The Brightness of Stars (Edexcel IGCSE Physics (Modular))

Absolute magnitude

Luminosity

• The luminosity of a star is defined as

The total amount of light energy emitted by the star

• Luminosity is a measure of a star's brightness or power output

Apparent magnitude

• The brightness, or apparent magnitude, of a star depends on two main factors:

  • the luminosity of the star

  • the distance the star is from Earth (more distant stars are usually fainter than nearby stars)

• Apparent magnitude is defined as

The perceived brightness of a star as seen from Earth

• The apparent magnitude scale runs back to front:

  • the brighter the star, the lower the magnitude

  • the dimmer the star, the higher the magnitude

The apparent magnitude scale

Examples of the apparent magnitude of different astronomical bodies

Absolute magnitude

• Astronomers describe the brightness of stars at a standard distance using the absolute magnitude scale

  • a bright star which is far away can look the same as a dim star which is nearby

  • therefore, it is difficult to measure the brightness of stars directly

• Absolute magnitude is defined as

A measure of how bright stars would appear if they were all placed the same distance away from the Earth

• The standard distance astronomers use is 10 parsecs, 32.6 light-years or 3.04 × 10¹⁴ km away from the Earth