|Exploring Our Universe: |
From the Classroom to Outer Space
Fact Sheet: How Astronomers Study Light
Fig.1 Wave model of a photon (credit Patricia Knezek)
These two models of light are related in the following way: the amount of energy a photon contains depends on its wavelength. The smaller the wavelength of the photon, the greater the energy it contains. When we see visible light, it appears white. This is because we are seeing many different wavelengths of light all at once. If we separate white light by using a prism, we see a rainbow of colors. The colors of a rainbow are always in the same order. We normally divide the rainbow into seven main colors: red, orange, yellow, green, blue, indigo and violet (you can remember their order with the acronym ROY G. BIV). But a rainbow is really a continuous range of colors, since the Sun emits light at all different wavelengths. Each color corresponds to a specific wavelength of light.
Fig.2 The Visible Spectrum
The wavelengths of visible light are usually measured in nanometers (nm), or in Angstroms (Å). A nanometer is one-billionth of a meter! And 1 Angstrom is one tenth of a nanometer. Our eyes can see wavelengths of light that range from 400 nm for violet light up to 700 nm for red light. Photons of violet light have more energy than photons of red light, because violet light has a shorter wavelength.
Fig.3 The Electromagnetic Spectrum
Gamma rays have the shortest wavelengths, about the size of the nucleus of an atom! Gamma ray photons carry a tremendous amount of energy (about ten billion times as much as a photon of visible light!) and therefore can be very harmful to living organisms. X-ray light also has very short wavelengths and very high energy. X-rays are able to pass directly through many materials. While this lets us use X-rays to take images of the bones inside our bodies, it also makes X-ray light harmful to our body cells. Ultraviolet light (which means "beyond violet") is responsible for giving us suntans and sunburns. The fact that ultraviolet light is able to "burn" our skin reminds us that this type of light has a high level of energy. Too much ultraviolet light is dangerous to most living organisms. Fortunately most ultraviolet light emitted by the sun never reaches us. It is blocked primarily by the ozone layer of the Earth's atmosphere. Glass is also able to block, or absorb, ultraviolet light- which is why no one sunbathes indoors! Infrared, microwaves and radio waves are kinds of light that have lower amounts of energy, because the photon wavelengths get longer and longer. Even still, we are able to use them in many ways. We experience infrared light ( which means "below red") as heat- another form of energy! We sometimes see heat lamps in restaurants used to keep food warm: they emit infrared light. Rescue workers use infrared detectors after earthquakes to locate people who are trapped inside collapsed buildings. Microwaves are used to heat food and to send communication signals for TV and telephone without using wires. Satellite dish antennas are devices that send or receive microwaves. Radio waves have the least energy of all, yet we can send them over long distances to transmit voice and music signals. AM radios receive light waves that are longer than a football field!
One of the remarkable things about light is that it always travels at a constant speed in a vacuum (space). Light moves incredibly fast, nearly 300,000 kilometers per second! (That equals 186,000 miles per second!) Because the Sun is 150,000,000 kilometers (93 million miles) away from Earth, light from the Sun takes over eight minutes to reach our eyes. When we see the Sun in the sky, we are actually seeing how the Sun looked eight minutes ago. So the next time you watch the Sun set, keep in mind that it has actually been below the horizon for eight minutes! When we look at other stars and galaxies, it is like looking at a very old photograph. We are seeing them as they looked when they originally emitted the light. The age of this "photograph" depends on how long the light has had to travel through space to reach our eyes. Astronomers often describe the distance from stars and galaxies to Earth by calculating the time it took their light to reach us. Astronomers use a unit called a light-year. This is the distance that light travels during one year's time. A light-year is equal to about 9,500,000,000,000 kilometers (or 5.9 trillion miles)! Go to a place far away from bright city lights and look at the sky on a clear night. You will see the light from thousands of stars, even without a telescope. The nearest of these stars (other than the Sun, of course) is Alpha Centauri, about 4.3 light-years away from Earth. This means that the distance to this star is about 25 trillion miles. If we launched a spacecraft in the direction of Alpha Centauri, it would take the spacecraft over 25,000 years to reach the star! Since we can't yet send any spacecraft to nearby stars in our lifetime, the only way astronomers can study stars, or even more distant objects like other galaxies, is to study the light that these objects send us from across the universe.
When most people think about astronomy, they think about the pictures we see of celestial objects like the Moon and planets, comets, stars and galaxies. Many of these images are formed by the visible light that they give off. Yet these objects also emit light at all other wavelengths, including forms of invisible light such as X-ray, ultraviolet and infrared. In the same way we make photographs using visible light, we can also create images made from the invisible light that objects emit, often using electronic detectors. Astronomers can study objects that emit very little visible light- such as pulsars, quasars and black holes- by using telescopes that "see" the forms of light our eyes cannot. Astronomers are able to uncover information about many of the objects in the universe by comparing images taken in different regions of the electromagnetic spectrum.
Fig.4 Images of the sun taken in different regions of the em spectrum; (from left to right) x rays, ultraviolet, visible, and radio ( Photo credit: Association of Universities for Research in Astronomy, Inc. (AURA)/Space Telescope Science Institute (STScI) )
While people are often fascinated by pictures from outer space, the images alone do not always give scientists all the information they need about the object in the picture. In order to study the information contained in different forms of light, astronomers need to further separate light into individual wavelengths. This technique is known as spectroscopy (from the word "spectrum," with the suffix scopy meaning to see). To separate different wavelengths of light, scientists use prisms or diffraction gratings. Diffraction gratings work just like prisms, but are able to separate wavelengths of light into much finer detail. This allows scientists to study very specific parts of the electromagnetic spectrum one at a time. Scientists also need to measure the amount of light- called the intensity- coming from a certain location. We normally think of intensity as "brightness" of an object. To measure intensity, scientists use electronic devices that count the number of photons striking them. By collecting data on how much light is emitted by an object at specific wavelengths, scientists can obtain information about the properties of the object itself.
Scientists seek to answer many questions about physical objects by studying the amount of light they emit or absorb at different wavelengths. To do this, scientists must establish some relationship between various wavelengths of light and the light intensity they observe at these wavelengths. Relationships are seen most clearly on graphs, so scientists transfer the information of light spectra gathered by electronic detectors onto graphs. By doing this, they can compare numbers directly. If you look at a rainbow spectrum, the intensity of the different colors you see represent the amount of light at each wavelength. The brightness of each color is related to the intensity at that particular wavelength. In the example below, the intensity of the colors in the rainbow produced by sunlight is graphed according to wavelength. The graph shows that the light is most intense, or brightest, at a wavelength of about 550 nanometers, which is in the yellow region. This is what should be expected, since a rainbow is produced by sunlight, and the Sun appears to be yellow! The reason the Sun appears to be yellow is because it is at that color that the Sun emits the highest intensity.
Fig. 5 Continuum Spectrum
Other sources of light, such as gaseous nebulae, do not emit a continuous spectrum with all the colors of the rainbow; they emit light only at discrete wavelengths. These wavelengths of light are like fingerprints. They help us identify what type of substance is producing the light. Think about the neon signs we see in windows- does neon light look like sunlight? If we were to look at neon light through a diffraction grating, we would not see all the colors in the rainbow. Instead, we would only see a few strong lines of color. The graph below shows the light intensity of a neon sign is high only at a few specific wavelengths. Whenever scientists see this type of pattern on a graph, they know they are looking at neon. This pattern is called the emission spectrum of neon. The spikes or peaks on the graph are called emission lines. What is the relationship between the color of a neon sign and the wavelengths of light that neon emits?
To learn more about these topics, here are some other very useful pages: Using Light to Learn About the Universe
Fig.6 Neon line emission spectrumSometimes chemical elements can be identified by the types of light they absorb. Neon gas emits light in a neon sign because of the electricity passing through it. When the electric current is turned off, the neon sign stops glowing. Now think about sunlight passing through a neon sign that has been turned off. If we were to look at this sunlight through a diffraction grating, we would no longer see a complete rainbow- some of the colors would be missing! This is because the neon gas in the tubes actually absorbed some of the sunlight. If you look at the graph below, you see that the light intensity dips far down at certain wavelengths. This type of pattern is called an absorption spectrum. The dips on the graph are called absorption lines. What do you notice when you compare the absorption and emission lines for neon?
Fig.7 Neon line absorption spectrum
Every element in the periodic table, and every chemical compound, has its own unique pattern of spectral lines. This pattern is the "fingerprint" scientists use to identify the presence of that given chemical element. Spectroscopy, or the study of the light spectrum, can be used by scientists in many fields. Biologists use spectroscopy to study the pigments in plants; this helps them understand how photosynthesis works. Chemists can look at spectra to identify the types of atoms or molecules present in an unknown substance. And astronomers use spectra to learn about distant objects in the universe. Spectroscopy is often the sole method astronomers can use to do their work. Remember that astronomers can't bring the planets and stars they wish to study into their laboratories! The only way they can learn about the objects in the universe is to study the light they emit. Astronomers use spectroscopy to study properties of objects in the universe, besides chemical composition, such as the temperature, density, chemical composition and speed of a celestial object, and whether it is moving away from or towards us.
To help advance our understanding of the universe, NASA will launch a new space telescope in 1999 called FUSE- the Far Ultraviolet Spectroscopic Explorer. It will examine a part of the electromagnetic spectrum that has not yet been looked at extensively- the far ultraviolet region, where the wavelengths of light range from 90nm to 120nm. The FUSE satellite will orbit outside most of the Earth's atmosphere, just like the Hubble Space Telescope. This space telescope will be a valuable new tool for astronomers, because FUSE is designed to study wavelengths of light too short for Hubble to observe. One purpose of putting telescopes into space is to observe forms of light that do not reach the Earth's surface. (It's a good thing that most electromagnetic radiation is blocked by the Earth's atmosphere; life could not exist if we were constantly bombarded by gamma rays, X-rays and UV light!) The instruments on FUSE will use spectroscopic techniques to study light that does not penetrate Earth's atmosphere. Astronomers are very eager to observe the light in the far ultraviolet region. They will be able to observe many interesting spectral lines in this region. The data they collect will yield clues to how the universe was formed, and how it continues to evolve. People continue to be fascinated by what scientists have learned so far about the universe. Yet many questions remain unanswered. It is through our modern understanding of the behavior of light that we have learned so much already. Spectroscopy is an extremely important tool that astronomers and other scientists use to study light, and will help us explore the remaining mysteries of the universe.