Hertzsprung-Russell diagram

A Hertzsprung-Russell (HR) diagram places stars according to their surface temperature and their intrinsic brightness. Stars with similar properties appear in the same region of the diagram.

Reading the axes

The vertical axis shows luminosity or absolute magnitude. Luminosity measures the energy a star emits each second. Absolute magnitude also describes intrinsic brightness, but its scale runs in the opposite direction: a more negative absolute magnitude means a more luminous star. On a typical absolute-magnitude axis, values run from about $$-10$$ at the top to $$+15$$ at the bottom. More luminous stars therefore appear higher on the diagram.

The horizontal axis shows surface temperature or spectral class. Temperature decreases from left to right, typically from about $$50,000\ \mathrm{K}$$ to $$2,500\ \mathrm{K}$$. The corresponding spectral classes run from hotter to cooler in the order O, B, A, F, G, K, M.

Regions of the HR diagram

• Main sequence: a band running from hot, luminous stars at the upper left to cool, faint stars at the lower right.
• Giants: stars in the upper right. Their surfaces are relatively cool, but they are luminous.
• White dwarfs: stars in the lower left. Their surfaces are hot, but they are faint.

The Sun lies on the main sequence. Its surface temperature is approximately $$5,800\ \mathrm{K}$$, its absolute magnitude is about $$+4.8$$, and its spectral class is G.

Evolutionary track of a Sun-like star

A Sun-like star changes in temperature and luminosity as it forms, ages and eventually cools. These changes can be followed on a Hertzsprung–Russell (HR) diagram, which plots luminosity vertically and surface temperature horizontally. More luminous stars are higher on the diagram; hotter stars are to the left.

From protostar to main sequence

A Sun-like star begins in a cloud of gas and dust. Gravity pulls material together, causing the cloud to collapse and form a protostar. As the protostar contracts, its interior heats up and it approaches the main sequence on the HR diagram.

When the core becomes hot enough, hydrogen fusion supplies the energy that sustains the star. The star has reached the main sequence.

From red giant to white dwarf

After hydrogen in the core is depleted, the star expands into a red giant. Its surface cools, but its luminosity increases. On the HR diagram, it therefore moves towards the upper right: cooler but more luminous. Helium fusion occurs during the giant stages.

The star later sheds its outer layers, leaving its hot, dense core as a white dwarf. The white dwarf is initially hot but faint: its small surface area makes its luminosity low despite its high temperature. It lies in the lower-left region of the HR diagram.

A white dwarf has no sustained fusion to replenish its energy. It gradually cools and fades, moving towards lower temperatures and lower luminosities on the diagram.