The Spectral Types of Stars

When you look up at the stars at night either with your naked eyes or through a telescope, you are looking at history. Each of those twinkling little dots tells a story that science and technology have finally begun to understand. To understand what each star’s unique story is you must look at it through the spectrum of its color. This is done by using a chart and learning what the spectral types of stars truly mean.

There are several different spectral classes of stars which include O, B, A, F, G, K and M from hottest to coolest. The spectral chart we base research on today has been used since 1918 and it seems to work much better than all methods before it.

The White Hot End of the Spectrum

The most common spectral type of stars you see in the sky with your bare eye is classified as a class “A” though other stars in this area of the spectrum can be referred to as type “O” and “B” if based solely on color. It is the Sirius, Vega or Altair type of star. They are bluish white to white in color due to their strong hydrogen lines and lines of ionized metals. This type star is the hottest and brightest star in the sky and their temperature is between 7,500 and 60,000 K. This type of star is the youngest type. They burn hot and burn out quickly by astrological standards. Typically they will only burn for less than 100 million years.

The Cooler Side of the Spectrum

Three more classifications of stars, in varying shades of yellow, include the classes “F”, “G”, and “K”. Being relatively rare to see makes them unique, especially considering that this group also holds the sun, which is categorized under class G. These stars can be somewhat unstable, and don’t live very long into the supergiant stage. All three classes hold stars that have very weak hydrogen lines and ionized metals. This means that the star is either too hot or too cold for the lines to form and for them to glow. At only 3,500 to 6,000 K in temperature, it is safe to say the stars are simply too cold for hydrogen. Some examples of these stars include Polaris, our sun, and Algol B.

Finally, for avid star gazers who own telescopes you have class “M”. This type cannot be seen with the naked eye even though this one class accounts for 76% of all stars. They have low luminosities, so none are bright enough to be seen without the proper equipment handy. A lot of these stars are red dwarfs, but this class also hosts giants and even few supergiants. There are no hydrogen lines, and the temperature can be anything under 3,500K.

The spectral types of stars are not an all-inclusive list or set of categories for the stars in our night’s sky. Each of the stars in the sky can also have other letters beyond the basic seven. These other sub types can mean a wide range of things. For example if you see a star classified as a “G” and then it has lower case letters beside it, such as “neb” and “nn”. All these codes mean something and now you know that you have a yellow star that is like the sun, that it has a nebula spectrum that is mixed with the stars own glow and that it has very broad lines due to its fast rotation.

Take some time and look at the stars for yourself. See if you can find colors that fit into the certain spectral types of stars listed and see if you can learn more about the stars you see every night.

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