Table 5.1 lists the basic regions of the electromagnetic spectrum and describes objects typically studied in each frequency range. Bear in mind that the list is far from exhaustive and that many astronomical objects are now routinely observed at many different electromagnetic wavelengths. As we proceed through the text, we will discuss more fully the wealth of information that high-precision astronomical instruments can provide us.

TABLE 5.1  Astronomy at Many Wavelengths
WAVELENGTH/FREQUENCY RANGE GENERAL CONSIDERATIONS COMMON APPLICATIONS (CHAPTER REFERENCE)
Radio Can penetrate dusty regions of interstellar space Radar studies of planets (2,9)
Planetary magnetic fields (11)
Earth's atmosphere largely transparent to these wavelengths Interstellar gas clouds (18)
Center of the Milky Way Galaxy (23)
Can be detected in the daytime as well as at night Galactic structure (23, 24)
Active galaxies (25)
High resolution at long wavelengths requires very large telescopes or interferometers Cosmic background radiation (27)
Infrared Can penetrate dusty regions of interstellar space Star formation (19,20)
Cool stars
Earth's atmosphere only partially transparent to IR radiation, so some observations must be made from space Center of the Milky Way Galaxy (23)
Active galaxies (25)
Large-scale structure in the universe (24, 27)
Visible Earth's atmosphere transparent to visible light Planets (7-14)
Stars and stellar evolution (17, 20, 21)
Galactic structure (23, 24)
Galaxies (24)
Active Galaxies (25)
Large-scale structure in the universe (24, 27)
Ultraviolet Earth's atmosphere opaque to UV radiation, so observations must be made from space Interstellar medium (19)
Hot stars
X ray Earth's atmosphere opaque to X-rays, so observations must be made from space Stellar atmospheres (16)
Neutron stars and black holes (22)
Special mirror configurations needed to form images Hot gas in galaxy clusters (24)
Active galactic nuclei (25)
Gamma ray Earth's atmosphere opaque to gamma rays, so observations must be made from space Neutron stars (22)
Cannot form images Active galactic nuclei (25)

It is reasonable to suppose that the future holds many further improvements in both the quality and the availability of astronomical data and that many new discoveries will be made. The current and proposed pace of technological progress presents us with the following very exciting prospect: early in the 21st century, if all goes according to plan, it will be possible, for the first time ever, to make simultaneous high-quality measurements of any astronomical object at all wavelengths, from radio to gamma ray. The consequences of this development for our understanding of the workings of the universe may be little short of revolutionary.

As a preview of the sort of comparison that full-spectrum coverage allows, Figure 5.34 shows a series of images of our own Milky Way Galaxy. They were made by several different instruments, at wavelengths ranging from radio to gamma ray, over a period of about 5 years. By comparing the features visible in each, we immediately see how multiwavelength observations can complement one another, greatly extending our perception of the universe around us.

Figure 5.34 The Milky Way Galaxy, as it appears (from top to bottom) at (a) radio, (b) infrared, (c) visible, (d) X-ray, and (e) gamma-ray wavelengths. Each frame is a panoranic view covering the entire sky. The center of our Galaxy, which lies in the direction of the constellation Sagittarius, is at the center of each map.