star types · HR diagram · stellar life cycles · galaxy classification · the Milky Way
Stars are enormous nuclear furnaces — they convert hydrogen to helium through fusion, releasing the energy that powers life on Earth. The Hertzsprung-Russell diagram is astronomers' most important tool for classifying and understanding stars.
Every atom heavier than hydrogen was forged inside a star. Carbon, oxygen, iron — everything your body is made of was produced in stellar cores and scattered by supernovae. We are, quite literally, made of stardust.
On the H-R diagram, click Betelgeuse and then Sirius B. They're both in the "cool + bright" and "hot + dim" extremes. What do these positions tell you about the star's size? Then check the Life Cycles tab to see how each type will eventually end.