The dimensions of the Milky Way galaxy

In 1930, when Lick Observatory astronomer Robert Trumpler proved that obscuring dust particles are widespread in the interstellar medium, our gauges of galactic dimensions began to attain modern standards of accuracy. Previously (as we saw in chapters 7, 8, and 9), estimates of stellar distances based on apparent brightness tended to run too high, because the unsuspected dust dimmed the visible light reaching us from the stars. Accounting for dust allowed astronomers to correct the scales of Kapteyn's and Shapley's stellar systems. Beginning in the 1950s, observations in the radio wavelengths that are not absorbed by dust allowed us to probe the outer limits of the Galaxy and to form a more complete picture of the Galaxy's size.3 The Milky Way has three major components (figure 10.3). The Milky Way's central bulge, whose mass is about 10 billion suns, is roughly 3000 light-years in diameter in the plane of the disk. It extends to about 1800 light-years above and below the

Milky Way Bulge Size

Figure 10.3 Components of the Milky Way. At the center of the disk of the Milky Way is the bulge, a bright region some 3000 light-years across where the stars are very densely packed. The visible disk itself, only some 1000 light-years thick, has a radius of about 80 000 light-years. Globular clusters fill the space around the disk, known as the halo. (Credit: Layne Lundstrom.)

Figure 10.3 Components of the Milky Way. At the center of the disk of the Milky Way is the bulge, a bright region some 3000 light-years across where the stars are very densely packed. The visible disk itself, only some 1000 light-years thick, has a radius of about 80 000 light-years. Globular clusters fill the space around the disk, known as the halo. (Credit: Layne Lundstrom.)

disk; it is rather cigar shaped. The visible disk, on the other hand, with a similar mass, is flattened like a pancake. It is only 1000 light-years thick, and extends to 50 000 light-years from the center. A thin layer of invisible hydrogen gas continues beyond the limit of the stars in the spiral arms and forms a disk of about 80 000 light-years in radius from the center.

Globular clusters, spherical agglomerations of stars, populate a third component of the Galaxy's structure, a volume called the stellar halo. This space encloses the Galaxy like a bubble that is big enough to contain the disk. Any star or cluster found outside this volume, or roughly 50 000 light-years from the center of the Milky Way, is about as likely to ''belong'' to a nearby galaxy as to the Milky Way. A case in point is the faint Sagittarius dwarf galaxy, which lies only 50 000 light-years from the center of the Milky Way. At this distance, the gravitational pull of the Milky Way competes against the satellite galaxy's own gravity; the satellite system is being stretched apart, or ''tidally disrupted,'' by its proximity to our more massive system.4

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