Classification by Luminosity

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Classification by Luminosity

Historically stars were only classified by how bright they appear to be.

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Hipparcos scale

  • The brightness of stars, or their luminosity, as recorded by the eye or a photograph is referred to as the star’s apparent magnitude.
  • The Hipparcos scale defined the dimmest category of visible stars to have an absolute magnitude of 6.
  • The brightest category of stars in the sky were given an absolute magnitude of 1.
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Logarithmic scale

  • Brightness, however, is a subjective scale.
  • When measurements of intensity were recorded and analysed, the scale was reformed as a logarithmic scale.
  • Each difference of magnitude of 1 on the scale is equal to an intensity ratio of 2.51.
  • This comes from the idea that a difference in 5 magnitudes corresponds to a ratio of 100.
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Logarithmic scale 2

  • This meant that the very brightest stars had absolute magnitudes much lower than magnitude 1.
    • e.g. Sirius has a magnitude of -1.4.
  • The dimmest objects observable through telescopes have magnitudes greater than magnitude 6.
    • e.g. the planet Neptune has a magnitude of +8.

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