Astronmy Glossary
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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.