The Milky Way Revealed

''What we know is little, what we do not know is immense.''

Pierre-Simon Laplace, 18271

The Andromeda galaxy, which we perceive with our unaided eyes only as a faint oval nebula, is both neighbor and kin to our home galaxy, the Milky Way. Viewed with binoculars or a small telescope, the Andromeda galaxy distinguishes itself from the foreground stars of our own galaxy as a glowing ball of light encircled by a wide, thin disk. The light from the disk shines feebly compared to that from the central part of the Galaxy; the disk appears as insubstantial as a skirt of tulle, although the ''skirt'' is actually a retinue of billions of stars.

To appreciate the disk's whirlpool structure, we must use larger telescopes or capture the image with long photographic exposures. A photograph of the Andromeda galaxy taken with a short exposure shows mainly the glow from the center of the Galaxy. The same galaxy taken with a long exposure brings out the disk (see figure 10.1).

Such is the makeup of spiral galaxies: the visible light is concentrated in two components, a bright central bulge and a relatively faint disk composed of more or less prominent spiral arms. A third component, called the stellar halo, is mostly dark. The Milky Way follows the same basic plan.

If we could gain a vantage point outside the Milky Way — perhaps erecting a telescope on a planet orbiting one of the billions of stars in the disk of the Andromeda galaxy—we would take in at one glance the features of our galaxy that astronomers have had to uncover more painstakingly by mapping the skies in all directions and using light from the entire electromagnetic spectrum. We

Figure 10.1 Disk and bulge. The first view is a short-exposure image of the Andromeda galaxy, showing the bright central region. The dusty spiral arms show up in the second view, taken with a longer exposure. (Credit: Jason Paul Lisle, Sommers-Bausch Observatory, University of Colorado.)

would probably be dazzled by the glare from the Milky Way's central bulge; our view would not be as obscured by dust and gas as it is from our actual position within the Milky Way's disk. We would be able to trace the Milky Way's spiral arms, arcs of stars and bright star clusters delineated by dark inter-arm dust lanes. We might marvel at the proximity of the Large and Small Magellanic clouds, the brightest of several satellite galaxies hovering relatively near ours in our corner of the cosmos.

Four Arm Spiral

Figure 10.2 The four-arm Milky Way. Although they are not as distinct as illustrated here, four major spiral arms can be discerned in our Milky Way galaxy. Smaller arms or spurs also appear. The naming of the arms varies by source, but the spiral arm in which the sun resides is known as the Perseus arm, or, according to some authorities, the Orion spur of the Perseus arm. (Credit: Layne Lundstrom.)

Figure 10.2 The four-arm Milky Way. Although they are not as distinct as illustrated here, four major spiral arms can be discerned in our Milky Way galaxy. Smaller arms or spurs also appear. The naming of the arms varies by source, but the spiral arm in which the sun resides is known as the Perseus arm, or, according to some authorities, the Orion spur of the Perseus arm. (Credit: Layne Lundstrom.)

The Milky Way galaxy is often depicted with four principal spiral arms (figure 10.2). Astronomers generally agree on the naming of the nearest two, the Perseus arm and the Sagittarius or Sagittarius-Carina arm. The Sun is located in the minor Orion arm, sometimes called the Orion "spur," which lies between the Perseus and Sagittarius arms. The third major arm is often called the Centaurus arm. A fourth arm goes by the name Outer arm, or sometimes Cygnus arm.2

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