The HR diagram plots luminosity (y-axis

Astrophysics & Cosmology - OCR A-Level Physics

The HR diagramThe Hertzsprung–Russell diagram: plots stellar luminosity against surface temperature (or spectral class). plots luminosityThe total powerThe rate of energy transfer. Measured in watts (W). radiated by a star across all wavelengths. Measured in watts (W). (y-axis, logarithmic) against surface temperature (x-axis, decreasing left to right).
The main sequenceThe diagonal band on the HR diagram where stars spend most of their lives, fusing hydrogen into helium in their cores. is a diagonal band from top-left (hot, luminous, massive) to bottom-right (cool, dim, low mass). About 90% of stars are on the main sequence.
Red giantsCool, luminous stars that have exhausted hydrogen in their cores and expanded. Found in the upper-right of the HR diagram. and red supergiantsVery large, luminous, cool stars — the most massive stars in the red giant phase before supernova. are in the top-right: cool but very luminous (large radius compensates for low temperature in L = $4\pi$r^$2 \sigma$$T^{4}$).
White dwarfsHot, dim remnants of low/medium mass stars after they shed their outer layers. Found in the lower-left of the HR diagram. are in the bottom-left: hot but very dim (extremely small radius).
A star's position on the HR diagram tells you its temperature, luminosityThe total powerThe rate of energy transfer. Measured in watts (W). radiated by a star across all wavelengths. Measured in watts (W)., and approximate size.
Stars spend most of their life on the main sequence, fusing hydrogen to helium in the core.
The temperature axis runs BACKWARDS: hottest on the left, coolest on the right (historical convention from spectral classification O B A F G K M).

Common Mistake MEDIUM

Wrong: Drawing the HR diagram with temperature increasing left to right.
Right: Temperature DECREASES from left to right on an HR diagram. The hottest, blue stars are on the LEFT.