The H-R Diagram
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The Hertzsprung-Russell Diagram

For centuries, astronomers wondered about the life cycles of stars. Do stars burn forever, or do they have lifetimes? How are they born, and how do they die? 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 burn even longer. Compared to the average human lifespan of about 100 years, stars seem to live forever. A human could never watch a star go through its complete life cycle.

So how can humans learn about the life cycles of stars? By looking at a large number of stars, we can see them 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, as you may have learned in the Color project, 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 also a measurement of temperature (see the Spectral Types project for more information), 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 your own Hertzsprung-Russell diagrams. You will learn what the various points represent, and you will also learn some of the difficulties in making H-R diagrams.