Classification by Luminosity
Physics: AQA A Level, topic 9.2.1
Classification by Luminosity
Historically stars were only classified by how bright they appear to be.
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.
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.
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.
Test yourself on Classification by Luminosity
Which of these statements are true about the Hipparcos scale?
What are the values of intensity for bright and dim objects in the sky?