Stellar Classification

Stars are not all the same. They vary enormously in size, temperature, colour, and luminosity. Classifying stars systematically allows astronomers to understand their physical properties, their life stages, and the underlying physics that governs them. This lesson covers the key physical quantities used to describe stars and the classification system that organises them.

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Luminosity

Luminosity L is the total power output of a star — the total energy radiated per second in all directions, measured in watts (W).

The Sun's luminosity is approximately L☉ = 3.85 × 10²⁶ W. Other stars range from about 10⁻⁴ L☉ (faint red dwarfs) to over 10⁶ L☉ (the most luminous supergiants).

Luminosity is an intrinsic property of the star — it does not depend on how far away the star is from the observer. This distinguishes it from brightness, which decreases with distance.

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Apparent Magnitude and Absolute Magnitude

Apparent magnitude (m) is a measure of how bright a star appears from Earth. It depends on both the star's luminosity and its distance from us. A star can have a low apparent magnitude (appear bright) either because it is very luminous or because it is very close.