Speed of light 
300 000 km/sec. It takes light about one and a half seconds to reach the Moon. 

Light-year 
The distance light travels in one year, which is about 9 460 800 000 000 km. The light from the Sun takes about 8 minutes to reach Earth, and the light from the nearest star, Proxima Centaurus, takes over four years to reach us. The Hubble Space Telescope can see galaxies over 10 billion light years away. This also means you are seeing these galaxies as they where 10 billion years ago. So next time you are out in the Sun light, you'll know the light reaching you is already 8 minutes old. 

Astronomical Unit (AU) 
A unit of length equal to the average radius of the Earth's orbit (the average distance of the Earth from the Sun) 149 600 000 km. One light year = 63 300 AU. 

 Black hole 
A collapsed star with the mass greater than two Suns. The Black hole does not emit light because the speed of light can not escape the immense gravitational pull. 

Celestial Sphere 
An imaginary sphere of infinite radius, in the centre of which the observer is located, and against which all celestial bodies appear to be projected, similar to that of a planetarium. 

Constellation 
A defined part of the celestial sphere. The names boundaries of the constellations are mostly derived form ancient mythology, for example the constellation of Gemini. 

Zodiac 
An imaginary belt in the heavens, centred on the ecliptic and which encompasses the paths of the Sun, Moon and the planets. 

Ecliptic 
1. The apparent annual path of the Sun on the celestial sphere. The Sun transverses the whole ecliptic in one year. 
2. The path of the Earth's orbit round the Sun. 

Star 
Our Sun which is one star among billions in our galaxy, is self luminous as a result of thermonuclear reaction within the interior of the star. The same processes that are used so destructively in the explosion of a hydrogen bomb. 

Supernova 
An explosion of a star during which its brightness increases up to 10 thousand million times. The explosion occurs during the gravitational collapse of a star of above average mass, which eventually becomes a neutron star or possibly a pulsar. If the mass of the star is sufficient it may collapse and become a Black Hole. 

Pulsar 
A rapidly pulsating source of radio (and occasionally visible light or x-ray source) radiation. Pulsars are rapidly rotating neutron stars emitting radiation in a narrow beam like a light house. 

Galaxy 
A giant assembly of stars, gas, and dust into which most of the visible matter in the universe is concentrated. Our Galaxy the Milky Way, of which our Sun is only one star among 200 billion stars in our Milky Way. And the Milky Way is one galaxy among billions of galaxies in the Universe.