3. Radiation - Information From The Cosmos

Information From The Cosmos

The Ring Nebula in the constellation Lyra is one of the most magnificent objects in our galaxy. Seen here glowing in the light of its own fading radiation, the nebula is actually the expanding atmosphere of a nearly dead star. That dying dwarf star can be seen at the center of the ring of hydrogen-rich gas, shown here in true color. Owing to the nebula's distance of some 5000 light-years, its apparent size is just 1/100th that of the full Moon. It is too dim to see with the naked eye. (STScI)The Big Picture: Stars evolve from birth to maturity to death, much like living things, but on vastly longer timescales. Our own star, the Sun, is about Mid-way through its evolution. In another 5 billion years, the Sun will swell late in life to resemble the object shown here. By that time, humanity will be long gone from Earth—either voluntarily, as our descendants move out into the wider Universe, or involuntarily, having perished in the atmosphere of our dying parent star. (Astronomical Society of the Pacific)

		LEARNING GOALS
		Studying this chapter will enable you to:
		Discuss the nature of electromagnetic radiation, and tell how that radiation transfers energy and information through interstellar space.
		Describe the major regions of the electromagnetic spectrum and explain how the properties of Earth's atmosphere affect our ability to make astronomical observations at different wavelengths.
		Explain what is meant by the term "blackbody radiation" and describe the basic properties of such radiation.
		Tell how we can determine the temperature of an object by observing the radiation that it emits.
		Show how the relative motion of a source of radiation and its observer can change the perceived wavelength of the radiation, and explain the importance of this phenomenon to astronomy.

Astronomical objects are more than just things of beauty in the night sky. Planets, stars, and galaxies are of vital significance if we are to fully understand our place in the big picture—the "grand design" of the universe. Each object is a source of information about the material aspects of our universe—its state of motion, its temperature, its chemical composition, even its past history. When we look at the stars, the light we see actually began its journey to Earth decades, centuries—even millennia—ago. The faint rays from the most distant galaxies have taken billions of years to reach us. The stars and galaxies in the night sky show us the far away and the long ago. In this chapter we begin our study of how astronomers extract information from the light emitted by astronomical objects. These basic concepts of radiation are central to modern astronomy.

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