Prime-Time Stars: The HR Diagram

In the prime of their lives, when stars burn hydrogen in their core, there’s a clear and simple relationship between a star’s color and brightness. Nearly a century ago, astronomers developed a way to illustrate this relationship with what’s now called the H-R Diagram, a critical tool for understanding how stars evolve. Here’s how it works.

Basics

• In a past article, you learned how astronomers classify stars according to their color and surface temperature. The massive bright O and B-type stars burn hot, blue, and bright; mid-sized A, F, and G-type stars are cooler and emit white-to-yellow light; and K and M-type stars burn cool and red like a lump of coal in a campfire.

• Around 1911-1913, the Danish astronomer Ejnar Hertzsprung and American Henry Norris Russell studied star clusters, in which all the stars are roughly the same age, and noticed a clear and surprising relationship between the stars’ brightness and color. They plotted the color and temperature of each star on a graph and came up with something that, in the modern day, looks like this:

• As you can see, most of the stars lie in a band from the upper left to the lower right. In this band, blue stars are brighter and red stars are fainter, with white and yellow stars in between. This band is called the “main sequence”. Although Hertzsprung and Russell didn’t know it at the time, stars on the main sequence are in their youth and middle age during which they burn hydrogen in their cores.

Deeper Look

• Stars spend most of their lives on the main sequence. At one time, astronomers believed stars evolved along the main sequence, moving from hotter to cooler as they expelled energy over their lifetimes. But this is not how it works. Once a star begins burning hydrogen through nuclear fusion, it settles onto a particular spot on the main sequence and stays there until the hydrogen runs out.

• For historical reasons, stars along the main sequence are called “dwarfs” and are given the additional symbol “V”. So the Sun is a G2-type dwarf star, or G2V. Our sun has absolute magnitude of +5.0.

• As a star begins to burn helium and heavier elements in the core, it quickly evolves off the main sequence into other types of stars like giants, supergiants, and eventually white dwarfs. We’ll cover that in a future issue.

Good To Know

Here is something interesting. An astronomer can measure the light from a star to determine its spectral type. If the star lies on the main sequence, the astronomer determines its absolute magnitude (the true brightness) from a standard HR diagram. He can measure the star’s apparent magnitude. Then, using a simple mathematical relationship, he can calculate the distance to the star.

Is this not amazing? Simply by measuring the color of light from a star, we can find the distance to the star using the HR diagram. For you keeners, this is called the method of “spectroscopic parallax”.

Personal View

Armed with this knowledge, you’re in a much better position to understand what you’re looking at through your telescope and how astronomers know what they know. And you can dip into more complex and rewarding books like Burnham’s Celestial Handbook and more complex articles in Sky and Telescope without getting terrified. Knowledge is power, dear reader.