Editing Absolute magnitude

{{Short description|Measure of the luminosity of celestial objects}}
{{About|the brightness of stars|the science fiction magazine|Absolute Magnitude (magazine)}}
{{Use dmy dates|date=April 2020}}

In [[astronomy]], '''absolute magnitude''' ('''{{mvar|M}}''') is a measure of the [[luminosity]] of a [[celestial object]] on an inverse [[Logarithmic scale|logarithmic]] [[Magnitude (astronomy)|astronomical magnitude]] scale; the more luminous (intrinsically bright) an object, the lower its magnitude number. An object's absolute magnitude is defined to be equal to the [[apparent magnitude]] that the object would have if it were viewed from a distance of exactly {{convert|10|pc|ly|1|abbr=off|lk=on}}, without [[Extinction (astronomy)|extinction]] (or dimming) of its light due to absorption by [[Interstellar medium|interstellar matter]] and [[cosmic dust]]. By hypothetically placing all objects at a standard reference distance from the observer, their luminosities can be directly compared among each other on a magnitude scale. For [[Solar System]] bodies that shine in reflected light, a different definition of [[Absolute magnitude#Solar System bodies (H)|absolute magnitude (H)]] is used, based on a standard reference distance of one [[astronomical unit]].

Absolute magnitudes of stars generally range from approximately −10 to +20. The absolute magnitudes of galaxies can be much lower (brighter).

The more luminous an object, the smaller the numerical value of its absolute magnitude. A difference of 5 magnitudes between the absolute magnitudes of two objects corresponds to a ratio of 100 in their luminosities, and a difference of n magnitudes in absolute magnitude corresponds to a luminosity ratio of 100<sup>n/5</sup>. For example, a star of absolute magnitude M<sub>V</sub> = 3.0 would be 100 times as luminous as a star of absolute magnitude M<sub>V</sub> = 8.0 as measured in the V filter band. The [[Sun]] has absolute magnitude M<sub>V</sub> = +4.83.<ref name="SunAbs"/> Highly luminous objects can have negative absolute magnitudes: for example, the [[Milky Way]] galaxy has an absolute [[UBV photometric system|B magnitude]] of about −20.8.<ref name="Karachentsev"/>

As with all astronomical [[magnitude (astronomy)|magnitudes]], the absolute magnitude can be specified for different [[wavelength]] ranges corresponding to specified [[Filter (optics)|filter]] bands or [[passband]]s; for stars a commonly quoted absolute magnitude is the '''absolute visual magnitude''', which uses the visual (V) band of the spectrum (in the [[UBV photometric system]]). Absolute magnitudes are denoted by a capital M, with a subscript representing the filter band used for measurement, such as M<sub>V</sub> for absolute magnitude in the V band.

An object's absolute ''bolometric'' magnitude (M<sub>bol</sub>) represents its total [[luminosity]] over all [[wavelengths]], rather than in a single filter band, as expressed on a logarithmic magnitude scale. To convert from an absolute magnitude in a specific filter band to absolute bolometric magnitude, a [[bolometric correction]] (BC) is applied.<ref name="Flower1996"/>

==Stars and galaxies==
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). A star at 10 parsecs has a [[parallax]] of 0.1″ (100 [[minute of arc|milliarcseconds]]). 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 were 10 parsecs away.

Some stars visible to the naked eye have such a low absolute magnitude that they would appear bright enough to outshine the [[planet]]s and cast shadows if they were at 10 parsecs from the Earth. Examples include [[Rigel]] (−7.8), [[Deneb]] (−8.4), [[Zeta Puppis|Naos]] (−6.2), and [[Betelgeuse]] (−5.8). For comparison, [[Sirius]] has an absolute magnitude of only 1.4, which is still brighter than the [[Sun]], whose absolute visual magnitude is 4.83. The Sun's absolute bolometric magnitude is set arbitrarily, usually at 4.75.<ref name="Strobel"/><ref name="Casagrande"/>
Absolute magnitudes of stars generally range from approximately −10 to +20. 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). Some [[active galactic nuclei]] ([[quasars]] like [[CTA-102]]) can reach absolute magnitudes in excess of −32, making them the most luminous persistent objects in the observable universe, although these objects can vary in brightness over astronomically short timescales. At the extreme end, the optical afterglow of the gamma ray burst [[GRB 080319B]] reached, according to one paper, an absolute [[Photometric system|r magnitude]] brighter than −38 for a few tens of seconds.<ref>{{Cite journal|last1=Bloom|first1=J. S.|last2=Perley|first2=D. A.|last3=Li|first3=W.|last4=Butler|first4=N. R.| last5=Miller|first5=A. A.|last6=Kocevski|first6=D.|last7=Kann|first7=D. A.|last8=Foley|first8=R. J.|last9=Chen|first9=H.-W.| last10=Filippenko|first10=A. V.|last11=Starr|first11=D. L.|title=Observations of the Naked-Eye GRB 080319B: Implications of Nature's Brightest Explosion|date=2009-01-19|journal=The Astrophysical Journal|language=en| volume=691|issue=1| pages=723–737| doi=10.1088/0004-637x/691/1/723|arxiv=0803.3215|bibcode=2009ApJ...691..723B|issn=0004-637X|doi-access=free}}</ref>

=== Apparent magnitude ===
{{Main|Apparent magnitude}}

The Greek astronomer [[Hipparchus]] established a numerical scale to describe the brightness of each star appearing in the sky. The brightest stars in the sky were assigned an apparent magnitude {{math|1=''m'' = 1}}, and the dimmest stars visible to the naked eye are assigned {{math|1=''m'' = 6}}.<ref name="Carroll"/> The difference between them corresponds to a factor of 100 in brightness. For objects within the immediate neighborhood of the Sun, the absolute magnitude {{mvar|M}} and apparent magnitude {{mvar|m}} from any distance {{mvar|d}} (in [[parsec]]s, with 1 pc = 3.2616 [[light-year]]s) are related by
<math display="block"> 100^{\frac{m-M}{5}}=\frac{F_{10}}{F} = \left(\frac{d}{10\;\mathrm{pc}}\right)^{2}, </math>
where {{mvar|F}} is the radiant flux measured at distance {{mvar|d}} (in parsecs), {{math|''F''<sub>10</sub>}} the radiant flux measured at distance {{math|10 pc}}. Using the [[common logarithm]], the equation can be written as
<math display="block"> M = m - 5 \log_{10}(d_\text{pc})+5 = m - 5 \left(\log_{10}d_\text{pc}-1\right),</math>
where it is assumed that [[Extinction (astronomy)|extinction from gas and dust]] is negligible. Typical extinction rates within the [[Milky Way]] galaxy are 1 to 2 magnitudes per kiloparsec, when [[Dark nebula|dark clouds]] are taken into account.<ref name="Unsoeld2013"/>

For objects at very large distances (outside the Milky Way) the luminosity distance {{math|''d''<sub>L</sub>}} (distance defined using luminosity measurements) must be used instead of {{mvar|d}}, because the [[Euclidean space|Euclidean]] approximation is invalid for distant objects. Instead, [[general relativity]] must be taken into account. Moreover, the [[cosmological redshift]] complicates the relationship 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 {{mvar|M}} can also be written in terms of the apparent magnitude {{mvar|m}} and [[stellar parallax]] {{mvar|p}}:
<math display="block"> M = m + 5 \left(\log_{10}p+1\right),</math>
or using apparent magnitude {{mvar|m}} and [[distance modulus]] {{mvar|μ}}:
<math display="block"> M = m - \mu.</math>

==== Examples ====
[[Rigel]] has a visual magnitude {{math|''m''<sub>V</sub>}} of 0.12 and distance of about 860 light-years:
<math display="block">M_\mathrm{V} = 0.12 - 5 \left(\log_{10} \frac{860}{3.2616} - 1 \right) = -7.0.</math>

[[Vega]] has a parallax {{mvar|p}} of 0.129″, and an apparent magnitude {{math|''m''<sub>V</sub>}} of 0.03:
<math display="block">M_\mathrm{V} = 0.03 + 5 \left(\log_{10}{0.129} + 1\right) = +0.6.</math>

The [[Black Eye Galaxy]] has a visual magnitude {{math|''m''<sub>V</sub>}} of 9.36 and a distance modulus {{mvar|μ}} of 31.06:
<math display="block">M_\mathrm{V} = 9.36 - 31.06 = -21.7.</math>

=== Bolometric magnitude ===
{{See also|Apparent bolometric magnitude}}

The absolute [[Bolometer|bolometric]] magnitude ({{math|''M''<sub>bol</sub>}}) takes into account [[electromagnetic radiation]] at all [[wavelengths]]. It includes those unobserved due to instrumental [[passband]], the Earth's atmospheric absorption, and [[Extinction (astronomy)|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:<ref name="Carroll"/>
<math display="block">M_\mathrm{bol,\star} - M_\mathrm{bol,\odot} = -2.5 \log_{10} \left(\frac{L_\star}{L_\odot}\right)</math>
which makes by inversion:
<math display="block">\frac{L_\star}{L_\odot} = 10^{0.4\left(M_\mathrm{bol,\odot} - M_\mathrm{bol,\star}\right)}</math>
where
*{{math|''L''<sub>⊙</sub>}} is the Sun's luminosity (bolometric luminosity)
*{{math|''L''<sub>★</sub>}} is the star's luminosity (bolometric luminosity)
*{{math|''M''<sub>bol,⊙</sub>}} is the bolometric magnitude of the Sun
*{{math|''M''<sub>bol,★</sub>}} is the bolometric magnitude of the star.

In August 2015, the [[International Astronomical Union]] passed Resolution B2<ref name="IAU_XXIX"/> defining the [[Zero Point (photometry)|zero points]] of the absolute and apparent [[bolometric magnitude]] scales in SI units for power ([[watt]]s) and irradiance (W/m<sup>2</sup>), 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.<ref name="IAU2015B2"/> Combined with incorrect assumed absolute bolometric magnitudes for the Sun, this could lead to systematic errors in estimated stellar luminosities (and other stellar properties, such as radii or ages, which rely on stellar luminosity to be calculated).

Resolution B2 defines an absolute bolometric magnitude scale where {{math|1=''M''<sub>bol</sub> = 0}} corresponds to luminosity {{math|1=''L''<sub>0</sub> = {{val|3.0128|e=28|u=W}}}}, with the zero point [[luminosity]] {{math|''L''<sub>0</sub>}} set such that the Sun (with nominal luminosity {{val|3.828|e=26|u=W}}) corresponds to absolute [[bolometric magnitude]] {{math|1=''M''<sub>bol,⊙</sub> = 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 {{math|''m''<sub>bol</sub> {{=}} 0}} corresponds to [[irradiance]] {{math|1=''f''<sub>0</sub> = {{val|2.518021002|e=-8|u=W/m<sup>2</sup>}}}}. Using the IAU 2015 scale, the nominal total [[solar irradiance]] ("[[solar constant]]") measured at 1 [[astronomical unit]] ({{val|1361|u=W/m<sup>2</sup>}}) corresponds to an apparent bolometric magnitude of the [[Sun]] of {{math|1=''m''<sub>bol,⊙</sub> = −26.832}}.<ref name="IAU2015B2" />

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:
<math display="block">M_\mathrm{bol} = -2.5 \log_{10} \frac{L_\star}{L_0} \approx -2.5 \log_{10} L_\star + 71.197425</math>
where
*{{math|''L''<sub>★</sub>}} is the star's luminosity (bolometric luminosity) in [[watt]]s
*{{math|''L''<sub>0</sub>}} is the zero point luminosity {{val|3.0128|e=28|u=W}}
*{{math|''M''<sub>bol</sub>}} 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 {{math|1=''M''<sub>bol</sub> = 4.74}}, a value that was commonly adopted by astronomers before the 2015 IAU resolution.<ref name="IAU2015B2" />

The luminosity of the star in watts can be calculated as a function of its absolute bolometric magnitude {{math|''M''<sub>bol</sub>}} as:
<math display="block">L_\star = L_0 10^{-0.4 M_\mathrm{bol}}</math>
using the variables as defined previously.

{{anchor|Solar System|Solar system|Solar System bodies|Solar system bodies}}

== Solar System bodies ({{mvar|''H''}}) ==
{{anchor|Solar System bodies (H)}}<!-- don't change title unless link is also changed in Template:Infobox_planet -->
{{For|an introduction|Magnitude (astronomy)}}

{| class="wikitable floatright" style=" text-align:center; font-size:0.9em;"
|+ Abs Mag (H)<br />and Diameter<br />for asteroids<br />([[Albedo#Astronomical albedo|albedo]]=0.14)<ref>[https://cneos.jpl.nasa.gov/tools/ast_size_est.html CNEOS Asteroid Size Estimator]</ref>
! H !! Diameter
|-
| 10 || 36&nbsp;km
|-
| 12.7 || 10&nbsp;km
|-
| 15 || 3.6&nbsp;km
|-id=1km
| 17.7 || 1&nbsp;km
|-
| 19.2 || 510&nbsp;m
|-
| 20 || 360&nbsp;m
|-id=PHA
| 22 || 140&nbsp;m
|-
| 22.7 || 100&nbsp;m
|-
| 24.2 || 51&nbsp;m
|-
| 25 || 36&nbsp;m
|-id=Chelyabinsk
| 26.6 || 17&nbsp;m
|-
| 27.7 || 10&nbsp;m
|-
| 30 || 3.6&nbsp;m
|-id=1m
| 32.7 || 1&nbsp;m
|}

For [[planet]]s and [[asteroid]]s, a definition of absolute magnitude that is more meaningful for non-stellar objects is used. The absolute magnitude, commonly called <math>H</math>, 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, and in conditions of ideal solar opposition (an arrangement that is impossible in practice).<ref name="Luciuk"/> Because Solar System bodies are illuminated by the Sun, their brightness varies as a function of illumination conditions, described by the [[phase angle (astronomy)|phase angle]]. This relationship is referred to as the [[Phase curve (astronomy)|phase curve]]. The absolute magnitude is the brightness at phase angle zero, an arrangement known as [[opposition (astronomy)|opposition]], from a distance of one AU.

=== Apparent magnitude ===
[[File:Phase angle explanation.png|thumb|right|250px|The phase angle <math>\alpha</math> can be calculated from the distances body-sun, observer-sun and observer-body, using the [[law of cosines]].]]
The absolute magnitude <math>H</math> can be used to calculate the apparent magnitude <math>m</math> of a body. For an object [[reflection (physics)|reflecting]] sunlight, <math>H</math> and <math>m</math> are connected by the relation
<math display="block">m = H + 5 \log_{10}{\left(\frac{d_{BS} d_{BO}}{d_0^2}\right)} - 2.5 \log_{10}{q(\alpha)},</math>
where <math>\alpha</math> is the [[phase angle (astronomy)|phase angle]], the angle between the body-Sun and body–observer lines. <math>q(\alpha)</math> is the [[Bond albedo#Phase integral|phase integral]] (the [[integral|integration]] of reflected light; a number in the 0 to 1 range).<ref name="Karttunen2016"/>

By the [[law of cosines]], we have:
<math display="block">\cos{\alpha} = \frac{ d_\mathrm{BO}^2 + d_\mathrm{BS}^2 - d_\mathrm{OS}^2 } {2 d_\mathrm{BO} d_\mathrm{BS}}.</math>

Distances:
* {{math|''d''<sub>BO</sub>}} is the distance between the body and the observer
* {{math|''d''<sub>BS</sub>}} is the distance between the body and the Sun
* {{math|''d''<sub>OS</sub>}} is the distance between the observer and the Sun
* {{math|''d''<sub>0</sub>}}, a [[unit conversion]] factor, is the constant 1&nbsp;[[Astronomical Unit|AU]], the average distance between the Earth and the Sun

=== Approximations for phase integral {{serif|''q''(''α'')}} ===
The value of <math>q(\alpha)</math> depends on the properties of the reflecting surface, in particular on its [[Surface roughness|roughness]]. In practice, different approximations are used based on the known or assumed properties of the surface. The surfaces of terrestrial planets are generally more difficult to model than those of gaseous planets, the latter of which have smoother visible surfaces.<ref name="Karttunen2016"/>

==== Planets as diffuse spheres ====
[[File:Diffuse reflector sphere disk.png|thumb|right|240px|Diffuse reflection on sphere and flat disk]]
[[File:Diffuse reflection model phase functions.svg|thumb|240px|Brightness with phase for diffuse reflection models. The sphere is 2/3 as bright at zero phase, while the disk can't be seen beyond 90 degrees.]]
Planetary bodies can be approximated reasonably well as [[Lambertian diffuse lighting model|ideal diffuse reflecting]] [[sphere]]s. Let <math>\alpha</math> be the phase angle in [[Degree (angle)|degrees]], then<ref name="Whitmell1907"/>
<math display="block">q(\alpha) = \frac23 \left(\left(1-\frac{\alpha}{180^{\circ}}\right)\cos{\alpha}+\frac{1}{\pi}\sin{\alpha}\right).</math>
A full-phase diffuse sphere reflects two-thirds as much light as a diffuse flat disk of the same diameter. A quarter phase (<math>\alpha = 90^{\circ}</math>) has <math display="inline">\frac{1}{\pi}</math> as much light as full phase (<math>\alpha = 0^{\circ}</math>).

By contrast, a ''diffuse disk reflector model'' is simply <math>q(\alpha) = \cos{\alpha}</math>, which isn't realistic, but it does represent the [[opposition surge]] for rough surfaces that reflect more uniform light back at low phase angles.

The definition of the [[geometric albedo]] <math>p</math>, a measure for the reflectivity of planetary surfaces, is based on the diffuse disk reflector model. The absolute magnitude <math>H</math>, diameter <math>D</math> (in [[kilometer]]s) and geometric albedo <math>p</math> of a body are related by<ref name="sizemagnitude"/><ref name="Mag_formula"/><ref name="H_derivation"/>
<math display="block">D = \frac{1329}{\sqrt{p}} \times 10^{-0.2H} \mathrm{km},</math> or equivalently,
<math display="block">H = 5\log_{10}{\frac{1329}{D\sqrt{p}}}.</math>

Example: The [[Moon|Moon's]] absolute magnitude <math>H</math> can be calculated from its diameter <math>D=3474\text{ km}</math> and [[geometric albedo]] <math>p = 0.113</math>:<ref name="Albedo"/>
<math display="block">H = 5\log_{10}{\frac{1329}{3474\sqrt{0.113}}} = +0.28.</math>
We have <math>d_{BS}=1\text{ AU}</math>, <math>d_{BO}=384400\text{ km}=0.00257\text{ AU}.</math>
At [[lunar phases|quarter phase]], <math display="inline">q(\alpha)\approx \frac{2}{3\pi}</math> (according to the diffuse reflector model), this yields an apparent magnitude of <math display="block">m = +0.28+5\log_{10}{\left(1\cdot0.00257\right)} - 2.5\log_{10}{\left(\frac{2}{3\pi}\right)} = -10.99.</math> The actual value is somewhat lower than that, <math>m=-10.0.</math> This is not a good approximation, because the phase curve of the Moon is too complicated for the diffuse reflector model.<ref name="Luciuk2"/> A more accurate formula is given in the following section.

==== More advanced models ====

Because Solar System bodies are never perfect diffuse reflectors, astronomers use different models to predict apparent magnitudes based on known or assumed properties of the body.<ref name="Karttunen2016"/> For planets, approximations for the correction term <math>-2.5\log_{10}{q(\alpha)}</math> in the formula for {{mvar|m}} have been derived empirically, to match [[phase curve (astronomy)|observations at different phase angles]]. The approximations recommended by the [[Astronomical Almanac]]<ref name="Mallama_and_Hilton"/> are (with <math>\alpha</math> in degrees):

{| class="wikitable"
|-
! Planet
! Referenced calculation<ref name="IMCCE">{{cite web | title=Encyclopedia - the brightest bodies | website=IMCCE | url=https://promenade.imcce.fr/en/pages5/572.html | access-date=2023-05-29}}</ref>
! <math>H</math>
! Approximation for <math>-2.5\log_{10}{q(\alpha)}</math>
|-
| [[Mercury (planet)|Mercury]]
| −0.4
| −0.613
| <math>+6.328\times10^{-2}\alpha - 1.6336\times10^{-3}\alpha^{2}+3.3644\times10^{-5}\alpha^{3}-3.4265\times10^{-7}\alpha^{4}+1.6893\times10^{-9}\alpha^{5}-3.0334\times10^{-12}\alpha^{6}</math>
|-
| [[Venus (planet)|Venus]]
| −4.4
| −4.384
|
* <math>-1.044\times10^{-3}\alpha+3.687\times10^{-4}\alpha^{2}-2.814\times10^{-6}\alpha^{3}+8.938\times10^{-9}\alpha^{4}</math> (for <math>0^{\circ}<\alpha \le 163.7^{\circ}</math>)
* <math>+240.44228-2.81914\alpha+8.39034\times10^{-3}\alpha^{2}</math> (for <math>163.7^{\circ}<\alpha<179^{\circ}</math>)
|-
| [[Earth]]
| −
| −3.99
|<math>-1.060\times10^{-3}\alpha+2.054\times10^{-4}\alpha^{2}</math>
|-
| [[Moon]]<ref>{{Cite book|first=A.N.|last=Cox|year=2000|title=Allen's Astrophysical Quantities, fourth edition|publisher=Springer-Verlag|pages=310}}</ref>
| 0.2
| +0.28
|
* <math>+2.9994\times10^{-2}\alpha-1.6057\times10^{-4}\alpha^{2}+3.1543\times10^{-6}\alpha^{3}-2.0667\times10^{-8}\alpha^{4}+6.2553\times10^{-11}\alpha^{5}</math> (for <math>\alpha\le150^{\circ}</math>, before full Moon)
* <math>+3.3234\times10^{-2}\alpha-3.0725\times10^{-4}\alpha^{2}+6.1575\times10^{-6}\alpha^{3}-4.7723\times10^{-8}\alpha^{4}+1.4681\times10^{-10}\alpha^{5}</math> (for <math>\alpha\le150^{\circ}</math>, after full Moon)
|-
| [[Mars (planet)|Mars]]
| −1.5
| −1.601
|
* <math>+2.267\times10^{-2}\alpha-1.302\times10^{-4}\alpha^{2}</math> (for <math>0^{\circ}<\alpha\le50^{\circ}</math>)
* <math>+1.234-2.573\times10^{-2}\alpha+3.445\times10^{-4}\alpha^{2}</math> (for <math>50^{\circ}<\alpha\le120^{\circ}</math>)
|-
| [[Jupiter (planet)|Jupiter]]
| −9.4
| −9.395
|
* <math>-3.7\times10^{-4}\alpha+6.16\times10^{-4}\alpha^{2}</math> (for <math>\alpha\le12^{\circ}</math>)
* <math>-0.033-2.5\log_{10}{\left(1-1.507\left(\frac{\alpha}{180^{\circ}}\right)-0.363\left(\frac{\alpha}{180^{\circ}}\right)^{2}-0.062\left(\frac{\alpha}{180^{\circ}}\right)^{3}+2.809\left(\frac{\alpha}{180^{\circ}}\right)^{4}-1.876\left(\frac{\alpha}{180^{\circ}}\right)^{5}\right)}</math> (for <math>\alpha>12^{\circ}</math>)
|-
| [[Saturn (planet)|Saturn]]
| −9.7
| −8.914
|
* <math>-1.825\sin{\left(\beta\right)}+2.6\times10^{-2}\alpha-0.378\sin{\left(\beta\right)}e^{-2.25\alpha}</math> (for planet and rings, <math>\alpha<6.5^{\circ}</math> and <math>\beta<27^{\circ}</math>)
* <math>-0.036-3.7\times10^{-4}\alpha+6.16\times10^{-4}\alpha^{2}</math> (for the globe alone, <math>\alpha\le6^{\circ}</math>)
* <math>+0.026+2.446\times10^{-4}\alpha+2.672\times10^{-4}\alpha^{2}-1.505\times10^{-6}\alpha^{3}+4.767\times10^{-9}\alpha^{4}</math> (for the globe alone, <math>6^{\circ}<\alpha<150^{\circ}</math>)
|-
| [[Uranus (planet)|Uranus]]
| −7.2
| −7.110
|<math>-8.4\times10^{-4}\phi'+6.587\times10^{-3}\alpha+1.045\times10^{-4}\alpha^{2}</math> (for <math>\alpha < 3.1^{\circ}</math>)
|-
| [[Neptune (planet)|Neptune]]
| −6.9
| −7.00
|<math>+7.944\times10^{-3}\alpha+9.617\times10^{-5}\alpha^{2}</math> (for <math>\alpha < 133^{\circ}</math> and <math>t > 2000.0</math>)
|}
{{Multiple image
| header            = The different halves of the Moon, as seen from Earth
| image1            = Daniel Hershman - march moon (by).jpg
| caption1          = Moon at first quarter
| image2            = Waning gibbous moon near last quarter - 23 Sept. 2016.png
| caption2          = Moon at last quarter
}}
Here <math>\beta</math> is the effective inclination of [[Saturn's rings]] (their tilt relative to the observer), which as seen from Earth varies between 0° and 27° over the course of one Saturn orbit, and <math>\phi'</math> is a small correction term depending on Uranus' sub-Earth and sub-solar latitudes. <math>t</math> is the [[Common Era]] year. Neptune's absolute magnitude is changing slowly due to seasonal effects as the planet moves along its 165-year orbit around the Sun, and the approximation above is only valid after the year 2000. For some circumstances, like <math>\alpha \ge 179^{\circ}</math> for Venus, no observations are available, and the phase curve is unknown in those cases. The formula for the Moon is only applicable to the [[near side of the Moon]], the portion that is visible from the Earth.

Example 1: On 1 January 2019, [[Venus (planet)|Venus]] was <math>d_{BS}=0.719\text{ AU}</math> from the Sun, and <math>d_{BO} = 0.645\text{ AU}</math> from Earth, at a phase angle of <math>\alpha=93.0^{\circ}</math> (near quarter phase). Under full-phase conditions, Venus would have been visible at <math>m=-4.384+5\log_{10}{\left(0.719 \cdot 0.645\right)}=-6.09.</math> Accounting for the high phase angle, the correction term above yields an actual apparent magnitude of <math display="block">m = -6.09 + \left(-1.044 \times 10^{-3} \cdot 93.0 + 3.687\times10^{-4} \cdot 93.0^{2} - 2.814 \times 10^{-6} \cdot 93.0^{3} + 8.938 \times 10^{-9} \cdot 93.0^{4}\right) = -4.59.</math> This is close to the value of <math>m=-4.62</math> predicted by the Jet Propulsion Laboratory.<ref name="JPLHorizonsVenus"/>

Example 2: At [[first quarter|first quarter phase]], the approximation for the Moon gives <math display="inline">-2.5\log_{10}{q(90^{\circ})}=2.71.</math> With that, the apparent magnitude of the Moon is <math display="inline">m = +0.28+5\log_{10}{\left(1\cdot0.00257\right)}+2.71= -9.96,</math> close to the expected value of about <math>-10.0</math>. At [[last quarter]], the Moon is about 0.06 mag fainter than at first quarter, because that part of its surface has a lower albedo.

Earth's [[albedo]] varies by a factor of 6, from 0.12 in the cloud-free case to 0.76 in the case of [[altostratus clouds|altostratus cloud]]. The absolute magnitude in the table corresponds to an albedo of 0.434. Due to the variability of the [[weather]], Earth's apparent magnitude cannot be predicted as accurately as that of most other planets.<ref name="Mallama_and_Hilton"/>

==== Asteroids ====
[[File:Ceres opposition effect.png|thumb|right|240px|Asteroid [[1 Ceres]], imaged by the [[Dawn (spacecraft)|Dawn]] spacecraft at phase angles of 0°, 7° and 33°. The strong difference in brightness between the three is real. The left image at 0° phase angle shows the brightness surge due to the [[opposition effect]].]]
[[File:Asteroid HG phase integrals.svg|thumb|240px|Phase integrals for various values of G]]
[[File:Slope parameter G.png|thumb|right|240px|Relationship between the slope parameter <math>G</math> and the opposition surge. Larger values of <math>G</math> correspond to a less pronounced opposition effect. For most asteroids, a value of <math>G = 0.15</math> is assumed, corresponding to an opposition surge of <math>0.3\text{ mag}</math>.]]
If an object has an atmosphere, it reflects light more or less isotropically in all directions, and its brightness can be modelled as a diffuse reflector. Bodies with no atmosphere, like asteroids or moons, tend to reflect light more strongly to the direction of the incident light, and their brightness increases rapidly as the phase angle approaches <math>0^{\circ}</math>. This rapid brightening near opposition is called the [[opposition effect]]. Its strength depends on the physical properties of the body's surface, and hence it differs from asteroid to asteroid.<ref name="Karttunen2016"/>

In 1985, the [[International Astronomical Union|IAU]] adopted the [[semi-empirical]] <math>HG</math>-system, based on two parameters <math>H</math> and <math>G</math> called ''absolute magnitude'' and ''slope'', to model the opposition effect for the [[ephemeris|ephemerides]] published by the [[Minor Planet Center]].<ref name="MPC1985"/>
<math display="block">m = H + 5\log_{10}{\left(\frac{d_{BS}d_{BO}}{d_{0}^{2}}\right)}-2.5\log_{10}{q(\alpha)},</math>

where
*the phase integral is <math>q(\alpha)=\left(1-G\right)\phi_{1}\left(\alpha\right)+G\phi_{2}\left(\alpha\right)</math> and
*<math display="inline">\phi_{i}\left(\alpha\right) = \exp{\left(-A_i \left(\tan{\frac{\alpha}{2}}\right)^{B_i}\right)}</math> for <math>i = 1</math> or <math>2</math>, <math>A_{1}=3.332</math>, <math>A_{2}=1.862</math>, <math>B_{1}=0.631</math> and <math>B_2 = 1.218</math>.<ref name="Lagerkvist"/>

This relation is valid for phase angles <math>\alpha < 120^{\circ}</math>, and works best when <math>\alpha < 20^{\circ}</math>.<ref name="dymock"/>

The slope parameter <math>G</math> relates to the surge in brightness, typically {{val|0.3|u=mag}}, when the object is near opposition. It is known accurately only for a small number of asteroids, hence for most asteroids a value of <math>G=0.15</math> is assumed.<ref name="dymock"/> In rare cases, <math>G</math> can be negative.<ref name="Lagerkvist"/><ref name="JPLdoc"/> An example is [[101955 Bennu]], with <math>G=-0.08</math>.<ref name="Bennu"/>

In 2012, the <math>HG</math>-system was officially replaced by an improved system with three parameters <math>H</math>, <math>G_1</math> and <math>G_2</math>, which produces more satisfactory results if the opposition effect is very small or restricted to very small phase angles. However, as of 2022, this <math>H G_1 G_2</math>-system has not been adopted by either the Minor Planet Center nor [[Jet Propulsion Laboratory]].<ref name="Karttunen2016"/><ref name="Shevchenko2016"/>

The apparent magnitude of asteroids [[Light curve|varies as they rotate]], on time scales of seconds to weeks depending on their [[rotation period]], by up to <math>2\text{ mag}</math> or more.<ref name="lc"/> In addition, their absolute magnitude can vary with the viewing direction, depending on their [[axial tilt]]. In many cases, neither the rotation period nor the axial tilt are known, limiting the predictability. The models presented here do not capture those effects.<ref name="dymock"/><ref name="Karttunen2016"/>

=== Cometary magnitudes ===
The brightness of [[comet]]s is given separately as ''total magnitude'' (<math>m_{1}</math>, the brightness integrated over the entire visible extend of the [[Coma (cometary)|coma]]) and ''nuclear magnitude'' (<math>m_{2}</math>, the brightness of the core region alone).<ref name="MPES"/> Both are different scales than the magnitude scale used for planets and asteroids, and can not be used for a size comparison with an asteroid's absolute magnitude {{mvar|H}}.

The activity of comets varies with their distance from the Sun. Their brightness can be approximated as
<math display="block">m_{1} = M_{1} + 2.5\cdot K_{1}\log_{10}{\left(\frac{d_{BS}}{d_0}\right)} + 5\log_{10}{\left(\frac{d_{BO}}{d_0}\right)}</math>
<math display="block">m_{2} = M_{2} + 2.5\cdot K_{2}\log_{10}{\left(\frac{d_{BS}}{d_0}\right)} + 5\log_{10}{\left(\frac{d_{BO}}{d_0}\right)},</math>
where <math>m_{1,2}</math> are the total and nuclear apparent magnitudes of the comet, respectively, <math>M_{1,2}</math> are its "absolute" total and nuclear magnitudes, <math>d_{BS}</math> and <math>d_{BO}</math> are the body-sun and body-observer distances, <math>d_{0}</math> is the [[Astronomical Unit]], and <math>K_{1,2}</math> are the slope parameters characterising the comet's activity. For <math>K=2</math>, this reduces to the formula for a purely reflecting body (showing no cometary activity).<ref name="Meisel1976"/>

For example, the lightcurve of comet [[C/2011 L4|C/2011 L4 (PANSTARRS)]] can be approximated by <math>M_{1}=5.41\text{, }K_{1}=3.69.</math><ref name="COBS 2011L4"/> On the day of its perihelion passage, 10 March 2013, comet PANSTARRS was <math>0.302\text{ AU}</math> from the Sun and <math>1.109\text{ AU}</math> from Earth. The total apparent magnitude <math>m_{1}</math> is predicted to have been <math>m_1 = 5.41 + 2.5\cdot3.69\cdot\log_{10}{\left(0.302\right)}+5\log_{10}{\left(1.109\right)} = +0.8</math> at that time. The Minor Planet Center gives a value close to that, <math>m_{1} = +0.5</math>.<ref name="MPC2011L4"/>

{| class="wikitable sortable" style="font-size: 0.9em;"
|+Absolute magnitudes and sizes of comet nuclei
! Comet
! Absolute<br />magnitude <math>M_{1}</math><ref name="kidger"/>
! Nucleus<br />diameter
|-
|[[Comet Sarabat]] || −3.0 || ≈100&nbsp;km?
|-
|[[Comet Hale-Bopp]] || −1.3 || 60 ± 20&nbsp;km
|-
|[[Comet Halley]] || 4.0 || 14.9 x 8.2&nbsp;km
|-
|average new comet || 6.5 || ≈2&nbsp;km<ref name="Hughes"/>
|-
|[[C/2014 UN271 (Bernardinelli-Bernstein)|C/2014 UN<sub>271</sub> (Bernardinelli-Bernstein)]] || 6.7<ref name="Bernardinelli">{{cite web|type = 2021-08-08 last obs.|title = JPL Small-Body Database Browser: (2014 UN271)|url = https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=54161348|publisher = [[Jet Propulsion Laboratory]]|accessdate = 15 September 2021}}</ref> || 60–200&nbsp;km?<ref>{{cite news |title=The Largest Comet Ever Found Is Making Its Move Into a Sky Near You |url=https://www.nytimes.com/2021/06/28/science/comet-largest-ever-seen.html|date=28 June 2021 |work=[[The New York Times]]|accessdate=1 July 2021 }}</ref><ref name="ATel14759">{{cite web|title = Comet C/2014 UN271 (Bernardinelli-Bernstein) exhibited activity at 23.8 au |url = https://www.astronomerstelegram.org/?read=14759|first1 = Tony|last1 = Farnham|work = The Astronomer's Telegram|date = 6 July 2021|accessdate = 6 July 2021}}</ref>
|-
|[[289P/Blanpain]] (during 1819 outburst) || 8.5<ref name="Yoshida"/> || 320 m<ref name="Jewitt"/>
|-
|289P/Blanpain (normal activity) || 22.9<ref name="JPL_289"/> || 320 m
|}

The absolute magnitude of any given comet can vary dramatically. It can change as the comet becomes more or less active over time or if it undergoes an outburst. This makes it difficult to use the absolute magnitude for a size estimate. When comet [[289P/Blanpain]] was discovered in 1819, its absolute magnitude was estimated as <math>M_{1} = 8.5</math>.<ref name="Yoshida"/> It was subsequently lost and was only rediscovered in 2003. At that time, its absolute magnitude had decreased to <math>M_{1} = 22.9</math>,<ref name="JPL_289"/> and it was realised that the 1819 apparition coincided with an outburst. 289P/Blanpain reached naked eye brightness (5–8 mag) in 1819, even though it is the comet with the smallest nucleus that has ever been physically characterised, and usually doesn't become brighter than 18 mag.<ref name="Yoshida"/><ref name="Jewitt"/>

For some comets that have been observed at heliocentric distances large enough to distinguish between light reflected from the coma, and light from the nucleus itself, an absolute magnitude analogous to that used for asteroids has been calculated, allowing to estimate the sizes of their nuclei.<ref name="Lamy2004"/>

== Meteors ==
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 name="IMO"/><ref name="SSD_Glossary"/>

== See also ==
* [[Araucaria Project]]
* [[Hertzsprung–Russell diagram]] – relates absolute magnitude or [[luminosity]] versus spectral color or surface [[temperature]].
* [[Jansky]] - the preferred unit for radio astronomy – linear in power/unit area
* [[List of most luminous stars]]
* [[Photographic magnitude]]
* [[Surface brightness]] – the ''magnitude'' for extended objects
* [[Zero point (photometry)]] – the typical calibration point for star flux

== References ==

{{Reflist |colwidth=30em |refs=

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<ref name="Mag_formula">The <math>1329\text{ km}</math> factor can be computed as <math>2\text{ AU}\cdot10^{H_{\text{Sun}}/5}</math>, where <math>H_{\text{Sun}}=-26.76</math>, the absolute magnitude of the Sun, and <math>1\text{ AU}=1.4959787\times10^{8}\text{ km}.</math></ref>

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<ref name="Yoshida">{{Citation
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<ref name="JPL_289">{{Citation
 | url = https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=289P#phys_par
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 | title = 289P/Blanpain (2013-07-17 last obs.)
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 | publisher = Jet Propulsion Laboratory}}</ref>

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}}

== External links ==
* [http://astro.pas.rochester.edu/~aquillen/ast142/costanti.html Reference zero-magnitude fluxes] {{Webarchive|url=https://web.archive.org/web/20030222000548/http://astro.pas.rochester.edu/~aquillen/ast142/costanti.html |date=22 February 2003 }}
* [http://www.iau.org/ International Astronomical Union]
* [http://www.fxsolver.com/solve/share/knc8CRLY7WMX324G2I7GXg==/ Absolute Magnitude of a Star calculator]
* [http://www.astronomynotes.com/starprop/s4.htm The Magnitude system]
* [http://webcitation.org/query.php About stellar magnitudes] {{Webarchive|url=https://web.archive.org/web/20211027163807/https://webcitation.org/query.php |date=27 October 2021 }}  
* [http://simbad.u-strasbg.fr/sim-fid.pl Obtain the magnitude of any star] – [[SIMBAD]]
* [http://www.minorplanetcenter.org/iau/lists/Sizes.html Converting magnitude of minor planets to diameter]
* [https://web.archive.org/web/20010302182040/http://neo.jpl.nasa.gov/glossary/h.html Another table for converting asteroid magnitude to estimated diameter]

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[[Category:Observational astronomy]]