STARS AND THEIR SPECTRAL TYPE
When in 1872 Dr. Henry Draper placed a very small wet plate in the
camera of his spectroscope and, by careful following, on account of the
necessarily long exposure, secured the first photographic spectrum of a
star ever taken, he could hardly have anticipated the wealth of the new
field of research which he was opening. His wife, Anna Palmer Draper,
was his enthusiastic assistant in both laboratory and observatory, and
on his death in 1882, she began to devote her resources very
considerably to the amplification of stellar spectrum photography. At
first with the cooperation of Professor Young of Princeton, and later
through extension of the facilities of Harvard College Observatory,
whose director, the late Professor Edward C. Pickering, devoted his
energies in very large part to this matter, all the preliminaries of the
great enterprise were worked out, and a comprehensive program was
embarked upon, which culminated in the "Henry Draper Memorial," a
catalogue and classification of the spectra of all the stars brighter
than the ninth magnitude, in both the northern and southern hemispheres.
One very remarkable result from the investigation of large numbers of
stars according to their type is the close correlation between a star's
luminosity and its spectral type. But even more remarkable is the
connection between spectral type and speed of motion. As early as 1892
Monck of Dublin, later Kapteyn, and still later Dyson, directed
attention to the fact that stars of the Secchi type II had on the
average larger proper motions than those of type I. In 1903 Frost and
Adams brought out the exceptional character of the Orion stars, the
radial velocities of twenty of which averaged only seven kilometers per
second.
Soon after, with the introduction of the two-stream hypothesis, a wider
generalization was reached by Campbell and Kapteyn, whose radial
velocities showed that the average linear velocity increases continually
through the entire series B, A, F, G, K, M, from the earliest types of
evolution to the latest. The younger stars of early type have velocities
of perhaps five or six kilometers per second, while the older stars of
later type have velocities nearly fourfold greater.
The great question that occurs at once is: How do the individual stars
get their motions? The farther back we go in a star's life history, the
smaller we find its velocity to be. When a star reaches the Orion stage
of development, its velocity is only one-third of what it may be
expected to have finally. Apparently, then, the stars at birth have no
motion, but gradually acquire it in passing through their several types
or stages of development.
More striking still is the motion of the planetary nebulae, in excess of
25 kilometers per second, while type A stars move 11 kilometers, type G
15 kilometers, and type M 17 kilometers per second. Can the law
connecting speed of motion and spectral type be so general that the
planetary nebula is to be regarded as the final evolutionary stage?
Stars have been seen to become nebulae, and one astronomer at least is
strongly of the opinion that a single such instance ought to outweigh
all speculation to the contrary, as that stars originate from nebulae.
In his discussion of stellar proper motions, Boss has reached a striking
confirmation of the relation of speed to type, finding for the cross
linear motion of the different types a series of velocities closely
paralleling those of Kapteyn and Campbell.
Concerning the marked relation of the luminosities of the stars to their
spectral types, there is a pronounced tendency toward equality of
brightness among stars of a given type; also the brightness diminishes
very markedly with advance in the stage of evolution. There has been
much discussion as to the order of evolution as related to the type of
spectrum, and Russell of Princeton has put forward the hypothesis of
giant stars and dwarf stars, each spectral type having these two
divisions, though not closely related. One class embraces intensely
luminous stars, the other stars only feebly luminous. When a star is in
process of contraction from a diffused gaseous mass, its temperature
rises, according to Lane's law, until that density is reached where the
loss of heat by radiation exceeds the rise in temperature due to
conversion of gravitational energy into heat. Then the star begins to
cool again. So that if the spectrum of a star depends mainly on the
effective temperature of the body, clearly the classification of the
Draper catalogue would group stars together which are nearly alike in
temperature, taking no note as to whether their present temperature is
rising or falling.
Another classification of stars by Lockyer divides them according to
ascending and descending temperatures. Russell's theory would assign
the succession of evolutionary types in the order, M_{1}, K_{1}, G_{1},
F_{1}, A_{1}, B, A_{2}, F_{2}, G_{2}, K_{2}, M_{2}, the subscript 1
referring to the "giants," and 2 to the dwarf stars. In large part the
weight of evidence would appear to favor the order of the Harvard
classification, independently confirmed as it is by studies of stellar
velocities, Galactic distribution, and periods of binary stars both
spectroscopic and visual, where Campbell and Aiken find a marked
increase in length of period with advance in spectral type. At the same
time, a vast amount of evidence is accumulating in support of Russell's
theory. Investigations in progress will doubtless reveal the ground on
which both may be harmonized.
The publication of the new Henry Draper Catalogue of Stellar Spectra is
in progress, a work of vast magnitude. The great catalogue of thirty
years ago embraced the spectra of more than ten thousand stars, and was
a huge work for that day; but the new catalogue utterly dwarfs it, with
a classification much more detailed than in the earlier work, and with
the number of stars increased more than twenty-fold. This work,
projected by the late director of the Harvard Observatory, has been
brought to a conclusion by the energy and enthusiasm of Miss Annie J.
Cannon through six years of close application, aided by many assistants.
The catalogue ranges over the stars of both hemispheres, and is a
monument to masterly organization and completed execution which will be
of the highest importance and usefulness in all future researches on the
bodies of the stellar universe.