The disk

Of the three main components of the Galaxy — central bulge, disk and halo—the disk, with its spiral arms, is the most visually complex. The spiral arms have been described in many beautiful ways: like pinwheels, or like cream poured into stirred coffee.

We know that the arms ''trail'' — that is, they follow the rotation of the Galaxy like the folds of a skirt winding around a spinning dancer. But exactly what the Milky Way's spiral arms would look like, if we could see them from a distance of millions of miles away, is an intriguing question.

In the visible wavelengths of light, the spiral arms look like concentrations of stars, leaving apparently little in the dark inter-arm lanes. Indeed, the mass of gas and stars per unit volume is enhanced in the arms, although stars and large amounts of dust and gas do collect in the inter-arm regions as well. The density of stars in the arms is only about 10% greater than in the rest of the disk, but the arms stand out because the stars there tend to be bright.

Besides the density enhancement in spiral arms, a factor that determines how prominent they look is the regularity of the spiral structure. At one extreme in this approach to classifying galaxies are the so-called ''grand design'' spirals, such as M51 and M81. These have a regular spiral structure, generally composed of two arms that one may easily trace from the center of the Galaxy to the outer limits of the visible arms. At the other extreme are ''flocculent'' spirals such as M63. These show fluffy, small-scale spiral structures throughout the disk (figures 10.4a and b). The Milky Way is probably of an intermediate type, with multiple arms and some ''feathering'' between adjacent arms.

The regularity of spiral arms appears to depend on various physical parameters. The proportion of mass in a galaxy's disk compared to the mass in the halo varies from one galaxy to another, so that one may speak of ''light'' or ''heavy'' disks. The proportion of mass in stars compared to that in gas may vary. The stars in the disk may appear settled, or ''cool,'' or they may zoom about with a high degree of random motion, in which case astronomers call the disk ''hot.'' According to recent theory, grand design spirals form in galaxies that have cool, light stellar disks and are relatively gas-poor. The Milky Way galaxy is evidently too rich in gas to form a stunning grand design spiral.

The origin of spiral arms is not understood in great detail. So much is clear: the spiral pattern is very likely the result of a ripple or wave that propagates through the disk. The wave, called a

Figure 10.4 (a) ''Grand Design'' galaxy. The spiral arms are prominent and extend in an unbroken line from the center to the extremities in this example of a so-called ''grand design'' galaxy, M51, the ''Whirlpool'' galaxy. (The bright round patch shown in Lord Rosse's drawing at the end of one of the spiral arms (figure 6.2) is just off the top edge of this image.) (See color section.) (b) ''Flocculent'' galaxy. The spiral pattern of this galaxy is more patchy, or flocculent, than that of a ''grand design'' galaxy.

Figure 10.4 (a) ''Grand Design'' galaxy. The spiral arms are prominent and extend in an unbroken line from the center to the extremities in this example of a so-called ''grand design'' galaxy, M51, the ''Whirlpool'' galaxy. (The bright round patch shown in Lord Rosse's drawing at the end of one of the spiral arms (figure 6.2) is just off the top edge of this image.) (See color section.) (b) ''Flocculent'' galaxy. The spiral pattern of this galaxy is more patchy, or flocculent, than that of a ''grand design'' galaxy.

density wave, arises naturally from the interplay of forces in the disk. Both it and the stars and gas clouds that make up the disk rotate around the center of the Galaxy, but not at the same speed. The stars and gas clouds rotate at varying speeds depending on their distance from the center, with those closest in orbiting fastest. The density wave rotates at a slower rate than even the outermost stars do. The stars and gas clouds overtake the density wave, and this is what gives rise to the spiral arm pattern.

The situation is often compared to that of cars entering and passing through a knot of traffic on the highway, with the stars as vehicles and a peak in the density wave as the region of traffic congestion. As the stars pass through the peak in the density wave, they slow down and become more bunched together. The combined effect of stars at varying distances from the galactic center slowing down and entering a ''traffic jam'' creates the spiral arms.6

Our Sun passes through a spiral arm every 200 million years or so, and is currently headed for the Perseus arm—in another 140 million years. Some astronomers have put forward the interesting hypothesis that mass extinctions on Earth, like the demise of the dinosaurs, correlate with the solar system's passage through spiral arms.7 In this scenario, the solar system takes a heavier pelting from comets and asteroids during a passage through a spiral arm, and the impacts on Earth lead to extreme climatic changes.

The spiral arms are delineated, on their trailing edges, by bright star-forming regions. That's because the gas in the galaxy's disk is compressed as it enters a spiral arm, and the compression leads to favorable conditions for star formation. The ''Great Nebula'' in Orion is at the star-forming edge of one such spiral arm in the Milky Way, the Orion arm. We have a great view of it from our Sun's position between the Sagittarius arm and the Orion and Perseus arms (see figure 10.2).

In the 1950s, astronomers used the Orion nebula and others of the same type, called HII (''H-two'') regions or regions of ionized hydrogen gas, to trace the contours of the nearer spiral arms of the Milky Way. Since then, we have mapped more distant parts of the galaxy using the techniques of radio astronomy and using other features, such as molecular gas clouds, as tracers.

The Orion nebula (chapter 2, figure 2.7), is in many ways typical of the glowing pink nebulae lining the inner edges of spiral arms. The diameter of the gas cloud associated with it is more than 20 000 times that of the solar system. The mass of gas composing the cloud—mostly hydrogen—is enough to form tens of thousands of stars. Hundreds of young stars, formed within the last million years, already burn within this stellar nursery. Four of the young stars form a bright cluster, the Trapezium, that one can see with binoculars or a small telescope. These O and B type stars radiate intensely in the ultraviolet part of the spectrum, which allows electrons to separate from hydrogen atoms in the gas. When the electrons have an opportunity to recombine with hydrogen atoms, they emit red light. This fluorescence process gives the Orion nebula, and other HII regions, a characteristic pink cast on photographic images.

Associated with the Orion nebula is a dense cloud of molecular hydrogen called the BN-KL infrared nebula. Star formation there is at an even more primitive stage. The turbulent roiling of the cloud and the effect of the cloud's own gravity is causing hundreds of pockets of molecular material to condense and take shape as stars and embryonic solar systems. In 1993, the Wide Field and Planetary Camera 2 on the Hubble Space Telescope first captured images of these condensing stars and their associated proto-planetary disks in the Orion region. In 1995, the same camera found even more spectacular views of star formation in progress, in the Eagle nebula (also known as M16) (figure 10.5).8

While stars are forming in the spiral arms, and particularly along the trailing edges of the arms, older stars created in earlier epochs of star formation are dying. The Perseus arm of the Milky Way has provided notable examples of dying stars over the past few thousand years of recorded history. The event that led to the Crab nebula is a well-known instance.

In 1054, Chinese and Japanese astronomers witnessed the death of a star by supernova near the horns of the bull in the constellation Taurus. They described the appearance of a new or ''guest'' star about as brilliant as the full moon, visible in the daytime for about a month and visible at night for more than a year. Curiously, European astronomers seem not to have noted it. Native Americans in Arizona may have observed the

Figure 10.5 Star-forming region. This 1995 image from the Hubble Space Telescope shows that the Eagle nebula, associated with the open cluster M16, is home to great pillars of molecular hydrogen gas and dust, many light-years in length. Within these pillars, gas is condensing under the influence of gravity, forming new stars. At the ends of the pillars, radiation from stars that have recently turned on their nuclear burning is clearing away some of the pillar material and exposing dense globules where stars are at an earlier stage of formation. (Credit: NASA.)

Figure 10.5 Star-forming region. This 1995 image from the Hubble Space Telescope shows that the Eagle nebula, associated with the open cluster M16, is home to great pillars of molecular hydrogen gas and dust, many light-years in length. Within these pillars, gas is condensing under the influence of gravity, forming new stars. At the ends of the pillars, radiation from stars that have recently turned on their nuclear burning is clearing away some of the pillar material and exposing dense globules where stars are at an earlier stage of formation. (Credit: NASA.)

supernova, according to one interpretation of pictographs found at White Mesa and Navajo Canyon.9

The visible output of the supernova declined below the threshold of naked-eye observations. Then, in the 1700s, European astronomers scanning the skies with telescopes found the supernova's gaseous remnant, without knowing what it was. Lord Rosse (see chapters 5 and 6) named the remnant the Crab nebula because of its resemblance to a crab's claw. In 1942, astronomers translated the Chinese accounts of a ''guest'' star into English and made the connection between the supernova and the glowing remnant. In the 1960s, the left-over star at the heart of the Crab was one of the first to be identified as a pulsar, a very dense, rapidly spinning star that emits a beacon of radio waves and x-rays.

Stars are born and die in the disks of spiral galaxies. But the disks play host to a rich variety of objects besides star-forming nebulae and supernovae. In the Milky Way's disk we observe stars in various stages of evolution; stars in pairs or triplets or in irregularly shaped galactic clusters; and planetary nebulae, the remnants of expired low-mass stars. We see large and small clouds of dust and gas, some in the form of filaments, knots or sheets. Our position in the plane of the disk, about two-thirds of the way out from the center of the Galaxy, grants us a good view of these diverse objects.

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