Hertzsprung–Russell Diagram
The HR diagram is the most important graph in stellar astrophysics. It plots luminosity against surface temperature and reveals that stars fall into distinct groups: the main sequence, red giants, supergiants, and white dwarfs. Click any star to see its properties, and toggle evolutionary tracks to watch how stars change over their lifetimes.
L = 4πR²σT⁴ • Stefan–Boltzmann Law
About this lab
The Hertzsprung–Russell diagram, independently developed by Ejnar Hertzsprung (1911) and Henry Norris Russell (1913), plots stars by their luminosity (vertical axis, in solar luminosities) against surface temperature (horizontal axis, in Kelvin — reversed, with hot stars on the left).
- Main Sequence: A diagonal band from hot, luminous blue stars (upper-left) to cool, dim red dwarfs (lower-right). Stars spend most of their lives here, fusing hydrogen into helium. The Sun is a main-sequence star.
- Red Giants: When a star exhausts its core hydrogen, it expands and cools, moving to the upper-right. These stars are luminous despite lower temperatures because of their enormous size.
- Supergiants: The most massive stars become supergiants — extremely luminous and large. Betelgeuse and Rigel are famous examples.
- White Dwarfs: The remnant cores of low- and intermediate-mass stars. Small, hot, and faint, they cluster in the lower-left.
Spectral classes (O, B, A, F, G, K, M) classify stars by temperature, from the hottest O-type (>30,000 K, blue) to the coolest M-type (<3,500 K, red). The Sun is a G2V star at 5,778 K.
Evolutionary tracks show how a star moves across the HR diagram during its lifetime. A 1 M☉ star like the Sun spends ~10 billion years on the main sequence, then becomes a red giant, and finally a white dwarf. More massive stars live shorter, more dramatic lives.