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
) away from Earth. On this scale the Sun has an absolute magnitude of +4.8 whilst
it has an apparent magnitude
of -26.7 because it is so close.
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.
A process by where the atmosphere melts away and removes the surface material of an incoming
The process whereby dust and gas accumulate into larger bodies such as stars and
A stone meteorite that contains no chondrules.
The reflective property of a non-luminous object. A perfect mirror has an albedo of 100% while a black
hole has an albedo of 0%.
A dark or light marking on the surface of an object that may be a geological or topographical feature.
The angular distance of an object above the horizon.
Matter consisting of particles with charges opposite that of ordinary matter. In antimatter, protons have a negative charge while electrons have a
A point that is directly opposite the opposing side of a planet.
The point of greatest separation of two stars, for example in a
binary star system.
The size of the opening through which light passes in an optical instrument (including a camera or telescope).
The higher the number the smaller the opening represented.
The point in the orbit of a planet, or other celestial body,
where it is furthest from the Sun.
The point in the orbit of the Moon, or other satellite,
where it is furthest from the Earth.
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 less bright objects
have a higher apparent magnitude.
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.
A branch of science that explores chemical interactions between dust and gas interspersed between the stars.
Astronomical Unit (AU)
A unit of measure equal to the average distance between the Earth and the Sun, approximately 93 million miles.
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.
A glow in a planet's ionosphere caused by the interaction between the planet's
magnetic field and charged particles from the Sun. This phenomenon is known as the Aurora Borealis
in the Earth's northern hemisphere and the Aurora Australis in the Earth's Southern Hemisphere.
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.
Also known as the northern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the northern hemisphere. It is caused by charged
particles from the Sun as they interact with the Earth's magnetic field.
An imaginary line through the poles and centre of rotation of an object.
The angular distance of an object around or parallel to the horizon from a predefined zero point.
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.
The theory 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
is said to account for the current expansion of the Universe.
A system of two stars that revolve around a common centre of
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.
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.
A term used to describe an exceptionally bright meteor. Bolides typically will produce a sonic boom.
A type of volcanic crater that is extremely large, usually formed by the collapse of a volcanic cone or by a violent volcanic explosion.
A series or chain of craters.
A hollow, irregular depression.
An imaginary line that divides the celestial sphere into a northern and southern hemisphere.
The North and South poles of the celestial sphere.
An imaginary sphere around the Earth on which the stars and planets
appear to be positioned.
A variable star whose light pulsates in a regular cycle. The period of fluctuation is linked to
the brightness of the star with brighter Cepheids having a longer period.
A distinctive area of broken terrain.
Another name for a canyon.
A meteorite that contains chondrules.
Small, glassy spheres commonly found in meteorites.
The part of the Sun's atmosphere which is just above the surface.
A star that never sets but always stays above the horizon, depending on the location of the observer. The
further South, the fewer circumpolar stars. Polaris, the North Star, is circumpolar in most of the northern hemisphere.
A torus or ring-shaped accumulation of gas, dust or other debris in orbit around a star in different phases of its life cycle.
An area of dust or gas surrounding the nucleus of a comet.
A gigantic ball of ice and rock that orbit the Sun in a highly eccentric orbit. Some comets have an orbit
that brings them close to the Sun where they form a long tail of gas and dust as they are heated by the Sun's rays.
An event that occurs when two or more celestial objects appear close together in the sky.
A grouping of stars that make an imaginary shape or illustration in the sky.
The outer part of the Sun's atmosphere. The corona is visible from Earth during a total
solar eclipse. It is the bright outer glow seen in most solar eclipse photos.
Atomic nuclei (mostly protons) that are observed to strike the Earth's atmosphere
with extremely high amounts of energy.
A tube-like 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 a centimetre but its length would extend from one end of the visible Universe to the other.
The study of celestial systems, including the Solar System, stars, galaxies
and galactic clusters.
A branch of science that deals with studying the origin, structure and nature of the Universe.
A bowl-shaped depression formed by the impact of an asteroid or meteoroid.
Also the depression around the opening of a volcano.
A term used to describe matter in the Universe that cannot be seen, but can be detected by its gravitational
effects on other bodies.
A ring-shaped circumstellar disk of dust and debris in orbit around a star. Debris disks can be created as the next phase in planetary system
development following the protoplanetary disk phase. They can also be formed by collisions
The angular distance of an object in the sky from the celestial equator.
The amount of matter contained within a given volume. Density is measured in grams per cubic centimeter
(or kilograms per litre). The density of water is 1.0, iron is 7.9 and lead is 11.3.
The surface of the Sun or other celestial body projected against the sky.
Two asteroids that revolve around each other and are held together by the
gravity between them. Also called a binary asteroid.
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 has a shorter wavelength (blue) while an object moving away has a longer (red) wavelength. The Doppler
effect can be used to estimate an object's speed and direction.
A grouping of two stars. This grouping can be apparent, where the stars only seem close together, or physical,
such as a binary system.
A celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of
planetesimals and is not a satellite. It has to have sufficient mass to overcome rigid body forces and achieve
hydrostatic equilibrium. Pluto is nowadays considered a dwarf planet.
The measure of how an object's orbit differs from a perfect circle. Eccentricity defines the shape of an
The total or partial blocking of one celestial body by another.
A binary system where one object passes in front of the other, cutting off some or all of its light.
An imaginary line in the sky traced by the Sun as it moves in its yearly path through the sky.
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 appear as a lighter colour than
the surrounding surface.
Another term for light. Light waves created by fluctuations of electric and magnetic fields in space.
The full range of frequencies, from radio waves to gamma waves, that characterises light.
An ellipse is an oval shape. Johannes Kepler discovered that the orbits of the planets were elliptical
in shape rather than circular.
A galaxy whose structure shaped like an ellipse and is smooth and lacks complex structures such as spiral arms.
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.
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
throughout each year.
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.
The speed required for an object to escape the gravitational pull of a planet or other body.
The invisible boundary around a black hole past which nothing can escape the gravitational pull
- not even light.
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.
The apparent dimming of star or planet when low on the horizon due to
absorption by the Earth's atmosphere.
A term that means outside of or beyond our own galaxy, the Milky Way.
A term used to describe anything that does not originate on Earth.
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.
Bright patches that are visible on the Sun's surface, or photosphere.
A strand of cool gas suspended over the photosphere by magnetic fields,
which appears dark as seen against the disk of the Sun.
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.
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.
A faint red star that appears to change in brightness due to explosions on its surface.
The name given to the spherical region surrounding the centre, or nucleus, of a galaxy.
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 centre of many galaxies.
A large grouping of stars. 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 that their light takes millions of years to reach the Earth.
The name given to Jupiter's four largest moons, Io, Europa, Callisto and Ganymede. They were discovered independently by Galileo Galilei
and Simon Marius.
The highest energy, shortest wavelength form of electromagnetic
An orbit in which a satellite's orbital velocity is matched
to the rotational velocity of the planet. A spacecraft in geosynchronous orbit appears to hang motionless
above one position of a planet's surface.
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.
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.
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.
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.
A mutual physical force of nature that causes two bodies to attract each other.
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.
The point at which the solar wind meets the interstellar
medium or solar wind from other stars.
The space within the boundary of the heliopause containing the Sun and the Solar System.
An element consisting of one electron and one proton. Hydrogen is the lightest of the elements and is the building block of the universe.
Stars form from massive clouds of hydrogen gas.
The law of physics that states that the farther a galaxy is from us, the faster it is moving
away from us.
A state that occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the
opposite direction. Hydrostatic equillibrium is responsible for keeping stars from imploding and for giving planets their spherical shape.
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.
A term used to describe water or a number of gases such as methane or ammonia when in a solid state.
A measure of the tilt of a planet's orbital plane in relation to that of the Earth.
A conjunction of an inferior planet that
occurs when the planet is lined up directly between the Earth and the Sun.
A planet that orbits between the Earth and the Sun. Mercury
and Venus are the only two inferior planets in our solar system.
International Astronomical Union (IAU)
An international organization that unites national astronomical societies from around
the world and acts as the internationally recognised authority for assigning designations to celestial bodies and their surface features.
Interplanetary Magnetic Field
The magnetic field carried along with the solar wind.
The gas and dust that exists in open space between the stars.
A region of charged particles in a planet's upper atmosphere.
In Earth's atmosphere, the ionosphere begins at an altitude of about 25 miles and extends outward about 250.
A meteorite that is composed mainly of iron mixed with smaller amounts of nickel.
A galaxy with no spiral structure and no symmetric shape. Irregular galaxies are usually filamentary
or very clumpy in shape.
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
Kepler's First Law
A planet orbits the Sun in an ellipse with the Sun at one focus.
Kepler's Second Law
A ray directed from the Sun to a planet sweeps out equal areas in equal times.
Kepler's Third Law
The square of the period of a planet's orbit is proportional to the cube of that planet's semi major
axis; the constant of proportionality is the same for all planets.
A distance equal to 1000 parsecs.
Regions in the main belt of asteroids where few or no
asteroids are found. They were named after the scientist who first noticed them.
A large ring of icy, primitive objects beyond the orbit of Neptune. Kuiper 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.
French mathematician and astronomer Joseph Louis Lagrange showed that three bodies could 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.
A disk-shaped galaxy that contains no conspicuous structure within the disk. Lenticular galaxies tend to look more like elliptical galaxies than
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.
An astronomical unit of measure equal to the distance light travels in a year, approximately 5.8 trillion miles.
The outer edge or border of a planet or other celestial body.
A small group of about two dozen galaxies of which our own Milky Way galaxy is a member.
The amount of light emitted by a star.
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.
The average time between successive new or full moons. A lunar month is equal to 29 days 12 hours and 44 minutes. Also called a synodic month.
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.
Two small, irregular galaxies found just outside our own Milky Way galaxy. The Magellanic Clouds
are visible in the skies of the southern hemisphere.
A condition found in the region around a magnet or an electric current, characterised by the existence of a detectable magnetic force
at every point in the region and by the existence of magnetic poles.
Either of two limited regions in a magnet at which the magnet's field is most intense.
The area around a planet most affected by its magnetic field.
The boundary of this field is set by the solar wind.
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 one with a visual
magnitude of +6.
The area between Mars and Jupiter where most of the
asteroids in our solar system are found.
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.
A word meaning "sea" used as a term to describe a large, circular plain. On the Moon, the maria are the smooth, dark-coloured areas.
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
A word used to describe anything that contains mass.
An imaginary circle drawn through the North and South poles of the celestial equator.
A term used by astronomers to describe all elements except hydrogen and helium.
A small particle of rock or dust that burns away in the Earth's atmosphere. Meteors are also
referred to as shooting stars.
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.
An object, usually a chunk of metal or rock, that survives entry through the atmosphere to reach
the Earth's surface. Meteors become meteorites if they reach the ground.
A small, rocky object in orbit around the Sun, smaller than an asteroid.
A measure of atmospheric pressure equal to 1/1000 of a bar. Standard sea-level pressure on Earth is about 1013
A term used since the 19th century to describe objects, such as asteroids, that are in orbit around the Sun but are not planets or comets. In 2006
the International Astronomical Union reclassified minor planets as either
dwarf planets or small solar system bodies.
An interstellar cloud of molecular hydrogen containing trace amounts of other molecules such as carbon monoxide
A term used to describe a point directly underneath an object or body.
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.
A fundamental particle produced by the nuclear reactions in stars. Neutrinos are very hard to detect because the
vast majority of them pass completely through the Earth without interacting.
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.
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.
A star that flares up to several times its original brightness for some time before returning to its original state.
The 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. Nuclear fusion is the reaction that fuels the Sun, where hydrogen nuclei are fused
to form helium.
The angle between a body's equatorial plane and orbital plane.
A measure of flattening at the poles of a planet or other celestial body.
An event that occurs when one celestial body conceals or obscures another. A solar eclipse is an
occultation of the Sun by the Moon.
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.
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.
The position of a planet when it is exactly opposite the Sun in the sky as seen from Earth. A planet
at opposition is at its closest approach to the Earth and is best suitable for observing.
The path of a celestial body as it moves through space.
The apparent change in position of two objects viewed from different locations.
A large distance often used in astronomy. A parsec is equal to 3.26 light-years.
A shallow crater with a complex, scalloped edge.
The area of partial illumination surrounding the darkest part of a shadow caused by an eclipse.
The point in the orbit of the Moon or other satellite at which it is closest to the Earth.
The point in the orbit of a planet or other body where it is closest to the Sun.
To cause a planet or satellite to deviate from a
theoretically regular orbital motion.
The apparent change in shape of the Moon and inferior planets as seen from Earth as
they move in their orbits.
A particle of light composed of a minute quantity of electromagnetic energy.
The bright visible surface of the Sun.
A celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause
thermonuclear fusion, and has cleared its neighbouring region of planetesimals.
A shell of gas surrounding a small, white star. The gas is usually illuminated by the star, producing
a variety of colours and shapes.
A solid object that is believed to exist in protoplanetary disks and in
debris disks. Planetesimals are formed from small dust grains that collide and stick together and
are the building blocks that eventually form planets in new planetary systems.
A low plain.
A high plain or plateau.
A form of ionized gas in which the temperature is too high for atoms to exist in their natural state. Plasma is composed of free electrons
and free atomic nuclei.
The apparent shift of the celestial poles caused by a gradual wobble of the Earth's axis.
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.
The apparent angular motion across the sky of an object relative to the Solar System.
A rotating circumstellar disk of dense gas surrounding a young newly formed star. It is
thought that planets are eventually formed from the gas and dust within the protoplanetary disk.
Dense regions of molecular clouds where stars are forming.
A spinning neutron star that emits energy along its gravitational axis. This energy
is received as pulses as the star rotates.
A point in the orbit of a superior planet where it appears at right angles to the Sun as seem from
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.
Sometimes also called quasi-stellar source, this is a star-like object with a large redshift that gives off a strong source of radio waves. They
are highly luminous and presumed to be extragalactic.
The movement of an object either towards or away from a stationary observer.
A point in the sky from which meteors in a meteor shower seem to originate.
Energy radiated from an object in the form of waves or particles.
Regions of charged particles in a magnetosphere.
A galaxy that gives off large amounts of energy in the form of radio waves.
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.
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.
A state in which an orbiting object is subject to periodic gravitational perturbationsby another.
The phenomenon where a celestial body appears to slow down, stop, them move in the opposite direction. This motion is caused when the Earth
overtakes the body in its orbit.
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.
A galaxy that has a ring-like appearance. The ring usually contains luminous blue
stars. Ring galaxies are believed to have been formed by collisions with other galaxies.
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.
The spin of a body about its axis.
A natural or artificial body in orbit around a planet.
A line of cliffs produced by erosion or by the action of faults.
A main-sequence star that rotates rapidly, causing a loss of matter to an ever-expanding shell.
A type of star which is believed to be surrounded by a thin envelope of gas, which is often indicated by bright emission lines in its
A satellite that constrains the extent of a planetary ring through gravitational forces. Also known as a shepherd moon.
Of, 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.
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.
The period of revolution of a planet around the Sun or a satellite
around its primary.
The centre of a black hole, where the curvature of space time is maximal. At the
singularity, the gravitational tides diverge. Theoretically, no solid object can survive hitting the singularity.
Small Solar System Body
A term defined in 2006 by the International Astronomical Union
to describe objects in the Solar System that are neither planets or
dwarf planets. These include most of the asteroids, comets and other small bodies in the Solar System.
The approximately 11-year quasi-periodic variation in frequency or number of solar active events.
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, resulting in a ring of light around the Moon.
A bright eruption of hot gas in the Sun's photosphere. Solar prominences are usually only detectable
by specialised instruments but can be visible during a total solar eclipse.
The cloud of dust and gas out of which the Solar System was believed to have formed about 5 billion years ago.
A flow of charged particles that travels from the Sun out into the Solar System.
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.
An instrument connected to a telescope that separates light signals into different frequencies,
producing a spectrum.
The technique of observing the spectra of visible light from an object to determine its composition, temperature, density and speed.
Patterns of gas seen in the atmosphere of the Sun.
The range of colours produced when visible light passes through a prism.
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.
A giant ball of hot gas that creates and emits its own radiation through
A large grouping of stars, from a few dozen to a few hundred thousand, that are bound together
by their mutual gravitational attraction.
Steady State Theory
This theory suggests the universe is expanding but exists in a constant, unchanging state in the large scale. It states that
new matter is being continually being created to fill gaps left by expansion. It has been abandoned by most astronomers in favor of the
Big Bang theory.
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.
A meteorite that resembles a terrestrial rock
and is composed of similar materials.
A meteorite that contains regions resembling both a stone
meteorite and an iron meteorite.
Areas of the Sun's surface that are cooler than surrounding areas. The usually appear black on visible light photographs of the Sun.
Sunspots are usually associated disturbances in the Sun's electromagnetic field.
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 colour.
A conjunction that occurs when a superior
planet passes behind the Sun and is on the opposite side of the Sun from the Earth.
A planet that exists outside the orbit of the Earth. All of the planets in our solar
system are superior except for Mercury and Venus. These two planets are inferior planets.
A supernova is a cataclysmic explosion caused when a star exhausts its fuel and ends its life.
Supernovae are the most powerful forces in the universe. All of the heavy elements were created in supernova explosions.
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.
A period of rotation of a satellite about its axis
that is the same as the period of its orbit around its primary. This causes the satellite to
always keep the same face to the primary. Our Moon is in synchronous rotation around the Earth.
The interval between points of opposition of a superior
A small, glassy material formed by the impact of a large body, usually a meteor or
asteroid. Tektites are commonly found at the sites of meteor
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.
The boundary between the light side and the dark side of a planet or other body.
A term used to describe anything originating on planet Earth.
A name given to a planet with a composition mainly of rock and iron, similar to that of Earth.
The differential gravitational pull exerted on any extended body within the gravitational field of another body.
Frictional heating of a satellite's interior due to flexure caused by the gravitational pull
of its parent planet and/or other neighbouring satellites.
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.
An object orbiting in the Lagrange points of another (larger) object. The name derives
from a generalisation of the names of some of the largest asteroids in Jupiter's Lagrange points.
Saturn's moons Helene, Calypso and Telesto are also sometimes called Trojans.
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 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.
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
Stars that fluctuate in brightness, including eclipsing binaries.
Wavelengths of electromagnetic radiation that are
visible to the human eye.
A gigantic cluster of over 2,000 galaxies that is located mainly within the constellation of Virgo. This cluster is located about 60 million
light-years from Earth.
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.
The distance between consecutive crests of a wave. This serves as a unit of measure of electromagnetic
A very small, white star formed when an average sized star uses up its fuel supply and collapses. This process
often produces a planetary nebula, with the white dwarf star at its centre.
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.
The field of astronomy that studies celestial objects by the x-rays they emit.
A bright celestial object that gives off x-rays as a major portion of its
An ordinary star such as the Sun at a stable point in its evolution.
A point directly overhead from an observer.
An imaginary belt across the sky in which the Sun, Moon and all of the planets can always be found.
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.