Glossary Of Terms

A.U.

- Abbr. see Astronomical Unit.

Accretion Disk

- The theoretical nebular cloud from which the planets and the Sun coalesced into the solar system.

Apparent Magnitude

- The apparent brightness of an object as viewed from Earth, regardless of its intrinsic magnitude value.

Asterism

- A named collection of stars in the sky that is not part of an official constellation. Ex. The Big Dipper.

Asteroid

- A small, rocky, celestial body that revolves around the Sun with characteristic diameters between a few and several hundred kilometers. Also called a minor planet, or a planetoid.

Asteroid Belt

- A collection of asteroids in orbit about the Sun lying between the planets Mars and Jupiter. It is believed that this belt formed as the result of a planet breaking apart or failing to coalesce at all.

Astronomical Unit (AU)

- A unit for measuring distance based on the average distance from the Earth's center to the Sun's center. One astronomical unit is equal to about 149 million Km's or 93 million miles. AU's are often more convenient to use than kilometers when measuring large distances such as those in space. In this case kilometers are just too small; it would be like measuring the distance from Halifax to Vancouver in centimetres! We use Astronomical Units to measure distances within our solar system. AU's simply make a measurement easier to understand and give you something to compare it to. For example, Saturn's orbit around the Sun has an average radius of 9.5 AU, which means that Saturn is about ten times farther from the Sun than Earth is.

Astronomy

- The scientific study of matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena.

Atmosphere

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Binary Stars

- A star system in which two or more stars orbit each other around a center of mass. They can share material between themselves.

Blackhole

- An object in space who's gravity is so strong that nothing inside can escape, not even light. The center of the Milky Way (and many other galaxies) is thought to contain a large black hole. A theory of their origin is that they are collapsed stars. (see our section covering Blackholes)

Blue Giant

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Blue Moon

- The term for the second full Moon in a single month. This usually happens a couple times a year.

Brahe, Tycho

1546 - 1601

Brown Dwarf

- A star which does not have enough mass to ignite thermonuclear fusion in the core.

Cassini Division

- A major gap between the A and B rings around Saturn. It was first discovered by Giovanni Domenico Cassini in the 1600's.

Callisto

- Jupiter's second largest moon. It is composed of about 40% ice and 60% rock. It was discovered by Galileo in 1610.

Cassini, Giovanni Domenico

1625 - 1712

Celestial

- Of or relating to the sky or heavens.

Charon

- The only moon of the planet Pluto.

Chromosphere

- An incandescent, transparent layer of gas, primarily hydrogen, several thousand miles in depth, lying above and surrounding the photosphere of a star, such as the sun, but distinctly separate from the corona.

Comet

- A solar system object in orbit around the Sun that is composed of ice, rock, and other chemicals. They are often described as a "snowball" in space.

Constellation

- A group of stars that are used as a system of mapping the night sky. The entire sky is broken up into multiple constellations, such as Andromeda, Perseus, and Draco to name a few.

Copernicus, Nicolaus

1473 - 1543

Corona

- The luminous, irregular envelope of highly ionized gas outside the chromosphere of the Sun.

Cosmic Dust

- Interstellar dust that makes up much of the matter in galaxies.

Cosmic Wind

- A force that pushes debris and cosmic dust along through galaxies.

Cosmology

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Crater

- A depression on a body's (planet, moon, asteroid, etc.) surface originating from, among other things, the collision of another celestial body.

Dark Matter

- Unknown and theoretical substance in space whose presence is detected by its gravitational effect on celestial objects.

Da Vinci, Leonardo

1452 - 1519

Deimos

- One of two small moons orbiting Mars discovered in 1877 by American astronomer Asaph Hall.

Dreyer, John

1852 - 1926

Doppler Effect

- The process by which light or sound is altered in perceived frequency or wavelength by the motion of its source with respect to the observer. This is a key component in determining redshift.

Earth

- The third planet from the Sun. The only known planet to sustain life in it's carbon form as we know it. Type: Terra.

Eclipse

- The condition in which the Sun or the Moon is blocked from our view, either partially or fully. Also see "annular eclipse" and "total eclipse". In a "solar eclipse" the Moon passes in front of the Sun, and in a "lunar eclipse" the Earth's shadow passes between the Sun and the Moon, blocking the Moon from Earth's view.

Ecliptic

- The apparent path of the Sun and planets across the sky as they travel across the constellations from Earth's perspective. You can think of the Ecliptic as the top of a billiard table, with the planets represented by the balls moving across the surface. Of course, the Earth is one of those balls too, so our viewpoint is from the table looking around at the other balls. This arrangemnet exist because the solar system is essentially flat. The planets all orbit in the same direction and in the same plane.

Einstein, Albert

1879 - 1955

Elliptical Galaxy

- An ellipsoidally shaped galaxy that does not contain much interstellar matter. They appear as small "wads" or orbs in a telescope.

Emission Nebula

- Nebular gas and dust that glows as a result of atoms being excited by high energy radiation from hot, massive, young stars.

Encke Division

- Nebular gas and dust that glows as a result of atoms being excited by high energy radiation from hot, massive, young stars.

Europa

- A moon of Jupiter's discovered by Galileo in 1610. There is strong evidence that underneath its surface there may exist liquid water.

Full Moon

- A phase of the Moon in which its entire face is lighted from Earth's perspective. It is 180 degrees opposite from the Sun in its orbit of Earth at this time.

Galaxy

- A system of billions of stars, gas and dust that has coalesced to form a huge body. Ex. The Milky Way galaxy.

Galilei, Galileo

1564 - 1642

Ganymede

- A large rocky moon orbiting Jupiter that was discovered by Galileo in 1610.

Gas Giant

- A large planet composed primaily of gases, ususally Helium and Hydrogen. There are four examples in our own solar system, in order from sun: Jupiter, Saturn, Uranus, and Neptune.

Globular Cluster

- Gravitationally bound concentrations of approximately ten thousand to one million stars, spread over a volume of several tens to about 200 light years in diameter. Also see "star cluster" and "open cluster". Ex. M13 - the Great Hercules Globular Cluster.

Gravity

- The attracting force that exists between two or more objects. Every object with mass has gravity, even though its effects can be minimal and undetectable

Greenhouse Effect

Hawking, Stephen

1942 -

Helium

- An inert, monoatomic, gaseous element occurring in the atmosphere of the sun and stars, and in small quantities in the earth's atmosphere. Helium is by-product of the thermonuclear fusion of hydrogen occuring in the Sun.

Herschel, Sir William

1738 - 1822

Huygens, Christiaan

1629 - 1695

Hydrogen

- A colorless, highly flammable gaseous element, the lightest of all gases and the most abundant element in the universe. The thermonuclear fusion of hydrogen to helium in the Sun produces its heat and energy

Inner Planets

- The four planets in between Jupiter and the Sun.

Io

- One of Jupiter's four large moons discovered by Galileo in 1610. Io is one of the only known sources of active volcanism in the solar system.

Jupiter

- The fifth planet from the Sun, and the largest of all solar system planets. It is a gas giant that is surrounded by many orbiting Moons, some of which can be seen with a small telescope.

Kelvin

- A measurement system of thermodynamic temperature adopted under the System International d'Unites. Solar temperatures are often given in kelvin.

Kuiper Belt

- The Kuiper Belt is a disk-shaped region past the orbit of Neptune at roughly 30 - 100 Astronomical Units (AU) from the Sun. The Kuiper belt contains many small icy bodies, and is believed to be the primary source of short-period comets. The interaction of the larger planets and the belt may disturb some of these icy bodies and tug them in our direction, in which they will become comets. Kuiper Belt objects are believed to be debris leftover from the initial formation of the solar system and cleared out of the central part of the solar system by the gravitational pull of the gas giant planets. As the debris was gaining speed towards the outer planets, the speed became so great that the debris was cleared away from the inner solar system.

Light-Year

- The distance that light travels in one year. One light-year is about 5.9 trillion miles.

Luna

- The proper name for Earth's Moon.

Mars

- The fourth planet from the Sun. It's reddish in appearance and shows evidence of liquid water on its surface in the distant past.

Mercury

- A small, rocky planet closest to the Sun.

Messier, Charles

1730 - 1817, An 18th century comet hunter probably best known for compiling a list of 110 celestial objects known as the "Messier Catalog" that is still in use today.

Meteor

- A natural solar-system object that enters Earth's atmosphere and is subsequently destroyed in a flash of light before reaching the surface. Several regular meteor showers occur throughout the year including the Leonids and Perseids to name two.

Meteorite

- A meteoroid that has landed on the surface of Earth.

Meteoroid

- A small rock in space consisting of rock and/or iron that is probably a chip of an asteroid.

Moon

- Any large naturally occuring mass orbiting a planet; The large rocky body orbiting the Earth around every 28 days (approx.).

Nebula

- A large cloud of gas and dust in space, much smaller than a galaxy. They can actually be a star producing region. They can emit, reflect, and/or absorb light and radiation from nearby stellar radiation. Ex. M42 - The Orion Nebula.

Neutron Star

- An object only tens of miles across, but greater in mass than the Sun.

Neptune

- The eighth planet from the Sun. It is a gas giant.

New General Catalog

- Also known as the "NGC", this catalog was compiled by John Louis Emile Dreyer around 1887. It contains a variety of objects with a detailed legend for descriptions of each of them.

Newton, Sir Isaac

1643 - 1727

Nova

- see: "Supernova"

Occultation

- The process by which one celestial body passes in front of another, blocking it from the view of an observer as seen from Earth. Similar to an "eclipse".

Open Cluster

- Gravitationally bound concentrations of stars believed to originate from large cosmic gas and dust clouds. Smaller than a globular cluster and usually have a short lifespan as a cluster. Also see "star cluster" and "globular cluster". Ex. M45 - the Pleiades star cluster.

Opposition

- The positioning of two celestial bodies relative to a third (usually the Sun, Earth, and a third celestial body) such that they are 180 degrees apart in Right Ascension. Note that the bodies need not actually lie on a straight line since they may lie at different Declinations.

Orbit

- The path and process by which one object revolves or moves around another, held together by their mutual gravitation.

Oort Cloud

- A theoretical region that surrounds the solar system at about 50 to 100 astronomical units; the Oort Cloud is presumably a remnant of the solar system's accretion disk and it is thought to contain trillions comets.

Perihelion

- The point nearest the sun in the orbit of a planet or other celestial body.

Phobos

- One of two small moons orbiting Mars discovered in 1877 by American astronomer Asaph Hall.

Photosphere

- The visible outer layer of a star, especially of the Sun.

Planet

- A large, round object, larger than an asteroid, that is formed inside an accretion disk. Unlike a star, a planet does not produce its own energy via nuclear reactions. There are nine planets in our solar system. Ex. Earth.

Planetary Nebula

- An expanding gas cloud that was created as the result of gas being expelled from a sunlike star in the later stages of its life. Ex. M57 - The Ring Nebula.

Pluto

- Once known as the ninth and final known planet from the Sun. Pluto was downgraded to a 'dwarf planet' in early September 2006. Pluto was the smallest planet in diameter, now held by Mercury. It is rocky and despite being a dwarf planet it is orbited by one moon named Charon.

Pulsar

- A very fast-spinning and highly dense object that emits light, radio and/or X-rays in very tight beams, similar to a lighthouse.

Quasar

- A small, extremely bright object at the center of galaxies who's origin is believed to result from the expenditure of large amounts of energy from the surroundings of a giant black hole.

Red Giant

- A large, red sunlike star in its latter stages of life.

Red Shift

- An measurement of the increase in the wavelength of light often due to the Doppler effect. It is used as a means of more accurately predicting the distances to cosmic objects. Objects with higher observed redshifts are farther away than those with lower redshifts.

Roche Limit

- The altitude above a planet at which the difference in gravity between the end of an object closest to the planet and the end farthest from the planet is great enough to pull the object apart while not pulling the remains out of orbit.

Saturn

- The sixth planet from the Sun, and probably best known for its complex ring system which is visible through a small telescope. It is a gas giant.

Sol.

- See "the Sun".

Solar System

- The collection of planets, asteroids, comets, debris, etc. that are in orbit around our Sun.

Spectrum

- The distribution of energy emitted by a radiant source, as by an incandescent body, arranged in order of wavelengths such as radio, x-ray, light and gamma ray waves.

Star

- A large mass of gas held together by its own gravity which emits energy that is fueled by nuclear reactions. Ex. the Sun.

Star Cluster

- A group of stars held together by their mutual gravitational influences. Also see "open clusters" and "globular clusters". Ex. M44 - the Beehive Cluster.

Sun

- The star at the center of our solar system about which the Earth (and all other planets) orbit. Also called "Sol".

Supernova

- An immense explosion of a star in its later stages of life that may form a black hole or neutron star.

Telescope: (Astronomical)

- An instrument used by astronomers to explore space. They can be designed to detect light, radio or any other waves of the spectrum and can be built by professionals or amateurs.

Terminator

- The (imaginary) line separating the lighted and dark portions of a celestial body that is being lit by the Sun. Ex. The terminator line between illuminated and dark sides of the Moon as it proceeds through its monthly phases.

Terrestrial Planet

- Also known as a 'rocky planet'; one of the four inner planets: Mercury, Venus, Earth and Mars.

Tesla, Nikola

1856 - 1943

Theology

Titan

- The largest moon of Saturn discovered by Christian Huygens in 1655. It is larger than both Mercury and Pluto.

Transit

- The apparent movement of a smaller object across a larger object. Ex. A Mercury transit across the Sun.

Universe

- The entire conglomeration of everything. All known matter including all galaxies, stars, planets, etc...

Uranus

- The seventh planet from the Sun. It is a gas giant that also has a ring system, but it is not near as complex as Saturn's.

Van Allen Radiation Belts

- The belt of radiation around the Earth that shields us from harmful rays emitted by the Sun.

Venus

- The second planet from the Sun. It is a rocky (solid) planet and is always shrouded in cloud cover. It is also known as the "morning star" or "evening star" because it is usually the first "star" seen at night or the last visible at daybreak. Venus is also the planet with the highest surface temperature and largest temperature range due to it's rampant Greenhouse Effect

Variable Star

- A star that perceptibly changes its illumination. See "Cepheid variable".

White Dwarf

- A small, dense remnants of a sunlike star in its last phase of life. It shines from stored heat.

Zenith

- The point in the sky that is directly above the observer.