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analysis was applied to the light coming to us from the heavenly bodies, a new era in astronomical science was,opened up of such importance that the See also:

BODY

body of knowledge revealed by this method has sometimes been termed the " new See also:

ASTRONOMY (from Gr. &arpov, a star, and v ,usiv, to classify or arrange)

astronomy." The development of the method has been greatly assisted by See also:

photography, while the application of photometric measurements has been a powerful See also:

AUXILIARY (from Lat. auxilium, help)

auxiliary in the See also:

WORK

work. It has thus come about that astrophysics owes its See also:

RECENT

recent development, and its recognition as a distinct branch of , astronomical science, to the See also:

COMBINATION (Lat. combinare, to combine)

combination of the processes involved in the three arts of See also:

SPECTROSCOPY (from Lat. spectrum, an appearance, and Gr. a-sore y, to see)

spectroscopy, photography and See also:

PHOTOMETRY (from Gr. (Pews, xarrbs, light, µerpov, a measure)

photometry. The most See also:

general conclusions reached by this combination may be summed up as follows: I. The heavenly bodies are composed of like See also:

MATTER

matter with that which we find to make up our globe. The See also:

sun and stars are found to contain the more important elements with . which See also:

chemistry has made: us acquainted. , See also:

IRON [symbol Fe, atomic weight 55.85 (0=16)]

Iron, See also:

CALCIUM [symbol Ca, atomic weight 40.0 (o= x6)]

calcium and See also:

HYDROGEN [symbol H, atomic weight x-oo8 (0=16)]

hydrogen may be especially mentioned as three See also:

FAMILIAR (through the Fr. familier, from Lat. familiaris, of or belonging to the familia, family)

familiar chemical elements which enter largely into the constitution of all the matter of the heavens. It would be going too far to say that all the elements known to us exist. in the sun or 'the s-tars;- nor is the question whether the rarer ones can or cannot be found there of See also:

PRIME, PRIMER AND PRIMING

prime importance. The general fact of identity in the See also:

main constituents is the one of most fundamental importance. ` It would be going too far in the other direction' to claim that all the elements which compose the heavenly bodies are found on the See also:

earth.

There are many lines in the spectra of the stars, as well as of the nebulae, which are not certainly identified with those belonging to any elements known to our chemistry. The recent discoveries growing out of the investigation of newly discovered forms of See also:

RADIATION, THEORY OF

radiation See also:

lead to the conclusion thatthe question of. the forms of matter in the stars has far wider range than the See also:

SIMPLE

simple question whether any given See also:

ELEMENT (Lat. elementum)

element is or is not found outside our earth. The question is rather that of the infinity of forms that matter may assume; including that most attenuated See also:

FORM (Lat. forma)

form found in the nebulae, which seem to be composed of matter more refined than even the atoms supposed to make up the matter around us. 2. The second conclusion is that, as ageneral See also:

RULE

rule, the incandescent heavenly bodies are not masses of solid or liquid matter as formerly assumed, but mainly masses either of See also:

gas, or of substances gaseous in their- nature, so compressed by the See also:

GRAVITATION (from Lat. gravis, heavy)

gravitation of their superincumbent parts toward a See also:

COMMON

common centre that their properties combine those of the three forms of matter known to us. We have strong See also:

REASON (Lat. ratio, through French raison)

reason to believe that even the sun, though much denser than the general See also:

AVERAGE

average of819 the stars, may possibly be characterized as gaseous rather than solid. Probabilities also seem to favour the view that this-may, to a certain extent, be true of the four See also:

GREAT

great planets of our See also:

SYSTEM

system. The See also:

case of bodies like our earth and See also:

Mars, which are solid either superficially or throughout, is probably confined to the smaller bodies of the universe. 3. A third characteristic which seems to belong to the great bodies of the universe is the very high temperature of their interior. With a modification to be mentioned presently, we may regard them as intensely hot bodies, probably at a temperature higher than any we can produce by artificial means, of which the superficial portions have cooled off by radiation into space. A modification in this proposition which may hereafter be accepted involves an See also:

EXTENSION (Lat. ex, out ; tendere, to stretch)

extension of our ideas of temperature, and leads us to regard the interior See also:

HEAT (0. E. haktu, which like " hot," Old Eng. hat, is from the Teutonic type haita, hit, to be hot; cf. Ger. hitze, heiss; Dutch, hitte, heet, &c.)

heat of the heavenly bodies as due to a form of molecular activity similar to that of which See also:

RADIUM (from Lat. radius, ray)

radium affords so remarkable an instance.

This modification certainly avoids many difficulties connected with the question of the interior heat of the earth, sun, See also:

JUPITER

Jupiter and probably all the larger heavenly bodies. A limit is placed on our knowledge of astrophysics which, up to the See also:

PRESENT

present See also:

time, we have found no means of overstepping. This is imposed upon us by the fact that it is only when matter is in agaseous form that the spectroscope can give •us certain knowledge as to its physical See also:

CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

condition. So long as bodies are in the solid See also:

state the light which they emit, though different in different substances, has no characteristic so precisely marked that detailed conclusions can be See also:

DRAWN

drawn as to the nature of the substance emitting. it. Even in a liquid form, the spectrum of any See also:

KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

kind of matter is less characteristic than that of gas. More-over, a gaseous body of See also:

UNIFORM

uniform temperature, and so dense as to be non-transparent, does not radiate the characteristic spectrum of the gas of which it is composed. Precise conclusions are possible only when a gaseous body is transparent through and through, so that the gas emits its characteristic rays—or when the rays from an incandescent body of any kind pass through a gaseous envelope at a temperature See also:

LOWER

lower than that of the body itself. - In this case the revelations of the spectroscope relate only to the constitution of the gaseous envelope, and not to the body below the envelope, from which the light emanates. The outcome of this See also:

DRAWBACK

drawback is that our knowledge of the chemical constitution of the stars and planets is still confined to their atmospheres, and that conclusions as to the constitution of the interior masses which form them must be drawn by other methods than the spectroscopic one. When the spectroscope was first applied in astronomy, it was hoped that the light reflected from living matter might be found to possess some See also:

PROPERTY

property different from that found in light reflected from non-living -natter, and that we might thus detect the presence of See also:

LIFE

life on the See also:

SURFACE

surface of a See also:

PLANET (Gr. ssXavirrns, a wanderer)

planet by a study of its spectrum; but no See also:

hope of this kind has so far been realized. We have, in this brief view of the subject, referred mainly to the results of spectrum analysis. Growing out of, but beyond this method is the beginning of a great branch of research which may ultimately explain many heretofore enigmatical phenomena of nature.

The See also:

DISCOVERY

discovery of radio-activity may, by explaining the interior heat of the great bodies of the universe, solve a ;difficulty which since the See also:

MIDDLE

middle of the 19th century has been discussed by physicists and geologists—that of reconciling the long duration which geologists claim for the crust of the earth with the See also:

period during which physicists have deemed it possible that the sun should have radiated heat. See also:

EVIDENCE (Lat. evidentia, evideri, to appear clearly)

Evidence is also accumulating to show that the sun and stars are radio-active bodies, and that emanations proceeding from the sun, and reaching the earth, have important relations to the phenomena of Terrestrial See also:

Magnetism and the See also:

Aurora. The subject of Astrophysics does not admit of so definite a sub-See also:

DIVISION (from Lat. dividere, to break up into parts, separate)

division as that of Astrometry. The conclusions which researches See also:

RELATING

relating to it have so far reached are treated in the articles See also:

STAR

STAR; SUN; See also:

COMET (Gr. Koµieiis, long-haired)

COMET; NEBULA; AURORA POLARIS, &C. (S.

End of Article: ASTROPHYSICS

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