A
Absolute Magnitude
a scale for measuring the actual brightness of a celestial object
without accounting for the distance of the object. Absolute magnitude
measures how bright an object would appear if it were exactly
10 parsecs (about 33 light years) away from Earth. On this scale,
the Sun has an absolute magnitude of +4.8 while it has an apparent
magnitude of -26.7 because it is so close.
Absolute Zero
the temperature at which the motion of all atoms and molecules
stops and no heat is given off. Absolute zero is reached at 0
degrees Kelvin or -273.16 degrees Celsius.
Accretion
process of particles sticking together to form larger bodies;
for example, solar nebular dust accreted to form chondrules,
and planetesimals accreted to form planets.
Achondrite
a stony meteorite representing differentiated planetary material.
Albedo Feature
a dark or light marking on the surface of an object that may
or may not be a geological or topographical feature. Albedo is
the measure of the reflectivity of a planet, measured on a scale
from zero to one. An albedo of zero describes a planet that absorbs
all the light it receives. A planet with an albedo of one reflects
all the light that shines on it.
Alpha Centauri
the closest bright star to our solar system.
Altitude
angle in degrees above the horizon.
Angstrom
abbreviated Å. A unit of length equal to 10-8 cm (one-hundredth
of a millionth of a centimeter). An Angstrom is on the order
of the size of an atom.
Antimatter
matter consisting of particles with charges opposite that of
ordinary matter. In antimatter, protons have a negative charge
while electrons have a positive charge.
Antipodal point
the point that is directly on the opposite side of the planet;
e.g., the Earth's north pole is antipodal to its south pole.
Apastron
the point of greatest separation of two stars, such as in a binary
star system.
Aperture
the size of the opening through which light passes in an optical
instrument such as a camera or telescope. A higher number represents
a smaller opening while a lower number represents a larger opening.
Aphelion
the point in its orbit where a planet is farthest from the Sun.
Apoapsis
the point in orbit farthest from the planet.
Apogee
the point in orbit farthest from the Earth.
Apparent Magnitude
the apparent brightness of an object in the sky as it appears
to an observer on Earth. Bright objects have a low apparent magnitude
while dim objects will have a higher apparent magnitude.
Arc Second
abbreviated arcsec. A unit of angular measure in which there
are 60 arc seconds in 1 arc minute and therefore 3600 arc seconds
in 1 arc degree. There are 206,265 arcseconds per radian. One
arc second is equal to about 725 km on the Sun.
Arc Degree
a unit of angular measure in which there are 360 arc degrees
in a full circle.
Arc Minute
one 1/60 of a degree.
Asteroid
a small planetary body in orbit around the Sun, larger than a
meteoroid but smaller than a planet. Most asteroids can be found
in a belt between the orbits of Mars and Jupiter. The orbits
of some asteroids take them close to the Sun, which also takes
them across the paths of the planets.
Astrochemistry
the branch of science that explores the chemical interactions
between dust and gas interspersed between the stars.
Astronomical unit (AU)
the average distance from the Earth to the Sun; 1 AU is 149,597,870
kilometers (92,960,116 miles).
Atmosphere
one atmosphere is 14.7 pounds per square inch (105 Newtons per
square meter); the average atmospheric pressure at sea level
on Earth. Atmosphere is also a layer of gases surrounding a planet,
moon, or star. The Earth's atmosphere is 120 miles thick and
is composed mainly of nitrogen, oxygen, carbon dioxide, and a
few other trace gases.
Aurora
a glow in a planet's ionosphere caused by the interaction between
the planet's magnetic field and charged particles from the Sun.
Aurora Borealis
the Northern Lights caused by the interaction between the solar
wind, the Earth's magnetic field and the upper atmosphere; a
similar effect happens in the southern hemisphere where it is
known as the aurora australis.
Aurora Australis
also known as the southern lights, this is an atmospheric phenomenon
that displays a diffuse glow in the sky in the southern hemisphere.
It is caused by charged particles from the Sun as they interact
with the Earth's magnetic field. Known as the Aurora Borealis
in the northern hemisphere.
Autumnal equinox
the point on the celestial sphere where the sun crosses the celestial
equator from north to south. The time when the sun is at the
autumnal equinox defines the first day of autumn. This happens
on about September 22 each year.
Average orbital
speed around the Sun: This is a measure of how fast a planet
moves through space, in kilometers per hour.
Axial inclination
axial inclination is the angle at which a planet's axis of rotation
is tilted, with respect to that planet's own orbit. On Earth,
as well as other planets, this tilt is responsible for the seasons.
Axis
also known as the poles, this is an imaginary line through the
center of rotation of an object.
Azimuth
the angular distance of an object around or parallel to the horizon
from a predefined zero point.
B
Bar
a unit of measure of atmospheric pressure. One bar is equal to
0.987 atmospheres, 1.02 kg/cm2, 100 kilopascal, and 14.5 lbs/square
inch.
Barycenter
the center of mass of a system of bodies; e.g., the center of
mass of the solar system.
Basalt
a general term for dark-colored, igneous rocks composed of minerals
that are relatively rich in iron and magnesium.
Big Bang
the theory that suggests that the universe was formed from a
single point in space during a cataclysmic explosion about 18
billion years ago. The force of the explosion accounts for the
current expansion of the universe.
Binary Stars
a system of two stars that revolve around a common center of
gravity.
Black Hole
the collapsed core of a massive star. Stars that are very massive
will collapse under their own gravity when their fuel is exhausted.
The collapse continues until all matter is crushed out of existence
into what is known as a singularity. The gravitational pull is
so strong that not even light can escape.
Blueshift
a shift in the lines of an object's spectrum toward the blue
end. Blueshift indicates that an object is
moving toward the observer. The larger the blueshift, the faster
the object is moving.
Bolide
an exploding meteorite.
C
Caldera
a large, basin-shaped volcanic depression that is more or less
circular in form. Most volcanic calderas are produced by collapse
of the roof of a magma chamber due to removal of magma by voluminous
eruptions or subterranean withdrawal of the magma, although some
calderas may be formed by explosive removal of the upper part
of a volcano.
Carbonaceous chondrite
a type of primitive chondrite with evidence of nebular processes.
Celestial equator
the intersection of the earth's equatorial plane with the celestial
sphere.
Celestial pole
the north and south celestial poles are points on the celestial
sphere where earths axis of rotation intersects the celestial
sphere.
Celestial sphere
an imaginary sphere centered on the earth on which all of the
stars are imagined to be projected.
Cepheid
a pulsating variable
star. This type of star undergoes a rhythmic pulsation as indicated
by its regular pattern of changing brightness as a function of
time. The period of pulsation has been demonstrated to be directly
related to a Cepheid's intrinsic brightness making observations
of these stars one of the most powerful tools for determining
distance known to modern day astronomy.
CGS
centimeter-Gram-Second (abbreviated cm-gm-sec or cm-g-s). The
system of measurement that uses these units for distance, mass,
and time.
Chondrite
a meteorite containing chondrules and other components produced
in the solar nebula.
Chondrule
small, glassy spheres commonly found in meteorites.
Chromatic aberration
an optical lens defect causing color fringes, because the lens
material brings different colors of light to focus at different
points.
Chromosphere
the layer of the solar atmosphere that is located above the photosphere
and beneath the transition region and the corona. The chromosphere
is hotter than the photosphere but not as hot as the corona.
Circumpolar Star
a star that never sets but always stays above the horizon. This
depends on the location of the observer. The further South you
go the fewer stars will be circumpolar. Polaris, the North Star,
is circumpolar in most of the northern hemisphere.
Coma
a spherical cloud of material surrounding the head of a comet.
This material is mostly gas that the Sun has caused to boil off
the comet's icy nucleus. This gas shines both by reflected sunlight
and light emitted by excited molecules. A cometary coma can extend
up to a million miles from the nucleus.
Comet
a chunk of frozen gasses, ice, and rocky debris that orbits the
Sun. A comet nucleus is about the size of a mountain on earth.
When a comet nears the Sun, heat vaporizes the icy material producing
a cloud of gaseous material surrounding the nucleus, called a
coma. As the nucleus begins to disintegrate, it also produces
a trail of dust or dust tail in its orbital path and a gas or
ion tail pointing away from the Sun. Comet comas can extend up
to a million miles from the nucleus and comet tails can be millions
of miles long. There are thought to be literally trillions of
comets in our solar system out past Neptune and Pluto, but only
once per decade or so does one become near and bright enough
to see easily without binoculars or a telescope.
Conjunction
an event that occurs when two or more celestial objects appear
close close together in the sky.
Constellation
a grouping of stars that make an imaginary picture in the sky.
There are 88 constellations.
Convection Zone
a layer in a star in which convection currents are the main mechanism
by which energy is transported outward. In the Sun, a convection
zone extends from just below the photosphere to about seventy
percent of the solar radius.
Convection
the physical upwelling of hot matter, thus transporting energy
from a lower, hotter region to a higher, cooler region. A bubble
of gas that is hotter than its surroundings expands and rises.
When it has cooled by passing on its extra heat to its surroundings,
the bubble sinks again. Convection can occur when there is a
substantial decrease in temperature with height, such as in the
Sun's convection zone.
Corona
the outermost layer of the solar atmosphere. The corona consists
of a highly rarefied gas with a low density and a temperature
greater than one million degrees Kelvin. It is visible to the
naked eye during a solar eclipse.
Coronal Mass Ejections
are huge bubbles of gas threaded with
magnetic field lines that are ejected over the course of several
hours.They are often associated with solar flares and prominence
eruptions.
Cosmic Ray
atomic nuclei (mostly protons) that are observed to strike the
Earth's atmosphere with extremely high amounts of energy.
Cosmic String
a tubelike configuration of energy that is believed to have existed
in the early universe. A cosmic string would have a thickness
smaller than a trillionth of an inch but its length would extend
from one end of the visible universe to the other.
Cosmology
a branch of science that deals with studying the origin, structure,
and nature of the universe.
Crater
a bowl-shaped depression formed by the impact of an asteroid
or meteoroid. Also the depression around the opening of a volcano.
Crystalline
indicates a rock is composed of mineral crystals rather than
glass. In general, when igneous melts cool very fast they form
glass (like obsidian), but when they cool slower, mineral crystals
have an opportunity to grow.
D
Dark Matter
a term used to describe matter in the universe that cannot be
seen, but can be detected by its gravitational effects on other
bodies.
Declination
the angular distance of an object in the sky from the celestial
equator.
Density
the amount of matter contained within a given volume. Density
is measured in grams per cubic centimeter (or kilograms per liter).
The density of water is 1.0, iron is 7.9, and lead is 11.3.
Differentiated
when a (partially) molten body has been divided into two or more
fractions of dissimilar compositions. In the case of the Earth,
iron-nickel metal was differentiated from silicate material to
form the planet's core.
Disk
the surface of the Sun or other celestial body projected against
the sky.
Doppler Effect
the apparent change in wavelength of sound or light emitted by
an object in relation to an observer's position. An object approaching
the observer will have a shorter wavelength (blue) while an object
moving away will have a longer (red) wavelength. The Doppler
effect can be used to estimate an object's speed and direction.
Double Star
a grouping of two stars. This grouping can be apparent, where
the stars seem close together, or physical, such as a binary
system.
Double Asteroid
two asteroids that revolve around each other and are held together
by the gravity between them. Also called a binary asteroid.
Dust grains
not the dust one finds around the house, which is typically fine
bits of fabric, dirt, or dead skin cells. Rather interstellar
dust grains are much smaller clumps, on the order of a fraction
of a micron across, irregularly shaped, and composed of carbon
and/or silicates. Dust is most evident by its absorption, causing
large dark patches in regions of our Milky Way Galaxy and dark
bands across other galaxies.
E
Eclipse
the total or partial blocking of one celestial body by another.
Ejecta
material from beneath the surface of a body such as a moon or
planet that is ejected by an impact such as a meteor and distributed
around the surface. Ejecta usually appears as a lighter color
than the surrounding surface.
Electromagnetic Spectrum
the entire range of all the various kinds or wavelengths of electromagnetic
radiation, including (from short to long wavelengths) gamma rays,
x-rays, ultraviolet, optical (visible), infrared, and radio waves.
Electromagnetic Radiation
radiation that travels through vacuous space at the speed of
light and propagates by the interplay of oscillating electric
and magnetic fields. This radiation has a wavelength and a frequency.
Electron Flux
the rate of flow of electrons through a reference surface. In
cgs units, measured in electrons s-1, or simply s-1.
Electron
a negatively charged elementary particle that normally resides
outside (but is bound to) the nucleus of an atom.
Electron Volt
Abbreviated eV. A unit of energy used to describe the total energy
carried by a particle or photon. The energy acquired by an electron
when it accelerates through a potential difference of 1 volt
in a vacuum. 1 eV = 1.6 x 10-12 erg.
Ellipse
an ellipse is an oval shape. Johannes Kepler discovered that
the orbits of the planets were elliptical in shape rather than
circular.
Elliptical Galaxy
a galaxy whose structure shaped like an ellipse and is smooth
and lacks complex structures such as spiral arms.
Elongation
the angular distance of a planetary body from the Sun as seen
from Earth. A planet at greatest eastern elongation is seen in
the evening sky and a planet at greatest western elongation will
be seen in the morning sky.
Emission nebula
a type of nebula that shines by emitting light when electrons
recombine with protons to form hydrogen atoms. The electron frequently
approaches the proton in steps emitting energy as light as it
gets pulled in. In one of the most common "steps,"
the recombining electron emits a photon of red light. Since many
atoms in the nebula do this all at once, the nebula appears red
in color. This type of nebula is created when energetic ultraviolet
light from a hot star shines on a cloud of hydrogen gas, stripping
away electrons from the atoms (ionization). The free electrons
can then begin the process of recombination.
Energy Flux
the rate of flow of energy through a reference surface. In cgs
units, measured in erg s-1. Also measured in watts, where 1 watt
= 1 x 107 erg s-1. Flux density, the flux measured per unit area,
is also often referred to as "flux".
Enstatite
a type of primitive chondrite. That chondrite is dominated by
the silicate mineral enstatite.
Ephemeris
a table of data arranged by date. Ephemeris tables are typically
to list the positions of the Sun, Moon, planets and other Solar
System objects.
Equinox
the two points at which the Sun crosses the celestial equator
in its yearly path in the sky. The equinoxes occur on or near
March 21 and September 22. The equinoxes signal the start of
the Spring and Autumn seasons.
Erg
a cgs unit of energy equal to work done by a force of 1 dyne
acting over a distance of 1 cm.
107 (ten million) erg s-1 (ergs per second) = 1 watt. Also, 1
Calorie = 4.2 x 1010 (42 billion) ergs.
Escape Velocity
the speed required for an object to escape the gravitational
pull of a planet or other body.
Event Horizon
the invisible boundary around a black hole past which nothing
can escape the gravitational pull - not even light.
Evolved Star
a star that is near the end of its life cycle where most of its
fuel has been used up. At this point the star begins to loose
mass in the form of stellar wind.
Extragalactic
a term that means outside of or beyond our own galaxy.
Extraterrestrial
a term used to describe anything that does not originate on Earth.
Eyepiece
the lens at the viewing end of a telescope. The eyepiece is responsible
for enlarging the image captured by the instrument. Eyepieces
are available in different powers, yielding differing amounts
of magnification.
F
Faculae
bright patches that are visible on the Sun's surface, or photosphere.
Filament
a strand of cool gas suspended over the photosphere by magnetic
fields, which appears dark as seen against the disk of the Sun.
Finder
a small, wide-field telescope attached to a larger telescope.
The finder is used to help point the larger telescope to the
desired viewing location.
Fireball
an extremely bright meteor. Also known as bolides, fireballs
can be several times brighter than the full Moon. Some can even
be accompanied by a sonic boom.
Flare (Solar)
rapid release of energy from a localized region on the Sun in
the form of electromagnetic radiation, energetic particles, and
mass motions.
Flare Star
a member of a class of stars that show occasional, sudden, unpredicted
increases in light. The total energy released in a flare on a
flare star can be much greater that the energy released in a
solar flare.
Footpoint
the intersection of magnetic loops with the photosphere.
Free Electron
an electron that has broken free of it's atomic bond and is therefore
not bound to an atom.
Frequency
the number of repetitions per unit time of the oscillations of
an electromagnetic wave (or other wave). The higher the frequency,
the greater the energy of the radiation and the smaller the wavelength.
Frequency is measured in Hertz.
Fusion
a process where nuclei collide so fast they stick together and
emit a great deal of energy. In the center of most stars, hydrogen
fuses together to form helium. Fusion is so powerful it supports
the star's enormous mass from collapsing in on itself, and heats
the star so high it glows as the bright object we see today.
G
Galactic Nucleus
a tight concentration of stars and gas found at the innermost
regions of a galaxy. Astronomers now believe that massive black
holes may exist in the center of many galaxies.
Galactic Halo
the name given to the spherical region surrounding the center,
or nucleus of a galaxy.
Galaxy
a large system of
about 100 billion stars. Our Sun is a member of the Milky Way
Galaxy. There are billions of galaxies in the observable universe.
Exactly when and how galaxies formed in the Universe is a topic
of current astronomical research.
Galaxies are found in a variety of sizes and shapes. Our own
Milky Way galaxy is spiral in shape and contains several billion
stars. Some galaxies are so distant their light takes millions
of years to reach the Earth. Galaxies are classified in three
main groups; spirals, ellipticals and irregulars.
Galilean Moons
the name given to Jupiter's four largest moons, Io, Europa, Callisto
& Ganymede. They were discovered independently by Galileo
Galilei and Simon Marius.
Gamma Ray
the highest energy (shortest wavelength) photons in the electromagnetic
spectrum. Gamma rays are often defined to begin at 10 keV, although
radiation from around 10 keV to several hundred keV is also referred
to as hard x-rays.
Geomagnetic Storm
a worldwide disturbance of the Earth's magnetic field, associated
with solar activity.
Geosynchronous Orbit
the orbit of a satellite that travels above the Earth's equator
from west to east so that it has a speed matching that of the
Earth's rotation and remains stationary in relation to the Earth
(also called geostationary). Such an orbit has an altitude of
about 35,900 km (22,300 miles).
Giant Molecular Cloud (GMC)
massive clouds of gas in interstellar space composed primarily
of hydrogen molecules. These clouds have enough mass to produce
thousands of stars and are frequently the sites of new star formation.
Globular Cluster
a tight, spherical grouping of hundreds of thousands of stars.
Globular clusters are composed of older stars,
and are usually found around the central regions of a galaxy.
Granulation
a pattern of small cells that can be seen on the surface of the
Sun. They are caused by the convective motions of the hot gases
inside the Sun.
Gravitational Lens
a concentration of matter such as a galaxy or cluster of galaxies
that bends light rays from a background object. Gravitational
lensing results in duplicate images of distant objects.
Gravity
a mutual physical force of nature that causes two bodies to attract
each other.
Greenhouse Effect
an increase in temperature caused when incoming solar radiation
is passed but outgoing thermal radiation is blocked by the atmosphere.
Carbon dioxide and water vapor are two of the major gases responsible
for this effect.
H
Heliocentric
sun centered.
Heliopause
the point at which the solar wind meets the interstellar medium
or solar wind from other stars.
Heliosphere
the space within the broundary of the heliopause containing the
Sun and solar system.
Helium
the second lightest and second most abundant element. The typical
helium atom consists of a nucleus of two protons and two neutrons
surrounded by two electrons. Helium was first discovered in our
Sun. Roughly 25 percent of our Sun is helium.
Hemisphere
a half of the celestial sphere that is divided into two halves
by either the horizon, the celestial equator, or the ecliptic.
Hertz
abbreviated Hz. A unit of frequency equal to one cycle per second.
One kHz = 1000 Hz. One MHz = 106 (one million) Hz. One GHz =
109 Hz.
H II region
a region of hot gas surrounding a young star or stars that is
mostly ionized. The energetic light from these young stars ionizes
the existing gas. This region typically appears red as it glows
with the photons emitted when elections recombine with hydrogen
protons.
Hot Spot
center of persistent volcanism, thought to be the surface expression
of a rising hot plume in Earth's mantle.
Hour Angle
the telescope based coordinate specifying the angle, in the equatorial
plane, from the meridian to a plane containing the celestial
object and the north and south celestial poles.
H-R Diagram
The Color-Magnitude Diagram is a graph upon
which stars are plotted by spectral type and actual luminosity.
It is named for the two scientists Russell and Hertzsprung who
first used it in 1913.
Hubble's Law
the law of physics that states that the farther a galaxy is from
us, the faster it is moving away from us.
Hydrogen
the lightest and most
abundant element. A hydrogen atom consists of one proton and
one electron. A hydrogen nucleus is just a single proton. Hydrogen
composes about 75 percent of the Sun but only a tiny fraction
of the Earth. Hydrogen is the building
block of the universe. Stars form from massive clouds of hydrogen
gas.
Hydromagnetic Wave
a wave in which both the plasma and magnetic field oscillate.
Hypergalaxy
a system consisting of a spiral galaxy surrounded by several
dwarf white galaxies, often ellipticals. Our galaxy and the Andromeda
galaxy are examples of hypergalaxies.
I
Igneous rock
a rock that was once molten.
Impact
a collision between two planetary bodies. In the case when one
is much smaller than the other (like a meteoroid colliding with
the Earth), a crater may be produced on the larger body.
Impact melt spherule
spherules of shock-melted rock ejected from an impact crater.
Most of these objects cool rapidly in the Earth's atmosphere
and solidify to a glassy state. However, some may remain molten
until they splash onto the ground or into water where they are
quenched to form solidified particles of glass.
Inclination
a measure of the tilt of a planet's orbital plane in relation
to that of the Earth.
Infrared
light that is so red, humans cannot see it. A band of the electromagnetic
spectrum between the visible and the microwave. Photons of infrared
light are less energetic than photons of visible light.
Inferior Planet
a planet that orbits between the Earth and the Sun. Mercury and
Venus are the only two inferior planets in our Solar System.
Interplanetary Magnetic
Field
the magnetic field carried along with the solar wind.
Interstellar Medium
the gas and dust that exists in open space between the stars.
Ion
an atom that has lost or gained one or more electrons and has
become electrically charged as a result.
Ionization
the process by which ions are produced, typically occurring by
collisions with atoms or electrons ("collisional ionization"),
or by interaction with electromagnetic radiation ("photoionization").
Ionosphere
the region of the Earth's upper atmosphere containing a small
percentage of free electrons and ions produced by photoionization
of the constituents of the atmosphere by solar ultraviolet radiation.
The ionosphere significantly influences radiowave propagation
of frequencies less than about 30 MHz. In Earth's atmosphere,
the ionosphere begins at
an altitude of about 25 miles and extends outward about 250.
Iron Meteorite
a meteorite which is composed mainly of iron mixed with smaller
amounts of nickel.
Irregular Galaxy
a galaxy with no spiral structure and no symmetric shape. Irregular
galaxies are usually filamentary or very clumpy in shape.
Isotope
one of two or more atoms having the same number of protons in
its nucleus, but a different number of neutrons and, therefore,
a different mass.
J
Jovian planet
any of the four outer, gaseous planets: Jupiter, Saturn, Uranus,
and Neptune.
Julian Date (JD)
the interval of time in days and fraction of a day since 1 January
4713 BC, Greenwich noon.
K
Kelvin
a temperature scale used in sciences such as astronomy to measure
extremely cold temperatures. The Kelvin temperature scale is
just like the Celsius scale except that the freezing point of
water, zero degrees Celsius, is equal to 273 degrees Kelvin.
Absolute zero, the coldest known temperature, is reached at 0
degrees Kelvin or -273.16 degrees Celsius.
Kepler's Second Law
a ray directed from the Sun to a planet sweeps out equal areas
in equal times.
Kepler's First Law
a planet orbits the Sun in an ellipse with the Sun at one focus.
Kepler's Third Law
the square of the period of a planet's orbit is proportional
to the cube of that planet's semimajor axis; the constant of
proportionality is the same for all planets.
keV
one thousand electron volts.
Kilometer
abbreviated km. 1 km = 1000 meters = 105 cm = 0.62 mile.
Kiloparsec
a distance equal to 1000 parsecs.
Kirkwood Gaps
regions in the main belt of asteroids where few or no asteroids
are found. They were named after the scientist who first noticed
them.
Kuiper Belt
a large ring of icy, primitive objects beyond the orbit of Neptune.
Kupier Belt objects are believed to be remnants of the original
material that formed the Solar System. Some astronomers believe
Pluto and Charon are Kuiper Belt objects.
L
Lagrange Point
french mathematician and astronomer Joseph Louis Lagrange showed
that three bodies can lie at the apexes of an equilateral triangle
which rotates in its plane. If one of the bodies is sufficiently
massive compared with the other two, then the triangular configuration
is apparently stable. Such bodies are sometimes referred to as
Trojans. The leading apex of the triangle is known as the leading
Lagrange point or L4; the trailing apex is the trailing Lagrange
point or L5.
lava
molten rock that is erupted onto the surface of a planet and
is hot enough to flow.
Lenticular Galaxy
a disk-shaped galaxy that contains no conspicuous structure within
the disk. Lenticular galaxies tend to look more like elliptical
galaxies than spiral galaxies.
Libration
an effect caused by the apparent wobble of the Moon as it orbits
the Earth. The Moon always keeps the same side toward the Earth,
but due to libration, 59% of the Moon's surface can be seen over
a period of time.
Light Year
the distance light travels in a year, at the rate of 300,000
kilometers per second (671 million miles per hour); 1 light-year
is equivalent to 9.46053e12 km, 5,880,000,000,000 miles or 63,240
AU.
Limb
the outer edge or border of a planet or other celestial body.
Local Group
a small group of about two dozen galaxies of which our own Milky
Way galaxy is a member.
Luminosity
the amount of light emitted by a star.
Lunar Eclipse
a phenomenon that occurs when the Moon passes into the shadow
of the Earth. A partial lunar eclipse occurs when the Moon passes
into the penumbra, or partial shadow. In a total lunar eclipse,
the Moon passes into the Earth's umbra, or total shadow.
Lunar Month
the average time between successive new or full moons. A lunar
month is equal to 29 days 12 hours 44 minutes. Also called a
synodic month.
Lunation
the interval of a complete lunar cycle, between one new Moon
and the next. A lunation is equal to 29 days, 12 hours, and 44
minutes.
M
Mare
latin word for "sea." Galileo thought the dark featureless
areas on the Moon were bodies of water, even though the Moon
is essentially devoid of liquid water. The term is still applied
to the basalt-filled impact basins common on the face of the
Moon visible from Earth.
Magellanic Clouds
two small, irregular galaxies found just outside our own Milky
Way galaxy. The Magellanic clouds are visible in the skies of
the southern hemisphere.
Magnetic Field
a field of force that is generated by electric currents. The
Sun's average large-scale magnetic field, like that of the Earth,
exhibits a north and a south pole linked by lines of magnetic
force.
Magnetic Field Lines
imaginary lines that indicate the strength and direction of a
magnetic field. The orientation of the line and an arrow show
the direction of the field. The lines are drawn closer together
where the field is stronger. Charged particles move freely along
magnetic field lines, but are inhibited by the magnetic force
from moving across field lines.
Magnetic Pole
either of two limited regions in a magnet at which the magnet's
field is most intense.
Magnetosphere
the area around a planet most affected by its magnetic field.
The boundary of this field is set by the solar wind.
Magnitude
The degree of brightness of a star or other object in the sky
according to a scale on which the brightest star has a magnitude
-1.4 and the faintest visible star has magnitude 6. Sometimes
referred to as apparent magnitude. In this scale, each number
is 2.5 times the brightness of the previous number. Thus a star
with a magnitude of 1 is 100 times brighter than on with a visual
magnitude of 6.
Magma
very hot, fluid rock. Magma is used to describe molten rock both
below and on top of the surface of a planet and thus is a more
general term than lava. Magma may contain solid mineral crystals
which are suspended in the melt.
Main Belt
the area between Mars and Jupiter where most of the asteroids
in our Solar System are found.
Major Planet
a name used to describe any planet that is considerably larger
and more massive than the Earth, and contains
large quantities of hydrogen and helium. Jupiter and Neptune
are examples of major planets.
Mass
a measure of the total amount of material in a body, defined
either by the inertial properties of the body or by its gravitational
influence on other bodies.
Matter
a word used to describe anything that contains mass.
Megaton
an explosive force equal to one million metric tons of TNT. The
energy released in the explosion of one megaton of TNT is equal
to 4.2 x 1022 ergs.
Meridian
an imaginary circle drawn through the North and South poles of
the celestial equator.
Messier, Charles
while hunting for comets in the skies
above France, 18th century astronomer Charles Messier made a
list of the positions of about 100 fuzzy, diffuse looking objects
which appeared at fixed positions in the sky. Although these
objects looked like comets, Messier knew that since they did
not move with respect to the background stars they could not
be the undiscovered comets he was searching for. These objects
are now well known to modern astronomers to be among the brightest
and most striking gaseous nebulae, star clusters, and galaxies.
Objects on Messier's list are still referred to by their "Messier
number". For example the Andromeda Galaxy, the 31st object
on the list, is known as M31.
Metamorphic rock
a rock that has been heated and compressed so that it recrystallizes,
but does not melt.
Meteor
a small particle of rock or dust that burns away in the Earth's
atmosphere. Meteors are also referred to as shooting stars.
Meteor Shower
an event where a large number of meteors enter the Earth's atmosphere
from the same direction in space at nearly the same time. Most
meteor showers take place when the Earth passes through the debris
left behind by a comet.
Meteorite
an object, usually a chunk or metal or rock, that survives entry
through the atmosphere to reach the Earth's surface. Meteors
become meteorites if they reach the ground.
MeV
one million electron volts.
Millibar
a measure of atmospheric pressure equal to 1/1000 of a bar. Standard
sea-level pressure on Earth is about 1013 millibars.
Minor Planet
another name used to describe a large asteroid.
Molecular Cloud
an interstellar cloud of molecular hydrogen containing trace
amounts of other molecules such as carbon monoxide and ammonia.
N
Nadir
a term used to describe a point directly underneath an object
or body.
Nebula
a cloud of dust and gas in space, usually illuminated by one
or more stars. Nebulae represent the raw material the stars are
made of.
Neutrino
a fundamental particle supposedly produced in massive numbers
by the nuclear reactions in stars; they are very hard to detect
because the vast majority of them pass completely through the
Earth without interacting.
Neutron Star
a compressed core of an exploded star made up almost entirely
of neutrons. Neutron stars have a strong gravitational field
and some emit pulses of energy along their axis. These are known
as pulsars.
Neutron
an electrically neutral elementary particle. A neutron is 1839
times heavier than an electron.
Newton's First Law of Motion
a body continues in its state of constant velocity (which may
be zero) unless it is acted upon by an external force.
Newton's Second Law of
Motion
for an unbalanced force acting on a body, the acceleration produced
is proportional to the force impressed; the constant of proportionality
is the inertial mass of the body.
Newton's Third Law of Motion
in a system where no external forces are present, every action
force is always opposed by an equal and opposite reaction.
Nodule
a semi-spherical fragment of rock embedded in a matrix with a
different composition.
Nova
a star that flares up to several times its original brightness
for some time before returning to its original state.
Nuclear Fusion
a nuclear process whereby several small nuclei are combined to
make a larger one whose mass is slightly smaller than the sum
of the small ones. The difference in mass is converted to energy
by Einstein's famous equivalence E=mc2. Nuclear fusion is the
reaction that fuels the Sun, where hydrogen nuclei are fused
to form helium.
Nucleus
the positively charged core of an atom, consisting of protons
and neutrons (except for hydrogen), around which electrons orbit.
O
Oblateness
a measure of flattening at the poles of a planet or other celestial
body.
Obliquity
the angle between a body's equatorial plane and orbital plane.
Occulation
the blockage of light by the intervention of another object;
a planet can occult (block) the light from a distant star
Old
a planetary surface that has been modified little since its formation
typically featuring large numbers of impact craters; (compare
to young).
Oort Cloud
a theoretical shell of comets that is believed to exist at the
outermost regions of our Solar System. The Oort cloud was named
after the Dutch astronomer who first proposed it.
Open Cluster
a collection of young stars that formed together. They may or
may not be still bound by gravity. Some of the youngest open
clusters are still embedded in the gas and dust from which they
formed.
Opposition
the position of a planet when it is exactly opposite the Sun
as seen from Earth. A planet at opposition is at its closest
approach to the Earth and is best suitable for observing.
Orbit
the path of an object that is moving around a second object or
point.
Orbital Period
the amount of time it takes a spacecraft or other object to travel
once around it's orbit.
P
Paleozoic
a geological term denoting the time in Earth history between
570 and 245 million years ago.
Palimpsest
a circular feature on the surface of dark icy moons such as Ganymede
and Callisto lacking the relief associated with craters; Pamlimpsests
are thought to be impact craters where the topographic relief
of the crater has been eliminated by slow adjustment of the icy
surface.
Pallasite
a stony-iron meteorite in which nodules of olivine (a silicate
mineral) are surrounded by a network of iron-nickel metal.
Parallax
the apparent change in position of two objects viewed from different
locations.
Parsec
a large distance often used in astronomy. A parsec is equal to
3.26 light years.
Patera
shallow crater; scalloped, complex edge.
Peak ring
a central uplift characterized by a ring of peaks rather than
a single peak; peak rings are typical of larger terrestrial craters
above about 50 kilometers (30 miles) in diameter.
Penumbra
the area of partial illumination surrounding the darkest part
of a shadow caused by an eclipse.
Periapsis
the point in the orbit closest to the planet.
Perigee
the point in the orbit of the Moon or other satellite at which
it is closest to the Earth.
Perihelion
the point in the orbit of a planet or other body where it is
closest to the Sun.
Perturb
to cause a planet or satellite to deviate from a theoretically
regular orbital motion.
Phase
the apparent change in shape of the Moon and inferior planets
as seen from Earth as they move in their orbits.
Planet
a spherical ball of rock and/or gas
that orbits a star. The Earth is a planet. Our solar system has
nine planets. These planets are, in order of increasing average
distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus, Neptune, and Pluto.
Planetesimal
a rocky and/or icy body, a few to several tens of kilometers
in size, that was produced in the solar nebula.
Precambrian
a geological term denoting the time in Earth history prior to
570 million years ago.
Photon
a discrete quantity of electromagnetic energy. Short wavelength
(high frequency) photons carry more energy than long wavelength
(low frequency) photons.
Photosphere
The visible surface of the Sun; the upper surface of a convecting
layer of gases in the outer portion of the sun whose temperature
causes it to radiate light at visible wavelengths; sunspots and
faculae are observed in the photosphere.
Planet
a very large body in orbit around a star. Planets can be composed
mainly of rock or of dense gases.
Planetary Nebula
a shell of gas surrounding a small, white star. The gas is usually
illuminated by the star, producing a
variety of colors and shapes.
Planitia
a low plain.
Planum
a high plain or plateau.
Plasma
plasma consists of a gas heated to sufficiently high temperatures
that the atoms ionize. The properties of the gas are controlled
by electromagnetic forces among constituent ions and electrons,
which results in a different type of behavior. Plasma is often
considered the fourth state of matter (besides solid, liquid,
and gas). Most of the matter in the Universe is in the plasma
state.
Precession
the apparent shift of the celestial poles caused by a gradual
wobble of the Earth's axis.
Prominence
an explosion of hot gas that erupts from the Sun's surface. Solar
prominences are usually associated with sunspot activity and
can cause interference with communications on Earth due to their
electromagnetic effects on the atmosphere.
Proper Motion
the apparent angular motion across the sky of an object relative
to the Solar System.
Proton
a positively charged elementary particle. A proton is 1836 times
heavier than an electron.
Protostar
dense regions of molecular clouds where stars are forming.
Pseudocrater
a generally circular crater produced by a phreatic eruption resulting
from emplacement of a lava flow over wet ground.
Pulsar
a spinning neutron star (burnt-out star) that emits energy along
its gravitational axis. This energy is received as pulses as
the star rotates.
Pyroclastic
pertaining to clastic (broken and fragmented) rock material formed
by volcanic explosion or aerial expulsion from a volcanic vent.
Pumice
a light vesicular form of volcanic glass with a high silica content;
it is usually light in color and will float on water.
Q
Quadrature
a point in the orbit of a superior planet where it appears at
right angles to the Sun as seem from Earth.
Quasar
An unusually bright object found in the remote areas of the universe.
Quasars release incredible amounts of energy and are among the
oldest and farthest objects in the known universe. They may be
the nuclei of ancient, active galaxies.
R
Radial Velocity
the movement of an object either towards or away from a stationary
observer.
Radiant
a point in the sky from which meteors in a meteor shower seem
to originate.
Radiation
energy radiated in the form of waves or particles; photons.
Radiation Belt
a ring-shaped region around a planet in which electrically charged
particles (usually electrons and protons) are trapped. The particles
follow spiral trajectories around the direction of the magnetic
field of the planet. The radiation belts surrounding Earth are
known as the Van Allen belts.
Radio Galaxy
a galaxy that gives off large amounts of energy in the form of
radio waves.
Regolith
the layer of rocky debris and dust made by metoritic impact that
forms the uppermost surface of planets, satellites and asteroids.
Red Giant
a stage in the evolution of a star when the fuel begins to exhaust
and the star expands to about fifty times its normal size. The
temperature cools, which gives the star a reddish appearance.
Redshift
a shift in the lines of an object's spectrum toward the red end.
Redshift indicates that an object is moving away from the observer.
The larger the redshift, the faster the object is moving.
Reflection nebula
a type of nebula that shines by reflected light. Bright stars
near reflection nebulae emit light into the region that is reflected
by the large amount of dust there. The size of the dust grains
causes blue light to be reflected more efficiently than red light,
so these reflection nebulae frequently appear blue in color.
Retrograde
the rotation or orbital motion of an object in a clockwise direction
when viewed from the north pole of the ecliptic; moving in the
opposite sense from the great majority of solar system bodies.
Rhyolite
fine-grained extrusive igneous rock, commonly with phenocrysts
of quartz and feldspar in a glassy groundmass.
Rift
a fracture or crack in a planet's surface caused by extension.
On some volcanoes, subsurface intrusions are concentrated in
certain directions; this causes tension at the surface and also
means that there will be more eruptions in these "rift zones."
Rift Valley
an elongated valley formed by the depression of a block of the
planet's crust between two faults or groups of faults of approximately
parallel strike.
Right Ascension
the amount of time that passes between the rising of Aries and
another celestial object. Right ascension is one unit of measure
for locating an object in the sky.
Rima
a fissure.
Roche Limit
the smallest distance from a planet or other body at which purely
gravitational forces can hold together a satellite or secondary
body of the same mean density as the primary. At a lesser distance
the tidal forces of the primary would break up the secondary.
Rotation
the spin of a body about its axis.
Rupes
the term applied to scarps on planetary surfaces; many scarps
are thought to be the surface expression of faults within the
crust of the planetary object.
S
Satellite
a natural or artificial body in orbit around a planet.
Semimajor axis
one-half of the longest dimension of an ellipse.
Seyfert Galaxy
a main-sequence star which rotates rapidly, causing a loss of
matter to an ever-expanding shell.
Shield volcano
a volcano in the shape of a flattened dome, broad and low, built
by flows of very fluid lava.
Shock
unusually high pressures produced briefly by an impact. These
pressures may be sufficiently high to shatter, melt, and vaporize
rocky material.
Silicate
a rock or mineral whose structure is dominated by bonds of silicon
and oxygen atoms (ie. olivine).
Sidereal
relating to, or concerned with the stars. Sidereal rotation is
that measured with respect to the stars rather than with respect
to the Sun or the primary of a satellite.
Sidereal day
the interval of time between two consecutive transits of the
vernal equinox. More intuitively, it is the length of time required
for Earth to make one full rotation with respect to the celestial
sphere -- approximately four minutes shorter than the solar day.
Sidereal month
the average period of revolution of the moon around the earth
in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes
in units of mean solar time.
Sidereal period
the period of revolution of a planet around the Sun or a satellite
around its primary.
Singularity
the center of a black hole, where the curvature of spacetime
is maximal. At the singularity, the
gravitational tides diverge. Theoretically, no solid object can
survive hitting the singularity.
Solar Cycle
the approximately 11-year quasi-periodic variation in frequency
or number of solar active events.
Solar Flare
a bright eruption of hot gas in the Sun's photosphere. Solar
prominences are usually only detectable by specialized instruments
but can be visible during a total solar eclipse.
Solar Nebula
the disk of dust and gas of which the Solar System was believed
to have formed about 5 billion years ago.
Solar Atmosphere
the atmosphere of the Sun. An atmosphere is generally the outermost
gaseous layers of a planet, natural satellite, or star. Only
bodies with a strong gravitational pull can retain an atmosphere.
Atmosphere is used to describe the outer layer of the Sun because
it is relatively transparent at visible wavelengths. Parts of
the solar atmosphere include the photosphere, chromosphere, and
the corona.
Solar Eclipse
a phenomenon that occurs when the Earth passes into the shadow
of the Moon. A total solar eclipse occurs when the Moon is close
enough to completely block the Sun's light. An annular solar
eclipse occurs when the Moon is farther away and is not able
to completely block the light. This results in a ring of light
around the Moon.
Solar Wind
a flow of charged particles that travels from the Sun out into
the Solar System.
Solstice
the time of the year when the Sun appears furthest north or south
of the celestial equator. The solstices mark the beginning of
the Summer and Winter seasons.
South Atlantic Anomaly
the region over the South Atlantic Ocean where the lower Van
Allen belt of energetic, electrically charged particles is particularly
close to the Earth's surface. The excess energy in the particles
presents a problem for satellites in orbit around the Earth.
Spectral Line
a line in a spectrum due to the emission or absorption of electromagnetic
radiation at a discrete wavelength. Spectral lines result from
discrete changes in the energy of an atom or molecule. Different
atoms or molecules can be identified by the unique sequence of
spectral lines associated with them.
Spectrograph
an instrument that spreads light or other electromagnetic radiation
into it's component wavelengths (spectrum), recording the results
photographically or electronically.
Spectrometer
the instrument connected to a telescope that separates the light
signals into different frequencies,
producing a spectrum.
Spectroscopy
the technique of observing the spectra of visible light from
an object to determine its composition,
temperature, density, and speed.
Spectrum
electromagnetic radiation arranged in order of wavelength. A
rainbow is a natural spectrum of visible light from the Sun.
Spectra are often punctuated with emission or absorption lines,
which can be examined to reveal the composition and motion of
the radiating source.
Spicules
the range of colors produced when visible light passes through
a prism.
Spiral Galaxy
a galaxy that contains a prominent central bulge and luminous
arms of gas , dust, and young stars that wind
out from the central nucleus in a spiral formation. Our galaxy,
the Milky Way, is a spiral galaxy.
Star Cluster
a large grouping of stars, from a few dozen to a few hundred
thousand, that are bound together by their mutual gravitational
attraction.
Star
a giant ball of hot gas that creates and emits its own radiation
through nuclear fusion. Stars are arranged in various classes
by their spectral characteristics. The chief classes are identified
by the letters O, B, A, F, G, K, M. Each class contains ten subdivisions
numbered from 0 to 9. The classes define a temperature or color
sequence. Stars of type O and B are Blue-white and have high
temperatures (35,000°K / 20,000°K); A stars are white,
temperature 10,000°K; F and G stars are yellowish, temperature
7,000°K; K stars are orange, and M stars are red, temperature
3,000°K. Wolf-Rayet stars are hot, blue giants. They are
extremely turbulent and have a temperature of 50,000 °K and
higher
Steady State Theory
the theory that suggests the universe is expanding but exists
in a constant, unchanging state in the large scale. The theory
states that new matter is being continually being created to
fill the gaps left by expansion. This theory has been abandoned
by most astronomers in favor of the big bang theory.
Stellar Wind
the ejection of gas from the surface of a star. Many different
types of stars, including our Sun, have
stellar winds. The stellar wind of our Sun is also known as the
Solar wind. A star's stellar wind is
strongest near the end of its life when it has consumed most
of its fuel.
Stone Meteorite
a meteorite which resembles a terrestrial rock and is composed
of similar materials.
Stony Iron
a meteorite which contains regions resembling both a stone meteorite
and an iron meteorite.
Stratosphere
the cold region of a planetary atmosphere above the convecting
regions (the troposphere), usually without vertical motions but
sometimes exhibiting strong horizontal jet streams.
Subduction
the process of one lithospheric plate descending beneath another.
Sunspot
a temporary disturbed area in the solar photosphere that appears
dark because it is cooler than the surrounding areas. Sunspots
consist of concentrations of strong magnetic flux. They usually
occur in pairs or groups of opposite polarity that move in unison
across the face of the Sun as it rotates.
Supergiant
the stage in a star's evolution where the core contracts and
the star swells to about five hundreds times its original size.
The star's temperature drops, giving it a red color.
Superior planets
The planets Mars, Jupiter, Saturn, Uranus, Neptune and Pluto
are superior planets because their orbits are farther from the
Sun than Earth's orbit.
Superior Conjunction
a conjunction that occurs when a superior planet passes behind
the Sun and is on the opposite side of the Sun from the Earth.
Supernova Remnant
an expanding shell of gas ejected at high speeds by a supernova
explosion. Supernova remnants are often visible as diffuse gaseous
nebulae usually with a shell-like structure. Many resemble "bubbles"
in space.
Supernova
the death explosion
of a massive star, resulting in a sharp increase in brightness
followed by a gradual fading. At peak light output, supernova
explosions can outshine a galaxy. The outer layers of the exploding
star are blasted out in a radioactive cloud. This expanding cloud,
visible long after the initial explosion fades from view, forms
a supernova remnant.
Supernovae are the most powerful forces in the universe. All
of the heavy elements were created in supernova explosions.
Synchronous rotation
a satellite's rotational period is equal to its orbital period;
this causes the same side of a satellite to always face the planet.
Synchronous rotation occurs when a planet's gravity produces
a tidal bulge in its satellite. The gravitational attraction
and bulge acts like a torque, which slows down the satellite
until it reaches a synchronous rotation.
T
Tectonic
the deformation forces acting on a planet's crust.
Tektite
Spherule of molten rock ejected from an impact crater and then
cooled rapidly to produce glass.
Telescope
an instrument used to collect large amounts of light from far
away objects and increase their visibility to
the naked eye. Telescopes can also enlarge objects that are relatively
close to the Earth.
Terminator
the dividing line between the illuminated and the unilluminated
part of the moon's or a planet's disk.
Terrestrial
a term used to describe anything originating on the planet Earth.
Terrestrial Planet
a name given to a planet composed mainly of rock and iron, similar
to that of Earth.
Thermonuclear Fusion
the combination of atomic nuclei at high temperatures to form
more massive nuclei with the simultaneous release of energy.
Thermonuclear fusion is the power source at the core of the Sun.
Controlled thermonuclear fusion reactors, when successfully implemented,
could become an attractive source of power on the Earth.
Terra
an extensive land mass.
Tidal forces
the gravitational pull on planetary objects from nearby planets
and moons. When the tidal forces of a planet and several moons
are focused on certain moons, particularly if the orbits of the
various objects bring them into alignment on a repeated basis,
the tidal forces can generate a tremendous amount of energy within
the moon. The intense volcanic acivity of Io is the result of
the interaction of such tidal forces.
Tidal heating
the frictional heating of a satellite's interior due to flexure
caused by the gravitational pull of its parent planet and possibly
neighboring satellites.
Toroidal Radius
in a solar loop structure, it is the distance from the axis of
the loop to the center of the "semi-circle" that the
loop forms. Half of the distance from one loop footpoint to the
other loop footpoint. For a doughnut, it is the distance from
the center of the doughnut hole to the center (circular axis)
of the pastry. See also Poloidal Radius.
Transit
the passage of a celestial body across an observer's meridian;
also the passage of a celestial body across the disk of a larger
one.
Trojan satellites
satellites which orbit at the Lagrangian points, 60° ahead
of and 60° behind another satellite. For example, Telesto
and Calypso are trojans of Saturn's satellite Tethys.
Troposphere
the lower regions of a planetary atmosphere where convection
keeps the gas mixed and maintains a steady increase of temperature
with depth. Most clouds are in the troposphere.
Tuff
the general term for consolidated pyroclastic debris.
U
Ultraviolet
electromagnetic radiation at wavelengths shorter than the violet
end of visible light. The atmosphere of the Earth effectively
blocks the transmission of most ultraviolet light, which can
be deadly to many forms of life. The light that is so blue humans
cannot see it.
Umbra
the area of total darkness in the shadow caused by an eclipse.
Universal Time (UT)
also known as Greenwich Mean Time, this is local time on the
Greenwich meridian. Universal time is used by astronomers as
a standard measure of time.
V
Vallis
a sinuous valley.
Vastitas
widespread lowlands.
Van Allen Belts
radiation zones of charged particles that surround the Earth.
The shape of the Van Allen belts is determined
by the Earth's magnetic field.
Variable Star
a star that fluctuates in brightness. These include eclipsing
binaries.
Vent
the opening in the crust through which volcanic material erupts.
Vernal Equinox
the point on the celestial sphere where the sun crosses the celestial
equator from south to north. The time when the sun is at the
vernal equinox defines the first day of spring. This happens
on about March 20 each year.
Virgo Cluster
a gigantic cluster of over 2000 galaxies that is located mainly
within the constellation of Virgo. This
cluster is located about 60 million light years from Earth.
Visible Light
wavelengths of electromagnetic radiation that are visible to
the human eye.
Visual Magnitude
A scale used by astronomers to measure the brightness of a star
or other celestial object. Visual magnitude measures only the
visible light from the object. On this scale, bright objects
have a lower number than dim objects.
Volcano
(1) A vent in the planetary surface through which magma and associated
gases and ash erupt. (2) The form or structure produced by the
erupted materials.
W
Wavelength
the distance from crest to crest or trough to trough of an electromagnetic
wave (see electromagnetic radiation) or other wave.
White Dwarf
A very
small, white star that
is the remnant core of a star that has completed fusion in its
core. The sun will become a white dwarf. White dwarfs are typically
composed primarily of carbon, have about the radius of the earth,
and do not significantly evolve further.
White Light
visible light that includes all colors and, therefore, all visible
wavelengths.
Wolf-Rayet stars
are hot, blue giants. They are extremely
turbulent and have a temperature of 50,000 °K and higher
X
X-ray Astronomy
the field of astronomy that studies celestial objects by the
x-rays they emit.
X-ray
electromagnetic radiation of a very short wavelength and very
high-energy. X-rays have shorter wavelengths
than ultraviolet light but longer wavelengths than cosmic rays.Because
x-rays are absorbed by the Earth's atmosphere, x-ray astronomy
is performed in space.
X-ray Star
a bright celestial object that gives off x-rays as a major portion
of its radiation.
Y
Young
when used to describe a planetary surface, "young"
means that the visible features are of relatively recent origin,
i.e. that older features have been destroyed by erosion or lava
flows. Young surfaces exhibit few impact craters and are typically
varied and complex; in contrast, an "old" surface is
one that has changed relatively little over geologic time. The
surfaces of Earth and Io are young; the surfaces of Mercury and
Callisto are old.
Z
Zenith
a point directly overhead from an observer.
Zodiac
an imaginary belt across the sky in which the Sun, Moon, and
all of the planets can always be found.
Zodiacal Light
a faint cone of light that can sometimes be seen above the horizon
after sunset or before sunrise. Zodiacal light is caused by sunlight
reflecting off small particles of material in the plane of the
Solar System.