Have you ever looked up at the sky and tried to find the brightest star? You may imagine that the number of stars you can see is countless. But the most that can be seen without a telescope is about 6.000 stars, and one-quarter of them are too far south to be seen in North America.
Ever since the days of the Greek astronomers 2.000 years ago, the stars have been divided into classes according to their magnitude or brightness. Until the invention of the telescope, only six magnitudes, or degrees of brightness, were recognized.
Stars of the first magnitude are the brightest, and stars of the sixth magnitude the faintest. Stars fainter than the sixth magnitude cannot be seen without a telescope.
Today, stars can be photographed with modern telescopes down to the 21st magnitude. A star of any given magnitude is about two and a half times fainter than a star of the magnitude above it.
There are 22 stars of the first magnitude, the brightest stars, and the brightest star of all is Sirius, which has a magnitude of 1,6. This makes Sirius over 1.000 times brighter than the faintest star that can be seen with the naked eye.
The lower we go down in magnitude, the more stars there are in that class. Thus, there are 22 stars of the 1st magnitude and about 1.000.000.000 stars of the 20th magnitude.
Read: What Is the Milky Way?