When we look up into the night sky on a very clear night we can see up to about 5000 stars with the naked eye. There are of course many more stars up there but they are too faint to see. On the very clearest night at a very dark location away from any town lights a person with good eyesight can see stars as faint as the 6th magnitude but what does this mean.
Astronomers measure the brightness of stars in units called magnitudes but this is not a unit like a meter or a kilogramme. Each magnitude is two and a half times brighter than the previous magnitude which is in turn is two and a half times brighter than the previous magnitude to that. The larger the magnitude number the dimmer the star will appear. Very bright stars have negative (minus) numbers. There are two kinds of magnitude measurements used :-
This is how bright a star appears to be in our sky.
This is how bright stars would appear if they were all the same distance away from us. The standard distance for measuring absolute magnitude is 10 parsecs or 32.6 Light Years.
-8 156250 x as bright as our sun (7 Cass -8)
-7 62500 x as bright as our sun (Rigel -7.1)
-6 25000 x as bright as our sun (Betelgeuse -5.6)
-5 10000 x as bright as our sun (Polaris -4.6)
-4 3800 x as bright as our sun (Antares -4.4)
-3 1525 x as bright as our sun (Spica -3.5)
-2 610 x as bright as our sun (Alcyone -1.6)
-1 244 x as bright as our sun (Regulus -0.6)
0 97 x as bright as our sun (Arcturus -0.2)
1 39 x as bright as our sun (Sirius 1.4)
2 15.6 x as bright as our sun (Alcor 2.1)
3 6.25 x as bright as our sun
4 2.5 x as bright as our sun (Alpha Cent 4.4)
5 1 Our sun has an Absolute magnitude of 4.8
6 2.5 x less bright than our sun
7 6.25 x less bright than our sun
8 15.6 x less bright than our sun
It can be seen that a star two magnitudes brighter than another star will be 2.5 x 2.5 = 6.25 times brighter. Three magnitudes will be 2.5 x 2.5 x 2.5 = 15.6 times brighter. So a star with a magnitude of 13 will be 156250 times fainter than a star of magnitude 0. Very bright stars have a magnitude less than 0 and therefore have negative magnitudes for example Sirius in Canis Major which is the brightest star visible from Britain, has an apparent magnitude of –1.47. Venus has a maximum apparent magnitude of –4.5 and the Sun is -27.
Astronomers who study variable stars, use stars of a known magnitude to estimate the changing brightness of the star they are studying. Experts in this field can judge the brightness of a variable star to less than a tenth of a magnitude.