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Learn4PH1Paper 2P · Paper 2 (all content)AstrophysicsStars and the HR diagram
Astrophysics · Subtopic 2 of 3

Stars and the HR Diagram

The Hertzsprung–Russell diagram plots luminosity against surface temperature (hot on the left). Most stars lie on the main sequence; red giants and supergiants are top right; white dwarfs bottom left. Nearby distances come from parallax: dd (parsecs) =1p=\frac1p (arcseconds). AQA also uses magnitudes: m−M=5log⁡d10m-M=5\log\frac d{10}.

Edexcel 4PH1 Paper 2PEdexcel YPH11 WPH15AQA 7408 Paper 3B≈ 45 minRevision 1 · updated 2026-10-27
Section 1 of 14

Learning objectives

  • Sketch and interpret the HR diagram.
  • Describe the life cycles of Sun-like and massive stars.
  • Use parallax: d=1pd=\frac1p parsecs.
  • Use apparent and absolute magnitudes (AQA).
Pause and think

Hold a finger at arm's length and close each eye in turn. Your finger jumps against the background. How could this measure the distance to a star?

Section 2 of 14

Core idea

Start hereA map of the stars

Plot every star by how hot it is and how much power it gives out, and they don't scatter randomly. Most fall along one band, with a few groups elsewhere. This chart, the HR diagram, turned out to be a map of how stars live and die.

The big idea

On the HR diagram, a star's position tells its stage of life: main sequence (fusing hydrogen), giants (late life), white dwarfs (remnants). Distances to nearby stars come from parallax: d=1pd=\frac1p parsecs.

1. Reading the HR diagram

Why position shows size
  1. L=4πr2σT4L=4\pi r^2\sigma T^4
    Stefan–Boltzmann law.
  2. same T: L∝r2\text{same }T:\ L\propto r^2
    Higher up at the same temperature means bigger.
  3. same L: r∝1T2\text{same }L:\ r\propto\frac1{T^2}
    Further right (cooler) at the same luminosity means bigger.
Top right: giants. Bottom left: white dwarfs.
Stellar life cycles

Mass decides the ending.

2. Distances

Worked exampleParallax

A star's parallax is 0.25′′0.25''.

Goal
Distance in pc and m.
Strategy
d=1pd=\frac1p.
  1. d=10.25=4.0 pcd=\frac1{0.25}=4.0\text{ pc}
    Definition of the parsec.
  2. 4.0×3.09×1016=1.24×1017 m4.0\times3.09\times10^{16}=1.24\times10^{17}\text{ m}
    Convert.
Answer4.04.0 pc =1.24×1017=1.24\times10^{17} m (13 light years).
✓ CheckNearest stars are a few parsecs away ✓.
Worked exampleMagnitudes (AQA)

m=11m=11, M=1M=1.

Goal
Distance.
Strategy
m−M=5log⁡d10m-M=5\log\frac d{10}.
  1. 10=5log⁡d1010=5\log\frac d{10}
    Substitute.
  2. d10=102\frac d{10}=10^2
    Undo the log.
  3. d=1000 pcd=1000\text{ pc}
    Multiply.
Answer10001000 pc.
✓ Checkm>Mm>M means further than 1010 pc ✓.
Common misconceptionBackwards axes Temperature decreases to the right; magnitude decreases upwards (brighter = smaller number).
Key takeawaySpectral classes O B A F G K M, from about 30 00030\,000 K (blue) to 30003000 K (red). The Sun is a G star.
Pause and think

Why can't parallax measure distant galaxies?

Summary

  • HR: LL against TT (decreasing).
  • Main sequence → giant → remnant.
  • d=1pd=\frac1p.
  • m−M=5log⁡d10m-M=5\log\frac d{10}.
Key words
Parsec
Distance at which 1 AU subtends 1′′1''.
Main sequence
Hydrogen-fusing stage.
Absolute magnitude
Apparent magnitude at 10 pc.
HistoryAnnie Jump Cannon. Annie Jump Cannon classified about 350,000 stellar spectra by hand at Harvard, creating the O B A F G K M scheme still used today.
Fun factOh Be A Fine Girl/Guy, Kiss Me. Generations of students have remembered the spectral classes with this mnemonic.
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