Apparent magnitude (m) is how bright a star appears as seen from Earth. In general, for stars of the same type and class, the farther away the star is from Earth, the dimmer it appears. Using the apparent magnitude, a dimmer star will have a larger apparent magnitude.
- Vega was originally chosen as the zero point for apparent magnitude. After more measurements, Vega has an apparent magnitude of +0.3.
- A difference in magnitudes of 5 is equal to a difference in brightness by a factor of 100
- A star with m=1.0 is 100 times brighter than a star with m=6.0
- Then for a difference in magnitude of 1, a star with m=1.0 is 2.512 times brighter than a star with m=2.0 (2.512 is the fifth root of 100)
- Apparent magnitudes (as seen from Earth) for some common objects
- the Sun: m=-26.74 (obviously the brightest thing in the sky)
- Sirius (the brightest non-Sun star in the sky): m=-1.47
- Proxima Centauri (the closest star to the Sun): m=5.35 (too faint to see without the aid of a telescope or binoculars)
- Alpha Centauri A (the brightest star in the triple star system Alpha Centauri (including Proxima Centauri): -0.01
- Betelgeuse (Red Supergiant in Orion): 0.42
- Rigel (Blue Supergiant in Orion): 0.13
- Venus (the brightest planet at maximum): -4.89
- Full Moon (at perigee): -12.92
Absolute magnitude (M) is the standard for all stars. It is how bright a star would be at 10 parsecs (approximately 32.7 light years from the Sun). It is measured the same way as apparent magnitude with a difference of 5 equating to a factor of 100 in brightness.
- Absolute Magnitudes for some common stars:
- the Sun: M=4.83
- Rigel: M=-7.0
- Deneb (in Cygnus): M=-7.2
- Betelgeuse: M=-5.6
- Sirius: 1.4
- Absolute magnitudes are a measure of a stars luminosity as well, and can be used on the y-axis of the HR Diagram.
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