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Absolute magnitude and the HR diagram

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To compare how bright stars really are, astronomers imagine them all at the same distance. The HR diagram plots this brightness against temperature.

Absolute magnitude

How bright a star looks from Earth depends on how far away it is.
Absolute magnitude is how bright a star would be at a standard distance (10 parsecs) from Earth.
The scale runs backwards: the more negative the number, the brighter the star. A star of absolute magnitude −5 is brighter than one of +10.

The Hertzsprung-Russell diagram

The HR diagram plots absolute magnitude (vertical axis, brightest at the top) against surface temperature or colour (horizontal axis, hottest on the left).

Reading the diagram

Top left: hot, bright blue stars. Bottom right: cool, dim red stars.
The Sun sits in the middle of the main sequence.
As a Sun-like star evolves, it moves off the main sequence to the red giant region, then down to the white dwarf region.
Worked example

A star is in the bottom left of the HR diagram. Describe it.

  1. Left means a high surface temperature.
  2. Bottom means low brightness (dim).
  3. Hot but dim means it must be small.

Answer: A white dwarf: hot, dim and small.

Key idea

Absolute magnitude is a star's brightness at a standard distance; more negative means brighter. The HR diagram plots absolute magnitude against temperature, hottest on the left. Main sequence runs top left to bottom right; giants top right; white dwarfs bottom left.

The interactive lesson includes the diagrams for this topic.

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