Stars and the H-R Diagram

Summary

Stars differ in brightness and color. However, how bright a star appears from Earth depends on its distance. Astronomers account for distance to determine a star’s true brightness.

The Hertzsprung–Russell (H–R) diagram classifies stars and helps track their evolution. It plots a star’s absolute magnitude (true brightness) against its color, which is linked to its temperature and spectral type.

How Stars Evolve

• Stars form in nebulae, large clouds of gas and dust. Pockets of material contract and heat up, forming protostars.
• A protostar needs suitable mass and temperature to begin fusing hydrogen into helium. Once it does, it becomes a star.
• Stars that fuse hydrogen in their cores occupy the main sequence. The Sun is a main-sequence star.
• When a Sun-like star uses up the hydrogen in its core, it evolves into a red giant. Later, it sheds its outer layers as a planetary nebula, leaving a hot white dwarf rich in carbon and oxygen.
• More massive stars use their fuel faster. They can become red supergiants, fuse heavier elements, and end in a supernova. The remnant may be a neutron star or a black hole.

By locating a star on the H–R diagram using its true brightness and temperature, astronomers can infer its stage of evolution.

Researchers and Source

• Ejnar Hertzsprung
• Henry Norris Russell
• Royal Observatory Greenwich (video source)