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The Greeks devised a rough and ready brightness scale for stars. The brightest stars were designated 'stars of the first magnitude', less bright stars were designated 'stars of the second magnitude', etc., down to stars of the sixth magnitude, which were barely visible to the naked eye. The magnitude scale is still in use, but modern telescopes and photometers have made it very much more exact and rigorous. A star of magnitude 1.0 is now 2.51188643 times as bright as a magnitude 2.0 star, which is 2.51188643 times as bright as a magnitude 3.0 star, and so on. This number has been chosen so that a magnitude 1.0 star is exactly 100 times as bright as a magnitude 6.0 star, i.e. 2.51188643 = 10<sup>0.4</sup>.
The Greeks devised a rough and ready brightness scale for stars. The brightest stars were designated 'stars of the first magnitude', less bright stars were designated 'stars of the second magnitude', etc., down to stars of the sixth magnitude, which were barely visible to the naked eye. The magnitude scale is still in use, but modern telescopes and photometers have made it very much more exact and rigorous. A star of magnitude 1.0 is now 2.51188643 times as bright as a magnitude 2.0 star, which is 2.51188643 times as bright as a magnitude 3.0 star, and so on. This number has been chosen so that a magnitude 1.0 star is exactly 100 times as bright as a magnitude 6.0 star, i.e. 2.51188643 = 10<sup>0.4</sup>.
The brightness of a star as seen from Earth depends on two things, how luminous it really is, and how far away it is. Stars vary greatly in their luminosity and their distance from Earth. The apparent brightness of a star varies according to the [[inverse-square law]]. Consider two stars that are equal in actual brightness, but one is at a distance of 100 [[light-years]] and the other is at a distance of 200 light-years. The farther star is twice as far away, but it will not appear to be half as bright, it will be only one quarter as bright. More generally, if one star is ''x'' times further away than the other it will appear to be ''x''<sup>2</sup> times less bright.
The brightness of a star as seen from Earth depends on two things, how luminous it really is, and how far away it is. Stars vary greatly in their luminosity and their distance from Earth. The apparent brightness of a star varies according to the [[inverse-square law]]. Consider two stars that are equal in actual brightness, but one is at a distance of 100 [[light year]]s and the other is at a distance of 200 light-years. The farther star is twice as far away, but it will not appear to be half as bright, it will be only one quarter as bright. More generally, if one star is ''x'' times further away than the other it will appear to be ''x''<sup>2</sup> times less bright.
==Variable stars==
==Variable stars==