Lab Exercise 9
Gas And Dust Clouds
Read Section 9.6. Answer Questions 11, 12 and 13.
Stars, Dust, and Clouds (Click on image for large view)
Figure 9.12: Near infrared 2MASS image of the sky. Image is based
on 95,851,173 stars with Ks 13.5. What appears most prominently are
the Galactic plane and the Galactic bulge. The plane is cut 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 Clouds,
is a satellite of the Milky Way. Click here
to see the same image in the optical.
Dust
Figure 9.13: 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 Infrared Astronomical
Satelite (IRAS). 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. The hazy, horizontal S-shaped feature that crosses the image
is faint heat emitted by dust in the plane of the solar system.
Hydrogen Gas Clouds
Figure 9.14: NRAO radio imge. Interstellar space is filled with extremely
tenuous 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 exactly 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.