chromatic aberration
Active Galaxy
A galaxy that is a source of excess radiation,
usually radio waves, X rays, gamma rays or some combination
Active Galaxy Nucleus (AGN)
The central energy source of an active galaxy
Active Optics
Optical elements whose position or shape
is continuously controlled by computers
Adaptive Optics
Computer controlled telescope mirrors that
can adjust for some changes in seeing conditions
Alt-Azimuth Mounting
A telescope mounting capable of motion parallel
to and perpendicular to the horizon
Angstrom
1 x 10 -10 meters (used to measure wavelengths)
Angular Momentum
Measure of the rotation of the body around
some point.
Annual Eclipse
A type of total eclipse in which the moon
is too far from the earth to totally cover the suns surface. A ring of
the photosphere surrounds the moon during mideclipse. The corona is not
visible and neither are the prominence because of the glare from the photosphere.
Anorthosite
Rock of aluminum and calcium silicates found
in the lunar highlands.
Aphelion
Point in the earth's orbit where the earth
is farthest away from the Sun (summer for us)
Apogee
Orbital point of greatest distance from the
earth
Apparent Relative Orbit
The Orbit of one star in a visual binary
with respect to the other star as seen from earth
Apparent Visual Magnitude (mv)
Is the magnitude of the stars without compensating
for their distance from Earth.
Arachnoids
On Venus, one of a number of round networks
of fractures in the crust, resembles spider webs.
Archaeoastronomy
The study of astronomy by ancient peoples
Association
Groups of widely scattered stars (10 - 1000)
moving together through space; not gravitationally bound to clusters.
Asterism
Named group for stars not identified as constellations
Asteroids
Small Rocky worlds, most of which lie between
Mars and Jupiter in the asteroid belt.
Astrometric Binary
A binary star identified by its irregular
proper motion
Astronomical Unit (AU)
Is the average distance from the Earth to
the Sun. 1 AU = 93 million miles or 1.5 X 1011 meters or 1.5 X 109 km.
Astronomy
Is the study of the universe
Atmospheric Window
Wavelength regions in which our atmosphere
is transparent - at visual wavelengths, infrared and radio.
Aurora
The glowing light display that results when
a planet's magnetic field guides charged particles toward the north and
south magnetic poles, where they strike the upper atmosphere and excite
atoms to emit photons.
Autumnal Equinox
The place where the Sun crosses the celestial
equator heading south.
B
B Canis Majoris Variable
Short Period variables stars that do not
lie in the instability strip
Barred Spiral Galaxy
A spiral galaxy with an elongated nuclei
resembling a bar from which the arm originate.
Basalt
Dark, igneous rock characteristic of solidified
lava.
Belts
Dark bands of clouds that circle Jupiter
parallel to its equator; generally red, brown, or blue-green; believed
to be regions of descending gas.
Big Bang Theory
The theory that the universe began with a
violent explosion from which the expanding universe of galaxies formed.
Binary Stars
Pairs of stars that orbit around their common
center of mass
Binding Energy
The energy needed to pull an electron away
from its atom.
Bipolar Flow
Opposite directed jets of gas ejected by
some protostellar objects.
BL Lac Object
Objects resemble quasars; thought to highly
luminous cores of distant galaxies.
Black Body Radiation
Radiation emitted by a hypothetical perfect
radiator. The spectrum is continuous, and the wavelength of maximum emission
depends only on the body's temperature.
Black Dwarf
The end state of a white dwarf that has cooled
to a low temperature
Black Hole
A mass that has collapsed to such a small
volume that its gravity prevents the escape of all radiation; also, the
volume of space from which the radiation may not escape.
Blue and Red Shifts
Blue shifts: Shorting of wavelengths of observed
light when an object in moving toward the observer. Red Shifts: Lengthening
of wavelengths of light as the object moves away from the observer.
Bok Globules
Small, dark clouds only about 1 light year
in diameter that contain 10-1000 solar masses of gas and dust. Believed
to be related to star formation.
Bow Shock
The boundary between the undisturbed solar
wind and the region being deflected around the planet or comet
Breccia
A rock composed of fragments of earlier rocks
bonded together.
Brown Dwarf
A very cool, low luminosity star whose mass
is not sufficient to ignite nuclear fusion.
Burster
A source of bursts of X-rays or in some cases,
gamma rays; believed to be associated with neutron stars.
C
Capture Hypothesis
The theory that the moon formed elsewhere
in the solar system and was later captured by the earth.
Carbon Detonation
The explosive ignition of carbon burning
in some giant stars. A possible cause of some supernova explosions
Cassegrain Telescope
A reflecting telescope in which the secondary
mirror reflects light back down the tube through a hole in the center
of the primary mirror
Celestial Equator
An imaginary line around the sky which is
directly above the earth's equator.
Celestial Sphere
Imaginary sphere surrounding the Earth to
which the stars, planets, Sun and moon seem to be attached.
Center of Mass
Two bodies revolve around a common center,
called the balance point of the system.
Cepheid Variable Star
Variable stars with a period of 1-60 days.
Their period is related to luminosity.
Chandrasekhar Limit
The maximum mass of a white dwarf, about
1.4 solar masses. A white dwarf of greater mass can not support itself
and will collapse.
Charge-Coupled Device
A CCD is an electronic device consisting
of an array of photosensitive elements, used to record images.
Chromosphere
A line layer of gas just above the photosphere
of the sun, it is often marked by solar eruptions called solar flares
and prominence.
Chromatic Aberration
A distortion found in refracting telescopes
because lenses focus different colors at slightly different distances.
Images are consequently surrounded by color fringes
Circular Velocity
The lateral velocity an object must have
to remain in orbit
Circumpolar Constellation
Constellations which appear around the celestial
poles, which never seem to rise or set.
Closed Orbit
A circular orbit, one which returns back
on itself.
Closed Universe
A model universe in which the average density
is great enough to stop the expansion and make the universe contract.
Cluster Method
The method of determining the masses of galaxies
in a cluster.
CNO Cycle
A series of nuclear reactions that use carbon
as a catalyst to combine four hydrogen atoms to make one helium atom plus
energy; effective in stars more massive than the sun.
Co-Accretion Hypothesis
The theory that the moon and the earth formed
together.
Cocoon
The cloud of gas and dust around a contracting
protostar that conceals it at visible wavelengths.
Collisional Broadening
The smearing out of a spectrum line because
of collisions among the atoms of the gas.
Color Index
A numerical measure of the color of a star.
Comet
One of the small, icy bodies that orbit the
sun and produce tails of gas and dust when they near the sun.
Compact Object
A star that has collapsed to form a white
dwarf, neutron star or black hole.
Comparative Planetology
The study of planets by comparing the characteristics
of different examples.
Condensation Sequence
The sequence in which different materials
condense from the solar nebula as we move outward from the sun.
Constellations
Apparent arrangement of stars, usually named
after ancient gods, heroes, animals or mythological beings.
Continuity of Energy Law
One of the basic laws of stellar structure,
The amount of energy flowing out of the top of a shell must equal the
amount coming in at the bottom plus whatever energy is generated within
the shell
Continuity of Mass Law
One of the basic laws of stellar structure.
The total mass of the star must equal the sum of the masses of the shells,
and the mass must be distributed smoothly throughout the star.
Continuous Spectrum
A spectrum in which there are no absorption
or emission lines.
Corona
The faint outer atmosphere of the Sun that
is exposed during a total solar eclipse.
Coronae
On Venus, circular features, not caused by
impacts, they are domed plains caused by the rising plumes of molten rock
from below.
Coronagraph
A telescope designed to photograph the inner
corona of the sun.
Coronal Hole
An Area of the solar surface that is dark
at X-ray wavelengths; thought to be associated with divergent magnetic
fields and the source of the solar wind.
Cosmic Ray
Atomic nuclei that enter earth's atmosphere
at nearly the speed of light. Some originate in solar flares, and some
may come from supernova explosions, but their true nature is not well
understood.
Cosmological Principle
The assumption that any observer in any galaxy
sees the same general features of the universe.
Cosmology
The study of the nature, origin and evolution
of the universe.
Coude Focus
The focal arrangement of a reflecting telescope
in which mirrors direct the light o a fixed focus beyond the bounds of
the telescope's movement. typically in a separate room, used primarily
for spectroscopy.
Critical Density
The average density of the universe needed
to make its curvature flat.
Critical Point
The temperature and pressure at which vapor
and liquid phases of a material have the same density.
D
Dark Nebula
A nebula consisting of dust and gas blocking
our view of more distant stars.
Decameter Radiation
Radio signals from Jupiter with wavelengths
about 10m
Decimeter Radiation
Radio signals
from Jupiter with wavelengths about 0.1m
Deferent
The larger circle of
which an epicycle revolves (the earth in this case).
Degenerate Matter
Extremely high density matter in which pressure
no longer depends on temperature due to the quantum mechanical effects.
Density Wave Theory
Theory proposed to account for spiral arms
as compressions of the interstellar medium in the disk of the galaxy.
Diamond Ring Effect
Just as totality begins during a solar eclipse
a small portion of the Suns photosphere can peak out from behind the moon
through a valley at the edge of the lunar disk. It is not visible during
every solar eclipse.
Differential Rotation
The rotation of a body in which different
parts of the body have different periods of rotation. This is true of
the sun, Jovian planets, and the disk of the galaxy.
Differentiation
The separation of planetary material according
to density.
Diffraction Fringe
Blurred fringe surrounding and image caused
by wave properties of light. because of this no image detail smaller than
the fringe can be seen
Dirty Snowball Theory
The hypothesis
that comets are kilometer-size balls of ices with embedded impurities.
Disk Component
All material confined to the plane of the
galaxy.
Distance Indicator
Objects whose luminosities or diameters are
known; used to find the distance to a star cluster or galaxy.
Distance Modulus
The difference between the apparent and absolute
magnitude of a star. A measure of how far away the star is.
Diurnal Motion
The apparent daily rotation of the sky.
Doppler Broadening
The smearing of spectral lines because of
the motion of atoms in the gas.
Doppler Effect
A change in the wavelength of radiation due
to relative radial motion of the source and the observer.
Double Galaxy Method
A method of finding the masses of galaxies
from orbiting pairs of galaxies.
Double Stars
A pair of stars close together in the sky.
Not all double stars are necessarily in orbit around each other.
Double-Exhaust Model
The theory that double radio lobes are produced
by pairs of jets emitted in opposite directions from the centers of active
galaxies.
Double-Line Spectroscopic Binary
A spectroscopic binary star in which spectral
lines from both stars are visible in the spectrum
Dwarf Nova
A star that undergoes novalike explosions
every few days or weeks; believed to be associated with mass transfer
onto a white dwarf in a binary system.
Dynamo Effect
The theory that the earth's magnetic field
is generated in the conducting material of its molten core.
E
Eccentrics
An offcenter circular path
Eclipse Season
Is the season when the Sun is close enough
to a node for an eclipse to occur, An eclipse season is 32 days. Any new
moon during this period will cause a solar eclipse. For Lunar eclipses
the period is shorter only about 22 days. A full moon occurring during
this time will cause a lunar eclipse.
Eclipse Year
The 346.62 days it takes the sun to return
to a node
Eclipsing Binary
A binary star system in which the stars eclipse
each other.
Ecliptic
The apparent path of the Sun around the Sky.
Ejecta
Pulverized rock scattered by meteorite impacts
on a planetary surface.
Electroglow
The UV radiation produced in the upper atmosphere
of Jupiter, Saturn, Uranus by high-energy particles in the planet's magnetosphere.
Electromagnetic Radiation
Changing electric and magnetic fields that
travel through space and transfer energy from one place to another - light,
radio waves and magnetism
Electron
A low mass atomic particles carrying a negative
charge.
Ellipse
A closed curve enclosing two points (foci)
such that the total distance from one focus to any other point on the
curve back to the other focus equals a constant.
Elliptical Galaxy
A galaxy that is round or elliptical in outline.
It contains little gas and dust, no disk or spiral arms, and a few hot,
bright stars.
Emission Line
A bright line in a spectrum caused by the
emission of photons from atoms.
Emission Nebula
A cloud of gas excited by UV wavelengths
of hot stars,
Emission Spectrum
A spectrum containing emission lines
Energy Level
One of a number of states an electron may
occupy in an atom, depending on its binding energy.
Energy Machine
An object that releases energy. Commonly
used to refer to the source of energy in active galactic nuclei
Epicycle
An attempt to explain the retrograde loop
in the earth centered universe, by attaching the planets to epicycles
and having them revolve around that, and in turn around the earth.
Equant
Ptolemy placed the earth offcenter and the
opposite point from the deferent on this off center circle is called the
equant.
Equatorial Mounting
A telescope mounting that allows motion parallel
to and perpendicular to the celestial equator.
Ergosphere
The region surrounding a rotating black hole
within one could not resist being dragged around the black hole. It is
possible for a particle to escape from the erosphere and extract energy
from the black hole.
Escape Velocity
The velocity needed to escape from the surface
of a body
Event Horizon
The boundary of the region of a black hole
from which no radiation may escape. No event that occurs with the event
horizon is visible to a distant observer.
Excited Atom
An atom in which an electron has moved from
a lower to higher orbit.
Extinction
The dimming of light by intervening material;
commonly, dimming by the interstellar medium.
Eyepiece
A short focal length lens used to enlarge
the image in a telescope; the lens nearest the eye
F
False Color
A graphical representation of data in which
an images is colored to reveal additional detail
Field
A way of explaining action at a distance.
Filar Micrometer
An instrument that permits precise measurements
at the telescope of the position of visual binary stars and similar objects.
Filtergrams
A photograph (usually of the sun) taken in
the light of a specific region of the spectrum - e.g., an H-alpha filtergram.
Fission Hypothesis
The theory that the moon formed by breaking
away from the earth.
Flare
A violent eruption on the sun's surface.
Flash Spectrum
The emission spectrum of the chromosphere
that is visible for the few seconds during a total solar eclipse when
the moon has covered the photosphere but has not covered the chromosphere.
Flat Universe
A model of the universe in which space-time
is not curved.
Flatness Problem
In cosmology the circumstance that the early
universe must have contained almost exactly the right amount of matter
to close the space-time (to make space-time flat)
Flocculent
Woolly, fluffy; used to refer to certain
galaxies that have a woolly appearance.
Focal Length
A distance from a lens to a point where it
focuses parallel rays of light.
Focus
The points around which an ellipse is drawn
Forward Scattering
The optical property of finely divided particles
to preferentially direct light in the original direction of the light's
travel
Frequency
The number of times a given event occurs
in a given time; for a wave the number of cycles that pass the observer
in one second
G
Galactic Cannibalism
The theory that large galaxies absorb smaller
galaxies.
Galactic Corona
The low-density extensions of the halo of
a galaxy; now suspected to extend many times the visible diameter of the
galaxy.
Galilean Satellites
The four largest moons of Jupiter, named
after their discoverer Galileo.
Gauss
A unit used to measure the strength of a
magnetic field.
General Relativity
Observers can not distinguish locally between
inertial forces due to acceleration and uniform gravitational forces due
to the presence of a massive body. Mass tells space-time how to curve,
and the curvature of space-time (gravity) tells mass how to accelerate
Geocentric Universe
Aristotle believed the universe was divided
into two parts, the earth corrupt and the heavens perfect and immutable.
The geocentric universe described the universe with the earth at the center.
Giant Molecular Cloud
Very large, cool clouds of dense gas in which
stars form.
Giant Stars
Large, cool, highly luminous stars in the
upper right of the H-R diagram. Typically 10-100 times the diameter of
the sun.
Glacial Period
Time when glaciers advance and engulf huge
sheets of land
Glitch
A sudden change in the period of a pulsar.
Globular Star Cluster
A star cluster containing 50,000 to 1 million
stars in a sphere about 75 light years in diameter; generally old metal
poor and found in the spherical component of the galaxy.
Graben Rille
A linear feature on a planetary surface caused
by the faulting and sinking of portions of the crust.
Granulation
The fine structure visible on the solar surface
caused by rising currents of hot gas and sinking currents of cool gas
below the surface.
Grating
Material onto which microscopic parallel
lines are inscribed. used to create a spectrum of colors from light
Gravitational Lens Effect
The focusing of light from a distant galaxy
or quasar by an intervening galaxy to produce multiple images of the distant
body.
Gravitational Red Shift
A lengthening of the wavelength of a photon
due to its escape from a gravitational field.
Gravitational Wave
A transport of energy by the motion of waves
in a gravitational field; predicted by general relativity
Greenhouse Effect
The process by which a carbon dioxide atmosphere
traps heat and raises the temperature of a planetary surface.
Grooved Terrain
Regions of the surface of Ganymede consisting
of parallel grooves; believed to have formed by repeated fracture and
refreezing of the icy crust.
Ground State
The lowest permitted orbit of an electron
in an atom.
H
H II Region
A region of ionized hydrogen around a hot
star.
H-R Diagram
Half-Life
The time required for half of the atoms in
a radioactive sample to decay.
Halo
The spherical region of a spiral galaxy containing
a thin scattering of stars, star clusters, and small amounts of gas.
Head-Tail Radio Galaxy
A radio galaxy with a contour consisting
of a head and a tail; believed caused by the motion of an active galaxy
through the intergalactic medium.
Heat
Thermal energy present in a body as agitation
(motion) among its particles (atoms or molecules).
Heat of Formation
In planetology, the heat released by the
infall of matter during the formation of a planetary body.
Heliocentric Universe
The Sun is at the center of the universe
and everything revolves around that. Was first proposed by Copernicus.
Helioseismology
The study of the interior of the sun by the
analysis of its modes of vibration.
Helium Flash
The explosive ignition of helium burning
that takes place in some giant stars.
Herbig-Haro Object
Small nebula that vary irregularly in brightness;
believed associated with star formation.
Heterogeneous Accretion
The formation of a planet by the accumulation
of planetisimals of different composition - e.g. first iron particles,
then silicates
High-Velocity Star
A star with a large space velocity. Such
stars are halo stars passing through the disk of the galaxy at steep angles.
Homogeneity
The assumption that, on the large-scale,
matter is uniformly spread throughout the universe.
Homogeneous Accretion
The formation of a planet by the accumulation
of planetisimals of the same composition.
Horizontal Branch
In the H-R diagram of a globular cluster,
the sequence of stars extending from the red giants toward the blue side
of the diagram; includes RR Lyrae stars.
Horoscope
A chart showing the location of heavenly
bodies among the zodiacal signs and with respect to the horizon at the
persons birth.
Hot Spot
In radio astronomy, a bright spot in a radio
lobe.
Hubble's Law
The linear relation between the distance
to a galaxy and its radial velocity.
Hubble Constant (H)
A measure of the rate of expansion of the
universe; the average value of velocity of recession divided by distance.
Hydrostatic Equilibrium
The balance between weight of the material
pressing downward on a layer in a star and the pressure in that layer.
I
Infrared Cirrus
A fine network of filaments covering the
sky detected in the far infrared by the IRAS satellite; believed associated
with dust in the interstellar medium.
Infrared Outburst
A sudden brightening of an object at infrared
wavelengths
Infrared Radiation
Electromagnetic Radiation with wavelengths
intermediate between visible light and radio waves
Instability Strip
The region of the H-R diagram in which stars
are unstable to pulsation. A star passing through this strip becomes a
variable star.
Intercrater Plain
The relatively smooth terrain on Mercury.
Interglacial Period
Time when glaciers melt back (cycles are
roughly 40,000 years)
Interstellar Absorption Lines
Dark lines in some stellar spectra that are
formed by the interstellar gas.
Interstellar Medium
The gas and dust distributed between the
stars.
Inverse Square Law
Force of gravity decreases as the square
of the distance increases
Io Flux Tube
A tube of magnetic lines and electric currents
connecting Io and Jupiter.
Ion
An atom that has lost or gained one or more
electrons.
Ionization
The process in which atoms lose or gain electrons.
Irregular Galaxy
A galaxy with a chaotic appearance, large
clouds of gas and dust, and both population I and population II stars,
but without spiral arms.
Isotope
Atoms that have the same number of protons
but a different number of neutrons.
Isotropy
The assumption that in its general properties
the universe looks the same in every direction
J
Joule (J)
A unit of energy roughly equivalent to the
energy given up when an apple falls on the floor. Equivalent to the force
of 1 Newton acting over a distance of 1 meter; 1 joule per second = 1
watt of power.
Jovian Planets
Jupiterlike planets with large diameters
and low densities.
Julian Day
The Julian Day is the number of days since
the year -4712. The Julian Day begins at 12:00 Noon Greenwich mean time.
Jumbled Terrain
Strangely disturbed regions of the moon opposite
the locations of the Imbrium basin and Mare Oriental
K
Kelvin Temperature Scale
The temperature, in Celsius (Centigrade)
degrees, measured above absolute zero.
Keplerian Motion
Orbital motion in accord with Kepler's laws
of planetary motion.
Kerr Black Hole
A solution to the equations of general relativity
that describes the properties of a rotating black hole.
Kiloparsec (kpc)
A unit of distance equal to 1000 pc or 3260
ly.
Kirchoff's Laws
A set of laws that describes the Absorption
and emission of light by matter.
L
Lagrangian Point
Points of stability in the orbital plane
of a binary system, planet, or moon. One is located 60 degrees ahead and
one 60 degrees behind the orbiting bodies. Another is located between
the orbiting bodies.
Large-Impact Hypothesis
The theory that the moon formed from debris
ejected during a collision between the earth and a large planetisimal.
Light Curve
A graph of brightness versus time commonly
used in analyzing variable stars and eclipsing binaries.
Light Gathering Power
The ability of a telescope to collect light.
Proportional to the are of the telescope objective lens or mirror
Light-Year
Is the distance that light travels in one
year. Abbreviation is ly
Lighthouse Theory
A theory that a neutron star produces pulses
of radiation by sweeping radio beams around the sky as it rotates.
Limb
The edge of the apparent disk of a body,
as in the "limb of the moon".
Limb Darkening
The decrease in the brightness of the sun
or other body from its center to its limb.
Line of Nodes
The nodes of the moons orbit are the points
where it passes through the plane of the earth's orbit. An eclipse season
occurs whenever the line connecting these nodes (line of nodes) points
toward the sun.
Line Profile
A graph of light intensity verses wavelength
showing the shape of an absorption line
Liquid Metal Hydrogen
A form of hydrogen under high pressure that
is a good electrical conductor.
Lobate Scarp
A curved cliff such as those found on Mercury.
Local Hypothesis
The theory that quasars are not at great
distances but relatively nearby.
Long Period Variable
A variable star with a period ranging from
100 days to over 400 days.
Look Back Time
The amount by which we look into the past
when we look at a distant galaxy; a time equal to the distance to the
galaxy in light-years.
Luminosity
The total amount of energy a star radiates
in one second.
Luminosity Class
A category of stars of similar luminosity;
determined by the widths of lines in their spectra.
Lyman, Balmer and Paschen Series
Spectral lines in the UV spectrum of hydrogen
produced by transitions whose lowest orbit of electrons is at ground state
(Lyman). Spectral lines in the visible and near UV of hydrogen produced
by transitions whose lowest orbit is second (Balmer). Spectral lines in
the infrared spectrum of hydrogen produced by transitions whose lowest
orbit is the third. (Paschen)
M
Magellanic Cloud
Small irregular galaxies that are companions
to the Milky Way; visible in the southern sky.
Magnetosphere
The volume of space around a planet within
which the motion of charged particles is dominated by the planetary magnetic
field rather than the solar wind.
Magnifying Power
The ability of a telescope to make an image
larger
Magnitude Scale
Method developed by Hipparchus who divided
the stars into 6 classes. The brightest stars are first class and those
slightly fainter are second class. The sixth class is the faintest stars
that he cold see with the unaided eye. The magnitude scale is logarithmic
like the eye.
Main Sequence
The region of the H-R diagram running from
upper left to lower right, which includes roughly 90 percent of all stars.
Mantle
The layer of dense rock and metal oxides
that lies between the molten core and the surface of the earth; also,
similar layers in other planets.
Mare
One of the lunar lowlands filled by successive
flows of dark lava.
Mass
A measure of the amount of matter in an object
Mass Function
A measure of the ratio of the masses in a
single-line spectroscopic binary. Also includes the inclination, which
is unknown for some systems.
Maunder Butterfly Diagram
A graph showing the latitude of sunspots
versus time, first plotted by W.W. Maunder in 1904.
Maunder Minimum
A period of less numerous sunspots and other
solar activity from 1645-1715.
Megaparsec (Mpc)
A unit of distance equal to 1 million pc.
Metals
IN astronomical usage, all atoms heavier
than helium.
Meteor
A small bit of matter heated by friction
to incandescent vapor as it falls into the atmosphere
Meteorite
A meteor that has survived its passage through
the atmosphere and strikes the ground.
Meteoroid
A meteor in space before it enters the earth's
atmosphere
Midocean Rift
Chasms that split the midocean rises where
the crustal plates move apart.
Midocean Rise
One of the undersea mountain ranges that
push up from the seafloor in the center of the oceans.
Minute of Arc
A measurement of the sky which includes degrees,
minutes and seconds.. There are 60 minutes of arc in one degree.
Missing Mass
Unobserved mass in clusters of galaxies believed
to provide sufficient gravity to bind the cluster together.
Model
An intellectual concept of how nature works
Molecule
Two or more atoms bonded together.
Momentum
The measurement of the amount of motion.
the product of mass and velocity
Morning and Evening Stars
A planet visible in the Morning shortly before
sunrise is a morning star, a planet visible just before sunset is the
Evening Star.
N
Nanometer
(nm) 1 x 10-9 meters
Natural Motion
Objects move toward their proper place. earth
and water downward, fire and air upward
Nebula
A cloud of gas and dust in space.
Neutrino
A neutral massless atomic particle that travels
at the speed of light.
Neutron
An atomic particles with no charge and about
the same mass as a proton.
Neutron Star
A small highly dense star composed almost
entirely of tightly packed neutrons; radius about 10 km.
Newtonian Focus
A focal arrangement of a reflecting telescope
in which a diagonal mirror reflects light out the side of the telescope
for easier access
Node
The point twice a month where the moon crosses
the ecliptic. Once a month the moon crosses heading north and two week
later crosses again heading south.
North and South Celestial Poles
The north ands south pivots points around
which the sky appears to rotate.
Nova
From Latin meaning "new"; a sudden
brightening of a star, making it appear as a new star in the sky; believed
associated with eruptions on white dwarfs in binary systems.
Nuclear Bulge
The spherical cloud of stars that lies at
the center of spiral galaxies.
Nucleosynthesis
The production of elements heavier than helium
by the fusion of atomic nuclei in stars and during supernova explosions.
Nucleus
The central core of an atom, containing protons
and neutrons; carries a net positive charge.
O
Objective Lens
In a refracting telescope, the long focal
length lens that forms an image of the object viewed; the lens closest
to the object
Objective Mirror
In a reflecting telescope the principle mirror
(reflecting surface) that forms an image of the object viewed
Oblate Spheroid
A sphere flattened such that its polar diameter
is smaller than its equatorial diameter.
Oblateness
The flattening of a spherical body; usually
caused by rotation
Occultation
The passage of a larger body in front of
a smaller body.
Olber's Paradox
The conflict between observation and theory
as to why the night sky should or should not be dark.
Opacity
The resistance of a gas to the passage of
radiation.
Open (Escape) Orbit
An orbit which leads away from the central
body, never to return
Open Star Cluster
A cluster of 10 to 10,000 stars with an open,
transparent appearance. The stars are not tightly grouped. Usually relatively
young and located in the disk of the galaxy.
Open Universe
A model universe in which the average density
is less than the critical density needed to halt the expansion of the
universe.
Optical Binary
A binary star in which the stars are only
apparently associated. One star is nearby and one is more distant.
Oscillating Universe Theory
The theory that the universe begins with
a big bang, expands, slows by its own gravity, collapses to create another
big bang.
Outgassing
The release of gas from a planets interior.
Ozone Layer
In earth's atmosphere, the layer of oxygen
ions (O3) lying 15 to 30 km high that protects the surface by absorbing
ultraviolet rays.
P
Parallax
The apparent change in the position of an
object due to a change in the location of the observer. It was because
ancient astronomers did not observe parallax that they though the earth
was the center of the universe and everything in the heavens revolved
around it.
Parsec
A hypothetical distance to a star whose parallax
is one arcsecond; 1pc = 206,265 A.U. = 3.26 ly.
Partial Eclipse (lunar or Solar)
Partial eclipses are caused when the moon
passes through only part of the umbra shadow of the earth. Or when the
earth passes through the only part of the umbra shadow of the moon.
Path of Totality
The path of a total eclipse that is swept
out by the umbra shadow of the moon on the earth.
Penumbra
A Partial blocking of the Sun by the Earth
creates an Penumbra Shadow. The Sunlight is dimmed but not extinguished.
Penumbra Eclipse
When the moon passes only through the penumbra
shadow of the earth or when the earth passes through only the penumbra
shadow of the moon.
Perigee
Point of closet approach to the earth
Perihelion
Point in earth's orbit around the sun where
it is closest to the Sun (winter for us)
Period-Luminosity Diagram
A graph showing the relation-between period
of pulsation and intrinsic brightness among Cepheid variable stars.
Photometer
A device used to measure the intensity and
color of light
Photon
A quantum of electromagnetic energy Carries
an amount of energy that depends inversely on its wavelength
Photosphere
The bright disk of the sun that is covered
completely by a total Solar eclipse.
Planet Motion
Always found near the ecliptic (except Pluto)
Planetary Nebula
An expanding shell of gas ejected by a star
in the latter stages of its evolution.
Planetisimal
One of the small bodies that formed from
the solar nebula and eventually grew into protoplanets.
Plastic
A material with properties of a solid but
capable of flowing under pressure.
Plate Tectonics
The constant destruction and renewal of the
earth's surface by the motions of sections of the crust.
Polar Axis
The axis of a telescope around which the
celestial sphere rotates
Poor Galaxy Cluster
An irregularly shaped cluster that contains
fewer than 1000 galaxies, many spiral, and no giant ellipticals.
Population I
Stars rich in atoms heavier than helium;
nearly always relatively young stars found in the disk of the galaxy.
Population II
Stars poor in atoms heavier than helium;
nearly always relatively old stars found in the halo, globular clusters,
or the nuclear bulge.
Position Angle
The angular direction of one body with respect
to another; measured from north toward the east; typically used in the
study of visual binaries.
Precession
A wobbling of the Earth's Axis. It takes
26,000 years for the earth to complete one wobble.
Pressure (P) Waves
In geophysics, mechanical waves of compression
and rarefaction that travel through the earth's interior
Primary Minimum
In the light curve of an eclipsing binary,
the deeper eclipse.
Prime Focus
The point at which an object mirror forms
an image in a reflecting telescope
Primeval Atmosphere
Earth's first air, composed of gases from
the solar nebula.
Primordial Background Radiation
Radiation from the hot clouds of the big-bang
explosion. Because of its large red shift it appears to come from a body
whose temperature is only 2.7K
Prolate Spheroid
A sphere stretched along its polar axis so
its polar diameter is greater than its equatorial diameter.
Prominence
Eruptions of the solar surface. Visible during
total solar eclipses.
Proper Motion
The rate at which a star moves across the
sky. Measured in arc seconds per year.
Proton
A positively charged atomic particle contained
in the nucleus of an atom. The nucleus of hydrogen atom.
Proton-Proton Chain
A series of three nuclear reactions that
build a helium atom by adding together protons. The main energy source
in our sun.
Protoplanet
Massive object resulting from the coalescence
of planetisimals in the solar nebula and destined to become a planet.
Protostar
A collapsing cloud of gas and dust destined
to become a star.
Pulsar
A source of short, precisely times radio
bursts believed to be spinning neutron stars.
Q
Quantum Mechanics
The study of behavior of atoms and atomic
particles.
Quasar
Small powerful source of energy believed
to be the active core of very distant galaxies.
Quasi-Periodic Object (QPO)
Certain X-rays sources that "flicker"
rapidly for short intervals.
R
Radial Velocity
(Vr) That component of an object's velocity
directed away from or toward the earth.
Radial Velocity Curve
A graph of the velocity of recession or approach
of the stars in a spectroscopic binary.
Radiation Pressure
The force exerted on the surface of a body
by its absorption of light. Small particles floating in the solar system
can be blown outward by the pressure of sunlight.
Radio Galaxy
A galaxy that is strong source of radio signals.
Radio Interferometer
Two or more radio telescopes that combine
their signals to achieve the resolving power of a larger telescope
Ray
Ejecta from meteorite impacts forming white
streamers radiating from some lunar craters.
Recombination
The stage within 1 million years of the big
bang when the gas became transparent to radiation
Recurrent Nova
Stars that erupt as nova every few dozen
years.
Red Dwarf
Cool, low mass stars on the lower main sequence
Reflecting Telescope
A telescope which uses a concave mirror to
focus light into an image
Reflection Nebula
A nebula produced by starlight reflecting
off dust particles in the interstellar medium.
Refracting Telescope
A telescope that forms images by bending
light through an objective lens
Regolith
A soil made up of crushed rock fragments.
Relative Age
The age of a geological feature referred
to other features. For example, relative ages tell us the lunar maria
and younger than the highlands.
Relativistic Jet Model
An explanation of superluminal expansion
based on a high velocity jet from a quasar directed approximately toward
the earth.
Relativistic Red Shift
The red shift due to Doppler effect for objects
traveling near the speed of light.
Resolving Power
The ability of a telescope to reveal fine
detail. depends on the diameter of the telescope objective
Resonance
The coincidental agreement between two periodic
phenomena; commonly applied to agreements between orbital periods, which
can make orbits more or less stable.
Retrograde loop
The planets sometimes speed up in their movements,
slow down, stop and even reverse direction. The motion traces out a retrograde
loop.
Rich Galaxy Cluster
A cluster containing over 1000 galaxies,
mostly elliptical, scattered over a volume about 3 Mpc in diameter.
Rift Valley
A long, straight, deep valley produced By
the separation of crustal plates.
Ring Galaxy
A galaxy that resembles a ring around a bright
nucleus; believed to be the result of a head-on collision of two galaxies.
Roche Limit
The minimum distance between a planet and
a satellite that holds itself together by its own gravity. If a satellites
orbit brings it inside the Roche limit, tidal forces will break the satellite
up.
Rolling Plains
The most common type of terrain on Venus.
Rotation Curve
A graph of orbital velocity versus radius
in the disk of a galaxy.
RR Lyrae Variable
Variable stars with periods of 12-24 hours,
common in some globular clusters.
S
Sagittarius A
The powerful radio source located at the
core of the Milky Way.
Saros Cycle
After 18 years and 11 1/3 days the eclipse
pattern repeats The saros cycle does not mean the eclipse will occur in
the same place. Sine the cycle take one third of a day. The earth will
have rotated 8 hours westward from the original location. It takes 3 saros
cycles for the eclipse to repeat in the exact same place or 54 years and
1 month.
Schmidt Camera
A photographic telescope that take wide angle
photographs
Schmidt Cassegrain Telescope
A cassegrain telescope that uses a thin correcting
lens as in a schmidt camera
Schwartzchild Radius
The radius of the event horizon around a
black hole.
Scientific Notation
A method of writing large numbers in a simple
way. For example 380,000 is written 3.8 x106. In scientific notation a
number is always written as a x 10 h Where a >= 1 and a <10.
Seasons
Caused by the tilt of the earth's Axis 23.5
degrees
Second of Arc
See Minute of Arc. 60 seconds of arc make
up one minute.
Secondary Atmosphere
The gases outgassed from a planet's interior;
rich in carbon dioxide.
Secondary Minimum
In the light curve of an eclipsing binary,
the shallower eclipse
Secondary Mirror
In a reflecting telescope, the mirror that
reflects the image to a point for easy observation
Seeing
Atmospheric conditions on a given night When
the atmosphere is unsteady, producing blurred images the seeing is considered
poor
Seismic Waves
A mechanical vibration that travels through
the earth. Usually caused by an earthquake.
Seismograph
An instrument that records seismic waves.
Self-Sustaining Star Formation
The process by which the birth of stars compress
the surrounding gas clouds and triggers the formation of more stars, proposed
to explain spiral arms
Semimajor Axis
Half the long diameter of an ellipse
Seyfert Galaxy
An otherwise normal spiral galaxy with an
unusually bright, small core that fluctuates in brightness; believed to
indicate the core is erupting.
Shear (S) Waves
Mechanical waves that travel through earth's
interior by the vibration of particles perpendicular to the direction
of the wave travel.
Shepherd Satellite
A satellite that, by its gravitational field,
confines particles to a planetary ring.
Shield Volcanoes
Wide, low profile volcanic cones produced
by highly liquid lava.
Shock Wave
A sudden change in pressure that travels
as an intense sound wave.
Sidereal Drive
The motion and gears on a telescope that
turn westward to keep it pointing at a star
Sidereal Period
(Sidereal Month) 27.5 Days - Actual Lunar
orbit around the earth. 1 revolution with respect to the stars. Moon drifts
eastward by 13 degrees per day.
Single-Line Spectroscopic Binary
A spectroscopic binary in which lines of
one star are visible in the spectrum.
Singularity
The object of zero radius into which the
matter of a black hole is believed to fall.
Sinuous Rille
A narrow, winding valley on the moon caused
by ancient lava flows along narrow channels.
Smooth Plain
Apparently young plains on Mercury formed
by lava flows at or soon after the formation of the Caloris Basin
Solar Constant
A measure of the energy output of the sun.
The total solar energy striking 1 sq. meter just above the earth's atmosphere
in 1 second.
Solar Nebula Theory
The theory that the planets formed from the
same cloud of gas and dust that formed the sun.
Solar Wind
Rapidly moving atoms and ions that escape
from the solar corona and blow outward through the solar system.
Special Relativity
Observers can never detect their uniform
motion except relative to other objects. The velocity of light is constant
for all observers
Spectral Class
A star's position in the temperature classification
system O,B,A,F,G,K, and M. Based on the appearance of the star's spectrum.
Spectral Sequence
The arrangement of spectral classes (O,B,A,F,G,K,
and M) ranging from hot to cool stars.
Spectrograph
Device that separates light by wavelengths
to produce a spectrum
Supernova Type I
The explosion of a star believed to be caused
by mass transfer to a white dwarf.
Supernova Type II
The explosion of a star believed to be caused
by the collapse of a massive star.
Synchrotron Radiation
Radiation emitted when high speed electrons
move through a magnetic field.
Synodic Period
(Synodic Month) - 29.5 days - One revolution
with respect to the Sun. This is the time frame that determines lunar
phases. Used as the basis for the first Roman Calendar.
T
T Tauri Stars
Young stars surrounding by gas and dust.
Believed to be contracting toward the main sequence.
Temperature
A measure of the velocity of random motions
among atoms or molecules in a material.
Terminator
The dividing line between daylight and darkness
on a planet or moon.
Terrestrial Planets
Earthlike planets - small, dense, rocky.
Tidal Coupling
The locking of the rotation of a body to
its revolution around another body.
Tidal Heating
The heating of a planet or satellite because
of friction caused by tides.
Time Dilation
The slowing of time in curved space time,
believed to occur as one approaches the speed of light or crosses the
even horizon of a black hole.
Titius-Bode Rule
A simple series of steps that produces numbers
approximately matching the sizes of the planetary orbits.
Total Eclipse (lunar or Solar)
When the umbra part of the Earth's Shadow
(lunar eclipse) totally blocks the light being received by the moon, these
can only occur during a Full Moon. A Total Solar Eclipse is the earth
passing through the umbra shadow of the moon and can only occur during
a New moon.
Transition
The movement of an electron from one atomic
orbit to another.
Transverse Velocity
The velocity of a star perpendicular to the
line of sight.
Triaxial Ellipsoid
A geometrical solid whose three axes are
equal.
Triple Alpha Process
The nuclear fusion process that combines
three helium nuclei (alpha particles) to make one carbon nucleus.
True Relative Orbit
The orbit of one star in a visual binary
with respect to the other star after correction for orbital inclination.
Tuning Fork Diagram
A system of classification for elliptical,
spiral and irregular galaxies.
Turnoff Point
The point on the H-R diagram where a cluster's
stars turn off the main sequence and move toward the red giant region
revealing the approximate age of the cluster.
U
Ultraviolet Radiation
Electromagnetic Radiation with wavelengths
shorter than visible light but longer than X-rays
Umbra
Is the Earth shadow that is total. No part
of the Sun can be seen when in the umbra shadow
Uncompressed Density
The density a planet would have if its gravity
did not compress it.
Uniform circular motion
Plato argued that the most perfect form should
be a circle and therefore motions of the heavens should be made of up
combinations of circular motion. The most perfect motion would be uniform
circular motion, so astronomers of ancient times tried to describe the
motions of the heavens in these terms
Universality
The assumption that the physical laws observed
on earth apply throughout the universe.
V
Van Allen Belts
Radiation belts of high-energy particles
trapped in the earth's magnetosphere.
Variable Star
A star whose brightness changes periodically.
Velocity
A directed rate of motion
Velocity Dispersion Method
A method of finding a galaxy's mass by observing
the range of velocities within the galaxy.
Vernal Equinox
The place where the Sun crosses the celestial
equator heading North.
Very Long Baseline
The use of radio telescopes located thousands
of miles apart to resolve detail in radio sources.
Vesicular Basalt
A porous rock formed by solidified lava with
trapped bubbles.
Violent Motion
Motion other than natural motion
Visual Binary
A binary star system in which two stars are
separately visible in a telescope.
W
Wavelength
The distance between successive peaks or
troughs of a wave
Wavelength of Maximum
The wavelength at which a perfect radiator
emits the maximum amount of energy; depends only on the object's temperature.
White Dwarf Stars
Dying stars that have collapsed to the size
of the earth and are slowly cooling off; at the lower left of the H-R
diagram.
Winter Solstice
The point on the ecliptic where the Sun reaches
it most southern point.
Z
Zeeman Effect
The splitting of spectral lines into multiple
components when the atoms are in a magnetic field.
Zenith
Point in the Sky that is directly overhead.
Zero-Age Main Sequence (ZAMS)
The locus in the H-R- diagram where stars
first reach stability as hydrogen burning stars.
Zodiac
The 12 constellations near the ecliptic through
which the Sun passes.
Zone of Avoidance
A region around the Milky Way where almost
no galaxies are visible because our view is blocked by dust in our galaxy.
Zones
Yellow-white regions that circle Jupiter parallel to its equator; believed to be areas of rising gas.
copyright © Peoria Astronomical Society 1994-2004.