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Light is remarkable. It is something we take for granted every day, but it's not something we stop and think about very often or even try and define. Let's take a few minutes and try and understand some things about light.
Simply stated, light is nature's way of transferring energy through space. We can complicate it by talking about interacting electric and magnetic fields, quantum mechanics, and all of that, but just remember--light is energy.
Light travels very rapidly, but it does have a finite velocity. In vacuum, the speed of light is 186,282 miles per second (or nearly 300,000 kilometers per second), which is really humming along! However, when we start talking about the incredible distances in astronomy, the finite nature of light's velocity becomes readily apparent. It takes about two and a half seconds, for instance, for a radio communication travelling at the speed of light to get to the moon and back.
We should also highlight right up front that light is more generally referred to as electromagnetic radiation. Okay, we used a big word. It had to happen sooner or later. But too often when we say "light" it is mistaken to mean "optical light," which is roughly the radiation visible to our eyes. Visible light is a tiny portion of a huge smorgasboard of light called the electromagnetic spectrum. For our convenience, we break this smorgasboard up into different courses (appetizer, salad, etc.) and refer to them by name, such as gamma-rays, X-rays, ultraviolet, optical, infrared, and radio. However, it is important to remember that they are all justlight. There are no "breaks" and no hard boundaries in the electromagnetic spectrum--just a continuous range of energy.
Physics experiments over the past hundred years or so have demonstrated that light has a dual nature. In many instances, it is convenient to represent light as a "particle" phenomenon, thinking of light as discrete "packets" of energy that we call photons. Now in this way of thinking, not all photons are created equal, at least in terms of how much energy they contain. Each photon of X-ray light contains a lot of energy in comparison with, say, an optical or radio photon. It is this "energy content per photon" that is one of the distinguishing characteristics of the different ranges of light described above. Even though it is not strictly correct, it is hard not to think of a beam of light as a collection of little "light bullets" all strung together in a row.
It's the same way as we move throughout the electromagnetic spectrum. Each range of light we have defined above corresponds to a range of frequencies (or wavelengths) of light vibrations. These wavelengths are one of the primary indicators we use to describe light and spectra on a graph. Displaying a spectrum as a graph instead of just a color bar allows us to measure the light.
For instance, the "rainbow" of color shown in the figure above is what you see when you pass white light through a prism. What may not be obvious, however, is that the "intensity" or brightness of the light is also changing along with the colors. If we converted the "rainbow" into a graph of light intensity versus wavelength, it would look like this:
Notice that the spectrum is brightest in the middle (yellow-green region) and drops off in both directions (toward red and blue). This was not obvious from the rainbow version of the spectrum! Also notice that the "intensity" of the light in the graph does not stop at the "ends" of the rainbow spectrum that is visible to our eyes! The light continues beyond what we can see in both directions, which we can see in the graph but not by looking at the rainbow. Astronomers use graphical spectra most of the time because they can get more information out of the light this way, and because they can still plot and analyze light that is not directly visible to our eyes!
Now we mentioned that the energy of each photon of light was also a basic property. It turns out that there is a simple relationship between the energy of a photon and the corresponding wavelength of that photon:
It should come as no surprise to you that atoms and molecules (which are simply bound collections of two or more atoms) can absorb light (= energy!). If they didn't, you could simply flick a light on and off, and then sit back while the photons continued to bounce around the room! Likewise, infrared light (= heat = energy!) wouldn't do any good in heating up your home in the winter if it didn't get absorbed by matter. Higher energy light photons, like X-rays, tend to want to plow through more matter before they get absorbed. (Hence, their use in medical imaging: they can pass through your "soft" tissue, but are more readily absorbed in your bones, which are denser.) How and why do photons get absorbed by matter?
Well, it's time to develop another conceptual device to help us understand this process. In physics, we often find it helpful to pretend we are looking at a single atom. Atoms are made up of protons, neutrons, and electrons, and each chemical element has a specific number of them--that's what makes them different! Protons (and neutrons) are more massive than electrons, and so we sometimes visualize an atom as a miniature solar system, with the heavy particles at the center (the nucleus) and the electrons whizzing around in specific "orbits" like planets. (In reality, this picture is not very accurate. Electrons are not thought to be little balls "in orbit" around a nuclear "sun." However, if you get the idea that the electrons are only found at specific, discrete "distances" from the nucleus, and that each allowed distance corresponds to a different "energy level" for the electron, that would be closer to reality.)
Without delving into atomic physics and quantum mechanics too far, let us just take the following statement for granted for now: the electrons bound to any particular atom can only be found in certain, specific energy levels with respect to the atom's nucleus. The hydrogen atom only contains one proton and one electron, and is the simplest (and most common) element in the Universe, so let's use it as an example. The figure [TBD] shows a schematic hydrogen atom where instead of drawing the allowed "orbits" for the electron we draw vertically-displaced lines to represent the allowed energy levels for the electron.
If left undisturbed, our hydrogen atom likes to bind its electron as tightly as it can, and so we would find the electron in the lowest energy level, which is called the "ground state." However, if our atom is immersed in a beam of light from, say, a nearby star, sooner or later the atom will encounter a photon with an energy that is just the right amount to jump the electron up to the next higher energy level. Voila! The photon gets absorbed, and is "gone" from the beam of light coming from the star! Since the absorbed photon had a specific energy, this absorption occurs at a specific wavelength in the spectrum.
Now our hydrogen atom is in what is called an "excited" state, sort of like a kid right before Halloween. However, as all parents know, this is not the natural state of a child, and it's not the natural state of an atom either. If no other photons are absorbed by the atom, the electron will eventually drop back down to the lower energy ground state. However, the atom has to lose energy to do this, and so it releases a photon of the same energy as the one it absorbed (albeit most likely into some other direction from which it was absorbed). This process is called emission because a photon of light is emitted by the atom, again at a very specific wavelength.
Of course, the atom could have absorbed another photon with just the right energy to jump up another energy level, or even two or three or more. Likewise, after each of these possible excitations of the atom, the electron could jump back down one or more steps, emitting photons as it went. If a photon with a sufficiently large energy gets absorbed, it can even cause an electron to become unbound from its nucleus, a process that is calledionization. Our crippled hydrogen atom could then no longer absorb or emit light until it manages to capture a free electron back into a bound energy level.
We have been discussing one specific transition or "energy jump" in one atom, but of course in any physical system there are many atoms. In a hydrogen gas, all of the separate atoms could be absorbing and emitting photons corresponding to the whole group of "allowed" transitions between the various energy levels, each of which would absorb or emit at the specific wavelengths corresponding to the energy differences between the energy levels. This pattern of absorptions (or emissions) is unique to hydrogen--no other element can have the same pattern--and causes a recognizable pattern of absorption (or emission) lines in a spectrum.
Just about every astronomy textbook you will ever pick up will contain a phrase to the effect that the process of breaking light up into a spectrum is "like passing white light through a prism." This process, calleddispersion, arises because different colors (or wavelengths) of light bend by different amounts as they pass from, say, a low density medium (like air) into a higher density medium (like the glass in a prism). Hence, a narrow beam of "white" light will get spread out into a rainbow. Voila, a spectrum!
In practice, most spectrographs in astronomy, including those that operate in the optical part of the spectrum, use a totally different method for creating a spectrum out of the incoming light from the telescope--the process of diffraction. This process depends on the wave-like properties of the light, and uses a component called a diffraction grating to actually separate the light into its component wavelengths. A diffraction grating consists of a substrate (often made of glass, but stainless steel, plastic, or other materials are sometimes used) onto which are etched very narrowly-spaced lines. How narrowly-spaced? Well, a typical diffraction grating used in optical astronomy may have anywhere from several hundred to over one thousand lines etched per millimeter! A famous physicist from Johns Hopkins University, Henry A. Rowland, was the first person to make high quality diffraction gratings for use in scinece.
How does such a grating break a beam of light into its component wavelengths? [TBD. Hope to add this soon! But most Intro Physics text books give a description of this process. So until I get around to it, use the Library!]
A diffraction grating by itself is really no better than a prism for creating an astronomical spectrum. The grating must be built into a device called a spectroscope or spectrograph for this to be done. These are effectively the same thing except that a spectroscope is simply used for visual inspection (that is, your eye is the detector), while a spectrograph includes some means (photographic film or an electronic detector) for recording the spectrum for analysis. In professional astronomy these days there is very little need for a spectroscope (just as there is very little other observational work actually done with the naked eye, with the possible exception of staring at a computer monitor all day!).
Ok, so what is a spectrograph? In its simplest form, it is a light-tight box with a small (often narrow rectangular or adjustable) opening to let light in, a grating to break the light into its components, and a "detector" of some kind placed at the proper angle and distance from the grating to record the spectrum of the wavelength range of interest. Telescopes are used to gather the faint light from distant objects, and the spectrographs are placed at the focus of the telescope to analyze the light.
A detector is simply a device that senses and measures the incoming light. In a spectrograph, the detector has to perform this task across a range of wavelengths, measuring the amount of light as it changes from wavelength to wavelength. In an optical spectroscope, the detector is your eye, which senses the different colors and the presence of dark absorption lines or bright emission lines in the spectrum of the source being viewed. In a spectrograph, some other device is used to sense the light.
For many years the primary detector used in spectrographs was the photographic plate (basically film, although special astronomical emulsions placed on glass plates were used for greater sensitivity and stability). Often spectra recorded this way were then traced with a device called a (are you ready for this?) microdensitometer. This device would shine a steady, narrow beam of light through the photographic plate to a light sensitive photomultiplier tube. As the plate was stepped along the length of the spectrum, the photomultiplier tube would measure and record the amount of light at each wavelength. The resulting tracing would essentially be a graph of the intensity of light as a function of the position on the photographic plate (or as a function of wavelength in the case of a spectrum). This graphical representation of a spectrum is what astronomers find most useful in doing their work.
This picture shows an electronic detector called a charge-coupled device, or CCD. The small central rectangle contains a closely packed array of 320 by 512 light sensing diodes, each of which individually record the brightness of light and send the information to a computer. Imagine placing this device at the focus of a large telescope! It allows astronomers to "see" objects millions of times fainter than the unaided eye! (Click on the picture to see a larger version. Photo courtesy of the Smithsonian Astrophysical Observatory.)
Over the last 20 years or so even photographic recording of spectra has nearly become a thing of the past. Electronic recording of spectra is the most sensitive, quantitative, way of detecting the light, and it gets the spectrum directly into a digital form that can be handled on a computer (where the real work gets done). The detector used most often in astronomy these days is called a charge coupled device, or CCD. This device is basically an array of tiny, light-sensitive diodes and is also now commonly used in video cameras and digital still cameras. Astronomical CCDs, however, are often tweaked up to provide the best performance at faint light levels, in many cases recording the arrival of individual photons of light from distant sources in the Universe!
Every time an astronomer goes to a telescope to obtain spectra, he/she has to answer several questions about the goals needed for their investigation. For instance, one has to know exactly what spectral lines need to be observed, and hence, how much spectral coverage is necessary. Are all of the lines of interest in the red part of the spectrum, or is full spectral coverage from the blue through the red needed? The other basic question is how much resolving power is needed (basically, how much does the light need to be spread out to show the details in the spectrum)?
This last question involves several considerations. Are there spectral lines of interest that are close together in wavelength? If so, one must use sufficiently high dispersion to allow the lines to be separated; otherwise the lines will be blended together such that they cannot be measured individually.
Another consideration may be whether one is making velocity measurements. If so, what precision is needed for measuring the redshifts or blueshifts of the lines in the spectrum? For instance, let's say you wanted to measure velocities of expansion of a planetary nebula, which are typically about 10 kilometers per second, using lines in the red part of the optical spectrum (about 6500 Angstroms). The equation for Doppler shifts says you would want to make sure your spectrograph can make measurements to an accuracy of at least 0.2 Angstroms.
A difficulty sometimes arises when a project desires both high spectral dispersion and broad wavelength coverage. For a detector of fixed size, the more one spreads the light out (higher dispersion) the less the range of wavelengths that will fall on the detector (smaller spectral coverage). In cases where both spectral coverage and high spectral dispersion are needed, a special spectrograph called an echelle spectrograph can be used. This device contains two diffration gratings instead of one, a high dispersion grating to provide the desired spectral resolution, and a lower dispersion grating that spreads the overall spectrum out into an array of miniature spectra, each covering only a portion of the desired spectral range. While these spectrographs are not suitable for every observation, they make it possible in certain instances for a single observation to do the job of 50 or more observations with a regular spectrograph!
The violet and red "ends" of the optical spectrum are not really "ends" at all, but rather simply the limits to the portion of the EM spectrum to which our eyes are sensitive. Beyond red light lies the region known as the infrared, which is also simply known as heat radiation. (The fact that light is just energy may be most obvious in this portion of the spectrum!) The longest wavelength infrared radiation blends into the shortest wavelength radio waves, and the radio region extends out to the longest wavelengths we are able to measure.
Likewise, beyond the violet of the optical spectrum lies a broad region known as the ultraviolet, which blends into the X-ray region, followed by the the shortest wavelength radiation known, the gamma rays. Again, there is no edge or "end" of the spectrum at shortest wavelengths, although we reach a practical limit as to what can be measured. (The shortest wavelength gamma rays are on a par with the size of an atomic nucleus!)
There are no hard boundaries to each spectral region; they just blend together into a continuum of smoothly changing wavelengths. Even the boundaries themselves are ill-defined, which is why in the diagram above we show overlaps in some of the ranges. The spectral regions are just convenient definitions that are used for reference, and can be modified as needed. For example, for some purposes it is convenient to define a range of wavelengths between infrared and radio, which is called the microwave region. To make this region, scientists simply revised the assumed boundaries for the infrared and radio regions and inserted this newly defined region inbetween!
Scientists also find it convenient at times to refer to smaller "sub-regions" of these major spectral regions. However, these sub-regions are not always well-defined, and different conventions are sometimes followed. For instance, the infrared region is sometimes broken into the "near-infrared" (closest to the red optical spectrum) and the "far-infrared" (closest the the microwave or radio region). However, the ultraviolet spectral band more often gets broken into three sections: the near-ultraviolet (closest to violet optical light), the far-ultraviolet, and finally the extreme-ultraviolet (closest to the X-ray region).
Then in the X-ray region, an entirely different convention is used. In this region we refer to "soft X-rays" as those closest to the ultraviolet region, and "hard X-rays" as those closest to gamma rays! (Go figure.) Thus, an X-ray astronomer might say one spectrum is "harder" than another, meaning it has more short wavelength (high energy!) emission than a comparison spectrum.
Taken From: http://violet.pha.jhu.edu/~wpb/spectroscopy/basics.html