For our purposes, magnitude is apparent magnitude - the measure of the apparent brightness of a celestial object as seen by an observer on Earth. The lower the numerical value of magnitude, the brighter the object appears. Magnitude -1 is brighter than magnitude 0, which is in turn brighter than magnitude +1. The scale is logarithmic (as is the response of the eye to light), and a difference of 5 magnitudes means a brightness difference of exactly 100 times. A difference of one magnitude corresponds to a brightness difference of around 2.51 (the fifth root of 100).

This system was started by the ancient Greeks, who divided the stars into one of six magnitude groups, with stars of the first magnitude being the first ones to be visible after sunset. In modern times, the scale has been extended in both directions and more strictly defined. Normally the faintest object visible to the naked eye is of the 6th magnitude, but this is possible only under a very clear sky.

Example Magnitudes:
-26.80 Sun
-12.60 Full Moon
-04.60 Venus at brightest
-04.40 Faintest daylight object
-02.94 Jupiter at brightest
-02.90 Mars at brightest
-02.20 Mercury at brightest
-02.00 International Space Station
-01.45 Sirius (brightest star)
+00.43 Saturn at brightest
+03.00 Limit of human eye – urban area
+05.40 Uranus at brightest
+06.50 Limit of human eye – clear sky
+07.80 Neptune
+09.00 Limit of 10x50 binoculars
+12.60 Brightest quasar
+13.60 Pluto
+30.00 Limit of Hubble telescope
+38.00 Limit of OWL telescope

The Overwhelmingly Large Telescope (OWL) is a conceptual design by the European Southern Observatory organisation for a ground-based telescope intended to have a single aperture of 100 metres in diameter. It is hoped the telescope can be built by 2017 to 2020, with the ability to observe objects down to an apparent magnitude of +38, which is 1,000 times fainter than objects seen by the Hubble Space Telescope.

Apparent magnitude (m) should not be confused with Absolute magnitude (M). Absolute magnitude is a measurement of how bright objects actually are if distance was not a factor.

Reddish stars such as Antares are about 3,000ºC. Generally these stars are giant reds which are very old and have burned up most of their hydrogen. Young stars such as Vega are a bluish-white colour and burn at about 11,000ºC. Medium aged stars, such as our Sun, can be orange, yellow or white and burn at imtermediate temperatures between the two extremes.

Light travels at approximately 300,000 kilometres (186,000 miles) per second, which is just under 9,500,000,000,000 (9.5 trillion) kilometres per year. If a star is 1 million light years away, it takes 1 million years for that star's light to reach us. We see light that was created 1 million years ago, and the star as it was then, not today.

Around the year 5246BC a star in the constellation of Taurus exploded, about 6,300 light years from Earth. The explosion was eventually observed in 1054AD as a very bright supernova, which today has developed as the Crab Nebula. Our Sun is about 8 light minutes away, so any changes take 8 minutes to be seen. Our nearest star, other than the Sun, is in the Alpha Centauri system, 4.3 light years or 41 trillion kilometres away. The spiral galaxy in Andromeda is over 2 million light years away.