The Solar System Within the Universe
To try to understand the nature of the Solar System, its origin, and its evolution, it is indispensable to consider it in its context, that of a universe in expansion, consisting of stars and galaxies. First, it is necessary to establish the distance scal
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To try to understand the nature of the Solar System, its origin, and its evolution, it is indispensable to consider it in its context, that of a universe in expansion, consisting of stars and galaxies. First, it is necessary to establish the distance scale that allows us to place the Solar System within its immediate surroundings: the nearby stars and galaxies, and the Local Group. Second, we briefly describe the universe at we view it today: the Big-Bang model and primordial nudeosynthesis; galaxies; stars and stellar nudeosynthesis; stellar formation; and interstellar medium.
2.1 Distance Scales The standard unit for describing distances within the Solar System is the astronomical unit (AU), which is defined as the average distance between the Sun and the Earth (149.6 million km). The planets in the Solar System extend out to some forty AU. Beyond that we find the Kuiper Belt, a reservoir of small objects like Pluto and the periodic comets, which undoubtedly extends out to about one hundred AU; and finally, at some 40 000 AU, we have the Oort Cloud, a vast reservoir of comets, which were ejected into the outer Solar System by perturbations by the giant planets. To describe the distances of the stars, even the dosest, we need to change units. Two new units are used. The first is the light-year (ly), the distance covered by light in one year. The second is the parsec (pc), which is the distance at which a length of I AU appears to subtend an angle of I second of arc on the sky. This unit refers to the method used to measure the distance to the nearest stars, which is the parallax method (Fig. 2.1). This utilizes the fact that, because of the Earth's annual orbital motion around the Sun, a star lying at a finite distance from us, appears, over the same period, to describe a small ellipse against the background stars. If the semimajor axis of the ellipse is 1", the star lies at a distance of 2 x 105 AU, and also, by definition, at I pe, i.e., at 3.26Iy. As we ean see, this distance is about 5 times the distanee from the Sun to the Oort Cloud. The dosest star is Proxima Centauri at a distance of l.3 parsecs; it is part of a tripie system, the brightest star of which is Alpha Centauri. The parallax method only permits measuring the distances to the dosest stars. To determine yet greater distances, astronomers have recourse to another technique whieh relies up the intrinsic properties of certain variable stars, known as Cepheids. These are stars in which there is a relationship between period and luminosity. From T. Encrenaz et al., The Solar System © Springer-Verlag Berlin Heidelberg 2004
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2 The Solar System Within the Uni verse Earth
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