University of Michigan - Department of Astronomy

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Version: Intro


H-R diagram

Nothing has such power to broaden the mind as the ability to investigate systematically and truly all that comes under your observation in life.

--Marcus Aurelius

Overview

Introduction

Some of the most important or useful discoveries come from organization or systematic investigation of information. Scientists frequently use graphs to visually organize data. In 1911 Ejnar Hertzsprung and Henry Norris Russell independently began organizing observations of stellar properties onto a graph. The vertical axis was increasing intrinsic brightness, which they plotted as decreasing absolute magnitude. Modern charts are as likely to use increasing luminosity. On the horizontal axis, they plotted the temperature of the stars (see the spectroscopy activity), in decreasing order, or the spectral type of the star based on the Harvard spectral class system (see the Spectral Classification and the Pleiades activity), which organizes stars into O, B, F, G, K, and M types, from hottest to coolest. These brightness vs. temperature diagrams are now known as Hertzsprung-Russell or H-R diagrams.

Modern astronomers sometimes also plot color on the horizontal axis, creating a variation of an H-R diagram called a color-magnitude diagram. The color of a star is determined by measuring the apparent magnitude of the star through 2 different colored filters, then subtracting one value from the other. For example, the B-V color index is determined by subtracting the apparent magnitude through a yellow filter (middle of the Visible spectrum) from the apparent magnitude through a Blue filter. Hotter (bluer) stars still fall on the left, but the numbers on the axis depend on the color system used: 0 falls about half way along the axis in the B-V system, for example. Because the color is determined directly from observation, color magnitude diagrams are frequently used for star clusters to measure distance and extinction (see the Galactic Star Clusters activity)

No matter which quantities you choose to put on each axis, certain things hold true: hot blue stars are on the left, bright stars are at the top. This leads to some interesting observations. Hertzsprung and Russell found that most of the stars fell along a line that runs from the upper left to lower right. They called this the Main Sequence. Blue main sequence stars are a little larger around and more massive than red main sequence stars. Stars can be bright because they are hot, or because they are big around(see the Spectroscopy activity), so stars can be cool and bright if they are very big. Thus, stars that fall above the main sequence are bigger around than the main sequence stars, so they are called giants or, if very far from the main sequence, supergiants. Objects that fall below the main sequence are generally very compact objects that do NOT do fusion in their core, such as white dwarfs (not to be confused with small main sequence stars, also called dwarfs). Thus the location of a star on the H-R diagram can tell you something about what is going on inside, and where the star is in its lifetime.

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created: 4/18/08 by SAM

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