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Encyclopedia > Absolute magnitude

In astronomy, absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard luminosity distance away from us, in the absence of interstellar extinction. It allows the overall brightnesses of objects to be compared without regard to distance. A giant Hubble mosaic of the Crab Nebula, a supernova remnant Astronomy is the science of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earths atmosphere (such as auroras and cosmic background radiation). ... The apparent magnitude (m) of a star, planet or other celestial body is a measure of its apparent brightness as seen by an observer on Earth. ... Luminosity distance DL is defined in terms of the relationship between the absolute magnitude M and apparent magnitude m of an astronomical object: which gives: Calculating the luminosity distance of an object correctly from its real distance is quite complex, but there are a number of useful webpages for performing... Extinction is a term used in astronomy to describe the absorption of light from astronomical objects by matter between them and the observer. ...


The absolute magnitude uses the same convention as the visual magnitude, with a ~2.512 difference in brightness between step rates (because 2.5125 ≈ 100). The Milky Way, for example, has an absolute magnitude of about -20.5. So a quasar at 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. Brightness is an attribute of visual perception in which a source appears to emit a given amount of light. ... It has been suggested that Andromeda-Milky Way collision be merged into this article or section. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. ...

Contents

Absolute magnitude for stars and galaxies (M)

In stellar and galactic astronomy, the standard distance is 10 parsecs (about 32.616 light years, or 3×1014 kilometres). A star at ten parsecs has a parallax of 0.1" (100 milli arc seconds). Distance is a numerical description of how far apart objects are at any given moment in time. ... A parsec is the distance from the Earth to an astronomical object which has a parallax angle of one arcsecond. ... A light-year or lightyear (symbol: ly) is a unit of measurement of length, specifically the distance light travels in vacuum in one year. ... km redirects here. ... This does not adequately cite its references or sources. ...


In defining absolute magnitude it is necessary to specify the type of electromagnetic radiation being measured. When referring to total energy output, the proper term is bolometric magnitude. The bolometric magnitude can be computed from the visual magnitude plus a bolometric correction, Mbol = MV + BC. This correction is needed because very hot stars radiate mostly ultraviolet radiation, while very cool stars radiate mostly infrared radiation (see Planck's law). The dimmer an object (at a distance of 10 parsecs) would appear, the higher its absolute magnitude. The lower an object's absolute magnitude, the higher its luminosity. A mathematical equation relates apparent magnitude with absolute magnitude, via parallax. It has been suggested that this article or section be merged with light. ... Various meters Measurement is the estimation of a physical quantity such as length, temperature, or time. ... Black body spectrum For a general introduction, see black body. ... Luminosity has different meanings in several different fields of science. ... Euclid, Greek mathematician, 3rd century BC, as imagined by by Raphael in this detail from The School of Athens. ... An equation is a mathematical statement, in symbols, that two things are the same (or equivalent). ... In mathematics, a finitary relation is defined by one of the formal definitions given below. ...


Many stars visible to the naked eye have an absolute magnitude which is capable of casting shadows from a distance of 10 parsecs; Rigel (-7.0), Deneb (-7.2), Naos (-6.0), and Betelgeuse (-5.6). Shadows on pavement A shadow is a region of darkness where light is blocked. ... Rigel (pronounced ) (β Orionis) is the brightest star in the constellation Orion and the seventh brightest star in the sky, with visual magnitude 0. ... Deneb (α Cyg / α Cygni / Alpha Cygni) is the brightest star in the constellation Cygnus and one of the vertices of the Summer Triangle. ... Zeta Puppis (ζ Pup / ζ Puppis) is a star in the constellation of Puppis. ... Betelgeuse (Alpha (α) Orionis) is a semiregular variable star located 427 light-years away [1]. It is the second brightest star in the constellation Orion, and the ninth brightest star in the night sky. ...


For comparison, Sirius has an absolute magnitude of 1.4 and the Sun has an absolute visual magnitude of 4.83 (it actually serves as a reference point). The Sun's absolute bolometric magnitude is 4.75. Sirius (α CMa / α Canis Majoris / Alpha Canis Majoris) (IPA: ) is the brightest star in the night-time sky, with a visual apparent magnitude of −1. ... The Sun (Latin: Sol) is the star at the center of the Solar System. ...


Absolute magnitudes for stars generally range from -10 to +17. The absolute magnitude for galaxies can be much lower (brighter). For example, the giant elliptical galaxy M87 has an absolute magnitude of -22. In descriptive statistics, the range is the length of the smallest interval which contains all the data. ... An elliptical galaxy is a type of galaxy in the Hubble sequence characterized by the following physical properties: The giant elliptical galaxy NGC 4881 (the spherical glow at upper left) lies at the edge of the Coma Cluster of Galaxies. ... The jet emitted by M87 in this image is thought to be caused by a supermassive black hole at the galaxys center. ...


Computation

One can compute the absolute magnitude M!, of an object given its apparent magnitude m!, and luminosity distance D_L!,: The apparent magnitude (m) of a star, planet or other celestial body is a measure of its apparent brightness as seen by an observer on Earth. ... Luminosity distance DL is defined in terms of the relationship between the absolute magnitude M and apparent magnitude m of an astronomical object: which gives: Calculating the luminosity distance of an object correctly from its real distance is quite complex, but there are a number of useful webpages for performing...

M = m - 5 ((log_{10}{D_L}) - 1)!,

where D_L!, is the star's luminosity distance in parsecs, which are (≈ 3.2616 light-years) This article is about the unit of length. ... A light-year, symbol ly, is the distance light travels in one year: exactly 9. ...


For nearby astronomical objects (such as stars in our galaxy) the luminosity distance DL 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. Luminosity distance DL is defined in terms of the relationship between the absolute magnitude M and apparent magnitude m of an astronomical object: which gives: Calculating the luminosity distance of an object correctly from its real distance is quite complex, but there are a number of useful webpages for performing... Distance is a numerical description of how far apart objects are at any given moment in time. ... General relativity (GR) [also called the general theory of relativity (GTR) and general relativity theory (GRT)] is the geometrical theory of gravitation published by Albert Einstein in 1915/16. ...


In the Euclidean approximation for nearby objects, the absolute magnitude M!, of a star can be calculated from its apparent magnitude and parallax: The apparent magnitude (m) of a star, planet or other celestial body is a measure of its apparent brightness as seen by an observer on Earth. ... This does not adequately cite its references or sources. ...

M = m + 5 (log_{10}{pi} + 1)!,

where π is the star's parallax in arcseconds.


You can also compute the absolute magnitude M!, of an object given its apparent magnitude m!, and distance modulus mu!,: The distance modulus is a way of expressing distances which is often used in astronomy to express the distance to galaxies and clusters of galaxies. ...

M = m - {mu}!,

Example

Rigel has a visual magnitude of mV=0.18 and distance about 773 light-years.
MVRigel = 0.18 + 5*(1 + log10(3.2616/773)) = -6.7
Vega has a parallax of 0.133", and an apparent magnitude of +0.03
MVVega = 0.03 + 5*(1 + log10(0.133)) = +0.65
Alpha Centauri has a parallax of 10.750" and an apparent magnitude of -0.01
MVα Cen = -0.01 + 5*(1 + log10(0.750)) = +4.37
Black Eye Galaxy has a visual magnitude of mV=+9.36 and a distance modulus of 31.06.
MVBlack Eye Galaxy = 9.36 - 31.06 = -21.7

Rigel (pronounced ) (β Orionis) is the brightest star in the constellation Orion and the seventh brightest star in the sky, with visual magnitude 0. ... Vega (α Lyr / α Lyrae / Alpha Lyrae) is the brightest star in the constellation Lyra, and the fifth brightest star in the sky. ... Alpha Centauri (α Cen / α Centauri, also known to astronomers as Rigil Kentaurus), is the brightest star system in the southern constellation of Centaurus. ... This does not adequately cite its references or sources. ... The Black Eye Galaxy (aka Messier 64, M64, or NGC 4826) was discovered by Edward Pigott in March 1779, and independently by Johann Elert Bode in April of the same year, as well as by Charles Messier in 1780. ...

Apparent magnitude

Given the absolute magnitude M!,, for objects within our galaxy you can also calculate the apparent magnitude m!, from any distance d!,:

m = M + 5 (log_{10}{d} - 1)!,

For objects at very great distances (outside our galaxy) the luminosity distance DL must be used instead of d. Luminosity distance DL is defined in terms of the relationship between the absolute magnitude M and apparent magnitude m of an astronomical object: which gives: Calculating the luminosity distance of an object correctly from its real distance is quite complex, but there are a number of useful webpages for performing...


Given the absolute magnitude M!,, you can also compute apparent magnitude m!, from its parallax p!,: This does not adequately cite its references or sources. ...

m = M - 5 (log_{10}p + 1)!,

Also calculating absolute magnitude M!, from distance modulus mu!,: The distance modulus is a way of expressing distances which is often used in astronomy to express the distance to galaxies and clusters of galaxies. ...

m = M + {mu}!,

Absolute magnitude for planets (H)

For planets, comets and asteroids a different definition of absolute magnitude is used which is more meaningful for nonstellar objects. A planet (from the Greek πλανήτης, planetes or wanderers) is a body of considerable mass that orbits a star and that produces very little or no energy through nuclear fusion. ... Comet Hale-Bopp Comet West For other uses, see Comet (disambiguation). ... 253 Mathilde, a C-type asteroid. ...


In this case, the absolute magnitude is defined as the apparent magnitude that the object would have if it were one astronomical unit (au) from both the Sun and the Earth and at a phase angle of zero degrees. This is a physical impossibility, as it requires the observing telescope to be at the centre of the Sun, but it is convenient for purposes of calculation. The astronomical unit (AU or au or a. ... The Sun (Latin: Sol) is the star at the center of the Solar System. ... Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... Phase angle in astronomical observations is the angle between the light incident onto an observed object and the light reflected from the object. ...


To convert a stellar or galactic absolute magnitude into a planetary one, subtract 31.57. This factor also corresponds to the difference between the Sun's visual magnitude of -26.8 and its (stellar) absolute magnitude of +4.8. Thus, the Milky Way (galactic absolute magnitude -20.5) would have a planetary absolute magnitude of -52. The apparent magnitude (m) of a star, planet or other heavenly body is a measure of its apparent brightness; that is, the amount of light received from the object. ...


Apparent magnitude

The absolute magnitude can be used to help calculate the apparent magnitude of a body under different conditions.

m = H + 2.5 log_{10}{(frac{d_{BS}^2 d_{BO}^2}{p(chi) d_0^4})}!,

where


d_0!, is 1 au, chi!, is the phase angle, the angle between the Sun-Body and Body-Observer lines; by the law of cosines, we have: Phase angle in astronomical observations is the angle between the light incident onto an observed object and the light reflected from the object. ... Fig. ...

cos{chi} = frac{ d_{BO}^2 + d_{BS}^2 - d_{OS}^2 } {2 d_{BO} d_{BS}}!,

p(chi)!, is the phase integral (integration of reflected light; a number in the 0 to 1 range) The Bond albedo is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space. ...

Example: (An ideal diffuse reflecting sphere) - A reasonable first approximation for planetary bodies

p(chi) = frac{2}{3} ( (1 - frac{chi}{pi}) cos{chi} + (1/pi) sin{chi} )!, Lamberts cosine law is the statement that the total power observed from a Lambertian surface is directly proportional to the cosine of the angle θ made by the observers line of sight and the line normal to the surface. ... A sphere is a perfectly symmetrical geometrical object. ...

A full-phase diffuse sphere reflects ⅔ as much light as a diffuse disc of the same diameter
Distances:
d_{BO}!, is the distance between the observer and the body
d_{BS}!, is the distance between the Sun and the body
d_{OS}!, is the distance between the observer and the Sun

Example

Moon

H_{Moon}!, = +0.25
d_{OS}!, = d_{BS}!, = 1 au
d_{BO}!, = 384.5 Mm = 2.57 mau
How bright is the Moon from Earth?
Full Moon: chi!, = 0, (p(chi)!, ≈ 2/3)
m_{Moon} = 0.25 + 2.5 log_{10}{(frac{3}{2} 0.00257^2)} = -12.26!,
(Actual -12.7) A full Moon reflects 30% more light at full phase than a perfect diffuse reflector predicts.
Quarter Moon: chi!, = 90°, p(chi) approx frac{2}{3pi}!, (if diffuse reflector)
m_{Moon} = 0.25 + 2.5 log_{10}{(frac{3pi}{2} 0.00257^2)} = -11.02!,
(Actual approximately -11.0) The diffuse reflector formula does better for smaller phases.

See also

  • Hertzsprung-Russell diagram - Relates absolute magnitude or luminosity versus spectral color or surface temperature.
  • Jansky radio astronomer's preferred unit - linear in power/unit area

In stellar astronomy, the Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD) shows the relationship between absolute magnitude, luminosity, stellar classification, and surface temperature. ... Luminosity has different meanings in several different fields of science. ... Fig. ... In radio astronomy, the flux unit or jansky (symbol Jy) is a non-SI unit of electromagnetic flux equivalent to 10−26 watts per square metre per hertz. ...

External links

Absolute Magnitude may also refer to a science fiction magazine of that name.


  Results from FactBites:
 
Absolute magnitude - Wikipedia, the free encyclopedia (843 words)
In defining absolute magnitude it is necessary to specify the type of electromagnetic radiation being measured.
Many stars visible to the naked eye have an absolute magnitude which is capable of casting shadows from a distance of 10 parsecs; Rigel (−7.0), Deneb (−7.2), Naos (−7.3), and Betelgeuse (−5.6).
In this case, the absolute magnitude is defined as the apparent magnitude that the object would have if it were one astronomical unit (au) from both the Sun and the Earth and at a phase angle of zero degrees.
Apparent magnitude - Wikipedia, the free encyclopedia (1197 words)
The apparent magnitude (m) of a star, planet or other celestial body is a measure of its apparent brightness as seen by an observer on Earth.
The absolute magnitude, M, of a star or galaxy is the apparent magnitude it would have if it were 10 parsecs (~ 32 lightyears) away; that of a planet (or other solar system body) is the apparent magnitude it would have if it were 1 astronomical unit away from both the Sun and Earth.
The absolute magnitude of the Sun is 4.83 in the V band (yellow) and 5.48 in the B band (blue).
  More results at FactBites »

 
 

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