Classification by luminosity (AQA A-Level Physics): Revision Notes

9.2.1 Classification by luminosity

Luminosity

• Luminosity is the rate of light energy released or the power output of a star.
• It reflects the total energy emitted per second, measured in watts ($W$).

Intensity

• Intensity is the power received from a star per unit area on Earth, often referred to as the star's effective brightness.
• The unit of intensity is $W$ $m⁻²$.
• Intensity obeys the inverse square law, which states that the intensity of a star's light decreases with the square of the distance from the star. Therefore, if a star is twice as far away, its intensity will be one-fourth as much.
• This property is important in astronomy because it allows us to estimate distances to stars based on their observed brightness.

Apparent Magnitude

• The apparent magnitude (m) of an object is a measure of how bright the object appears from Earth.
• Apparent magnitude depends on both a star's intrinsic brightness (luminosity) and its distance from Earth.
• The Hipparcos scale is used to classify stars by their apparent magnitude:
  • Bright stars have lower apparent magnitudes. For instance, the brightest stars are given an apparent magnitude of $1$.
  • Faint stars, just visible to the naked eye, have higher apparent magnitudes around $6$.