Read section 8.6 and answer questions 11-13.
Stars, Dust, and Clouds
Figure 8.12: Near-infrared 2MASS Image of the Sky
The image below is based on 95,851,173 stars with Ks 13.5. The most prominent features you see are the Galactic plane and the Galactic bulge. The plane is interrupted by dark dust lanes and clouds, even in the near-infrared, and several dusty regions, including Orion, are conspicuous. One can also see the two Magellanic Clouds, Large and Small (LMC,SMC). Cutting almost north-south through one side of the bulge is the dwarf Sagittarius galaxy, which, along with the Magellanic Clouds, is a satellite of the Milky Way. See the same image in the optical.
The image below is an InfraRed Astronomical Satellite (IRAS) view of nearly the entire sky, as seen in infrared wavelengths and projected at one-half degree resolution, is shown in this image, assembled from six months of data from the IRAS mission. The bright horizontal band is the plane of the Milky Way, with the center of the Galaxy located at the center of the picture. (Because of its proximity, the Milky Way dominates our view of the entire sky, as seen in this image. IRAS data processed to show smaller regions of the sky, however, reveal thousands of sources beyond the Milky Way.) The colors represent infrared emission detected in three of the telescope’s four wavelength bands (blue is 12 microns; green is 60 microns, and red is 100 microns). Hotter material (mostly dust and gas) appears blue or white while the cooler material appears red. NOTE: The blue hazy, S-shaped feature that crosses the image is faint heat emitted by dust in the plane of the solar system, much closer to us than the rest of the sources of light in this image.
Hydrogen Gas Clouds
Figure 8.14: NRAO Radio Image
Interstellar space is filled with extremely tenuous (low-density) clouds of gas which are mostly Hydrogen. The neutral Hydrogen atom (HI in astronomer’s shorthand) consists of 1 proton and 1 electron. The proton and electron spin like tops but can have only two orientations; spin axes parallel or anti-parallel. It is a rare event for Hydrogen atoms in the interstellar medium to switch from the parallel to the anti-parallel configuration, but when they do they emit radio waves with a wavelength of 21 centimeters (about 8 inches) and a corresponding frequency of 1420 MHz. Tuned to this frequency, radio telescopes have mapped the neutral Hydrogen in the sky. The above image represents such an all-sky HI survey with the plane of our Milky Way Galaxy running horizontally through the center. In this false-color image, no stars are visible, just diffuse clouds of gas tens to hundreds of light years across which cluster near the plane. The gas clouds seem to form arching, looping structures, stirred up by stellar activity in the galactic disk.