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, based on the relative intensity of certain absorption lines occurring in the spectra.
The observed variations within
the sequence provide clues to the age of the different stars and
their stages of development.
(see Evolution of Stars)
Hertzsprung-Russell
Diagram
The H-R diagram compares
the brightness of a star with its temperature. The diagonal line running
from the upper left to the lower right is called the Main Sequence.
Stars lying on
the Main Sequence are blue when they are bright and red when they are dim.
Stars in the upper
right (called Red Giants) are very bright, but still appear red. Stars near the bottom
(known as White Dwarfs) are white, but not very bright. This diagram
was developed independently by Ejnar Hertzsprung,
a Danish astronomer, and Henry Norris Russell,
an American astronomer.
The various stages in the spectrum sequence, 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.
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.
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.
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.
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 d Aquilae.
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.
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.
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 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 interior temperature
of the average star is about 20,000,000° C (36,000,000° F).