Astronomy Jargon 101: Hertzsprung–Russell (HR) diagram

In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll soon have a better way to categorize today’s topic: the Hertzsprung–Russell diagram!

In the early 1900’s astronomy was in a bit of a mess (NB it’s still in a mess, but a completely different one). Astronomers had figured out the trick of spectroscopy, and were beginning some seriously large-scale surveys of our galactic neighborhood.

Those astronomers surveyed all sorts of amazing stars. Giant red ones. Giant blue ones. Small red ones. Medium white ones. The stars they saw had different colors, different temperatures, different specta, and different sizes.

And none of it made any sense.

Why were some stars red and big, while others were blue and big? And what about the small red ones? There needed to be some sort of classification system; some way to organize this giant flood of information. Astronomers had proposed various ideas, like the suggestion that stars start out big and hot and shrink as they age, but that didn’t fit all the data.

Around 1910, the astronomers Ejnar Hertzsprung and Henry Norris Russell, working independently, both came up with the same solution. They found that if they arranged stars according to their temperature and their luminosity, a remarkable pattern popped out.

This Hertzsprung-Russell diagram (more commonly called the “HR” diagram) showed that stars didn’t have just any combination of luminosity and temperature. Instead, stars tended to cluster along a narrow band, which came to be called the Main Sequence. Bluer stars tended to be brighter stars. Redder stars tended to be dimmer. And white stars tended to be in the middle.

In addition to the Main Sequence, there was a small clump of dim, white/blue stars (the white dwarfs), and bright, red stars (the red giants). And that…was it.

The Hertzsprung-Russell diagram may not seem like a big deal, but it was a total revolution in our understanding of stellar life cycles. It made sense of all the data. It crafted order out of the chaos. It showed that stars in our universe weren’t just random – there was a central organizing principle to the observable properties of stars.

If you wanted to develop a theory of stellar evolution, then you had to confront the Hertzsprung-Russell diagram. Your model of how stars worked had to explain it. Without the Hertzsprung-Russell diagram, we wouldn’t have known about this relationship between luminosity and temperature, and we would have had to work a lot harder to pin down the physics of stars.