Classification of Stellar Spectra

> Classification of Stellar Spectra

The photographic study of stellar spectra was initiated in 1885 by the American astronomer Edward Charles Pickering at the Harvard College Observatory and carried out principally by the American astronomer Annie J. Cannon. This research led to the important discovery that stellar spectra can be arranged in a continuous sequence on the basis of the relative intensity of certain absorption lines. The observed variations within the sequence provide clues to the ages of the different stars and their stages of development.

Hertzsprung-Russell Diagram

The position in the H-R diagram of the point representing a star corresponds to its brightness and temperature. Stars on the left of the diagram are blue because they are hot, while those on the right are red because they are cool. The diagonal band of stars running from the upper left to the lower right is called the main sequence. Stars in the upper right are called red giants: although they are cool and red, they are very bright, because they are big. Stars near the bottom (known as white dwarfs) are very hot, but not very bright, because they are small. This diagram was developed independently by Ejnar Hertzsprung, a Dane, and Henry Norris Russell, an American.

The various stages in the sequence of spectra, which are designated by the letters O, B, A, F, G, K, and M, are characterized especially by variations in the intensity of the hydrogen lines that occur throughout the sequence. In addition, the lines of other elements become prominent at different stages. Subscripts from 0 to 9 are used to denote gradations in the pattern within each class.

Contents:

1. Class O

2. Class B

3. Class A

4. Class F

5. Class G

6. Class K

7. Class M

Class O

This group is primarily characterized by the lines of helium, oxygen, and nitrogen, besides the hydrogen lines. The O group, which comprises extremely hot stars, includes those showing bright-line spectra of hydrogen and helium, as well as those exhibiting dark lines of the same elements.

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Class B

In this group the helium lines attain maximum intensity at the subdivision B2 and fade progressively in higher subdivisions. The intensity of the hydrogen lines steadily increases throughout the subdivisions. The group is typified by the star Epsilon (e) Orionis.

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Class A

This group comprises the so-called hydrogen stars with spectra dominated by the absorption lines of hydrogen. A typical star of this group is Sirius, the Dog Star.

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Class F

This group comprises stars in which the so-called H and K lines of calcium and the characteristic lines of hydrogen are strong. A notable star in this category is δ Aquilae.

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Class G

This group comprises stars with prominent H and K calcium lines and less prominent hydrogen lines. The spectra of many metals, notably iron, are also present. The Sun belongs to this group, and the G stars are therefore frequently called solar stars.

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Class K

This group comprises stars having strong calcium lines and lines indicating the presence of other metals. The violet light of the spectrum is less intense, compared with the red light, than in the classes previously mentioned. The group is typified by Arcturus.

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Class M

This group comprises stars with spectra dominated by bands resulting from the presence of metallic-oxide molecules, notably those of titanium oxide. The violet end of the spectrum is less intense than that in the K stars. The star Betelgeuse, a Orionis, is typical of this group.

All these characteristics are compatible with the conclusion that stars of these classes are all of similar chemical composition and are arranged in a temperature order from hottest to coolest. The absolute surface temperatures of the various star groups are approximately the following: O, 22,200° C (40,000° F); B, 13,900° C (25,000° F); A, 10,000° C (18,000° F); F, 6650° C (12,000° F); G, 5540° C (10,000° F); K, 3870° C (7000° F); and M, 1760° C (3200° F). The temperature at the centre of the average star is about 20,000,000° C (36,000,000° F).

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