$$\require{cancel}$$

# 10.2 Radiation and the Universe

By sending astronauts to the surface of the Moon or a robot past Neptune, we can learn directly about regions of space beyond the Earth. However, most parts of the universe are too far away to reach in person or with space probes. In fact, very little of the information in astronomy is derived from travel or direct contact. To obtain information about remote regions we often rely on messages from them in the form of light. When this light is dispersed, there are two aspects of the spectrum that are of importance to astronomers. First, radiation that is smoothly distributed in wavelength gives us evidence of the thermal properties of its source. The peak wavelength of this radiation measures the temperature of a remote body. Hotter objects have radiation with shorter peak wavelengths. Second, electrons moving from one energy state to another in atoms create a pattern of sharp lines arrayed in wavelength. Each chemical element has a unique pattern of sharp lines. The overall pattern of lines is like a "fingerprint" that reveals the chemical composition of a remote object.

Iron spectrum. Click here for original source URL

There is a broad spectrum of radiation of which visible light is just a small slice. For all of human history we have watched light from the Sun and the stars and the patterns of the night sky. Yet this light that the eye can see only spans a factor of two in wavelength. In the past 50 years, we have developed the technology to discover that the universe is filled with many kinds of invisible radiation. Now astronomers can explore the universe with radiation ranging from radio waves to gamma rays — a factor of 1015 or a thousand trillion in wavelength!

Diagram of the Electromagnetic Spectrum. Click here for original source URL

A few simple physical ideas govern a vast spectrum of radiation. All types of radiation travel at the same speed and that any type of radiation is specified by its wavelength. Any material that is not transparent — a solid, a liquid, or a dense gas — will radiate waves that are determined solely by its temperature. An object at a temperature of 5000 to 6000 Kelvin will glow with a yellowish color, whether it is molten metal in a furnace, the Sun itself, or a star like the Sun on the other side of the galaxy. Any low-density gas produces the same set of sharp spectral lines. The spectrum of helium is the same whether it is in a sealed tube in the lab, or in an enormous gas cloud that is so far away in the universe that its light has taken billions of years to reach us.

The broad idea of radiation illustrates the scope of the scientific method. Physicists study the interactions between matter and radiation using carefully controlled experiments in the laboratory. Astronomers apply those principles throughout the universe. This is induction on a large scale! We must remember that astronomers make a big assumption: the universe works the same everywhere and at all times.

Consideration of invisible forms of radiation gives us another perspective on the role of humans in the universe. Humans are sensitive to only a tiny fraction of the range of waves that the universe contains. After several thousand years of visual astronomy, spacecraft and new detector technologies have yielded a flood of information at other wavelengths. Imagine that you had to live with only one of your senses: the sense of sight. Think how many worlds would be closed to you: music, conversation, the taste and smell of food. You can probably understand then the excitement of astronomers who have new tools that extend their "senses" to explore the electromagnetic spectrum. These scientists are answering the intriguing question: how would the universe look if we had eyes that could see invisible rays.