Astronomy

Astronomers made classification for stars based on its spectra. This aimed to water down the perception and analysis to the stars. In the year 1863, this classification is made for the first time by astronomer Angelo Secchi. Later, his works developed again by a commission from Harvard observatory leaded by Annie J. Cannon.

Initially this spectral classification is made based on strength of hydrogen atom absorption line. Stars’ spectra classed starting from A class, representing stars with strong hydrogen line, and ended with P class, which have weakest hydrogen line. However, is then realized that the difference of spectra is also influenced by different temperature of pertinent star. Hence the classification is corrected by changing its sequence and eliminates some spectral classes pursuant to the star temperature.

It is finally decided that sequence of stars’ spectral classes is started from highest to lowest temperature stars, so that, the new sequence is O, B, A, F, G, K, and M respectively. Because this division is still too wide, then each classes divided again on several levels, signed by number of 0 to 9. For example, brightest B class stars classified as B0, feebler is B1, and so on to B9.

O class stars being the hottest stars with temperature between 30.000 to 60.000ºC; B class stars has temperature ranged from 10.000 to 30.000ºC; and A class stars has temperature between 7.500 to 10.000ºC. Dominant color for these three classes of stars is blue. F Class stars have its temperature among 6.000 until 7.500ºC, whereas G class with temperature in between 5.000 to 6.000ºC. Lower temperature stars classified as K class (3.500 to 5.000ºC), and M class (less than 3.500ºC). Our sun is classified as G class star. Astronomers believe that the G class star is most likely to contribute existence of the planet that can harbor life.

This classification still extended again to covering the stars with very typical spectra, so that cannot be packed into the classification. For example is C class spectra, which consist of carbon-rich stars. Some stars with rich compound of zirconium oxide classified as S class stars. Booth S and M class stars has relatively same temperature. Also there is another kind of spectral class named Wolf-Rayet class, which is actually O class stars with wide range of emission lines resulted from material released from the star.

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