In astronomy, magnitude is
the logarithmic measure of the brightness of an object, measured in a specific wavelength
or passband, usually in the visible or near-infrared spectrum. An imprecise but
systemic determination of the magnitude of objects was introduced in ancient
times by Hipparchus.
Astronomers use two different definitions of magnitude: apparent magnitude and absolute magnitude. The apparent magnitude (m, or vmag for the visible spectrum) is the brightness of an object as it appears in the night sky from Earth, while the absolute magnitude (Mv, V and H) describes the intrinsic brightness of an object as it would appear if it were placed at a certain distance from Earth. This distance is 10 parsecs for stars and 1 astronomical unit for asteroids and planets. The size of an asteroid is typically estimated based on its absolute magnitude.
The brighter an object appears, the lower the value of its magnitude, with the brightest objects reaching negative values. The Sun has an apparent magnitude of −27, the full moon −13, the brightest planet Venus measures −5, and Sirius, the brightest visible star in the night sky is at −1.5. An apparent magnitude can also be assigned to man-made objects in Earth orbit. The brightest satellite flares are ranked at −9, and the International Space Station appears at a magnitude of −6. Since the scale is logarithmic, each step of one magnitude changes the brightness by a factor of about 2.512. A magnitude 4 star is exactly a hundred times brighter than a magnitude 9 star, as the difference of five magnitude steps corresponds to (2.512)5 or 100.
Two of the main types of magnitudes distinguished by astronomers are:
Usually only apparent magnitude is mentioned since it can be measured directly.
The human eye is easily fooled, and Hipparchus's scale has had problems. For example, the human eye is more sensitive to yellow/red light than to blue, and photographic film more to blue than to yellow/red, giving different values of visual magnitude and photographic magnitude. Apparent magnitude can also be affected by factors such as dust in the atmosphere or light cloud cover absorbing some of the light.
Furthermore, many people find it counter-intuitive that a high magnitude star is dimmer than a low magnitude star.
Astronomers use two different definitions of magnitude: apparent magnitude and absolute magnitude. The apparent magnitude (m, or vmag for the visible spectrum) is the brightness of an object as it appears in the night sky from Earth, while the absolute magnitude (Mv, V and H) describes the intrinsic brightness of an object as it would appear if it were placed at a certain distance from Earth. This distance is 10 parsecs for stars and 1 astronomical unit for asteroids and planets. The size of an asteroid is typically estimated based on its absolute magnitude.
The brighter an object appears, the lower the value of its magnitude, with the brightest objects reaching negative values. The Sun has an apparent magnitude of −27, the full moon −13, the brightest planet Venus measures −5, and Sirius, the brightest visible star in the night sky is at −1.5. An apparent magnitude can also be assigned to man-made objects in Earth orbit. The brightest satellite flares are ranked at −9, and the International Space Station appears at a magnitude of −6. Since the scale is logarithmic, each step of one magnitude changes the brightness by a factor of about 2.512. A magnitude 4 star is exactly a hundred times brighter than a magnitude 9 star, as the difference of five magnitude steps corresponds to (2.512)5 or 100.
Apparent and Absolute
Magnitude
Two of the main types of magnitudes distinguished by astronomers are:
- Apparent magnitude, the brightness of an
object as it appears in the night sky. For example, Alpha Centauri has
higher apparent magnitude (i.e. lower value) than Betelgeuse, because it
is much closer to the Earth.
- Absolute magnitude, which measures the luminosity
of an object (or reflected light for non-luminous objects like asteroids);
it is the object's apparent magnitude as seen from a specific distance.
For stars it is 10 parsecs (32.6 light years). Betelgeuse has much higher
absolute magnitude than Alpha Centauri, because it is much more luminous.
Usually only apparent magnitude is mentioned since it can be measured directly.
Problems
The human eye is easily fooled, and Hipparchus's scale has had problems. For example, the human eye is more sensitive to yellow/red light than to blue, and photographic film more to blue than to yellow/red, giving different values of visual magnitude and photographic magnitude. Apparent magnitude can also be affected by factors such as dust in the atmosphere or light cloud cover absorbing some of the light.
Furthermore, many people find it counter-intuitive that a high magnitude star is dimmer than a low magnitude star.
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