Casper Planetarium

Human Eyes Detect Temperatures of Stars
02.06.2012

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Human Eyes Detect Temperature of Stars
By Rod Kennedy

February is not the best month to go out with a telescope. This has less to do with the temperature and more to do with the atmosphere. Between the howling wind and the turbulence caused by temperature variation, the atmosphere is chaos, even if the sky is perfectly clear. However, while this makes telescope observing impractical, it does not hinder naked eye observing. Only the most jaded skywatcher will say there is nothing to see with the naked eye. In reality we can learn a lot about the nature of stars just by looking at their color. February offers a great chance to do this by turning south and looking for a very large asterism known as the Winter Hexagon. An asterism is a group of stars that is easy to recognize but is not really a constellation.

Begin by looking for the constellation Orion the Hunter. The bright star at Orion’s lower right is the star Rigel. Rigel appears a bright blue-white color. This tells us that Rigel is very hot. The hottest stars appear bright blue. Think of a light bulb on a dimmer switch. When the light is dim it appears red or orange, but when it is brightest it appears blue white. From Rigel look down and to the East for Sirius in Canis Major. Sirius is not as hot as Rigel so it appears more white than blue. However, Sirius appears brighter in our sky than Rigel because Sirius is much closer to our solar system than Rigel. Sirius is less than ten light years away while Rigel is more than 700 light years away.

Moving higher in the sky we find the star Procyon in Canis Minor. Procyon is cooler still, appearing yellow-white. Moving still higher we find the star Pollux in Gemini the Twins. Pollux is very cool and appears orange. Its nearby neighbor, Castor is hotter and appears white like Sirius. Almost directly overhead is the star Capella in Auriga. Capella is a star much like our own sun and appears yellow. Stars like Capella and the Sun are very average in temperature, mass, diameter and life expectancy. From Capella we move down and to the West to find Aldebaran, in Taurus the Bull. Aldebaran is cool like Pollux, and so appears orange. In the center of the Winter Hexagon is the star Betelgeuse in the shoulder of Orion. Betelgeuse represents the coolest type of star, appearing red in color.

The colors of stars depend on their temperatures, and astronomers use this to classify stars into what are known as Spectral Classes. The modern classification scheme has seven classes each class representing a different temperature range of stars. The classes are O, B, A, F, G, K, and M. Type M stars are the coolest, and appear red. Type K stars are orange, type G stars are yellow, type F are yellow-white, type A are white, type B are blue-white and type O are bright blue. The coolest stars are typically less than 2,500 degrees Kelvin while the hottest stars are more than 30,000 degrees Kelvin. So why do we call this scheme a Spectral Classification? Objects that are hot give of most of their energy in different wavelengths of light depending on their temperature. Hot stars like Rigel will give off most of their energy in the UV wavelength, while cool stars like Betelgeuse give off most of their energy in the Infrared. This means that the naked eye color of a star can tell us something about the wavelengths of energy it gives off.

While February may not be the best month to get out and use a telescope, it is a great month to get out and view the sky with the original observing tool, the human eye. Simply looking at the stars tells us about their temperature, their energy output, their size, their age and in many cases their eventual deaths.