🔭 Stars & Galaxies

star types · HR diagram · stellar life cycles · galaxy classification · the Milky Way

CLICK A STAR ON THE DIAGRAM

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The H-R diagram plots stellar luminosity (brightness) vs surface temperature. Most stars fall on the Main Sequence diagonal band.

What is this?

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.

Why does it matter?

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.

Key terms

Main sequence — the life stage where stars fuse hydrogen; our Sun has been here for ~4.6 billion years Luminosity — total energy output of a star; measured relative to the Sun (L☉) Spectral class — OBAFGKM classification by temperature (hot to cool) Supernova — catastrophic explosion at the end of a massive star's life; briefly outshines entire galaxies Light-year — distance light travels in one year; ≈ 9.46 × 10¹² km

🎯 Try this challenge

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.

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