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What is Absolute Magnitude?

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  • Written By: Ray Hawk
  • Edited By: E. E. Hubbard
  • Last Modified Date: 01 December 2016
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Absolute magnitude is an astronomical term that refers to an object's true level of brightness in space, not what may be perceived as its brightness, which can be altered by the distance of the object, gravitational effects, and stellar material the light must pass through to reach the observer. Despite this clear definition, the term is relative. as an object's absolute magnitude brightness must be further broken down by defining the spectrum of electromagnetic radiation being measured. If making an observation based on the total energy output of a stellar object, the term bolometric magnitude is used, named after Samuel Langley who invented the bolometer in 1878 for measuring electromagnetic radiation.

Calculating absolute magnitude for any one object in space can be complicated, as its apparent magnitude must first be quantified or the brightness perceived by an Earth bound observer. Then, luminosity distance must be determined in parsecs, which is the actual distance of the object if it's located within the Milky Way galaxy. Redshift, or the effect of gravity on light for distant objects, must also be accounted for, with light shifting towards the red end of the spectrum as an object moves away from Earth. Finally, with objects beyond our local galaxy, general relativity calculations must be employed to determine absolute magnitude.

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Another process used in absolute magnitude determinations is to calculate an object's absolute magnitude temperature, with the colors of light being produced by the object broken down into the chemical signature that they indicate for photons emitted from various elements. The classification system for stars has an absolute magnitude temperature that ranges from “O” for the hottest with a blue color, to “M” as the coolest with a red color. O class stars are considered to be the rarest in space, only comprising around 0.00003% of the total, with red M-class stars accounting for the bulk at 76.45% of the total. The hottest burning O-class blue stars are also the most massive, and have the shortest lifespan, degrading eventually to red giants, with stars one-quarter the size of the sun degrading to the stage of a white dwarf.

The process of determining and classifying the brightness of objects in space can be traced back to the Greek astronomer Hipparchus, who devised the first magnitude system in 150 B.C.E. At the time, there were only six classifications for brightness based on what one could see with the naked eye. Today, absolute magnitude is a much more refined process, with adaptations to the original process giving negative magnitude values such as for our sun, with -26.74 being its apparent magnitude. Larger negative numbers on the scale indicate bright, nearby objects, with the star Sirius receiving a -1.4 apparent magnitude rating as one of the closest stars to Earth, the planet Venus a -4.4, and the Earth's Moon at a -12.6.

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