Absolute magnitude（Absolute magnitude,M）It is assumed thatfixed starPlace at the distanceearthThe brightness of a star measured at a distance of 10 seconds (32.6 light years), used to distinguish it fromApparent magnitude(Apparent brightness, m). It reflects the real luminous ability of celestial bodies.This method can objectively compare the luminosity of celestial bodies without being affected by distance.If the absolute magnitude is represented by M, and the apparent magnitude is represented by m, the distance of the star is converted into a second difference of r, then M=m+5-5lgr.
By assuming that all celestial bodies are placed at the standard reference distance from the observer, their brightness can be directly compared by magnitude.The magnitude is the same as that in all astronomy, and the absolute magnitude can be specified for different wavelength ranges corresponding to the specified filter band or passband;For stars, the absolute magnitude usually quoted isAbsolute apparent magnitude, which uses the optical V-belt of the spectrum (in the UBV photometric system).
The brighter the object, the smaller its absolute magnitude.Twocelestial bodiesThe absolute magnitude difference is 5, so the luminosity difference is 100 times, and the difference of n absolute magnitudes corresponds to the luminosity ratio of 100 (n/5).For example, the star with absolute magnitude M=3 will be 100 times brighter than the star with absolute magnitude M=8 measured in the V filter band.[1]High luminosity objects can have negative absolute magnitude values: for example, the absolute B star equivalent of the Milky Way is about - 20.8.
The absolute thermal magnitude of a celestial body indicates its total luminosity at all wavelengths, not in a single filter band, such as on the logarithmic amplitude scale.In order to convert from absolute magnitude to absolute thermal magnitude in a specific filter band, thermal correction is applied.
For objects in the solar system, the light is the reflected sunlight. We use different absolute magnitude (H), which is based on a standard reference of one astronomical unit.
Measuringfixed starAbsolute magnitude with the galaxy, the standard distance is set to 10Second gap, about 32.616Light yearOr 309 trillion kilometersparallaxThe value is 0.1 ".
When defining its absolute magnitude, you must specify which type ofelectromagnetic radiation。If calculated according to the released energy, the result will be calledRadiant heat intensity。The lower the magnitude, the brighter the celestial body.Absolute magnitude and apparent magnitude can be converted by parallax (distance).
Due to the distance, the absolute magnitude of many stars is much smaller than their apparent magnitude (actually brighter than it seems);Because some stars are close to us, their absolute magnitudes will be much larger than the visible magnitudes (they look brighter than the actual ones).
Below are some reference tables of absolute magnitude and apparent magnitude of stars:
The absolute magnitude of converted stars usually ranges from - 10 to+17.The absolute magnitude of galaxies is usually lower (bright), for example,Elliptical galaxyM87The absolute magnitude of is - 22.
conversion
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If the apparent magnitude m and distance d of the celestial body are known, then the absolute magnitude M of the celestial body can be obtained according to the following formula:
or
Among them,
10 seconds difference, i.e. 32.616 light years,
Is the parallax of the celestial body, in arc seconds.
If the absolute magnitude M and distance d of the celestial body are known, the apparent magnitude m of the celestial body can be obtained according to the following formula:
or
example:
RigelThe apparent magnitude of+0.18 is 773 light-years away, then its absolute magnitude is:
MRigel= 0.18 + 5*log10(32.616/773) = -6.7
VegaThe parallax of is 0.133 ", and the apparent magnitude+0.03, then the absolute magnitude is:
MVega= 0.03 + 5*(1 + log10(0.133)) = +0.65
Alpha With a parallax of 0.750 "and an absolute magnitude of+4.37, the apparent magnitude is:
mAlpha = 4.37 - 5*(1 + log10(0.750)) = -0.01
computing method
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aboutplanet，comet，asteroidFor non stellar objects, their absolute magnitude definitions are completely different.The definition of absolute magnitude of a star is not applicable to it.At this time, the absolute magnitude is defined as the distance between the celestial body and the sun and the earth is oneAstronomical unit(au), andphase angleAt 0 °Apparent magnitude。This is actually impossible, just for the convenience of calculation.
Calculate absolute magnitudeH:
Where is the apparent magnitude of the sun (- 26.73), is the geometric albedo of the celestial body surface (between 0 and 1), is the radius of the celestial body, and is an astronomical unit.
moon:
,
3476/2 km = 1738 km
Absolute magnitude ranking
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Here are the brightest stars with absolute magnitude
