A simple graph (usually with intimidating labels) offers a concise introduction to the varieties of stars and their stages of evolution.

Stars are sometimes described as being like the sun, only very far away. Like the sun, other stars are large balls of gas that radiate light and heat. However, stars come in a variety of sizes, temperatures and brightnesses. The differing temperatures give the stars different colors. When looked at in this way, few stars are “like the sun”.

Scientists can measure the brightness of a star and they can determine the temperature of the star by analyzing the spectrum of light that it emits. The astronomers Enjar Hertzsprung and Henry Norris Russell used the brightness and temperature of stars to create a chart now known as the H-R diagram. The H-R diagram has become very useful in understanding the varieties and life histories of stars.

The H-R diagram is a plot of the star temperature along the horizontal axis and the brightness along the vertical axis. The hottest temperature is at the left and decreases to the right. The vertical scale has brightest stars at the top and dimmest at the bottom. The Sun’s brightness is at the midpoint. An individual star is placed on the diagram according to its temperature and brightness. The sun falls just about in the middle.

H-R Diagram

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If stars were all “like the sun” we would see a cluster of stars in the diagram centered around the sun. If star brightness and star temperature were totally unrelated we would see a random scattering all over the diagram. In fact the H-R diagram shows neither pattern. Instead we see stars concentrated along the upper-left to lower-right diagonal with some stars also in the upper-right and lower-left corners.

The star temperature causes the star to have a particular color according the the principle of black-body radiation. The coolest stars are red, then they range to orange, yellow, white, and finally to blue as they get hotter. The temperature range is 3000K to 30,000K. We usually think of “red-hot” as being very hot – and 3000K is indeed dangerously hot. Yet a red star is the coolest temperature for a star. For comparison, 3000K is 4940F or 2727C.

The upper-left to lower-right diagonal is called the main sequence. Along the main sequence, stars are smaller dwarfs on the right and progress to large super-giants on the left. Simply put, the more gas a star has, the larger it is and the hotter it can get. The main-sequence stars are in the prime of their lives. Most of them attain this size soon after their formation and keep it until shortly before their demise. The red stars on the main sequence are called red dwarfs and are thought to keep this size indefinitely.

Stars in the upper-right are red giants and red super-giants. They represent what happens to yellow, white, and blue stars that begin to run low on hydrogen fuel. They cool down and expand in size. The red giants will eventually eject most of their mass as a planetary nebula. What remains after the nebulae form are the white dwarfs in the lower-left corner of the H-R diagram.

So, most stars are not like the sun. Over 90% of stars are tiny red dwarfs, invisible to the naked-eye star watcher. Many of the bright stars, such as Sirius, are hotter than the Sun. Those bright stars that are cooler are much larger, such as the red super-giant Betelgeuse or the red giant Aldebaran. The great distances to the stars hide these differences and render them as similar-looking points of light.


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