How to Read a Hertzsprung Russell Diagram

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Stars come in many varieties and ages. Red giants, white dwarfs, and the multitude of main sequence stars all have important differences. Anyone who knows how to read a Hertzsprung-Russell diagram can categorize a star by where it falls on the chart, and immediately know quite a bit about its past, its present nature, and its future.

A Hertzsprung-Russell diagram, an HRD, maps changes in stars as they age. To do this, it plots the relationship between each star’s luminosity and its temperature. A Hertzsprung-Russell diagram shows where each star belongs on a graph of brightness and heat.

Technically, luminosity is the amount of energy a star radiates in one second. However, since stars’ radiation mostly appears as light, as our sun’s radiation does, luminosity amounts to a measure of a star’s overall brightness. Some Hertzsprung-Russell diagrams use absolute magnitude instead. Absolute magnitude is a measure of brightness that corrects for the effects of distance on our perception of a star’s shine. Both luminosity and absolute magnitude are rated by using our sun as the standard unit.

Surface temperature is the other characteristic plotted on a Hertzsprung-Russell diagram. Temperature at the surface, measured in Kelvins, is used because no one can see into a star. Sometimes the color of stars is also charted, because color varies with temperature. Always, temperature is graphed with the highest temperature on the left, and the lowest on the right.

Therefore, a star in the upper left corner is hot and bright. A star in the lower right is cool and dim. Similar stars group together on the diagram, so a star’s position on the Hertzsprung-Russell Diagram yields information about its age, kind, and ultimate destiny.

When a group of stars is plotted, most of them will form a curved line through the center of the diagram. This is the main sequence. It contains the commonest stars, in the middle of their life cycles, and our sun is one of them. These stars are fusing hydrogen into helium to shine, and are in balance between the gas pressure and radiation that tends to expand them, and the gravity that tends to shrink them. They are in equilibrium, and probably will be for billions of years.

When a star has turned most of the hydrogen in its core to helium, what happens to it next depends on its mass. Stars that are smaller than the sun, red dwarfs, stay on the main sequence the longest. A star with much more mass than the sun will pass through a red supergiant phase and then blow up as a supernova, spreading heavy elements through space.

A star of mass similar to that of our sun will eventually begin to use helium as fuel in its core. Meanwhile, the outer layer of the star, still composed of hydrogen, will expand, cool, and continue to shine.

The star becomes a large relatively cool red giant. Its luminosity is relatively high because it is so large, not because it burns so brightly. In the same way, a huge chandelier makes more light than a light bulb, though each small bulb of it burns no brighter. Red giants are above the main sequence on the HRD, because they are so luminous.

After millennia, a red giant will go through a period of change that is rapid in stellar terms. It may become a particular type of variable star. A variable star changes its brightness and size in pulses. Eventually, it may throw off its outer shell, creating a beautiful planetary nebula, a glowing gas cloud. A planetary nebula has nothing to do with planets; it was misnamed because it looked like a gas giant planet through early telescopes.

Once a star loses its outer shell it may become a white dwarf, placed below and to the left of the main sequence. Its luminosity is low because of its small size, but its temperature is still high. A star that begins at about the size of the sun may end as a white dwarf about the size of the earth. A white dwarf will slowly cool until it becomes a black dwarf, but our universe is not old enough for any to exist yet.

Once someone knows how main sequence stars, red giants, and white dwarfs are arranged on an HRD, they will know a star’s type and its probable fate as soon as they see its position. Though the diagram cannot tell everything about a star, it does codify large amounts of useful information.

Ejnar Hertzsprung and Henry Norris Russell invented the Hertzsprung-Russell diagram, beginning in about 1910. Their work produced a tool that has ever since allowed astrophysicists to chart and explain stellar evolution: the origin, transformation, and slow death of stars.

Cosmos by Carl Sagan

More about this author: Janet Grischy

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