Stellar Classification

There are over 300 billion stars in the Milky Way galaxy and over 1 billion trillion stars in the observable Universe. Each of them vary in color, temperature, size and brightness. How do Astronomers classify trillions of stars with so many varying features? To make it easy to categorize/identify stars, Astronomers came up with the system of stellar classification, where they categorize stars based on their spectral characteristics.

The modern classification system for stars is the Morgan-Keenan classification. Under this classification system, stars are classified under the letters O, B, A, F, G, K, or M. Stars classified as ‘O-type’ are the hottest whereas stars that are classified ‘M-type’ are the coldest. Each letter is sub-divided into 10 categories of numbers, from 0 to 9 where 0 is the hottest and 9 is the coldest. Few classification examples- K8, O0 etc.

But, how do we know how to classify which Star under which category? We split the light from a Star using a prism analyze that light to categorize the Star. Radiation from a star is split using a prism which reveals a spectrum comprised of rainbow colors. When we use a magnifying instrument and observe this spectrum, we will find many dark lines in between. These are called spectral lines. Each dark line indicates a specific chemical element that is present in the Star. This is shown below.

Other features like the strength of the dark line indicate the abundance of the element in the star. The spectrum gives us information about the age, temperature, chemical composition of the star and allows us to categorize the star according to the table provided later in the article.

Classification table

ClassTemperatureColorMass (in solar masses)Radius (in solar radii)
O>30,000 KBlue>16>6.6
B10,000-30,000 KBlue white2.1-161.8-6.6
A7,500-10,000 KBlue white1.4-2.11.4-1.8
F6,000-7500 KWhite1.04-1.41.15-1.4
G5,200-6000 KYellowish White0.8-1.040.96-1.15
K3,700-5,200 KPale Yellow Orange0.45-0.80.7-0.96
M2,400-3,700 KLight Orange Red0.08-0.45<0.7

Other Stellar and Non-Stellar Classes

There are classifications of other types of stars as well. for example, white dwarf Stars are classified Classes D. L, T and Y where introduced to classify cool stars like Red and Brown Dwarfs. Class C was introduced to classify Carbon stars- Old red giant stars with a great abundance of carbon in atmosphere.

Finally, there are Classes for classification of non-stellar objects as well. For example, class-P is the classification of a Planetary Nebula. Class Q is the classification for Novae.

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