Using the spectrum and armed with the proper instrumentation, then, astronomers can accurately read the temperature of even very distant objects in space. And even without sophisticated equipment, you can startle your friends by letting them know that Betelgeuse (a reddish star) must have a lower surface temperature than the yellow sun.
Astronomers also use the spectrum to learn even more about distant sources. A spectroscope passes incoming light through a narrow slit and prism, splitting the light into its component colors. Certain processes in atoms and molecules give rise to emission at very particular wavelengths. Using such a device, astronomers can view these individual spectral lines and glean even more information about conditions at the source of the light. While ordinary white light simply breaks down into a continuous spectrum—the entire rainbow of hues, from red to violet, shading into one another—light emitted by certain substances produces an emission spectrum with discrete emission lines, which are, in effect, the fingerprint of the substance.
Hydrogen, for example, has four clearly visible spectral lines in the visible part of the spectrum (red, blue-green, violet, and deep violet). The color from these four lines (added together as light) is pinkish. These four spectral lines result from the electron that is bound to the proton in a hydrogen atom jumping between particular energy levels. There are many other spectral lines being emitted; it just so happens that only four of them are in the visible part of the spectrum. Hot hydrogen gas is the source of the pinkish emission from regions around young stars like the Orion.
In our hydrogen atom example, a negative electron is bound to a positive proton. The electron, while bound to the proton, can only exist in certain specific states or energy levels. Think of these energy levels as rungs on a ladder. The electron is either on the first rung or the second rung. It can’t be in between. When the electron moves from one energy level to another (say, from a higher one to a lower one), it gives off energy in the form of a photon. Since the levels that the electron can inhabit are limited, only photons of a few specific frequencies are given off. These particular photons are apparent as bright regions in the spectrum of hydrogen: the element’s spectral emission lines.
As Maxwell first described in the nineteenth century, all objects emit radiation at all times because the charged atomic particles of which they are made are constantly in random motion. As these particles move, they generate electromagnetic waves. Heat an object, and its atomic particles will move more rapidly, thereby emitting more radiation. Cool an object, and the particles will slow down, emitting proportionately less electromagnetic radiation. If we can study the spectrum (that is, the intensity of light from a variety of wavelengths) of the electromagnetic radiation emitted by an object, we can understand a lot about the source. One of the most important quantities we can determine is its temperature. Fortunately, we don’t need to stick a thermometer in a star to see how hot it is. All we have to do is look at its light carefully. But how?
All objects emit radiation, but no natural object emits all of its radiation at a single frequency. Typically, the radiation is spread out over a range of frequencies. If we can determine how the intensity (amount or strength) of the radiation emitted by an object is distributed across the spectrum, we can learn a great deal about the object’s properties, including its temperature.
Physicists often refer to a black body, an imaginary object that absorbs all radiation falling upon it and re-emits all the radiation that it absorbs. The way in
which this re-emitted energy is distributed across the range of the spectrum is drawn as a black-body curve. Now, no object in the physical world absorbs and radiates in this ideal fashion, but the black-body curve can be used as a reference index against which the peak intensity of radiation from real objects can be measured. The reason is that the peak of the blackbody curve shifts toward higher frequencies (and shorter wavelengths) as an object’s temperature increases.
Thus, an object or region that is emitting very short wavelength gamma ray photons must be much hotter than one producing longer wavelength radio waves. If we can determine the wavelengths of the peak of an object’s electromagnetic radiation emissions, we can determine its temperature.
Astronomers measure peak intensity with sophisticated scientific instruments, but we all do this intuitively almost every day. You have an electric kitchen range, let’s say. The knob for one of the heating elements is turned to off. The heating element is black in color. This tells you that it may be safe to touch it.
But if you were to turn on the element, and hold your hand above it, you would feel heat rising, and would know that it was starting to get hot. If you had infrared vision, you would see the element “glowing” in the infrared. As the element grows hotter, it will eventually glow red, and you would know that it was absolutely a bad idea to touch it (regardless of where the control knob happened to be pointing). At room temperature, the metal of the heating element is black, but as it heats up, it changes color: from dull red to bright red. If you had a very high-voltage electric range and a sufficiently durable heating element, you could crank up the temperature so that it became even hotter. It would emit most of its electromagnetic radiation at progressively higher frequencies.
Now, an object that omits most of its radiation at optical frequencies would be very hot. And a range will never (we hope) reach temperatures of 6000 K, like the sun. The red color you see from the range is in the “tail” of its black-body spectrum. Even when hot, it is still emitting most of its radiation in the infrared part of the spectrum.
The information—the news—we get from space is censored by the several layers of Earth’s atmosphere. In effect, our Earth is surrounded by a ceiling pierced by two skylights. A rather broad range of radio waves readily penetrates our atmosphere, as does a portion of infrared and most visible light, in addition to a small portion of ultraviolet. Astronomers speak of the atmosphere’s radio window and optical window, which allow passage of electromagnetic radiation of these types. To the rest of the spectrum—lower-frequency radio waves, some lower-frequency infrared, and, fortunately for us, most of the energetic ultraviolet rays, x-rays, and gamma rays—the atmosphere is opaque, an impenetrable ceiling.
In many ways, the partial opacity of our atmosphere is a very good thing, since it protects us from x-ray and gamma radiation. An atmosphere opaque to these wavelengths, but transparent to visible light and some infrared, is a big reason why life can survive at all on Earth.
For astronomers, however, there is a downside to the selective opacity of the earth’s atmosphere. Observations of ultraviolet, x-ray, and gamma ray radiation cannot be made from the surface of the earth, but must be made by means of satellites, which are placed in orbit well above the atmosphere. No wonder that the advent of the space age has led to such an explosion in the amount of information that we have about the universe.
Often, when people get excited, they run around, jump up and down, and shout without making a whole lot of sense. But when atomic particles get excited, they can produce energy that is radiated at a variety of wavelengths. In contrast to the babble of an excited human throng, this electromagnetic radiation can tell you a lot, if you have the instruments to interpret it.
Our eyes, one such instrument, can interpret electromagnetic radiation in the 400 to 700 nanometer (or 4000 to 7000 Angstrom) wavelength range. A nanometer (abbreviated nm) is one billionth of a meter, or 10–9 meter. An Angstrom (abbreviated A) is 10 times smaller, or 10–10 meter. But that is only a small part of the spectrum. What about the rest of the “keyboard”?
The Greek philosopher Democritus (ca. 460–ca. 370 B.C.E.) was partially right: matter does consist of atoms. But he would have been fascinated to know that the story doesn’t end there. Atoms can be further broken down into electrons, protons, and neutrons, and the latter two are made of even smaller things called quarks.
Electrons carry a negative electric charge, and protons a positive charge. Neutrons have a mass almost equal to a proton, but as their name implies, neutrons are neutral, with no positive or negative charge. Charged particles (like protons and electrons) that are not moving are surrounded by what we call an electric field; those in motion produce electromagnetic radiation.
James Clerk Maxwell (1831–1879) first explored what would happen if such a charged particle were to oscillate, or move quickly back and forth. He showed that a moving charged particle created a disturbance that traveled through space—without the need for any medium. Particles in space are getting banged around all the time. Atoms collide, electrons are accelerated by magnetic fields, and each time they move, they pull their fields along with them, sending “electromagnetic” ripples out into space.
In short, information about the particle’s motion is transmitted through space by a changing electric and magnetic field. But a field is not a substance. It is a way in which forces can be transmitted over great distances without any physical connection between the two places. The force of gravity, which we have discussed, can also be thought of as a field.
Let’s turn to a specific example: A star is made up of innumerable atoms, most of
which at unimaginably hot stellar temperatures are broken into innumerable charged
particles. A star produces a great deal of energy (by nuclear fusion. This energy causes particles to be in constant motion. In motion, the charged particles are the center points of electromagnetic waves (disturbances in the electromagnetic field) that move off in all directions. A small fraction of these waves reaches the surface of the earth, where they encounter other charged particles. Protons and electrons in our eyes, for instance, oscillate in response to the fluctuations in the electric field. As a result, we perceive light: an image of the star. If we happened to have, say, the right kind of infrared-detecting equipment with us, electrons if that equipment would respond to a different wavelength of vibrations originating from the same star.
Similarly, if we were equipped with sufficiently sensitive radio equipment, we might pick up a response to yet another set of proton and electron vibrations.
Remember, it is not that the star’s electrons and protons have traveled to Earth, but that the wave they generated so far away have excited other electrons and protons here. Call it an interstellar handshake.
If you don’t happen to like math, don’t panic. Just visualize stone-generated waves rippling across a pond, and you’ll understand the basic concept of waves. But wait a minute. There is something wrong with our ripples in a pond as a model of electromagnetic radiation.
Water is a medium, a substance, something through which waves are transmitted. Space, we have said, is very nearly a vacuum, nothing. How, then, do waves move through it?
This is a question that vexed physicists for centuries. They understood the concept of waves. But they also understood that sound, a wave, could not travel through a vacuum, whereas light, also a wave, could.
Why?
At first, most scientists believed that the very fact that light is transmitted through space means that space must not be empty. They knew it didn’t have air, as on Earth, but they suggested it was filled with another substance, which they called the ether. But this fictitious substance did not long vex physicists. A series of experiments in the late nineteenth century made it clear that ether didn’t exist and that although light could be studied as a wave, it was a different kind of wave than, say, sound.
We can understand how electromagnetic radiation is transmitted through space if we appreciate that it involves waves. What is a wave? The first image that probably jumps to mind is that of ocean waves. And ocean waves do have some aspects in common with the kind of waves that we use to describe electromagnetic radiation. One way to think of a wave is that it is a way for energy to be transmitted from one place to another without any physical matter being moved from place to place. Or you may think of a wave as a disturbance that carries energy and that occurs in a distinctive and repeating pattern. A row boat out in the ocean will move up and down in a regular way as waves pass it. The waves do transmit energy to the shore (think of beach erosion), but the row boat will stay put.
That regular up-and-down motion that the rowboat experiences is called harmonic motion. But there are two important differences with electromagnetic radiation: The sources of waves are things on atomic scales (electrons and the nuclei of atoms), and no medium is required for electromagnetic waves to travel through space. The “pond” of space consists only of electric and magnetic fields, and photons of light are ripples in that ghostly pond.
Waves come in various shapes, but they all have a common anatomy. They have crests and troughs, which are, respectively, the high points above and low points below the level of an undisturbed state (for example, calm water). The distance from crest to crest (or trough to trough) is called the wavelength of the wave. The height of the wave—that is, the distance from the level of the undisturbed state to the crest of the wave—is its amplitude. The amount of time it takes for a wave to repeat itself at any point in space is its period.
In other words, the period is the time between the passage of wave crests as seen by an observer in the bobbing row boat. The number of wave crests that pass a given point during a given unit of time is called the frequency of the wave. If many crests pass a point in a short period of time, we have a high-frequency wave. If few pass that point in the same amount of time, we have a low-frequencywave. The frequency and wavelength of a wave are inversely proportional to one another, meaning that as one gets bigger, the other gets smaller. High frequency radiation has short wavelengths.
Electromagnetic radiation sounds like dangerous stuff—and, in fact, some of it is. But that the word radiation need not set off sirens in your head. It just describes any way energy is transmitted from one place to another without the need for a physical connection between the two places. We use it as a general term to describe any form of light. It is important that radiation can travel without any physical connection, because space is essentially a vacuum; that is, much of it is empty. If you went on a space walk clicking a pair of castanets, no one, including you, would hear your little concert. Sound is transmitted in waves, but not as radiation. Sound waves require some medium to travel in. So despite what most science fiction movies would lead you to believe, explosions in space are silent. Light (and other forms of electromagnetic radiation) requires no such medium to travel, although many physicists tried in vain to detect a medium, which they called the ether. We’ll talk more about this fact in a moment.
The electromagnetic part of the phrase denotes the fact that the energy is conveyed in the form of fluctuating electric and magnetic fields. These fields require no medium to support or sustain them.
The light we receive from distant sources is generated on the tiniest of scales. To explore the largest objects, such as galaxies, we have to first understand the smallest of objects, atoms and the particles making up atoms. The photons that we detect with our eyes and catch with our telescopes were generated in many different ways: sometimes by electrons hopping between different orbital levels in an atom, or other times by the energetic collisions of atomic nuclei. We now explore the ways in which photons of light arise, how they get from there to here, and what they can tell us about the objects that we observe.
We have concentrated thus far on optical photons (the ones that we can see with our eyes). As it turns out, our eyes respond to “visible” wavelengths because that is where the peak of the emission from the sun is located in the electromagnetic spectrum. If our eyes were most sensitive to infrared radiation, for example, we would see some things we can’t now see (body heat), but would miss a lot of other useful stuff. In this chapter, we’re going to talk more about visible light and the electromagnetic spectrum, of which visible light is a tiny subset. Think of it this way: If the electromagnetic spectrum is represented by a piano keyboard, then the visible part of the spectrum is but a single key or note. In the cosmic symphony, there are many notes, and we want to be able to hear them all. If you’re concerned that this sounds more like physics than astronomy, you’re right. But don’t be intimidated. Most of astronomy involves applications of physics principles, and we are convinced that understanding what you are seeing when you look at a star greatly enhances the experience of looking. Remember this astounding fact : When you look at the light from our sun or a distant star, you are witnessing the product of nuclear fusion reactions that are, every second, releasing more energy than any atomic explosion Earth has ever witnessed. Yet it is not just brute energy, but also information from the sky. Let’s take a closer look.
