Staggering as they may seem to us, interplanetary distances are puny compared to those to reach stars. Our Solar System is located about two-thirds of the way from the center of our Galaxy towards the rim—about 25,000 light-years from the galactic centre, on the inner edge of the Orion arm. Our Galaxy has a diameter of approximately 100,000 light-years and is roughly shaped as a luminous disk 12,000 light-years thick near its hub, decreasing to about 1,000 light-years near the rim of its "arms". The presence of a black hole of mass corresponding to 2 to 3 million Sun masses, and long believed to be at its center [Cohen et al., 2003], seems confirmed by recent radiowave measurements using very long baseline interferometry [Reynolds, 2008].

Astrophysicists mapping 21 cm hydrogen radiation had previously thought that our Galaxy was a spiral galaxy with five major arms or spokes (Centaurus, Sagittarius, Orion, Perseus, and Cygnus). In fact, recent data from the Spitzer Telescope orbited by NASA have shown that our Galaxy has only two major arms, Perseus and Centaur: the density of stars in the other three was found lower than estimated in the past, and definitely much lower than in these two arms.

Using the distance of our Earth to the Sun, the astronomical unit, AU = 1.496 x 108 km, as yardstick, 1 light-year (9.46 x 1012 km) is approximately equal to 63,200 AU. Our Galaxy comprises some 250 million stars; their density decreases from the Galactic center towards the arms' ends, where average interstellar distances are of the order of many light years, see Chapter 1. The spiral structure of the Galaxy is such that the average distance between stars, were it a true homogeneous disk, would be about 50 light-years. In fact, stars are not uniformly distributed, their density increasing when going towards the galactic center and inside its five major arms. This explains why the Sun's nearest neighbor is only a few light-years away; see Figure 8.1.

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