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Disks and haloes

With modern instruments, astronomers can now penetrate the “smog” of the Milky Way by studying radio and infrared emissions from distant parts of the Galaxy. Measurements at these wavelengths (as well as observations of other galaxies like ours) have given us a good idea of what the Milky Way would look like if we could observe it from a distance.

[link] sketches what we would see if we could view the Galaxy face-on and edge-on. The brightest part of the Galaxy consists of a thin, circular, rotating disk of stars distributed across a region about 100,000 light-years in diameter and about 1000 light-years thick. (Given how thin the disk is, perhaps a CD is a more appropriate analogy than a wheel.) In addition to stars, the dust and gas from which stars form are also found mostly in the thin disk of the Galaxy. The mass of the interstellar matter is about 15% of the mass of the stars in this disk.

Schematic representation of the galaxy.

A schematic representation of the Milky Way Galaxy. On the left is the face-on view of the spiral disk, with the central bar in the center, the Cygnus spiral arm on the lower left, the Perseus arm labeled on the bottom, the smaller Orion spur labeled above that, and the Carina arm labeled on the right. On the right of the schematic is the edge-on view of the spiral disk, surrounded by serval globular clusters. The nuclear bulge is labeled in the center of both views, and the Sun is labeled on the Orion spur. The distance between the Sun and the nuclear bulge is labeled 26,000 light years.
The left image shows the face-on view of the spiral disk; the right image shows the view looking edge-on along the disk. The major spiral arms are labeled. The Sun is located on the inside edge of the short Orion spur.

As the diagram in [link] shows, the stars, gas, and dust are not spread evenly throughout the disk but are concentrated into a central bar and a series of spiral arms. Recent infrared observations have confirmed that the central bar is composed mostly of old yellow-red stars. The two main spiral arms appear to connect with the ends of the bar. They are highlighted by the blue light from young hot stars. We know many other spiral galaxies that also have bar-shaped concentrations of stars in their central regions; for that reason they are called barred spirals. [link] shows two other galaxies—one without a bar and one with a strong bar—to give you a basis for comparison to our own. We will describe our spiral structure in more detail shortly. The Sun is located about halfway between the center of the Galaxy and the edge of the disk and only about 70 light-years above its central plane.

Unbarred and barred spiral galaxies.

Unbarred and Barred Spiral Galaxies. Panel (a), at left, shows the beautifully symmetric spiral form of M74. The blue spiral arms and dust lanes spiral neatly into the bright nucleus at center. Panel (b), at right, shows the barred spiral NGC 1365. A bar of yellow stars projects out from the nucleus at center, with a nearly straight blue arm at each end of the bar.
(a) This image shows the unbarred spiral galaxy M74. It contains a small central bulge of mostly old yellow-red stars, along with spiral arms that are highlighted with the blue light from young hot stars. (b) This image shows the strongly barred spiral galaxy NGC 1365. The bulge and the fainter bar both appear yellowish because the brightest stars in them are mostly old yellow and red giants. Two main spiral arms project from the ends of the bar. As in M74, these spiral arms are populated with blue stars and red patches of glowing gas—hallmarks of recent star formation. The Milky Way Galaxy is thought to have a barred spiral structure that is intermediate between these two examples. (credit a: modification of work by ESO/PESSTO/S. Smartt; credit b: modification of work by ESO)
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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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