By some coincidence the brightest stars in the sky tend to appear in the winter. But how bright is bright? Or to put it another way, its obvious some stars are brighter than others; can we assign a value to the brightness so we can make comparisons between stars that might not be visible at the same time? The first person to do so was Hipparchus more than two millennia ago. He took the brightest twenty or so and said “these stars are of the first magnitude” the next hundred were “of the second magnitude” and so on. The faintest star visible in a dark sky at the zenith to a person of normal eyesight “is of the sixth magnitude.”
This worked well enough for many centuries; but when the telescope was invented millions of more stars were visible that were not covered by this scheme, this signaled that changes were required to the system. Then too, a star like Sirius was vastly brighter than the dimmest of Hipparchus’ first magnitude stars, Castor. This was a little uncomfortable also. With modern instruments it was determined the average first magnitude star was 2.5 times brighter than the average second magnitude star; and the average sixth magnitude star was one hundred times fainter than an average first magnitude star. As a mathematical convenience then, the five magnitude difference between +1.0 and +6.0 was defined to be exactly 100 and thus the difference between each magnitude was 2.512 (the fifth root of 100).
For many years the summer star Vega was the reference star and was assigned a magnitude of 0.0 as a starting point. Now that everything has been reduced to mathematical ratios one can now say Pollux has a magnitude of +1.15 while the dimmer Castor has been assigned a magnitude of +1.9 There is no mathematical reason to stop with positive numbers; Sirius has a magnitude of -1.45 and is thus 22 times brighter than Castor. It is very important to remember that the larger the magnitude the dimmer the star.
There is no reason to limit ourselves to stars when measuring magnitude. This morning Venus had a magnitude of -3.95, the just now rising Saturn has a magnitude of +0.84 and the asteroid Juno has a magnitude of +9.06 – which you now understand means its almost 16 times dimmer than the dimmest star than can be seen by the unaided eye. A pair of 7×50 binoculars could spot it though – 7×50’s enable you to see down to +9.5 magnitude objects or even +10.3 in the darkest skies.
The concept of magnitude is a vital one for serious stargazers, we will return to it next time. See you then!
Mr. Usher is President of the Everglades Astronomical Society which meets every second Tuesday at 7:00PM at the Norris Center, Cambier Park, Naples.