Absolute magnitude is the measure of intrinsic brightness of a celestial object. It is the hypothetical apparent magnitude of an object at a standard distance of exactly 10 parsecs (32.6 light years) from the observer, assuming no astronomical extinction of starlight. This places the objects on a common basis and allows the true energy output of astronomical objects to be compared without the distortion introduced by distance. As with all astronomical magnitudes, the absolute magnitude can be specified for different wavelength intervals; for stars the most commonly quoted absolute magnitude is the absolute visual magnitude, which uses only the visual (V) band of the spectrum (UBV system). Also commonly used is the absolute bolometric magnitude, which is the total luminosity expressed in magnitude units that takes into account energy radiated at all wavelengths, whether visible or not.
- Stars and galaxies M
- Apparent magnitude
- Bolometric magnitude
- Solar System bodies H
The brighter the celestial object the smaller its absolute magnitude. The magnitude scale extends downward through the positive numbers and into the negative numbers as brightness increases. A difference of 1.0 in absolute magnitude corresponds to a ratio of 100.4 ≈ 2.512 of absolute brightness. Therefore, a star of magnitude −2 is 100 (or 2.5125) times brighter than a star of magnitude +3. The Milky Way, for example, has an absolute magnitude of about −20.5, so a quasar with an absolute magnitude of −25.5 is 100 times brighter than the Milky Way. If this particular quasar and the Milky Way could be seen side by side at the same distance of one parsec and the Milky Way's stars reduced to a single point, the quasar would be 5 magnitudes (or 100 times) brighter than the Milky Way. Similarly, Canopus has an absolute visual magnitude of about −5.5, whereas Ross 248 has an absolute visual magnitude of +14.8, for a difference of about 20 magnitudes, i.e. Canopus would be seen as about 20 magnitudes brighter; stated another way, Canopus emits more than 100 million (108) times more visual power than Ross 248.
Stars and galaxies (M)
In stellar and galactic astronomy, the standard distance is 10 parsecs (about 32.616 light years, 308.57 petameters or 308.57 trillion kilometres). A star at 10 parsecs has a parallax of 0.1″ (100 milliarcseconds). Galaxies (and other extended objects) are much larger than 10 parsecs, their light is radiated over an extended patch of sky, and their overall brightness cannot be directly observed from relatively short distances, but the same convention is used. A galaxy's magnitude is defined by measuring all the light radiated over the entire object, treating that integrated brightness as the brightness of a single point-like or star-like source, and computing the magnitude of that point-like source as it would appear if observed at the standard 10 parsecs distance. Consequently, the absolute magnitude of any object equals the apparent magnitude it would have if it were 10 parsecs away.
The measurement of absolute magnitude is made with an instrument called a bolometer. When using an absolute magnitude, one must specify the type of electromagnetic radiation being measured. When referring to total energy output, the proper term is bolometric magnitude. The bolometric magnitude usually is computed from the visual magnitude plus a bolometric correction, Mbol = MV + BC. This correction is needed because very hot stars radiate mostly ultraviolet radiation, whereas very cool stars radiate mostly infrared radiation (see Planck's law).
Many stars visible to the naked eye have such a low absolute magnitude that they would appear bright enough to cast shadows if they were at 10 parsecs from the Earth: Rigel (−7.0), Deneb (−7.2), Naos (−6.0), and Betelgeuse (−5.6). For comparison, Sirius has an absolute magnitude of 1.4, which is brighter than the Sun, whose absolute visual magnitude is 4.83 (it actually serves as a reference point). The Sun's absolute bolometric magnitude is set arbitrarily, usually at 4.75. Absolute magnitudes of stars generally range from −10 to +17. The absolute magnitudes of galaxies can be much lower (brighter). For example, the giant elliptical galaxy M87 has an absolute magnitude of −22 (i.e. as bright as about 60,000 stars of magnitude −10).
The Greek astronomer Hipparchus established a numerical scale to describe the brightness of each star appeared in the sky. The brightest stars in the sky were assigned an apparent magnitude m = 1, and the dimmest stars visible to the naked eye are assigned m = 6. The difference between them corresponds to a factor of 100 in brightness. For objects within the Milky Way, the absolute magnitude M and apparent magnitude m from any distance d (in parsecs) is related by:
where F is the radiant flux measured at distance d (in parsecs), F10 the radiant flux measured at distance d = 10 pc. The relation can be written in terms of logarithm:
where the insignificance of extinction by gas and dust is assumed.
For objects at very large distances (outside the Milky Way) the luminosity distance dL must be used instead of d (in parsecs), because the Euclidean approximation is invalid for distant objects and general relativity must be taken into account. Moreover, the cosmological redshift complicates the relation between absolute and apparent magnitude, because the radiation observed was shifted into the red range of the spectrum. To compare the magnitudes of very distant objects with those of local objects, a K correction might have to be applied to the magnitudes of the distant objects.
The absolute magnitude M can also be approximated using apparent magnitude m and stellar parallax p:
or using apparent magnitude m and distance modulus μ:
Rigel has a visual magnitude mV of 0.12 and distance about 860 light-years
Vega has a parallax p of 0.129″, and an apparent magnitude mV of 0.03
Alpha Centauri A has a parallax p of 0.742″ and an apparent magnitude mV of −0.01
The Black Eye Galaxy has a visual magnitude mV of 9.36 and a distance modulus μ of 31.06
The bolometric magnitude Mbol, takes into account electromagnetic radiation at all wavelengths. It includes those unobserved due to instrumental pass-band, the Earth's atmospheric absorption, and extinction by interstellar dust. It is defined based on the luminosity of the stars. In the case of stars with few observations, it must be computed assuming an effective temperature.
Classically, the difference in bolometric magnitude is related to the luminosity ratio according to:
which makes by inversion:
whereL⊙ is the Sun's luminosity (bolometric luminosity) L★ is the star's luminosity (bolometric luminosity) Mbol,⊙ is the bolometric magnitude of the Sun Mbol,★ is the bolometric magnitude of the star.
In August 2015, the International Astronomical Union passed Resolution B2 defining the zero points of the absolute and apparent bolometric magnitude scales in SI units for power (watts) and irradiance (W/m2), respectively. Although bolometric magnitudes had been used by astronomers for many decades, there had been systematic differences in the absolute magnitude-luminosity scales presented in various astronomical references, and no international standardization. This led to systematic differences in bolometric corrections scales, which when combined with incorrect assumed absolute bolometric magnitudes for the Sun could lead to systematic errors in estimated stellar luminosities (and stellar properties calculated which rely on stellar luminosity, such as radii, ages, and so on).
Resolution B2 defines an absolute bolometric magnitude scale where Mbol = 0 corresponds to luminosity L0 = 7028301280000000000♠3.0128×1028 W, with the zero point luminosity L0 set such that the Sun (with nominal luminosity 7026382800000000000♠3.828×1026 W) corresponds to absolute bolometric magnitude Mbol,⊙ = 4.74. Placing a radiation source (e.g. star) at the standard distance of 10 parsecs, it follows that the zero point of the apparent bolometric magnitude scale mbol = 0 corresponds to irradiance f0 = 6992251802100199999♠2.518021002×10−8 W/m2. Using the IAU 2015 scale, the nominal total solar irradiance ("solar constant") measured at 1 astronomical unit (7003136100000000000♠1361 W/m2) corresponds to an apparent bolometric magnitude of the Sun of mbol,⊙ = −26.832.
Following Resolution B2, the relation between a star's absolute bolometric magnitude and its luminosity is no longer directly tied to the Sun's (variable) luminosity:
whereL★ is the star's luminosity (bolometric luminosity) in watts L0 is the zero point luminosity 7028301280000000000♠3.0128×1028 W Mbol is the bolometric magnitude of the star
The new IAU absolute magnitude scale permanently disconnects the scale from the variable Sun. However, on this SI power scale, the nominal solar luminosity corresponds closely to Mbol = 4.74, a value that was commonly adopted by astronomers before the 2015 IAU resolution.
The luminosity of the star in watts can be calculated as a function of its absolute bolometric magnitude Mbol as:
using the variables as defined previously.
Solar System bodies (H)
For planets and asteroids a definition of absolute magnitude that is more meaningful for nonstellar objects is used.
In this case, the absolute magnitude (H) is defined as the apparent magnitude that the object would have if it were one astronomical unit (AU) from both the Sun and the observer. Because the object is illuminated by the Sun, absolute magnitude is a function of phase angle and this relationship is referred to as the phase curve.
To convert a stellar or galactic absolute magnitude into a planetary one, subtract 31.57. A comet's nuclear magnitude (M2) is a different scale and can not be used for a size comparison with an asteroid's (H) magnitude.
The absolute magnitude can be used to help calculate the apparent magnitude of a body under different conditions.
where d0 is 1 AU, χ is the phase angle, the angle between the Sun–body and body–observer lines. By the law of cosines, we have:
p(χ) is the phase integral (integration of reflected light; a number in the 0 to 1 range).
Example: Ideal diffuse reflecting sphere. A reasonable first approximation for planetary bodies
A full-phase diffuse sphere reflects 2/3 as much light as a diffuse disc of the same diameter.
Note: because Solar System bodies are never perfect diffuse reflectors, astronomers use empirically derived relationships to predict apparent magnitudes when accuracy is required.
How bright is the Moon from Earth?
For a meteor, the standard distance for measurement of magnitudes is at an altitude of 100 km (62 mi) at the observer's zenith.