The Hertzsprung-Russell Diagram

For centuries, astronomers wondered about the life cycles of stars. Do stars burn forever, or do they eventually die? How are they born, and how do they change as they go through their lives?

However, these questions proved difficult to answer because stars live so long. Today, we know that even a relatively short-lived star burns for a few million years. Stars like our sun last about 10 billion years, and some stars last even longer. Compared to a good human lifespan of about 100 years, stars seem to live forever. A human could never watch a star go through its complete life cycle.

A section of the star cluster NGC 2420

So how can humans learn about the life cycles of stars? Look at the image to the right. It shows just a tiny area of sky, but contains hundreds of stars. By looking at so many stars, we are likely to see some stars in various stages of development. We see young hot stars, old cool stars, and stars that have ended their lives and left empty husks. If we look at enough stars of various ages, we can put together a complete picture of stellar evolution.

At the beginning of the 1900s, scientists closed in on a picture of stellar evolution. Physicists worked out the theory of nuclear fusion and realized that fusion provided enough energy to power stars. They realized that eventually, stars would run out of fuel for fusion and would burn out. So all stars would eventually die. But what would happen to stars during their lives?

The first clue came soon after astronomers on two continents. In 1911, Ejnar Hertzsprung, a Danish astronomer studying at the University of Leiden in the Netherlands, plotted the luminosities of stars against their colors. Luminosity is measure of how the total energy a star gives off. The color tells you the star's surface temperature. So, essentially, Hertzsprung graphed how much energy a star gave off as a function of the star's temperature. He noticed some distinct and unexpected patterns.

In 1913, Henry Norris Russell of Princeton University plotted the luminosities of stars against their spectral types. Spectral types are another measure of temperature, so essentially, Russell made the same diagram that Hertzsprung made. The diagram became known as the Hertzsprung-Russell (or "H-R") diagram. By studying H-R diagrams, later astronomers figured out the life cycles of stars.

In this project, you will make two simple H-R diagrams, one for bright stars and one for nearby stars. You will then dive into real astronomy data supplied by the NVO, making an H-R diagram that contains thousands of stars.