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14.19 Layout of the Milky Way

Go to a dark place and look at the night sky; it is not the same in all directions. The Milky Way is a band of stars (and gas and dust) that encircles our entire sky. It appears much wider toward the constellation Sagittarius in the south than it does in other directions. Also, the Milky Way itself has variations — its highest density of stars occurs in the southern sky. Finding our place in this star-filled space took several hundred years of speculation and solid observation detective work.


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The Milky Way's Bulge & Disk. Click here for original source URL.

In 1750, the English theologian Thomas Wright hypothesized that our galaxy must be a slab-like arrangement of stars. The German philosopher Immanuel Kant also elaborated on this idea in 1755, coining the term "island universe" in his writing. In the 1780s, Herschel put enormous effort into counting stars in different areas of sky. He assumed that the number of stars in a given direction directly reflected how deep the Milky Way is in that direction (with higher star counts meaning a greater depth of Milky Way Disk). His results suggested that the Sun was at the center of the Milky Way because he counted roughly equal numbers of stars all along that band of light. Unfortunately, Herschel was unaware of the obscuring effects of dust. It was not until the 1930s that Robert Julius Trumpler corrected for the dust and showed that the stars of the Milky Way were especially concentrated in the direction of the constellation Sagittarius. Astronomers can use the anisotropy of the stars in the night sky to help deduce the shape of the Milky Way Galaxy and our position within it. (When something is the same in all directions, it is isotropic. When it varies in different directions, it is anisotropic.)

The shape of the Milky Way is more easily revealed by looking at the distribution of star clusters rather than of stars. The clusters are not randomly distributed: the open clusters lie in the plane of the Milky Way and define the galactic disk, and the much more populous globular clusters form a spherical cloud around the disk called the galactic halo. Harvard astronomer Harlow Shapley said that the clusters reveal “the bony frame of our galaxy.” Shapley showed that globular clusters are distributed in a spherical swarm extending above and below the disk. He also showed that the halo is centered not on the Sun but on a distant point in the disk in the direction of the constellation Sagittarius. He identified this point as the center of the galaxy. Astronomers also realized that there was a concentration of stars toward the galactic center, roughly spherical in shape but much smaller than the halo. These stars appeared to be mostly old and red. This third component of the Milky Way is called the galactic bulge. Dutch astronomer Jan Oort also did important work on galactic structure in the 1920s and 1930s by mapping stellar motions. While his results and Hubble's differed in details, they painted similar ides: Our galaxy is a disk with the Sun offset from the center, which is located in the direction of the Sagittarius constellation.

Astronomers use a special coordinate system to describe locations within our galaxy. The galactic equator runs along the center of the Milky Way's disk. Galactic longitude, designated l, measures the angular distance around the Milky Way, starting at a zero point defined to lie at the galactic center in the constellation Sagittarius. The direction l = 90° lies toward the constellation Cygnus, near the top of the Northern Cross. Opposite the galactic center, at longitude l = 180°, is the direction of Taurus, near the Pleiades and Hyades clusters. The direction l = 270° is just south of Canis Major. Galactic latitude, designated b, is defined to be zero along the galactic equator. The direction b = +90° points “straight up” out of the disk in the northern sky.

The map of galactic coordinates allows us to align our view within the Milky Way Galaxy. Looking toward the center of the disk, at l = 0°, we see the largest concentration of stars. Looking away from the center, at l = 180° (sometimes called the anti-center), we see a lower concentration of stars in the galactic plane. At high galactic latitudes toward the center, we see a large number of globular clusters. At low galactic latitudes toward the center, we see a smaller number of globular clusters. If our view is confined to a few hundred parsecs, we see roughly equal numbers of stars in every direction. Most of the bright stars in the night sky are relatively close. From our position within the galactic disk, we see nearby disk stars more or less equally in every direction.

We view the Milky Way from within the enormous disk of stars. The disk is about 30,000 pc across, 400 pc thick, and packed with open clusters, individual stars, dust, and gas, mostly arranged in ragged spiral arms. Globular clusters surround the disk in a spherical swarm concentrated toward the center of the disk. This swarm of clusters, which also includes some sparsely scattered individual stars and gas, defines the galactic halo. According to current estimates, the Sun is about 8500 pc from the galactic center. Although the International Astronomical Union (IAU) has adopted 8500 pc as the distance to the center of the galaxy, recent studies of the space distribution of globular clusters and X-ray bursting stars suggest the true number may be somewhat lower.

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Immanuel Kant. Click here for original source URL.

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Thomas Wright. Click here for original source URL.

Objects in distant parts of our galaxy are at distances of thousands of parsecs. For this reason, astronomers often use a unit of distance still larger than astronomical units, light-years, and parsecs. This unit is a kilo parsec (abbreviation, kpc). Note the use of the same prefix, kilo-, that is used throughout the metric system to indicate 1000. Therefore, 1 kpc = 1000 pc, and astronomers say, for example, that we are 8.5 kpc from the galactic center and that our galaxy is roughly 30 kpc in diameter. These distances are difficult to comprehend. In a model of the Milky Way Galaxy the size of North America, stars like the Sun would be microscopic specks less than a thousandth of a centimeter across and scattered a block apart. The solar system would fit in a saucer. The quest to map out the size and shape of the Milky Way Galaxy is an important step in the Copernican revolution. We are truly dwarfed by the vastness of the galaxy that surrounds us.

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Jan Oort. Click here for original source URL.

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Our galaxy as we see it from Earth, using galactic coordinates. Click here for original source URL.

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Open cluster NGC 290 in the Small Magellanic Cloud. Click here for original source URL.

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The globular star cluster M80. Click here for original source URL.