Hertzsprung-Russell diagram and stellar life cycleEdexcel International A Level Physics: Revision notes

A Hertzsprung-Russell (HR) diagram is a plot of the luminosity of stars against their surface temperature.

• The vertical axis is luminosity, usually on a logarithmic scale, increasing upwards.
• The horizontal axis is surface temperature, which increases from right to left. The hottest stars are on the left.

Absolute magnitude or spectral class may be used in place of luminosity or temperature. Each star is a point, so the diagram shows how stars are distributed.

To sketch a simple HR diagram, draw these groups:

• Main sequence: a diagonal band from the top left (hot, very luminous) to the bottom right (cool, dim). Most stars, including the Sun, lie here.
• Red giants and supergiants: at the top right. They are cool but very luminous, so they must have a very large surface area.
• White dwarfs: at the bottom left. They are hot but dim, so they must be very small.

Luminosity depends on both surface area and temperature. Comparing stars of the same temperature, the more luminous star is the larger.

A nebula, a cloud of gas and dust, collapses under gravity. It heats up as it contracts and forms a protostar. When the core is hot enough, hydrogen fusion starts and the star joins the main sequence.

On the main sequence the outward pressure from the hot gas and radiation balances the inward force of gravity, so the star is stable. A star spends most of its life here.

The position on the main sequence depends on the mass. A star of greater mass has a greater luminosity and a higher surface temperature, and fuses hydrogen much faster, so its main sequence lifetime is much shorter.

When the core hydrogen is used up, fusion stops in the core and it contracts. The outer layers expand and cool, so the star moves up and to the right to become a red giant.

Eventually the outer layers are ejected as a planetary nebula. The hot core remains as a white dwarf, which has no fusion and slowly cools, moving down on the diagram.

A star of much greater mass starts at the top left of the main sequence. After the hydrogen runs out it becomes a red supergiant at the top right.

The core contracts and heats up, and heavier elements are formed by fusion. The core then collapses and the star explodes as a supernova, ejecting the outer layers.

What remains is a neutron star, or a black hole if the star was massive enough.

The mass of a star determines its place on the main sequence, its lifetime and its final remnant.