Originally printed in the Oct. 23, 2002 edition of the Illinois Valley News
Humans are good at grouping things, words, and even ideas into categories. Doesn’t every household have a silverware holder to keep the spoons , forks, and knives in separate compartments? There are good reasons why we do this but some people seem to be more fanatical about it than others. Should I hang shirts in the closet in order of color, or by fabric type, or even by their intended use, such as: church shirts, casual, or grubbies? Should I hang the church shirts next to the suit coats or should the pants go next to the suit coats?
Some people think that biologists are a little too fanatical about classification with phyla, orders, and species names to keep similar organisms together. Even astronomers get some static over whether a newly discovered object in space should be a planet or put in a different category. Stars, which produce their own light, are not planets which reflect sunlight. But even stars are subdivided into smaller groups.
Classification of stars is based on two main properties, brightness and color. The brightness, or magnitude, of a star is determined by its size, distance from the earth, its age, and the chemicals it contains. The color of a star, according to present theories, is primarily determined by its temperature with red being the coolest and blue-white being the hottest. Our sun is considered a medium size, medium temperature (yellow) star. It is the brightest star because it is so close to the earth. Astronomers can mathematically calculate what the brightness of stars would be if they were all at the same distance from the earth. This is then called the absolute magnitude of a star. In this comparison the sun would be very dim and some of the fainter stars would have a low magnitude. The lower the magnitude is, the brighter the star would be, with some stars having a negative magnitude because they would be so bright. Astronomers are not always logical.
In 1906 a Danish astronomer named Ejnar Hertzsprung made a graph with the absolute magnitude of stars on the vertical axis and temperature (color) on the horizontal axis. After plotting most of the known stars he found that the majority would be located on a diagonal zone through the graph. Most red stars are dim, yellow are brighter, the white stars brighter still, and the blue-white stars being at the top left corner of the graph as the brightest. This zone was called the “Main Sequence” stars. This idea was later supported by the work of an American astronomer, Henry N. Russell in 1913. The two men then working together corrected their graph to produce what today is called the H- R Diagram, using their last initials.
Today the H- R Diagram is an essential part of most astronomy textbooks and is well known by all astronomy students. The diagram is used by astronomers to determine approximate distances for stars, as well as temperature estimates of newly discovered stars. Some stars, such as red giants and white dwarfs are not on the main sequence zone in the diagram. This has led to much speculation about the way stars are formed , how long they will last, and how they will behave at the end of their existence.
Hertzsprung, born Oct. 8, 1873 in Copenhagen, Denmark, made many other important discoveries in astronomy. Calculating distances using astrophotography was another important contribution by Ejnar Hertzsprung. Henry N. Russell, born Oct. 25, 1877 in New York state, became the director of the Princeton University Observatory and developed principles of atomic physics into theories of star evolution. These two men created a star classification system which is widely used by astronomers of today.
