For most celestial objects, light is the astronomer's only subject for study. Light from celestial objects is packed with amazingly large amounts of information. Studying the distribution of brightness for each wavelength (color) which makes up the light provides the temperature of a source. A simple example of this comes from flame color comparison. Think of the color of a flame from a candle (yellow) and a flame from a chemistry class Bunson burner (blue). Which is hotter? The flame from the Bunson burner is hotter. By observing which color is dominant in the flame, we can determine which flame is hotter or cooler. The same is true for stars; by observing the color of stars, we can determine which stars are hot and which stars are cool. If we know the temperature of a star, and how far away it is (see the "Measuring Distances Using Parallax'' lab), we can determine how big a star is.
We can also use a device, called a spectroscope, to break-up the light from an object into smaller segments and explore the chemical composition of the source of light. For example, if you light a match, you know that the predominant color of the light from the match is yellow. This is partly due to the temperature of the match flame, but it is also due to very strong emission lines from sodium. When the sodium atoms are excited (heated in the flame) they emit yellow light.
In this lab, you will learn how astronomers can use the light from celestial objects to discover their nature. You will see just how much information can be packed into light! The close-up study of light is called spectroscopy.
This lab is split into three main parts:
Thus there are three main components to this lab, and they can be performed in any order. So one third of the groups can work on the computers, while the other groups work with the spectrographs and various light sources.
Blackbody radiation (light) is produced by any hot, dense object. By "hot'' we mean any object with a temperature above absolute zero. All things in the Universe emit radiation, since all things in the Universe have temperatures above absolute zero. Astronomers idealize a perfect absorber and perfect emitter of radiation and call it a "blackbody''. This does not mean it is black in color, simply that it absorbs and emits light at all wavelengths, so no light is reflected. A blackbody is an object which is a perfect absorber (absorbs at all wavelengths) and a perfect emitter (emits at all wavelengths) and does not reflect any light from its surface. Astronomical objects are not perfect blackbodies, but some, in particular, stars, are fairly well approximated by blackbodies.
The light emitted by a blackbody object is called blackbody radiation. This radiation is characterized simply by the temperature of the blackbody object. Thus, if we can study the blackbody radiation from an object, we can determine the temperature of the object.
To study light, astronomers often split the light up into a spectrum. A spectrum shows the distribution of brightness at many different wavelengths. Thus, a spectrum can be shown using a graph of brightness vs. wavelength. A simple example of this is if you were to look at a rainbow and record how bright each of the separate colors were. Figure 11.1 shows what the brightness of the colors in a hot flame or hot star might look like. At each separate color, a brightness is measured. By fitting a curve to the data points, and finding the peak in the curve, we can determine the temperature of the blackbody source.
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One question which you may have considered is: how do astronomers know what elements and molecules make up astronomical objects? How do they know that the Universe is made up mostly of hydrogen with a little bit of helium and a tiny bit of all the other elements we have discovered on Earth? How do astronomers know the chemical make up of the planets in our Solar System? They do this by examining the absorption or emission lines in the spectra of astronomical sources. [Note that the plural of spectrum is spectra.]
In the early part of this century, a group of physicists developed the
Quantum Theory of the Atom. Among these scientists was a Danish
physicist named Niels Bohr. His model of the atom, shown in the
figure below, is the easiest to understand. In the Bohr model, we
have a nucleus at the center of the atom, which is really much much
smaller relative to the electron orbits than is illustrated in our
figure. Almost all of the atom's mass is located in the nucleus. For
Hydrogen, the simplest element known, the nucleus consists of just one
proton. A proton has an atomic mass unit of 1 and a positive electric
charge. In Helium, the nucleus has two protons and two other
particles called neutrons which do not have any charge but do have
mass. An electron cloud surrounds the nucleus. For Hydrogen there is
only one electron. For Helium there are two electrons and in a larger
atom like Oxygen, there are 8. The electron has about
the mass of the proton but an equal and opposite
electric charge. So protons have positive charge and electrons have
negative charge. Because of this, the electron is attracted to the
nucleus and will thus stay as close to the nucleus as possible.
In the Bohr model, the electron is allowed to exist only at certain distances from the nucleus. This also means the electron is allowed to have only certain orbital energies. Often the terms orbits, levels, and energies are used interchangeably so try not to get confused. They all mean the same thing and all refer to the electrons in the Bohr model of the atom.
After much study, the physicists found out that the atom had taken-in energy from the collision or from the surrounding environment and that as it jumps downward in levels, it re-emits the energy as light. The light is a particular color because the electron really is allowed only to be in certain discrete levels or orbits. It cannot be halfway in between two energy levels. This is not the same situation for large scale objects like ourselves. Picture a person in an elevator moving up and down between floors in a building. The person can use the emergency stop button to stop in between any floor if they want to. An electron cannot. It can only exist in certain energy levels around a nucleus.
Now, since each element has a different number of protons and neutrons in its nucleus and a different number of electrons, you may think that studying "electron gymnastics'' would get very complicated. Actually, nature has been kind to us because at any one time, only a single electron in a given atom jumps around. This means that each element, when it is excited, gives off certain colors or wavelengths. This allows scientists to develop a color fingerprint for each element. This even works for molecules. These fingerprints are sometimes referred to as spectral lines. The light coming from these atoms does not take the shape of lines. Rather, each atom produces its own set of distinct colors. Scientists then use lenses and slits to produce an image in the shape of a line so that they can measure the exact wavelength accurately. This is why spectral lines get their name, because they are generally studied in a linear shape, but they are actually just different wavelengths of light.
Continuous spectra are the same as blackbody spectra, and now you know about spectral lines. But there are two types of spectral lines: absorption lines and emission lines. Emission lines occur when the electron is moving down to a lower level, and emits some light in the process. An electron can also move up to a higher level by absorbing the right wavelength of light. If the atom is exposed to a continuous spectrum, it will absorb only the right wavelength of light to move the electron up. Think about how that would affect the continuous spectrum. One wavelength of light would be absorbed, but nothing would happen to the other colors. If you looked at the source of the continuous spectrum (light bulb, core of a star) through a spectrograph, it would have the familiar Blackbody spectrum, with a dark line where the light had been absorbed. This is an absorption line.
The absorption process is basically the reverse of the emission process. The electron must acquire energy (by absorbing some light) to move to a higher level, and it must get rid of energy (by emitting some light) to move to a lower level. If you're having a hard time keeping all this straight, don't worry. Gustav Kirchoff made it simple in 1860, when he came up with three laws describing the processes behind the three types of spectra. The laws are usually stated as follows:
A blackbody produces a continuous spectrum. This is in agreement with Kirchoff's first law. When the light from this blackbody passes through a cloud of cooler gas, certain wavelengths are absorbed by the atoms in that gas. This produces an absorption spectrum according to Kichoff's third law. However, if you observe the cloud of gas from a different angle, so you cannot see the blackbody, you will see the light emitted from the atoms when the excited electrons move to lower levels. This is the emission spectrum described by Kirchoff's second law.
Kirchoff's laws describe the conditions that produce each type of spectrum, and they are a helpful way to remember them, but a real understanding of what is happening comes from the Bohr model.
In the second half of this lab you will be observing the spectral lines produced by several different elements when their gaseous forms are heated. The goal of this section of the lab is to observe these emission lines and to understand their formation process.
Light which has been split up to create a spectrum is called dispersed light. By dispersing light, one can see how pure white light is really made up of all possible colors. If we disperse light from astronomical sources, we can learn a lot about that object. To split up the light so you can see the spectrum, one has to have some kind of tool which disperses the light. In the case of the rainbow mentioned above, the dispersing element is actually the raindrops which are in the sky. Another common dispersing element is a prism.
We will be using an optical element called a diffraction grating to split a source of white light into its component colors. A diffraction grating is a bunch of really, really, small rectangular openings called slits packed close together on a single sheet of material (usually plastic or glass). They are usually made by first etching a piece of glass with a diamond and a computer driven etching matchine and then taking either casts of the original or a picture of the original.
The diffraction grating we will be using is located at the optical entrance of an instrument called a spectroscope. The image screen inside the spectroscope is where the dispersed light ends up. Instead of having all the colors land on the same spot, they are dispersed across the screen when the light is split up into its component wavelengths. The resultant dispersed light image is called a spectrum.
In part one of this lab, we will study a common blackbody in everyday use: a simple white light bulb. Your Lab TA will show you a regular light bulb at two different brightnesses (which correspond to two different temperatures). The light bulb emits at all wavelengths, even ones that we can't see with our human eyes. You will also use a spectroscope to observe emission line sources
In the second section of this lab, we will investigate how changing the temperature of a source changes the characteristics of the radiation which is emitted by the source. We will see how the measurement of the color of an object can be used to determine the object's temperature. We will also see how changing the temperature of a source also affects the source's brightness.
To do this, we will use a computer program (written by people at the University of Oregon) which will simulate a spectrum for objects at a given temperature. This program should already be running on the computer; if not, consult the instructor. The program just produces a graph of wavelength on the x-axis vs. brightness on the y-axis; you are looking at the relative brightness of different wavelengths.
The program is simple to use. There is a sliding bar on the left side of the graph which can be used to set the temperature of an object. There is another sliding bar on the right side which can be used to set the temperature of a different object; the spectrum for the two different objects will be shown with a black and a red line, respectively, to make it easier to compare the spectra of two different objects. Play around with it a bit to get the idea. Be aware that the y-axis scale of the plot will change to make sure that none of the spectrum goes off the top of the plot; thus if you are looking at two objects of different temperature, it will be hard to see details of the fainter, cooler, object.
Note that the temperature of the objects are measured in units called degrees Kelvin (K). These are very similar to degrees Centigrade/Celsius (C); the only difference is that: K = C + 273. So if the outdoor temperature is about 20 C (68 Fahrenheit), then it is 293 K. Temperatures of stars are measured in thousands of degrees Kelvin; they are much hotter than it is on Earth!
| Temperature | Peak Wavelength |
| 4000 | |
| 5000 | |
| 6000 | |
| 12000 |
The instrument we will be using is called a spark tube. It is very similar to the equipment used to make neon signs. Each tube is filled with gas of a particular element. The tube is placed in a circuit and electricity is run through the circuit. When the electrons pass through the gas they collide with the atoms causing them to become excited. So the electrons in the atoms jump to higher levels. When these excited electrons cascade back down to the lower levels, they emit light which we can record as a spectrum.
For the third, and final section of this lab you will be using the spectrographs to look at the spark tubes that are emission line sources.
Now your TA will show you one of the elements again, but won't tell you which one. This time you will be using a higher quality spectroscope (the large gray instrument) to try to identify which element it is by comparing the wavelengths of the spectral lines with those in a data table. The gray, table-mounted spectrograph is identical in nature to the handheld spectrographs, except it is heavier, and has a more stable wavelength calibration. When you look through the gray spectroscope you will see that there is a number scale at the bottom of the spectrum. These are the wavelengths of the light in "nanometers'' (1 nm = 10-9 meter). Look through this spectrograph at the unknown element and write down the wavelengths of the spectral lines that you can see in the table below, and note their color.
Now, compare the wavelengths of the lines in your data table to each of the three elements listed below. In this next table we list the wavelengths (in nanometers) of the brightest emission lines for hydrogen, helium and argon. Note that most humans cannot see light with a wavelength shorter than 400 nm or with a wavelength longer than 700 nm.
Which element is the unknown element? _____________________ (5 points)
(35 points) Summarize the important ideas covered in this lab. Some questions to answer are:
Use complete sentences, and proofread your summary before handing in the lab.
