| Magnitude |
Most methods of counting and measuring things work logically. When the item you're measuring increases, the number gets larger. Star magnitudes however, don't follow this rule and count backwards the brighter the magnitude gets.
The Greek astronomer Hipparchus produced the first recognised star catalogue. He ranked his stars in a simple way. The brightest stars were "of the first magnitude," simply meaning "the biggest." Stars not so bright he called "of the second magnitude," second biggest. The faintest stars he could see were called "of the sixth magnitude." This system was copied by Ptolemy in his own catalogue of stars. Sometimes Ptolemy added the words "greater" or "smaller" to distinguish between stars that fell within a magnitude class.
Next came along Galileo, he made the first change to the system. On turning his new telescope to the sky, Galileo discovered that stars existed that were fainter than Ptolemy's sixth magnitude and the magnitude scale became open-ended.
As telescopes got bigger and better, astronomers kept adding more magnitudes to the bottom of the scale. Today a pair of 10X50 binoculars will show stars of about 9th magnitude, a 6-inch telescope will reveal objects to 13th, and the Hubble Space Telescope has seen objects as faint as 30th magnitude.
Astronomers had already determined that a 1st-magnitude star shines with about 100 times the light of a 6th-magnitude star. Hence a difference of five magnitudes is defined as a brightness ratio of exactly 100 to 1.
| Magnitudes | |
|---|---|
|
Difference |
ratio |
|
0
|
1 to 1
|
|
0.1
|
1.1 to 1 |
| 0.2 | 1.2 to 1 |
| 0.3 | 1.3 to 1 |
| 0.4 | 1.4 to 1 |
| 0.5 | 1.6 to 1 |
| 0.6 | 1.7 to 1 |
| 0.7 | 1.9 to 1 |
| 0.8 | 2.1 to 1 |
| 0.9 | 2.3 to 1 |
| 1.0 | 2.5 to 1 |
| 1.5 | 4.0 to 1 |
| 2 | 6.3 to 1 |
| 2.5 | 10 to 1 |
| 3 | 16 to 1 |
| 4 | 40 to 1 |
| 5 | 100 to 1 |
| 6 | 251 to 1 |
| 7.5 | 1,000 to 1 |
| 10 | 10,000 to 1 |
| 15 | 1,000,000 to 1 |
| 20 | 100,000,000 to 1 |
Now star magnitudes were ranked on a precise scale, however another problem became apparent. Some "1st-magnitude" stars were much brighter than others. Astronomers had no choice but to extend the scale to describe brighter values as well as fainter ones. Thus Rigel, Capella, Arcturus, and Vega are magnitude 0, an awkward statement that might sound like they have no brightness at all. It was too late to start the magnitude scale again so the scale was extended into negative numbers: Sirius shines at magnitude -1.5, Venus reaches -4.4, the full Moon is about -12.5, and the Sun blazes at magnitude -26.7. So a negative magnitude means a brighter object.