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Contents
Aberration
An effect caused by the Earth's motion which slightly changes the positions
of stars. Aberration of starlight was discovered by James Bradley in 1728.
This was the first direct proof of the Earth's movement around the Sun.
Absolute Magnitude
A measurement that is used to compare the total light output of stars.
Astronomers use this to determine how far away a star is.
accretion
The gradual accumulation of matter in one location, usually due to gravitational
action
Altitude
The angle between a celestial object and the horizon.
Alpha Beta Gamma Theory
An explanation of how primordial hydrogen was partly converted into helium
during the Big Bang to provide the raw material from which stars were
made. This theory predicts the existence of background radiation, which
was later discovered to fill the universe.
Alpha decay
The process in which the nucleus of an atom emits an alpha particle and
is transformed into a nucleus with four less atomic mass units and two
fewer units of charge.
Alpha Particle
Two protons and two neutrons held together by nuclear interactions to
make a stable nucleus.
Andromeda
A constellation in the northern hemisphere of the sky, located next to
Pegasus. Andromeda is 5,000 light-years away. At the heart of the constellation
is the Andromeda galaxy.
Andromeda galaxy
The most distant object in space that is visible to the naked eye. It
is a separate spiral galaxy in space, twice the diameter of the Milky
Way. This galaxy appears as a fuzzy oval patch to the naked eye, but telescopes
show that it is composed of approximately 300 billion stars. It is about
2.2 million light-years away, making it the closest major galaxy to our
own.
Antimatter
Matter which consists of antiparticles, elementary particles of ordinary
matter, such as protons and electrons, but have an opposite electrical
charge. For example, an antiproton has a negative charge while a proton
has a positive charge. When a particle of matter and a particle of antimatter
come into contact with each other they are destroyed, releasing energy.
Aperture synthesis
A technique used in astronomy to make radio telescopes simulate the power
of much larger instruments. Antennae are linked together to observe the
same part of the sky. The information received by all the telescopes are
combined to produce a final image of the sky.
Apollo program
An American space project that landed 12 astronauts on the Moon between
1966 and 1972.
Apparent magnitude
How bright a star appears to be from Earth.
Asteroid
A small, rocky body that moves in an elliptical orbit around the Sun.
They are too small to have atmospheres, and occasionally fall to Earth
as Meteorites. On January 1, 1801, an Italian astronomer, Giuseppe Piazzi,
became the first person to discover an asteroid which was later named
Ceres.
Astrometry
The measurement of objects' positions in space.
Astronomy
The science of the celestial bodies: the Sun, the Moon, the planets, the
stars and galaxies, and every other object in the universe. Astronomy
includes the positions, motions, distances, physical conditions, and the
origin and evolution of the celestial bodies.
Astrophysics
The application of physics to the study of the universe.
Astronomical Unit (AU)
The semimajor axis of the Earth's orbit; the average distance between
the Earth and the Sun
Autumnal equinox
The intersection of the ecliptic and the celestial equator where the Sun
crosses the equator from north to south
Baby Universes
Regions of space time that are connected to one another by wormholes.
Background radiation
Radiation that comes from space at radio and infrared wavelengths.
Barnard's star
The second-closest star to the Sun, about 5.9 light-years away. It is
named after Edward E. Barnard who discovered it in 1916. This star has
the fastest proper motion of any star that has been discovered.
Beta decay
The process in which a neutron emits an electron and an anti-neutrino
as it transforms into a proton. When this happens inside a nucleus, the
charge of the nucleus is increased by one unit.
Big Bang
The event which is believed to have started the universe's expansion.
It is believed that the Big Bang occurred 18 billion years ago when all
the matter in the universe was squeezed into a fireball. The fireball
exploded, rapidly expanding the universe and sending particles into the
newly created space.
Black hole
A theoretical object which has a gravitational pull so strong that nothing
can escape, not even light.
BL Lac objects
These are bright, compact sources of intense energy located in the middle
of some galaxies.
Bursters
Very powerful gamma-ray sources.
Celestial mechanics
The study of the motion and gravitational interaction of bodies in space.
It is used to calculate the orbits of objects in space and predict their
motion.
Celestial Equator
A great circle on the celestial sphere 90 degrees from the celestial poles
Celestial Poles
Points about which the celestial sphere appears to rotate
Center of Mass
That point in an isolated system that moves at a constant velocity in
accordance to Newton's laws of motion
Cepheid variable
A star which enlarges and contracts in size, varying in brightness as
it does so. They can be used to measure distances to objects in space.
Ceres
The largest and the first asteroid to be discovered, by Guiseppe Piazzi
in 1801. It is one of the few asteroids to be visible to the naked eye.
Chromosphere
A layer of gas about 10,000 miles (16,000 kilometers) thick above the
Sun's surface. It is a pinkish-red color because of the light emitted
from hydrogen atoms.
Comet
A small, icy body embedded in a cloud of gas which orbits around the Sun.
When they orbit close to the Sun they heat up, releasing gas, which appears
as a tail always pointing away from the Sun.
Coriolis Effect
The deflection of moving objects on a rotating surface
Cosmic rays
The nuclei of atoms which are stripped of all their electrons that move
through space at speeds close to that of light.
Cosmology
The study of the origin of the Universe.
Crab Nebula
An expanding cloud of gas that was ejected by a star that exploded in
July, 1054 AD. It is abut 6,300 light-years away from the Sun and emits
powerful radiation at all of the wavelengths.
Craters
Bowl-shaped depressions formed by the striking of Meteorites.
Curvature of space
A distortion of space that is caused by the presence of matter. For example,
the Earth is spherical, but on a map it is shown to be flat with the Polar
regions "stretched out."
Declination
Angular distance of a celestial object north or south of the celestial
equator
Degenerate Matter
Matter that is at such a high density that quantum effects dominate its
behavior. The outward pressure is much greater than the pressure appropriate
to that density of material.
density
The ratio of the mass of an object to its volume
Dirty Snowball
A term used to describe the core (nucleus) of a comet which is made of
ice and dust.
differential rotation
The rotation of a nonridgid object in which parts adjacent to each oterh
do not always stay close togehter. Examples are Saturn and Jupiter's clouds.
Doppler effect
The change in frequency of waves emitted by an object as it moves. For
example, as an emergency vehicle approaches the sirens get louder and
as the vehicle moves away the sirens fade out.
Double star
Also called a binary star, two stars that are linked together by gravity.
The first double star was identified in 1650. About 46 percent of all
stars are double stars, 15 percent are single stars, and 39 percent are
multiple systems of three or more stars.
Dwarf stars
Ordinary stars, like the Sun, that are in the prime of their lives, converting
hydrogen into helium in their centers.
Eccentricity
Measure of how much an ellipse differs from a circle. Eccentricity ranges
from o to 1 where 0 is a perfect circle and 1 is a straight line.
Eclipse
The passage of one astronomical body into the shadow of another, but this
term is usually applied to the passage of the Moon in front of the Sun,
called a solar eclipse. A Lunar eclipse is when the Moon passes into the
Earth's shadow, which can only happen during a full Moon.
Electromagnetic radiation
The range of radiation, from gamma rays through to the spectrum of visible
light, and to radio waves.
Ellipse
A shape that looks like a squashed circle that is produced by cutting
a cone at an angle.
Emission
The production of Electromagnetic Radiation. Radiation is emitted when
atomic particles accelerate or decelerate.
Fermions
Elementary particles that are conserved during particle interactions.
The archetypal fermion is the electron because an electron cannot be created
or destroyed in a particle interaction.
Fireball
A meteor that has a magnitude of -5 or brighter. Occasionally they are
as bright as magnitude -25. The Moon has a magnitude of -12.
Flare
A burst of light, lasting only a few hours, that occurs near a Sunspot.
Frequency
The number of waves that pass through a specific point in a certain period
of time.
Galactic clusters
Shapeless star clusters found in the spiral arms of the Galaxy. The individual
stars are scattered within the structure.
Galaxies
Systems of billions of stars that are bound together by their own gravity,
such as the Milky Way.
Gamma-ray astronomy
The study of radiation emitted by celestial objects at wavelengths shorter
than X-rays. Gamma rays are absorbed by the Earth's atmosphere so they
can only be studied by rockets and satellites.
General Theory of Relativity
A theory of gravity developed by Albert Einstein in the early part of
the 20th century. It tells us how the universe evolved from an initial
singularity and was born during the Big Bang. This theory gives us a physical
picture of how gravity and the accelerated motions work.
Giant star
A large, bright star. The brightness of a star depends on its size and
temperature. A giant star is brighter than a dwarf star of the same temperature.
Gravity
A property of matter which produces a mutual attraction between all the
bodies present.
Greenhouse effect
The warming of a planet due to short-wavelength radiation reaching the
surface of the planet.
Gregorian telescope
The earliest proposed form for a reflecting telescope, first suggested
by James Gregory. The image is formed by a concave primary mirror refocused
by a second mirror, which reflects the light back through a central hole
in the primary mirror.
Halley's comet
The first comet to be identified, and the largest of its type.
Helioseismology
The technique of investigating the interior structure of the Sun by studying
the way in which its surface moves.
Hertzsprung-Russell Diagram
A type of graph in which the temperature of each star is plotted against
its absolute magnitude. The position of a star depends on its mass and
age. These diagrams help astrophysicists work out how stars evolve.
Hubble's constant
The rate at which the Universe is expanding. It is measured to be 10 miles
per second per million light-years. For example, this means that a galaxy
would recede at a rate of 10 miles per second for every million light-years
of its distance.
Hubble's law
The relationship between a galaxy's size and its speed of recession.
Infrared astronomy
The study of radiation from space between the wavelengths of red light
and one millimeter, where microwave radio astronomy begins.
Interferometry
A technique used in radio astronomy in which two or more separate antennae
are linked together to study the same astronomical source.
Interferometer
A pair of receiving devices that are linked together to make the equivalent
of a larger receiver.
Interstellar absorption
The dimming of light because of dust particles in space.
Ions
Atoms which have either lost or gained electrons, so they have gained
a positive or a negative charge.
Ionosphere
The region of the Earth's atmosphere that contains atoms and molecules
that have had electrons removed by solar radiation, making them electrically
charged.
Kepler's laws
Three laws to the motions of the planets in their orbits around the Sun:
1. Every planet orbits the Sun in an ellipse, with the Sun at one focus
of that ellipse.
2. The radius vector, a line from the Sun to the planet, sweeps out equal
areas within the ellipse in equal times.
3. A fixed ratio exists between the time taken to complete an orbit and
the size of the orbit, and this ratio is the same for every planet.
Kelvin temperature scale
A scale in which zero degrees Celsius corresponds to 273.15 Kelvin (K).
Laser
A device that produces an intense beam of light. Lasers have allowed astronomers
to find the Earth-Moon distance to within a few centimeters.
Lens
An optical glass component which converges or diverges a beam of light
when it passes through the lens. A converging lens can form a real image
of an object on a screen. A diverging lens cannot do this, and is used
only for special purposes.
Light
The part of the spectrum of electromagnetic radiation to which the human
eye is sensitive.
Light, velocity of
The speed at which light travels through a vacuum.
Light-year
The distance traveled by light in a vacuum during one year. It is approximately
5.8786 trillion miles (9.4607 trillion kilometers).
Luminosity
The total amount of radiation emitted by a star or other glowing object
per second.
Magellanic Clouds
Two small galaxies that orbit the Milky Way.
Magnetic stars
Stars which have very strong magnetic fields.
Magnitude
A measure of an object's brightness.
Maksutov telescope
A telescope system using both a lens and a mirror. This type of telescope
is expensive, so few are in existence.
MASER
This name stands for Microwave Amplification by Stimulated Emission of
Radiation. When an atom is in a suitable energetic state, the passage
of an electromagnetic wave triggers it to give up energy in the form of
electromagnetic radiation. This reinforces the passing wave, which allows
excited atoms to build up an intense pulse of radiation with a defined
frequency.
Meteor
The streak of light that is produced when a solid particle from space,
a meteoroid, enters the Earth's atmosphere at a high speed and burns up
to produce a fine dust. Meteors bright enough to be visible to the naked
eye are usually called shooting stars, while brighter meteors are called
fireballs.
Meteorite
A lump of rock or metal from space that falls to Earth.
Meteoroid
A solid object that is moving in interplanetary space, but is larger than
a single molecule and smaller than an asteroid. Meteoroids that are larger
than 6 inches can fall to Earth as meteorites.
Micrometeorite
A particle from space which is small enough to be decelerated in the Earth's
atmosphere, but isn't vaporized. Approximately 50 micrometeorites per
square yard fall to Earth each day.
Milky Way
A spiral galaxy of approximately 100 billion stars.
Mills cross
A type of radio telescope made of two lines of antennae at right angles
to one another.
Mirror
A reflecting component in any astronomical instrument.
Moon
The Earth's only natural satellite. The surface is alternately scorched
by the Sun and then frozen. The Moon is 2,160 miles (3,476 kilometers)
in diameter, less than a third of the diameter of Earth.
Morning star
Not actually a star, but the planet Venus shining brightly in the morning
sky.
NASA
The National Aeronautics and Space Administration, a federal agency established
in 1958 to administer all non-military aeronautical research and space
programs in the United States.
Navigation satellites
Satellites that help aircrafts and ships to identify their exact locations.
Nebula
A region of gas and dust in a galaxy. They appear to be fuzzy.
Neutron star
A very small, dense star that is so tightly packed together that the protons
and electrons have been compressed to form neutrons.
Nova
A faint star that suddenly becomes bright, becoming visible from where
no star had been seen before. Only two or three novae are discovered each
year. Most novae are binary stars.
Nuclear fission
The process in which the nucleus of a heavy atom splits into two or more
parts, releasing energy and two or three free neutrons.
Nuclear fusion
The process in which light nuclei fuse together to make one heavier nucleus,
releasing energy as they do so.
Open cluster
A shapeless, loosely packed cluster of stars.
Orbit
The path that an astronomical body follows as it moves around another
astronomical body.
Phase
The portion of any bright body that is visible to the observer.
Photometer
A device that is used to measure the brightness of an object.
Photometry
The measurement of an object's brightness.
Photon
A particle of light.
Photosphere
The visible surface of the Sun.
Planet
A body that shines by reflecting sunlight. Planets can be made out of
rock and metal, like Earth, or out of gas, like Jupiter.
Planetary nebula
A gaseous shell surrounding a hot central star. They often look like planets,
but they are much hotter: the surface temperature is anywhere from 30,000
to 150,000 degrees Celsius.
Plasma A hot gas that consists of negatively charged electrons and positively
charged atomic nuclei, or ions. The atoms of gas are broken into these
parts by collisions between the atoms, or as a result of being struck
by high-energy ultraviolet radiation from a nearby star.
Polar distance
The angle between an object and the celestial pole.
Proper motion
The motion of a star across the sky. It is expressed in arc seconds per
year.
Protoplanet
A cloud of gas, rocks, and dust particles that are in the process of forming
into a planet.
Protostar
The early stage in a star's formation before the onset of nuclear burning
at the core of the star.
Pulsar
A radio source that emits short pulses of radiation at regular intervals.
Quantum Theory
The set of physical laws that apply primarily on a very small scale, the
size of atoms or smaller.
Quasar
An object which appears as a star-like point of light, but emits more
energy than an entire galaxy. Many quasars also emit radio waves.
Radar astronomy
The study of the bodies in the solar system by radio waves reflecting
off of them. A radio telescope is used to transmit a powerful pulse of
radio waves, and then to detect the faint echo that returns a few seconds
later.
Radio astronomy
The study of the universe at radio wavelengths.
Radio galaxies
Distant galaxies that are sources of powerful radio waves. Their radio
output is up to a million times that of our own galaxy.
Radio telescopes
Instruments for receiving radio waves from space. A radio telescope consists
of an antenna, an amplifying system, and a recorder to preserve the observations.
Red dwarf
A star that has a low surface temperature, 2,000 to 3,000 degrees Celsius,
and a diameter about half that of the Sun.
Red giant
A star that has a low surface temperature, 2,000 to 3,000 degrees Celsius,
and a diameter between 10 and 100 times that of the Sun.
Redshift
The amount that wavelengths of light and radiation from distant galaxies
and quasars are increased because of the expansion of the Universe.
Reflecting telescope
A telescope that has a concave mirror which collects and focuses the light
from a celestial object. The majority of modern telescopes are reflectors
because they are inexpensive and compact.
Refracting telescope
A telescope that has a large lens that collects and focuses the light
from the object being observed.
Relativity
A theory to the structure of space and time, and its relation to gravity
and motion. This theory was first introduced by Albert Einstein in 1905.
Resolution
The ability of a telescope to identify fine details.
Satellite
Any small object orbiting a larger body.
Schmidt telescope
A wide-angle photographic telescope that was first constructed by Bernhard
Voldemar Schmidt in 1930. This telescope uses a spherical mirror with
a correcting lens in front which increases the field of view to make it
larger than normal reflecting telescopes.
Scintillation
The "twinkling" of a celestial object or the rapid variations in a star's
brightness. This twinkling is caused by changes of the air temperature
in the Earth's atmosphere. Changes in air temperature causes turbulence
which varies the bending of the light entering the atmosphere.
Sideral day
The interval between successive meridian pasges of the vernal equionox.
Sideral month
The period of the Moon's revolution abou the Earth with respect to the
stars.
Sideral time
Time recokoned by the location of the vernal equionox
Sideral year
The orbital period of the Earth about the Sun with respect to the stars
Siderite
An iron meteorite.
Siderolite
A stony-iron meteorite which contains nickel, iron and stony minerals.
Solar system
The group of planets, comets, and asteroids that orbit the Sun and give
a gravitational pull that dominates space in all directions.
Solar wind
A continuous stream of protons and electrons which is emitted by the Sun
in all directions.
Space probe
Any instrument-carrying device sent from Earth to study another celestial
body, or to take measurements of the conditions in space.
Space station
A large orbiting spacecraft made for long-term human habitation in space.
Special Theory of Relativity
A description of the relationships and interactions between moving objects.
The 'special' theory only applies to the special case of objects moving
at constant speeds in straight lines. It does not deal with accelerated
motions.
Speckle interferometry
A technique that is used to reconstruct star images that have been distorted
by atmospheric turbulence.
Spectroscope
A device used to observe the spectrum.
Spectroscopy
The study of the spectra of heavenly bodies.
Spectrum
The entire range of electromagnetic radiation, from gamma rays to radio
waves.
Spiral galaxy
A type of galaxy in which many of the stars and nebulae lie in spiral
arms.
Star
A bright ball of gas, such as the Sun.
Star cluster
A collection of stars that are held together by their own gravity.
Steady State hypothesis
The idea that the universe has the same appearance to observers wherever
they are in it.
Summer Solstice
The point on the ecliptic where the Sun is fartherst north of the celestial
equator
Sun
The star that is the central body of our solar system.
Sunspot
A relatively cool, dark area on the surface of the Sun.
Supergiant stars
The brightest, largest stars that are formed when stars heavier than the
Sun move into old age. They burn out very quickly so they are rarely seen.
Supernova
A star that explodes and loses most of its mass.
Synodic month
The period of revolution of the Moon with respect to the Sun; the length
of one cycle of lunar phases
Synodic period
The interval between successive occurrences of the same configuration
of a planet
Telescope
A device that collects and magnifies light.
Titan
The largest satellite of Saturn, and the only moon in our solar system
that has a substantial atmosphere.
Transit
The moment when a celestial body crosses the meridian, the north-south
line in the sky.
UFO
Unidentified Flying Object.
Ultraviolet astronomy
The study of the universe in the ultraviolet part of the electromagnetic
spectrum, which is located between visible light and X-rays.
Uncertainty Principle
A feature of the quantum theory which says that for any object certain
pairs of properties, such as position and momentum, are linked so that
they cannot both be precisely determined at the same time.
Variable stars
Stars whose light reaching the Earth varies in brightness.
Vernal Equinox
The point on the ecliptic where the Sun crosses the celestial equator
from south to north
Very Long Baseline Interferometry (VLBI)
A technique of linking distant radio telescopes together to form an interferometer.
Wavelength
The term that is used to describe a wave's motion.
White dwarf
A star which has about the same mass as the Sun, but occupies a volume
about the same as the Earth.
Winter Solstice
The point on the ecliptic where the Sun reaches its greatest distance
south of the celestial equator
X-ray astronomy
The study of the universe at X-ray wavelengths.
Zenith
The piont of the celestial sphere opposite to the direction of gravity
Zodiac
A band of 12 constellations around the sky centered on the ecliptic
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