The Milky

On clear, moonless nights a nebulous band of light can be seen stretching across the sky. This is the Milky Way (Fig. 17.1). The name is used both for the phenomenon in the sky and for the large stellar system causing it. The Milky Way system is also called the Galaxy — with a capital letter. The general term galaxy is used to refer to the countless stellar systems more or less like our Milky Way.

The band of the Milky Way extends round the whole celestial sphere. It is a huge system consisting mostly of stars, among them the Sun. The stars of the Milky Way form a flattened disc-like system. In the direction of the plane of the disc huge numbers of stars are visible, whereas relatively few are seen in the perpendicular direction. Thefaint light of distant stars merges into a uniform glow, and therefore the Milky Way appears as a nebulous band to the naked eye. A long-exposure photograph reveals hundreds of thousands of stars (Fig. 17.2).

In the early 17th century Galileo Galilei, using his first telescope, discovered that the Milky Way consists of innumerable stars. In the late 18th century William Herschel attempted to determine the size and shape of the Milky Way by means of star counts. Only early in the 20th century did the Dutch astronomer Jacobus Kapteyn obtain the first estimate of the size of the Milky Way. The true size of the Milky Way and the Sun's position in it became clear in the 1920's from Harlow Shaple/s studies of the space distribution of globular clusters.

In studying the structure of the Milky Way, it is convenient to choose a spherical coordinate system so that the fundamental plane is the symmetry plane of the Milky Way. This is defined to be the symmetry plane of the distribution of neutral hydrogen, and it agrees quite closely with the symmetry plane defined by the distribution of stars in the solar neighbourhood (within a few kpc).

Optical Telescope Map Cosmic
Fig. 17.1. The nebulous band of the Milky Way stretches across the entire sky. (Photograph M. and T. Keskiila, Lund Observatory)

Hannu Karttunen et al. (Eds.), The Milky Way.

In: Hannu Karttunen et al. (Eds.), Fundamental Astronomy, 5th Edition. pp. 347-366 (2007) DOI: 11685739_17 © Springer-Verlag Berlin Heidelberg 2007

The basic direction in the fundamental plane has been chosen to be the direction of the centre of the Milky Way. This is located in the constellation Sagittarius (a = 17 h 45.7 min, S = - 29° 00', epoch 2000.0) at a distance of about 8.5 kpc. The galactic latitude is counted from the plane of the Galaxy to its pole, goingfrom 0° to + 90°, and to the galactic south pole, from 0° to - 90°. The galactic coordinate system is shown in Fig. 17.3 (see also Sect. 2.8).

Fig. 17.2. A section of about 40° of the Milky Way between the constellations of Cygnus and Aquila. The brightest star at the upper right is Vega (a Lyrae). (Photograph Palomar Observatory)

Fig. 17.2. A section of about 40° of the Milky Way between the constellations of Cygnus and Aquila. The brightest star at the upper right is Vega (a Lyrae). (Photograph Palomar Observatory)

17.1 Methods of Distance Measurement

Fig. 17.3. The directions to the galactic centre and the North galactic pole (NGP) in equatorial coordinates. The galactic longitude l is measured from the galactic centre along the galactic plane. The coordinates of the Galactic centre are precessed from the defining equinox 1950 and are not very accurate (see A.P. Lane (1979), PASP, 91, 405)

Fig. 17.3. The directions to the galactic centre and the North galactic pole (NGP) in equatorial coordinates. The galactic longitude l is measured from the galactic centre along the galactic plane. The coordinates of the Galactic centre are precessed from the defining equinox 1950 and are not very accurate (see A.P. Lane (1979), PASP, 91, 405)

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