Hertzsprung-Russell Diagram


The Hertzsprung-Russell Diagram or, the H-R Diagram for short, is a graph which plots stars according to their temperature and absolute magnitude. This graph reveals a pattern, which in fact is quite interesting. The H-R Diagram is named for the two astronomers, Ejnar Hertzsprung and Henry Russell, who discovered this pattern of stars. These two astronomers independently discovered that comparing magnitudes and spectral class (color) of stars yielded a lot of information about them.

One key purpose of the H-R diagram is to show the relationship between temperature and absolute magnitude of stars. The type of temperature measurement used is Kelvin, where the zero point is equal to -273.16 C. On the H-R Diagram, the temperature of degrees Kelvin ranges from 3,000 to 30,000. The absolute magnitude of stars on the H-R Diagram ranges from +15 to -10. Absolute magnitude is how bright stars would appear if they were positioned at 32.6 light years away from earth. On this scale, the lower the number, the brighter the star. Thus, a start with an absolute magnitude of -10 would be much brighter than a star with an absolute magnitude of +15.

The two astronomers found many patterns after developing their graph. They found that 90% of stars graphed fell within a band that ran through the middle of the graph. These stars range from cool, dim, red stars at the lower right of the H-R Diagram to hot, bright, blue stars at the upper left corner of the H-R Diagram. The stars that fall into to this band are known as main-sequence stars. Stars such as the sun, and almost every start visible in the night sky fall within this band of main sequence stars.

There is another group of stars which are cool and bright that appear near the upper right corner of the H-R Diagram. These stars are very large and therefore have very big surface areas. These large surface areas give off large amounts of light and this makes the stars bright. Most of these stars are known as red giants. Some are so large however that they are referred to as supergiants. Red giants have a temperature of about 3,500 degrees Kelvin and an absolute magnitude of around 0. Supergiants have a temperature of around 3,000 degrees Kelvin and an absolute magnitude of about -7.

Another group of stars, which are rather small, is found near the bottom left of the H-R Diagram. These stars are known as white dwarfs. These stars are quite hot, around 20,000 degrees Kelvin. However, because of their small size, they are not very bright. A white dwarfs absolute magnitude is around +13.

There is some other information that the H-R diagram can give us about stars including spectral class (color) and luminosity. The possible colors that a star can be classified under are blue, white, yellow, orange, red, brown, and black. A black star is one that is known to be completely dead. A brown star describes a star that never reached a temperature of 18 million degrees F (10,000,255 K).