Apparent Magnitude
Under the modern logarithmic magnitude scale, two objects whose intensities (brightnesses) measured from Earth in units of power per unit area (such as Watts per square metre or Wm−2) are I1 and I2 and will have magnitudes m1 and m2 related by;
Using this formula, the magnitude scale can be extended beyond the ancient magnitude 1–6 range, and it becomes a precise measure of brightness rather than simply a classification system. Astronomers can now measure differences as small as one-hundredth of a magnitude. Stars between magnitudes 1.5 and 2.5 are called second-magnitude; there are some 20 stars brighter than 1.5, which are first-magnitude stars (see the list of brightest stars). For example, Sirius is magnitude −1.46, Arcturus is −0.04, Aldebaran is 0.85, Spica is 1.04, and Procyon (the little Dog) is 0.34. Under the ancient magnitude system, all of these stars might have been classified as "stars of the first magnitude".
Magnitudes can also be calculated for objects far brighter than stars (such as the Sun and Moon), and for objects too faint for the human eye to see (such as Pluto). What follows is a table giving magnitudes for objects ranging from the Sun to the faintest object visible with the Hubble Space Telescope (HST):
| Apparent magnitude |
Brightness relative to magnitude 0 |
Example | Apparent magnitude |
Brightness relative to magnitude 0 |
Example | Apparent magnitude |
Brightness relative to magnitude 0 |
Example | ||
|---|---|---|---|---|---|---|---|---|---|---|
| −27 | 6.3×1010 | Sun | −7 | 630 | SN 1006 supernova | 13 | 6.3×10−6 | 3C 273 quasar | ||
| −26 | 2.5×1010 | −6 | 250 | ISS (max) | 14 | 2.5×10−6 | Pluto (max) | |||
| −25 | 1.0×1010 | −5 | 100 | Venus (max) | 15 | 1.0×10−6 | ||||
| −24 | 4.0×109 | −4 | 40 | 16 | 4.0×10−7 | Charon (max) | ||||
| −23 | 1.6×109 | −3 | 16 | Jupiter (max), Mars (max) | 17 | 1.6×10−7 | ||||
| −22 | 6.3×108 | −2 | 6.3 | Mercury (max) | 18 | 6.3×10−8 | ||||
| −21 | 2.5×108 | −1 | 2.5 | Sirius | 19 | 2.5×10−8 | ||||
| −20 | 1.0×108 | 0 | 1.0 | Vega, Saturn (max) | 20 | 1.0×10−8 | ||||
| −19 | 4.0×107 | 1 | 0.40 | Antares | 21 | 4.0×10−9 | Callirrhoe (satellite of Jupiter) | |||
| −18 | 1.6×107 | 2 | 0.16 | Polaris | 22 | 1.6×10−9 | ||||
| −17 | 6.3×106 | 3 | 0.063 | Cor Caroli | 23 | 6.3×10−10 | ||||
| −16 | 2.5×106 | 4 | 0.025 | Acubens | 24 | 2.5×10−10 | ||||
| −15 | 1.0×106 | 5 | 0.010 | Vesta (max), Uranus (max) | 25 | 1.0×10−10 | Fenrir (satellite of Saturn) | |||
| −14 | 4.0×105 | 6 | 4.0×10−3 | typical limit of naked eye | 26 | 4.0×10−11 | ||||
| −13 | 1.6×105 | Full Moon | 7 | 1.6×10−3 | Ceres (max) | 27 | 1.6×10−11 | visible light limit of 8m telescopes | ||
| −12 | 6.3×104 | 8 | 6.3×10−4 | Neptune (max) | 28 | 6.3×10−12 | ||||
| −11 | 2.5×104 | 9 | 2.5×10−4 | 29 | 2.5×10−12 | |||||
| −10 | 1.0×104 | 10 | 1.0×10−4 | typical limit of 7x50 binoculars | 30 | 1.0×10−12 | ||||
| −9 | 4.0×103 | Iridium flare | 11 | 4.0×10−5 | 31 | 4.0×10−13 | ||||
| −8 | 1.6×103 | 12 | 1.6×10−5 | 32 | 1.6×10−13 | visible light limit of HST |
Read more about this topic: Magnitude (astronomy)
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