by D. Vogt

Created on: June 15, 2010

Determining the number of stars in the sky is actually a surprisingly difficult problem, because it leads directly into a second problem: determining the size of the universe. The European Space Agency says there are between 10 sextillion and one septillion stars in the universe (respectively a number 1 followed by 22 zeroes, and a number 1 followed by 24 zeroes). The fact that one of the world's premiere space organizations is effectively admitting their estimate may be off by as much as a factor of one hundred indicates how unsure people are. However, an

Australian survey done in 2003, which tried to be more specific, came up on the low end of this spectrum: 70 sextillion (7 followed by 22 zeroes).

On any given clear, dark night, of course, the human eye cannot detect the vast majority of these stars. On a good night in a rural area, you might be able to make out as many as several thousand stars. In a city, where ubiquitous lights on the ground ruin night vision and obscure the sky, you would be lucky to make out a fraction of those, probably a few hundred of the brightest stars. However, this "number of stars" is somewhat misleading. Some of the apparent stars are actually distant galaxies. Others, including some of the bright ones, are actually two or more stars orbiting closely together. The brightest star visible from Earth, Sirius, is just such a binary system, consisting of one large white main-sequence star (brighter than our own Sun) and one white dwarf, the glowing ember of a dying star which has exhausted its hydrogen resources.

Simply counting the number of stars in our own Milky Way Galaxy is a more daunting task. Not even all of these stars can be found, even with powerful telescopes: when one peers directly along the galactic plane, gas and dust in the interstellar medium means it becomes impossible to make out actual stars beyond a distance of about 6000 light-years. Moreover, dim red dwarf stars would be hard to make out at significant distances, too, at least with currently operating technology. However, astronomers currently believe that the Milky Way Galaxy is home to around 100 billion stars, though the real number may be as high as 400 billion. 

Around the Milky Way Galaxy orbit a handful of so-called dwarf galaxies, small galaxies with only a fraction of the number of stars. However, among the major galaxies, the Milky Way Galaxy is at best average; our nearest neighbour, the Andromeda Galaxy, is several times as large. About

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