The bulge

The boundary between the galaxy's disk, comprising the arms and inter-arm dust lanes, and the amorphous central bulge is a blurry one. Lacking a clear-cut transition or precise definition, astronomers commonly refer to the bulge as the luminous mound in the middle of a galaxy that would be left over if one subtracted the disk. In the case of the Milky Way, most of the light from the bulge is contained within a radius of about 1500 light-years from the galactic center. The bulge becomes difficult to separate from the disk at about 10 000 light-years from the galactic center.10

The mystery of bulges is how they relate to galaxy formation. In spiral galaxies, did the bulges form first, before the disks? Or did they build up over time, under particular conditions? Is it a coincidence that bulges look like elliptical galaxies? Despite the difficulties associated with studying the dense heart of galaxies, astronomers are interested in what clues bulges might provide.

Bulges are a defining characteristic in the phenomenological system Edwin Hubble devised to classify galaxies—the ''Hubble system'' (see chapter 9). In the Sa category of spiral galaxies, the bulge is relatively large compared to the disk. As one progresses to the Sc category and beyond, the size of the central bulge decreases relative to the size of the disk. Other characteristics vary in tandem with the bulge-to-disk ratio—for example, a galaxy's current star formation activity appears to increase as the relative bulge size decreases, and the arms become less tightly wound.

This "sequence" does not necessarily imply an evolution in time (say, from large bulges to small ones), as was once thought. It does suggest that the Hubble type of a particular galaxy, and the relative size of the bulge, derive somehow from the physical conditions under which the Galaxy formed.

The Milky Way bulge is a good place to start studying bulges in general, because of its proximity. However, dust and gas in the plane of the disk dim the visible light from the central regions of the Galaxy by a factor of about a trillion. When we look to the bulge, toward the constellation Sagittarius, we see only that the band of light that we also call the ''Milky Way'' in that direction is thick with stars, and that the star clouds appear somewhat yellower than in other dense regions, reflecting a difference in average color of bulge stars.

In the 1940s, Mount Wilson astronomer Walter Baade discovered the first known of a few small tunnels into the bulge — lines of sight that happen to skirt the irregular concentrations of obscuring dust and gas. His line of sight from the Earth passes within 1800 light-years of the galactic center, and gives us a rare glimpse into the bulge in the visible wavelengths (figure 10.6). The view through ''Baade's window,'' as it is called, is the subject of intense study. Baade himself used it to obtain a first measure of the distance from the Earth/Sun system to the galactic center. The current value is about 26 000 light-years.11

Infrared wavelengths travel through dust with less dimming, and so provide another perspective on the bulge. The wavelength of visible light is comparable to the scale of molecules and dust particles. This similarity of scale means the interstellar matter can dim visible light; infrared and radio wavelengths are longer, and they pass through space unimpeded. (The disadvantage of the longer wavelengths is that they generally provide a coarser view, or they may reveal a different aspect of the source.)

Figure 10.6 Baade's Window. Named for the astronomer who discovered it, Walter Baade, the ''window'' is a relatively unobstructed view deep into the heart of our galaxy. The sky in the direction of the constellations Sagittarius and Scorpius is full of dust and gas, but a line of sight through Baade's window penetrates to the rich star fields close to the galactic center. The ''window'' can be found near the spout of the ''teapot,'' which many recognize in the constellation Sagittarius. (Credit: Layne Lundstrom.)

Figure 10.6 Baade's Window. Named for the astronomer who discovered it, Walter Baade, the ''window'' is a relatively unobstructed view deep into the heart of our galaxy. The sky in the direction of the constellations Sagittarius and Scorpius is full of dust and gas, but a line of sight through Baade's window penetrates to the rich star fields close to the galactic center. The ''window'' can be found near the spout of the ''teapot,'' which many recognize in the constellation Sagittarius. (Credit: Layne Lundstrom.)

In 1990, NASA's Cosmic Background Explorer Satellite (COBE) took a photograph of the disk of the Milky Way in the near-infrared wavelengths (figure 10.7). The image was the first to show the bulge directly, as a bulbous thickening in the middle of the disk. The shape of our galaxy in this view is reminiscent of visible-light images of some distant spiral galaxies seen edge-on.

The COBE image shows the bulge in two dimensions. Studies of the kinematics of stars — how they move in complex orbits around the center of the Galaxy — provide some information on their distribution in three dimensions. An intriguing result that has emerged in the last decade is that the bulge is in the shape of a bar, albeit a ''mild'' bar, about twice as long as it is wide.

Figure 10.7 The Milky Way galaxy from inside. This view of our galaxy, taken in the infra-red portion of the spectrum using instruments on the Cosmic Background Explorer (COBE) satellite, shows our galaxy's thin disk of stars, and dust (which appears red or orange in this image) within the disk. Infra-red light penetrates dust and gas much better than visible light, so the image reveals much more of the central swath of the Galaxy than we could see with traditional telescopes. (See color section.) (Copyright Edward L. Wright. Used with permission.)

Figure 10.7 The Milky Way galaxy from inside. This view of our galaxy, taken in the infra-red portion of the spectrum using instruments on the Cosmic Background Explorer (COBE) satellite, shows our galaxy's thin disk of stars, and dust (which appears red or orange in this image) within the disk. Infra-red light penetrates dust and gas much better than visible light, so the image reveals much more of the central swath of the Galaxy than we could see with traditional telescopes. (See color section.) (Copyright Edward L. Wright. Used with permission.)

The long axis points nearly toward our Sun's position in the disk. If the Milky Way does have a bar, it is in good company. (No, the bar at the center of the Milky Way is not a good place to have a drink!) Recent research suggests at least one third of all spiral galaxies are barred, although the bar may be more pronounced when the galaxy is imaged using a colored filter.12

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