* Space Based Atronomy.b/w 2/28/01 8:54 AM Page 23 UNIT 2 THE ELECTROMAGNETIC SPECTRUM Introduction Contrary to popular belief, outer space is not empty space. It is filled with electro- magnetic radiation that crisscrosses the universe. This radiation comprises the spectrum of energy ranging from radio waves on one end to gamma rays on the other. It is called the electromagnetic spectrum because this radiation is associat- ed with electric and magnetic fields that transfer energy as they travel through space. Because humans can see it, the most familiar part of the electromagnetic spectrum is visible light—red, orange, yellow, green, blue, and violet. Like expanding ripples in a pond after a pebble Electromagnetic radiation has properties of both has been tossed in, electromagnetic radiation waves and particles. What we detect depends on the travels across space in the form of waves. These method we use to study it. The beautiful colors that waves travel at the speed of light—300,000 kilo- appear in a soap film or in the dispersion of light meters per second. Their wavelengths, the dis- from a diamond are best described as waves. The tance from wave crest to wave crest, vary from light that strikes a solar cell to produce an electric thousands of kilometers across (in the case of current is best described as a particle. When the longest radio waves) to fractions of a described as particles, individual packets of electro- nanometer, in the cases of the smallest x-rays magnetic energy are called photons. The amount of and gamma rays. energy a photon of light contains depends upon its wavelength. Electromagnetic radiation with long Space-Based Astronomy Activity Guide for Science, Mathematics, and Technology Education 23 * Space Based Atronomy.b/w 2/28/01 8:54 AM Page 24 billionth. Thus 700 nanometers is a distance equal to 700 billionths or 7 x 10-7 meter.) Visible light is a very narrow band of radiation ranging from 400 to 700 nanometers. For comparison, it would take 50 visible light waves arranged end to end to span the thickness of a sheet of household plastic wrap. Below visible light is the slightly broader band of ultraviolet light that lies between 10 and 300 nanometers. X-rays follow ultraviolet light and diminish into the hundred-billionth of a meter range. Gamma rays fall in the trillionth of a meter range. The wavelengths of x-rays and gamma rays are so tiny that scientists use another unit, the electron volt, to describe them. This is the energy that an electron gains when it falls through a potential difference, or voltage, of one volt. It works out that one electron volt has a wavelength of about 0.0001 centimeters. X-rays range from 100 elec- tron volts (100 eV) to thousands of electron volts. Gamma rays range from thousands of elec- tron volts to billions of electron volts. Using the Electromagnetic Spectrum All objects in space are very distant and difficult for humans to visit. Only the Moon has been vis- ited so far. Instead of visiting stars and planets, astronomers collect electromagnetic radiation These two views of the constellation Orion dramatically illustrate the differ- from them using a variety of tools. Radio dishes ence between what we are able to detect in visible light from Earth’s surface capture radio signals from space. Big telescopes and what is detectable in infrared light to a spacecraft in Earth orbit. Photo on Earth gather visible and infrared light. Credits: Akira Fujii—visible light image; Infrared Astronomical Satellite— Interplanetary spacecraft have traveled to all the infrared image. planets in our solar system except Pluto and have landed on two. No spacecraft has ever brought wavelengths contains little energy. Electro-magnet- back planetary material for study. They send ic radiation with short wavelengths contains a great back all their information by radio waves. amount of energy. Virtually everything astronomers have learned Scientists name the different regions of the elec- about the universe beyond Earth depends on the tromagnetic spectrum according to their wave- information contained in the electromagnetic lengths. (See figure 1.) Radio waves have the radiation that has traveled to Earth. For example, longest wavelengths, ranging from a few centime- when a star explodes as in a supernova, it emits ters from crest to crest to thousands of kilometers. energy in all wavelengths of the electromagnetic Micro-waves range from a few centimeters to spectrum. The most famous supernova is the about 0.1 cm. Infrared radiation falls between stellar explosion that became visible in 1054 and 700 nanometers and 0.1 cm. (Nano means one produced the Crab Nebula. Electromagnetic National Aeronautics 24 and Space Administration * Space Based Atronomy.b/w 2/28/01 8:54 AM Page 25 Figure 1: Electromagnetic Spectrum radiation from radio to gamma rays has been have yet to understand the highly energetic detected from this object, and each section of the engines in the centers of these strange objects. spectrum tells a different piece of the story. Ultraviolet light studies have mapped the hot gas For most of history, humans used only visible near our Sun (within about 50 light years). The light to explore the skies. With basic tools and high energy end of the spectrum—x-rays and the human eye, we developed sophisticated gamma rays—provide scientists with information methods of time keeping and calendars. about processes they cannot reproduce here on Telescopes were invented in the 17th century. Earth because they lack the required power. Astronomers then mapped the sky in greater Nuclear physicists use strange stars and galaxies as a detail––still with visible light. They learned laboratory. These objects are pulsars, neutron stars, about the temperature, constituents, distribu- black holes, and active galaxies. Their study helps tion, and the motions of stars. scientists better understand the behavior of matter at extremely high densities and temperatures in the In the 20th century, scientists began to explore the presence of intense electric and magnetic fields. other regions of the spectrum. Each region provid- ed new evidence about the universe. Radio waves Each region of the electromagnetic spectrum pro- tell scientists about many things: the distribution of vides a piece of the puzzle. Using more than one gases in our Milky Way Galaxy, the power in the region of the electromagnetic spectrum at a time great jets of material spewing from the centers of gives scientists a more complete picture. For some other galaxies, and details about magnetic example, relatively cool objects, such as star-form- fields in space. The first radio astronomers unex- ing clouds of gas and dust, show up best in the pectedly found cool hydrogen gas distributed radio and infrared spectral region. Hotter objects, throughout the Milky Way. Hydrogen atoms are such as stars, emit most of their energy at visible the building blocks for all matter. The remnant and ultraviolet wavelengths. The most energetic radiation from the Big Bang, the beginning of the objects, such as supernova explosions, radiate universe, shows up in the microwave spectrum. intensely in the x-ray and gamma ray regions. Infrared studies (also radio studies) tell us about There are two main techniques for analyzing molecules in space. For example, an infrared starlight. One is called spectroscopy and the search reveals huge clouds of formaldehyde in other photometry. Spectroscopy spreads out the space, each more than a million times more mas- different wavelengths of light into a spectrum for sive than the Sun. Some ultraviolet light comes study. Photometry measures the quantity of light from powerful galaxies very far away. Astronomers in specific wavelengths or by combining all Space-Based Astronomy Activity Guide for Science, Mathematics, and Technology Education 25 * Space Based Atronomy.b/w 2/28/01 8:54 AM Page 26 Transparency of Earth’s Atmosphere RADIO MICROWAVE INFARED VISIBLE UV X-RAYS GAMMA RAYS 400 200 50 12 6 3 SEA LEVEL 42 -2 -4 -6 -8 -10 -12 10 101101010 10 10 10 Wavelengths (meters) Figure 2: Transparency of Earth’s Atmosphere wavelengths. Astronomers use many filters in Unit Goals their work. Filters help astronomers analyze par- • To investigate the visible light spectrum. ticular components of the spectrum. For exam- • To demonstrate the relationship between energy ple, a red filter blocks out all visible light wave- and wavelength in the electromagnetic spectrum. lengths except those that fall around 600 nanometers (it lets through red light). Teaching Strategy Unfortunately for astronomical research, Earth’s Because of the complex apparatus required to study atmosphere acts as a filter to block most wave- some of the wavelengths of the electromagnetic lengths in the electromagnetic spectrum. (See spectrum and the danger of some of the radiation, Unit 1.) Only small portions of the spectrum only the visible light spectrum will be studied in the actually reach the surface. (See figure 2.) More activities that follow. Several different methods for pieces of the puzzle are gathered by putting displaying the visible spectrum will be presented. observatories at high altitudes (on mountain Some of the demonstrations will involve sunlight, tops) where the air is thin and dry, and by flying but a flood or spotlight may be substituted. For best instruments on planes and balloons. By far the results, these activities should be conducted in a best viewing location is outer space. room where there is good control of light. National Aeronautics 26 and Space Administration * Space Based Atronomy.b/w 2/28/01 8:54 AM Page 27 ACTIVITY: Simple Spectroscope Description: A basic hand-held spectroscope is made from a diffraction grating and a paper tube.