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1. WHY ELECTROMAGNETIC RADIATION IS SO IMPORTANT FOR ASTRONOMY.

We have actually sent probes to the surfaces of a few planets, including the Moon, Venus, Mars, and Jupiter. But it will be many centuries, at least, before mankind sends a probe to the nearest star (beyond the Sun). Therefore, almost everything we know about the universe beyond the solar system, we have learned by remote sensing of electromagnetic radiation. (I say almost, because we have in fact learned some important facts about distant cosmic sources from the measurement of subatomic particles such as protons, electrons, and neutrinos, as you will learn later in this course.)

The most familiar form of electromagnetic radiation is optical (visible light). For almost all of history, optical radiation was the only type of radiation by which we knew the sky. There are three simple reasons:

our eyes are extremely precise and sensitive devices to measure optical light;
the Earth's atmosphere is transparent to optical light; and
most stars, including the Sun, radiate most of their energy in optical light. (If you are clever, you will realize that (1) may be a consequence of (2) and (3).)

In fact, humans had learned a great deal about the cosmos through optical observations, even before Galileo first observed the sky with a telescope in 1610 AD. By 1930, with large optical telescopes, astronomers had discovered that the universe is expanding, had measured its size, and had a pretty good idea of how many stars and galaxies it contains, what stars are made of, and how they burn. We learned all this through observations of visible light alone.

But optical radiation covers only a small part of the electromagnetic spectrum (see Fig. 1 below), and many of the most exotic systems in the universe radiate very faintly at optical wavelengths. Therefore, to discover these objects we must observe the sky at wavelengths bands that our eyes cannot see.

Figure 1: The Electromagnetic Spectrum. Source: Chandra X-ray Observatory

As Figure 1 shows, the wavelength band at which an object radiates most of its power depends primarily on its temperature. Cooler objects emit longer wavelengths, and hotter objects radiate shorter wavelengths. For example, objects with temperatures of a few thousand degrees K*, such as the Sun's surface or the filament of a light bulb, radiate mostly optical radiation. Objects with temperatures of a few hundred K, such as your body, radiate mostly infrared radiation. You can't see infrared radiation, but you can build a camera that will take an infrared picture, such as the one below.

*We shall measure temperatures on the Kelvin scale, for which the lowest possible temperature is 0 K (= -273 C = -459 F), the freezing temperature of water is 273 K (= 0 C = 32 F), and the boiling temperature of water is 373 K (= 100 C = 212 F).

Figure 2: Infrared image of a man holding a candle. The man's skin is warm (shown as red), while his glasses and necktie are cooler (shown as green and blue). Source: IPAC Learning Center

You should make a list of some of the distinguishing characteristics of the different types of electromagnetic radiation. To do this, look at The Electromagnetic Spectrum from NASA's Imagine, where you will find an excellent summary of the various types of electromagnetic radiation.

In 1931, Karl Jansky of the Bell Telephone Laboratories in New Jersey made the first observation of radio waves from the sky. The technology for detecting radio waves (for radar) developed extremely rapidly during World War II, and after the war this technology was quickly adapted for radio astronomy. The sky looks very different at radio wavelengths than it does at optical wavelengths. The normal stars and galaxies that dominate the optical sky are very faint at radio wavelengths. Instead, the radio sky is dominated by the expanding debris of exploding stars and galaxies.

Figure 3 below shows that much of the electromagnetic spectrum is blocked by the Earth's atmosphere. The Earth's atmosphere is fully transparent only to optical and radio waves, and to certain bands of infrared radiation. To see the sky in other wavelength bands we must place telescopes above most of the atmosphere, with high altitude airplanes and balloons (good enough for infrared and gamma rays) and rockets and satellites (required for X-rays and ultraviolet). Astronomical observations of the sky at infrared, ultraviolet, X-ray, and gamma rays began seriously in the 1960s. They have now reached a fairly highly developed stage, characterized by a number of "Great Observatories" in space, some currently operational and some under development (see Lesson 2).

Figure 3: Transmission of the Earth's Atmosphere. The upper boundary of the white region is the minimum altitude required for a telescope to see the sky at the wavelength band noted. From NASA's Imagine.

The sky (and every object in it) appears different at every band. At each wavelength band we discover new objects that are almost invisible at other wavelength bands. This is nicely illustrated in the Multiwavelength Milky Way, by NASA, which shows how the Milky Way appears at each different band of the electromagnetic spectrum. Play with this a bit. Find objects that are bright at X-ray or gamma ray wavelengths but not at other wavelength bands. Do the same for radio and infrared. We will return to this site many times during this course. You may also find multiwavelength images of selected astronomical sources at higher magnification in The Multiwavelength Messier Museum and the Chandra Photo Album.

The tremendous progress in modern astronomy is a result of our ability to observe and analyze the sky in detail in all of these wavelength bands. This progress results from three concurrent developments:

Laboratory experiments in the first half of this century that led us to a deep understanding of the properties of electromagnetic radiation of all kinds; and
The development of sophisticated devices to detect and analyze electromagnetic radiation of all kinds.
The development of space technology, which enables to place telescopes above the Earth's atmosphere.

Figure 4: To the left is an image of the Orion nebula taken by the Hubble Space Telescope in visible light. To the right is an image of a part of the same region, taken in infrared light. The infrared light pierces deep into the molecular cloud, revealing forming stars deep inside.

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