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The Greek astronomer named Hipparchus was able to devise a way to catalog approximately 850 stars visible to the unaided eye before the birth of Christ. He used a scale which assigned a magnitude to each star based on its brightness as seen from Earth, with the brightest stars having magnitudes of one and the ones barely able to be seen having magnitudes of six. With the advent of inventions, such as the telescope and the photometer, a newer scale of magnitude was developed. It was re-calibrated to show that a star that is magnitude two for example is more than two and a half times more luminous than a magnitude three star. Using this formula, that one magnitude is two and a half times brighter than the next, meant that the brightest objects in the heavens actually could have negative magnitudes.
Apparent
What Hipparchus developed was a system to determine apparent magnitudes, which means how bright the star appears to someone standing on Earth and looking at it in the night sky. With the upgrades to his system, the Sun was eventually given a magnitude of minus 26.72, with the Moon having one of minus 12.5. The brightest star in the night sky is Sirius in the constellation Canis Major, which stands at minus 1.46. Only Canopus in Carina, Alpha Centauri in Centaurus, and Arcturus in Bootes are rated as being bright enough to earn negative magnitude values.
Absolute
As astronomy began to unlock some of the secrets of space, it was obvious that the brightness of stars depended on factors, such as how much energy it produces and how far it is from the Earth. Sirius for example is one of the closest stars to us at "just" a little more than eight and half light years from Earth, which allows it to appear as bright as it does in the sky. Astronomers finally decided that a star should also have an absolute magnitude and came up with the idea to rate the star's brightness as it would look from a distance of 10 parsecs away. In such a case Sirius would decrease drastically in luminosity, coming in with an absolute magnitude of plus 1.4.
Distance
A light year is a term astronomers needed to come up with to make the incredibly vast distances between objects in space more readily understandable. It is simply the distance that light would travel from one point to another in the course of a full year. With light traveling at 186,000 miles per second it would go 5,880,000,000,000 miles in one year. The parsec is 3.26 light years, making 10 parsecs 32.6 light years away.
Sun
Putting the Sun at 10 parsecs from Earth instead of the 93,000,000 miles away that it actually is would make it seem as if it were a fourth magnitude star. With an absolute magnitude of plus 4.8, the Sun is an average star in terms of brightness. It would be easily visible to a person on Earth but would by no means standout at all in the darkness.