Magnitude (astronomy) - Apparent Magnitude

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

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