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Glossary

GLOSSARY

Absolute Magnitude
A figure that indicates the true light output, or luminosity, of a star. It is the magnitude that the star would appear if it were placed at a standard distance, chosen as 10 parsecs (32.6 light-years).

Altitude
The angular distance of an object above the horizon, in degrees.

Big Bang Theory
The explosive event that marked the origin of the Universe about 13.7 billion years ago.

Binary Star
A pair of stars linked by gravity, orbiting around their common centre of mass.

Chromosphere
A layer of gas above the Sun's visible surface, or photosphere. The cromosphere can be seen only when the brighter photosphere is blocked out.

Comet
A small body consisting of ice and dust. When far from the Sun, a comet is frozen solid. Closer to the Sun, it warms up and releases dust and gas to form a large glowing head(the coma) and sometimes a tail.

Doppler Effect
The change in the frequency of waves (of sound or radiation) that reach an observer when the source is moving closer or further away.

Declination
The angular distance of an object north or south of the celestial equator, measured in degrees. It is equivalent of latitude on Earth.

Eclipse
Effect caused by one celestial object casting a shadow on another. A lunar eclipse happens when Earth's shadow falls on the Moon. A solar eclipse occurs when the Moon passes between Earth ad the Sun, casting the shadow on Earth.

Emission Line
Bright line in a spectrum caused by atoms giving out energy of a certain wavelength. Hot gas in a nebula often produces emission lines.

Filter
Thin material that is transparent to only a narrow range of wavelength of light.

Flares
A sudden eruption of energy on the solar disk lasting minutes to hours, from which radiation and particles are emitted.

Galaxy
A mass of stars held together by gravity. Galaxies are two main types: spirals, which have arms; and ellipticals, which do not. Diameters of galaxies range from about a thousand light-years to hundreds of thousands of light-years.

Granulation
Bright spots of convection on the Sun's surface 700 to 1000 km across forming a honeycomb pattern. Formed from hot. Bright gas rising from below in the center of a granule and cooler, dimmer gas falling back down at the edge of a granule.

Horizon
The horizon is an imaginary circle that delimits the sky and the Earth, or an extension of the plane of the observer (at an altitude of 0 degree).

Hubble Law
The relationship between a galaxy's recession speed from other galaxies and the distance between them: the recession speed = H x distance, where H is the Hubble constant. The recession speed is derived from redshift of the galaxy spectra and with the Hubble Law, it can be used to find the distance to the farthest galaxies.

Ionosphere
Electrically charged region of the Earth's atmosphere between 50 and 600 km above the surface.

Isotope
A sub-group of an element in which the atomic nucleus has the same number of neutrons, as well as, the same number protons. All of the atoms of an element will have very nearly the same chemical properties, but the isotope can have very different nuclear properties.

Joule
A joule is the amount of work dome by a force of a Newton acting through one meter.

Jovian Planet
Any of the four biggest planets: Jupiter, Saturn, Uranus and Neptune.

Kiloparsec (kpc)
A distance of one thousand parsecs.

Kinetic Energy
The energy something has because of its motion = ½ x mass x velocity2.

Light-year
The distance covered by a beam of light in a calender year, 9460 billion km (5878 billion miles)

Lysithea
Lysithea is Jupiter's eleventh moon. Lysithea is 15 miles (24km) in diameter and orbits 7,200,000 miles (11,720,000 km) from Jupiter. Lysithea has a mass of 8 x 1016 kg. It orbits Jupiter in 259,22 (Earth) days. Very little is known about Lysithea. Lysithea was discovered by S. Nicholson in 1938.

Magnitude
Used to quantify brightness. Based on the ancient system of Hipparchus but refined and quantified for measurements today such that a ratio of 100 in brightness corresponds to a magnitude difference of 5. Fainter objects have larger, positive magnitudes (closer to positive infinity), while brighter objects have lower magnitudes (closer to negative infinity).

Magnetometer
An instrument used to measure the strength and direction of a magnetic field.

Neutron Star
Collapsed star composed mainly of neutrons - the most common aftermath of a supernova explosion.

Nova
Faint star that suddenly becomes thousands of times brighter. It is a white dwarf star in a binary system that pulls material off its companion star and collects an atmosphere. It flares up when nuclear processes take place in this atmosphere and release energy.

Observatory
Place where astronomers make observations of celestial objects and phenomena.

Opposition
The occasion when a body in the Solar System lies in the opposite direction to the Sun, as seen from Earth.

Parsec
Distance at which a star or other object has a parallax of 1 arc second, equivalent to 3.26 light years.

Pulsar
A neutron star that emits pulses of radio waves and other radiation as it spins.

Quasar
The highly luminous core of a distant galaxy, thought to be caused by ultra-hot gas circulating around a massive black hole at the galaxy's centre.

Quarter Moon
A quarter moon looks like half a circle. This moon has completed one quarter of an orbit around the Earth from its full or new position, and one quarter of the moon's surface is visible from Earth. It is also called half moon.

Red Giant
A star that has become larger and cooler as it nears the end of its life.

Red Shift
A shift in the lines in a spectrum towards longer wavelengths, caused by the movement of the emitting object away from us.

Satellite
A body that's orbits another, usually a moon of a planet.

Solar wind
A stream of atomic particles from the Sun, mostly protons and electrons, which flows outwards through the Solar System.

Telescope
Device used to gather and focus electromagnetic radiation. A telescope extends the power of human vision by making objects brighter, sharper and larger, as well as, imaging objects in wavelengths that are not detectable by the human eye.

Terminator
The line separating the illuminated and illuminated parts of a celestial body; the dividing line between day and night as observed from a distance.

Universe
Everything that exists, including all matter, space and time. The Universe is thought to have begun in a Bing Bang about 13.7 billion years ago.

Umbra
Region of total shadow; the light source is totally blocked- The umbra is the inner dark, cool (3700K = 6600oF = 3400 oC) region of a sunspot. The umbra of a sunspot can be up to 12,000 miles (20,000 km) wide. In the umbra, the Sun's magnetic field is very strong.

Variable Star
Any star that appears to change in brightness.

Vesta
The brightest of all minor planets, at times approaching naked-eye visibility. It was discovered by Olbers in 1807.

White Dwarf
A small, dense star with a mass similar to that of the Sun, but only about the diameter of the Earth. White dwarfs are the shrunken remains of the stars such as the Sun that have burnt out.

Winter Solstice
The solstices are the days when the Sun reaches its farthest northen and southern declinations. The winter solstice occurs on Dec 21 and marks the beginning of winter (this is the shortest day of the year) in The Northern Hemisphere. The summer solstice occurs on June 21 and marks the beginning of summer (this is the longest day of the year) in the Northern Hemisphere.

X-Ray Binary Star
X-ray binary star are a special type of binary star in which one of the stars is a collapsed object such as white dwarf, neutron star or black hole. As matter is stripped from the normal star, it falls into the collapsed star, producing X-rays.

Zodiac
The band of sky either side of the ecliptic through which the Sun and planets move.

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