Albrecht Unsöld

The Structure and Evolution of Stars

The significance of his diagram for investigating stellar evolution was clearly realized by H.N. Russell as early as 1913. But its deeper understanding, and thus a theory of stellar evolution founded on observational facts, became possible only in connection with the study of the innerstructure of stars. The older works oft. H. Lane (1870), A. Ritter (1878–89), R. Emden (the “Spheres of Gas” appeared in 1907) and others could be based only on classical thermodynamics. A.S. Eddington succeeded in combining these approaches with the theory of radiation equilibrium and with Bohr’s theory of atomic structure, which had in the meantime been formulated; his book “The Internal Constitution of Stars” (1926) gave the start signal for the whole development of modern astrophysics.

Classical Astronomy. The Solar System

We shall begin our treatment of astronomy with a historical overview of classical astronomy (Sect. 2.1) from ancient times up through the introduction of the heliocentric system by Nicholas Copernicus, Tycho Brahe, Johannes Kepler, Galileo Galilei and their contemporaries, and the founding of celestial mechanics by Isaac Newton at the close of the 17th century. We then turn in Sect. 2.2 to the description of motions on the celestial sphere and the coordinate systems used to describe the positions of objects in the sky. In Sect. 2.3, we treat the motions of the Earth its rotation around its own axis and its revolution around the Sun, which make themselves evident as motions on the celestial sphere; in this section, we also take up the sidereal time scale. Following these preparatory remarks, we make the acquaintance of the objects of our Solar System beginning in Sect. 2.4 with the Moon, its motions, its phases, and with lunar and solar eclipses. In Sect. 2.5, we survey the motions of the planets, the comets, and other bodies in the Solar System, and the methods for determining distances between them. After reviewing the fundamentals of mechanics and the theory of gravitation, we discuss in Sect. 2.6 some of their applications to celestial mechanics and, in Sect. 2.7, to the calculation of the orbits of artificial satellites and space probes. Section 2.7 also contains a brief chronology of space-research missions. Finally, we treat the physical structure of the objects in the Solar System: the planets, their moons, and the asteroids in Sect. 2.8; and the comets, meteors, and meteorites in Sect. 2.9.

Interstellare Materie und Sternentstehung

Zwischen den Sternen des Milchstrasensystems fein verteilte Materie trat in den Gesichtskreis der Astronomen zuerst in Gestalt der Dunkelwolken, welche das Licht der hinter ihnen befindlichen Sterne durch Absorption schwachen und roten. Aber erst 1930 konnte R. J. Trumpler zeigen, das auch auserhalb der erkennbaren Dunkelwolken interstellare Extinktion und Verfarbung in der ganzen Milchstrase bei der photometrischen Messung von Entfernungen uber wenige hundert parsec keineswegs zu vernachlassigen sind. Schon 1922 hatte E. Hubble erkannt, das die galaktischen (diffusen) Reflexionsnebel (wie sie z. B. die Plejaden umgeben) durch Streuung des Lichtes relativ kuhler Sterne an kosmischen Staubwolken entstehen, wahrend in den galaktischen (diffusen) Emissionsnebeln interstellares Gas durch die Strahlung heiser Sterne zur Emission eines Linienspektrums angeregt wird. Daraufhin kam in den Jahren 1926/27 die Erforschung des interstellaren Gases rasch in Gang. Zwar hatte schon 1904 J. Hartmann die „stationaren“ Ca lI-Linien entdeckt, welche in den Spektren von Doppelsternen die Bahnbewegung nicht mitmachen, aber erst 1926 entwickelten A.S. Eddington von, der Theorie, O. Struve, J. S. Plaskett u. a. von der Beobachtung her die Vorstellung, das die interstellaren Ca II-, Na I-,... Linien in einer durch die Strahlung der Sterne teilweise ionisierten Gasschicht entstehen, welche die ganze Scheibe der Milchstrase erfullt und auch an deren Rotation teilnimmt. Auf der anderen Seite gelang 1927 I.S. Bowen die lange gesuchte Identifikation der „Nebuliumlinien” in den Spektren der Gasnebel als verbotene Ubergange in den Spektren von [011], [0111], [N II],..., und H. Zanstra entwickelte die Theorie des Nebelleuchtens. Erst etwa zehn Jahre spater erkannte man, das auch im interstellaren Gas — wie in den Sternatmospharen — der Wasserstoff das weitaus uber-wiegende Element ist. O. Struve und seine Mitarbeiter entdeckten mit Hilfe ihres sehr lichtstarken Nebelspek-trographen, das viele O- und B-Sterne von einer ziemlich scharf begrenzten Region umgeben sind, die in der roten Rekombinationslinie Hα des Wasserstoffs leuchtet. Hier mus der interstellare Wasserstoff also ionisiert sein. Die Theorie dieser H II-Regionen hat dann 1938 B. Stromgren entwickelt.

Entfernungen und Zustandsgrößen der Sterne

Wir beginnen mit der Diskussion des uns nachsten Sterns, der Sonne. In Abschn. 6.1 stellen wir ihre Daten zusammen, die haufig fur die Sterne als Einheiten verwendet werden, und lernen die Energieverteilung ihrer Strahlung aus der Photosphare kennen: ein Kontinuum mit zahlreichen Absorptionslinien.

The Cosmogony of the Solar System

We turn back now from the depths of interstellar space to our own Solar System, and the old question of how it came into existence. The daring thought that this question cannot be answered by the handing-down of ancient myths, but only through our own probing, was proposed in France as early as 1644 by Rene Descartes in his whirlpool theory. In Germany, still by 1755, Immanuel Kant had to publish the first edition of his “Allgemeine Naturgeschichte and Theorie des Himmels” anonymously, for fear of the (protestant) theologians; in it, he treated the origin of the Solar System for the first time “according to Newtonian principles”. Kant assumed a rotating, flattened primordial nebula, from which the planets and later their satellites were formed. The description offered independently somewhat later by S. Laplace in 1796 in his popular “Exposition du Systeme du Monde” was based on a similar hypothesis.

The Distances and Fundamental Properties of the Stars

We shall begin with a discussion of our nearest star, the Sun. In Sect. 6.1, we collect the data concerning the Sun, which are often used to define units for describing the properties of other stars, and introduce the energy distribution of the solar radiation from the photosphere: a continuum with numerous absorption lines.

The Physical Structure of the Objects in the Solar System

The study of the planets and their satellites, exploiting the possibilities of space research, has developed in recent years into one of the most interesting but also most difficult branches of astrophysics. In particular, understanding the observations has required all the available resources of physical chemistry and the Earth sciences (geology, mineralology, etc.).

Interstellar Matter and Star Formation

The matter which is finely distributed between the stars of the Milky Way at first came to the attention of astronomers in the form of dark clouds, which weaken and redden the light of those stars which are behind them, due to absorption and scattering. But it was only in 1930 that R. J. Trumpler was able to show that even outside the recognizable dark clouds, interstellar extinction and reddening are by no means negligible in the photometric determination of distances of a few hundred parsec throughout the Milky Way Galaxy. Already in 1922, E. Hubble had recognized that the galactic (diffuse) reflection nebulae (like the one which surrounds the Pleiades, for example) are due to scattering of the light from relatively cool stars in cosmic dust clouds, while in the galactic (diffuse) emission nebulae, interstellar gas is excited bythe radiation from hot stars and therefore emits line spectra. Following his observations, the investigation of the interstellar gas quickly gained momentum in the years 1926/27. The “stationary” Call lines had already been discovered in 1904 by J. Hartmann; they occur in the spectra of binary stars but do not show Doppler shifts corresponding to the orbital motion. Only in 1926 was an explanation developed, theoretically by A.S. Eddington, and based on observations by O. Struve, J. S. Plaskett, and others: the interstellar Ca II, Na I,... lines are produced in a gas layer which is partially ionized by the stellar radiation. This gas layer fills the entire disk of the Milky Way and participates in its rotation. About the same time, in 1927, I. S. Bowen succeeded in making the long-sought identification of the “nebulium lines” in the spectra of gaseous nebulae, finding that they are due to forbidden transitions in the spectra of [O II], [0 III], [N II],...; and H. Zanstra developed the theory of nebular luminescence. Only about ten years later was it recognized that in the interstellar gas, as in stellar atmospheres, hydrogen is the strongly predominant constituent. O. Struve and his coworkers discovered with the aid of their nebula spectrograph, which had great light-gathering power, that many O and B stars are surrounded by well-defined regions which fluoresce in the red hydrogen recombination line, Hα. Here, the interstellar hydrogen must thus be ionized. The theory of these H II regions was formulated in 1938 by B. Stromgren.

Galaxies and Clusters of Galaxies

The leap into deep space outside our Milky Way Galaxy, into the realm of the distant galaxies (or the extragalactic nebulae, as they were formerly called), and the beginnings of a cosmology based on observations, will be considered throughout history to be one of the most important achievements of the 20th century.

Aufbau und Entwicklung der Sterne

Schon 1913 war sich H. N. Russell uber die Bedeutung seines Diagramms fur die Erforschung der Sternentwicklung durchaus im klaren. Aber dessen Deutung und damit eine auf dem Boden der Beobachtung begrundete Theorie der Sternentwicklung wurde erst moglich im Zusammenhang mit dem Studium des inneren Aufbaus der Sterne. Die alteren Arbeiten von J. H. Lane (1870), A. Ritter (1878–89), R. Emden (die „Gaskugeln“ erschienen 1907) u. a. konnten sich im wesentlichen nur auf die klassische Thermodynamik stutzen. A.S. Eddington gelang es dann, diese Ansatze mit der Theorie des Strahlungsgleichgewichtes und mit der inzwischen entstandenen Bohrschen Theorie des Atombaus zu verschmelzen; sein Buch „The Internal Constitution of the Stars“ (1926) gab den Auftakt fur die ganze Entwicklung der modernen Astrophysik.