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| {{About|the brightness of stars|the science fiction magazine|Absolute Magnitude (magazine)}}
| | Jayson Berryhill is how I'm called and my wife doesn't like it at all. My day occupation is an invoicing officer but I've already utilized for another 1. To play domino is some thing I really enjoy doing. North Carolina is the location he enjoys most but now he is considering other options.<br><br>Here is my web site ... online psychic reading; [http://dunhua2015.com/protect/node/11444 linked here dunhua2015.com], |
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| '''Absolute magnitude''' is the measure of a celestial object's intrinsic brightness. It is the hypothetical [[apparent magnitude]] of an object at a standard [[luminosity distance]] of exactly 10.0 [[parsecs]] or about 32.6 [[light year]]s away from the [[Observation|observer]], assuming no [[Extinction (astronomy)|astronomical extinction]] of starlight. This allows the true energy output of astronomical objects to be compared without regard to their variable distances. As with all astronomical [[magnitude (astronomy)|magnitudes]], the absolute magnitude can be specified for different [[Filter (optics)#Bandpass|wavelength intervals]]; for stars the most commonly quoted absolute magnitude is the '''absolute visual magnitude''', which is the absolute magnitude in the visual (V) band of the [[UBV photometric system|UBV system]]. Also commonly used is the '''absolute bolometric magnitude''', which is the total [[luminosity]] expressed in magnitude units; it takes into account energy radiated at all wavelengths, whether observed or not.
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| The absolute magnitude uses the same conventions as the visual magnitude: brighter objects have smaller magnitudes, and 5 magnitudes corresponds exactly to a factor of 100, so a factor of 10<sup>0.4</sup> (≈2.512) ratio of [[brightness]] corresponds to a difference of 1.0 in magnitude. 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 our [[galaxy]]. If this particular quasar and our galaxy could be seen side by side at the same distance, the quasar would be 5 magnitudes (or 100 times) brighter than our galaxy. Similarly, [[Canopus]] has an absolute visual magnitude of about -5.5, while [[Ross 248]] has an absolute visual magnitude of +14.8, for a difference of slightly more than 20 magnitudes, so if the two stars were at the same distance, Canopus would be seen as about 20 magnitudes brighter; stated another way, Canopus gives off slightly more than 100 million (10<sup>8</sup>) times more visual power than Ross 248.
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| == Stars and galaxies (''M'') ==
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| In stellar and galactic astronomy, the standard distance is 10 parsecs (about 32.616 light years, 308.57 Petameters or 308.57 [[Orders_of_magnitude_(numbers)#1012|trillion]] kilometres).
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| A star at 10 parsecs has a [[parallax]] of 0.1" (100 milli [[minute of arc|arc seconds]]).
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| Galaxies (and other [[nebula|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 was 10 parsecs away.
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| In using an absolute magnitude one must specify the type of [[electromagnetic radiation]] being measured. When referring to total energy output, the proper term is [[bolometer|bolometric]] magnitude. The bolometric magnitude usually is computed from the visual magnitude plus a [[bolometric correction]], <math>M_{bol}=M_V+BC</math>. This correction is needed because very hot stars radiate mostly ultraviolet radiation, while very cool stars radiate mostly infrared radiation (see [[Planck's law]]).
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| 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 only 10 parsecs from the Earth: [[Rigel]] (−7.0), [[Deneb]] (−7.2), [[Zeta Puppis|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.<ref>{{Citation
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| | last = Cayrel de Strobel
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| | first = G.
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| | title = Stars resembling the Sun
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| | journal = Astronomy and Astrophysics Review
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| | volume = 7
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| | issue = 3
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| | pages = 243–288
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| | year = 1996
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| | doi = 10.1007/s001590050006
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| |bibcode = 1996A&ARv...7..243C }}
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| </ref>
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| <ref>
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| {{Citation
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| | last = Casagrande | first = L. | coauthors = Portinari, L., and Flynn, C.
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| |date=November 2006
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| | title = Accurate fundamental parameters for lower main-sequence stars
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| | journal = MNRAS | volume = 373 | issue = 1 | pages = 13–44
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| | doi =10.1111/j.1365-2966.2006.10999.x
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| | bibcode = 2006astro.ph..8504C
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| <!--| format = Abstract |arxiv = astro-ph/0608504-->}}
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| </ref>
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| 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]] [[Elliptical Galaxy M87|M87]] has an absolute magnitude of −22 (i.e. as bright as about 60,000 stars of magnitude −10).
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| === Computation ===
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| For a negligible [[Extinction (astronomy)|extinction]], one can compute the absolute magnitude <math>M\!\,</math> of an object given its apparent magnitude <math>m\!\,</math> and luminosity distance <math>D_L\!\,</math>:
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| :<math> M = m - 5 ((\log_{10}{D_L}) - 1)\!\,</math><!-- See the distance modulus article for support for this formula. -->
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| where <math>D_L\!\,</math> is the star's luminosity distance in parsecs, where 1 parsec is 206,265 [[astronomical unit]]s, approximately 3.2616 light-years. For very large distances, the [[cosmological redshift]] complicates the relation between absolute and apparent magnitude, because the radiation observed at one wavelength was radiated at a significantly different one. For comparing 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.
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| For nearby astronomical objects (such as stars in our galaxy) luminosity distance ''D<sub>L</sub>'' is almost identical to the real distance to the object, because spacetime within our galaxy is almost Euclidean. For much more distant objects the Euclidean approximation is not valid, and [[General Relativity]] must be taken into account when calculating the luminosity distance of an object.
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| In the Euclidean approximation for nearby objects, the absolute magnitude <math>M\!\,</math> of a star can be calculated from its apparent magnitude and parallax:
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| :<math> M = m + 5 (1 + \log_{10}{p})\!\,</math>
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| where p is the star's parallax in arcseconds.
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| You can also compute the absolute magnitude <math>M\!\,</math> of an object given its apparent magnitude <math>m\!\,</math> and [[distance modulus]] <math>\mu\!\,</math>:
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| :<math> M = m - {\mu}.\!\,</math>
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| ==== Examples ====
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| Rigel has a visual magnitude of <big><math>m_V = 0.12</math></big> and distance about 860 light-years
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| :<math>M_V = 0.12 - 5 \cdot (\log_{10} \frac{860}{3.2616} - 1) = -7.02.</math>
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| [[Vega]] has a parallax of 0.129", and an apparent magnitude of +0.03
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| :<math>M_V = 0.03 + 5 \cdot (1 +\log_{10}{0.129}) = +0.6.</math>
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| [[Alpha Centauri A]] has a parallax of 0.742" and an apparent magnitude of −0.01
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| :<math>M_V = -0.01 + 5 \cdot (1 +\log_{10}{0.742}) = +4.3.</math>
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| The [[Black Eye Galaxy]] has a visual magnitude of m<sub>V</sub>=+9.36 and a distance modulus of 31.06.
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| :<math>M_V = 9.36 - 31.06 = -21.7.</math>
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| === Apparent magnitude ===
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| {{Main|Apparent magnitude}}
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| Given the absolute magnitude <math>M\!\,</math>, for objects within our galaxy you can also calculate the apparent magnitude <math>m\!\,</math> from any distance <math>d\!\,</math> (in parsecs):
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| :<math> m = M - 5 (1-\log_{10}{d}).\!\,</math>
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| For objects at very great distances (outside our galaxy) the luminosity distance ''D<sub>L</sub>'' must be used instead of ''d'' (in parsecs).
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| Given the absolute magnitude <math>M\!\,</math>, you can also compute apparent magnitude <math>m\!\,</math> from its parallax <math>p\!\,</math>:
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| :<math> m = M - 5 ( 1+ \log_{10}p).\!\,</math>
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| Also calculating absolute magnitude <math>M\!\,</math> from distance modulus <math>\mu\!\,</math>:
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| :<math> m = M + {\mu}.\!\,</math>
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| === Bolometric magnitude ===
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| [[Bolometer|Bolometric]] magnitude corresponds to [[luminosity]], expressed in magnitude units; that is, after taking into account all [[electromagnetic radiation|electromagnetic]] wavelengths, including those unobserved due to instrumental pass-band, the Earth's atmospheric absorption, or extinction by interstellar dust. For stars, in the absence of extensive observations at many wavelengths, it usually must be computed assuming an [[effective temperature]].
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| The difference in bolometric magnitude is related to the luminosity ratio according to:
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| :<math>M_{bol_{\rm star}} - M_{bol_{\rm Sun}} = -2.5 \log_{10} {\frac{L_{\rm star}}{L_{\odot}}}</math>
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| which makes by inversion:
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| :<math>\frac{L_{\rm star}}{L_{\odot}} = 10^{((M_{bol_{\rm Sun}} - M_{bol_{\rm star}})/2.5)}</math>
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| where
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| :<math>L_{\odot}</math> is the Sun's (sol) luminosity (bolometric luminosity)
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| :<math>L_{\rm star}</math> is the star's luminosity (bolometric luminosity)
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| :<math>M_{bol_{\rm Sun}}</math> is the bolometric magnitude of the Sun
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| :<math>M_{bol_{\rm star}}</math> is the bolometric magnitude of the star.
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| {{anchor|Solar System|Solar system|Solar System bodies|Solar system bodies}}
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| == Solar System bodies (''H'') ==
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| For [[planets]] and [[asteroid]]s a different definition of absolute magnitude is used which is more meaningful for nonstellar objects.
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| 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 (astronomy)|phase angle]] and this relationship is referred to as the [[Phase curve (astronomy)|phase curve]].
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| 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. | |
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| === Apparent magnitude ===
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| The absolute magnitude can be used to help calculate the apparent magnitude of a body under different conditions.
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| :<math>m = H + 2.5 \log_{10}{\left(\frac{d_{BS}^2 d_{BO}^2}{p(\chi) d_0^4}\right)}\!\,</math>
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| where <math>d_0\!\,</math> is 1 au, <math>\chi\!\,</math> is the [[phase angle (astronomy)|phase angle]], the angle between the Sun–Body and Body–Observer lines. By the [[law of cosines]], we have:
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| :<math>\cos{\chi} = \frac{ d_{BO}^2 + d_{BS}^2 - d_{OS}^2 } {2 d_{BO} d_{BS}}.\!\,</math>
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| <math>p(\chi)\!\,</math> is the [[phase integral]] (integration of reflected light; a number in the 0 to 1 range).
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| Example: [[Lambertian diffuse lighting model|Ideal diffuse reflecting]] [[sphere]]. A reasonable first approximation for planetary bodies
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| :<math>p(\chi) = \frac{2}{3} \left( \left(1 - \frac{\chi}{\pi}\right) \cos{\chi} + \frac{1}{\pi} \sin{\chi} \right).\!\,</math>
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| A full-phase diffuse sphere reflects ⅔ as much light as a diffuse disc of the same diameter.
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| Distances:
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| * <math>d_{BO}\!\,</math> is the distance between the observer and the body
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| * <math>d_{BS}\!\,</math> is the distance between the Sun and the body
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| * <math>d_{OS}\!\,</math> is the distance between the observer and the Sun
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| Note: because Solar System bodies are never perfect diffuse reflectors, astronomers use empirically derived relationships to predict apparent magnitudes when accuracy is required.<ref>{{citation |url=http://www.shatters.net/forum/viewtopic.php?t=8160 |title=Planetary magnitudes Calculator - update |deadurl=no |accessdate=2013-05-16}}</ref>
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| ==== Example ====
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| Moon:
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| * <math>H_{Moon}\!\,</math> = +0.25
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| * <math>d_{OS}\!\,</math> = <math>d_{BS}\!\,</math> = 1 au
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| * <math>d_{BO}\!\,</math> = 384.5 Mm = 2.57 mau
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| How bright is the Moon from Earth?
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| * Full Moon: <math>\chi\!\,</math> = 0, (<math>p(\chi)\!\,</math> ≈ 2/3)
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| ** <math>m_{Moon} = 0.25 + 2.5 \log_{10}{\left(\frac{3}{2} 0.00257^2\right)} = -12.26\!\,</math>
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| ** (Actual −12.7) A full Moon reflects 30% more light at full phase than a perfect diffuse reflector predicts.
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| * Quarter Moon: <math>\chi\!\,</math> = 90°, <math>p(\chi) \approx \frac{2}{3\pi}\!\,</math> (if diffuse reflector)
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| ** <math>m_{Moon} = 0.25 + 2.5 \log_{10}{\left(\frac{3\pi}{2} 0.00257^2\right)} = -11.02\!\,</math>
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| ** (Actual approximately −11.0) The diffuse reflector formula does for smaller phases.
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| == Meteors ==
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| For a [[meteor]], the standard distance for measurement of magnitudes is at an altitude of {{convert|100|km|0|abbr=on}} at the observer's [[zenith]].<ref>{{citation | url=http://www.imo.net/glossary#term66 | contribution=Absolute magnitude of meteors | title=Glossary | publisher=International Meteor Organization |deadurl=no |accessdate=2013-05-16}}</ref><ref>{{citation | url=http://ssd.jpl.nasa.gov/?glossary&term=H | contribution=Absolute magnitude of Solar System bodies | publisher=NASA Jet Propulsion Laboratory | title=Solar System Dynamics Glossary |deadurl=no |accessdate=2013-05-16}}</ref>
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| == See also ==
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| * [[Photographic magnitude]]
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| * [[Hertzsprung–Russell diagram]] – Relates absolute magnitude or [[luminosity]] versus spectral color or surface [[temperature]].
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| * [[Jansky]] radio astronomer's preferred unit – linear in power/unit area
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| * [[Surface brightness]]- The ''magnitude'' for extended objects
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| * [[List of most luminous stars]]
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| == References==
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| {{Reflist}}
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| == External links ==
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| * [http://astro.pas.rochester.edu/~aquillen/ast142/costanti.html Reference zero-magnitude fluxes]
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| * [http://www.iau.org/ International Astronomical Union]
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| * [http://www.astronomynotes.com/starprop/s4.htm The Magnitude system]
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| * [http://csep10.phys.utk.edu/astr162/lect/stars/magnitudes.html About stellar magnitudes]
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| * [http://simbad.u-strasbg.fr/sim-fid.pl Obtain the magnitude of any star] – [[SIMBAD]]
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| * [http://www.minorplanetcenter.org/iau/lists/Sizes.html Converting magnitude of minor planets to diameter]
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| * [http://neo.jpl.nasa.gov/glossary/h.html Another table for converting asteroid magnitude to estimated diameter]
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| {{Star}}
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| {{DEFAULTSORT:Absolute Magnitude}}
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| [[Category:Observational astronomy]]
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