Because stars and most other objects in the universe are so far away, it is impossible to visit them. So, we must use the information we have available to study astronomical objects. Most of the information about distant object that we receive comes in the form of electromagnetic energy. As a result most of what we know about distant object is derived from light emitted by stars and other bodies.
Light can be used to determine three basic facts about electromagnetic energy emitting objects. First, the light can be used to determine the composition of the object. Second, in many cases, the light can be used to determine the temperature of the object. Finally, the electromagnetic energy can be used to determine the object's approximate position and motion.
Before I begin the discussion about using starlight, I need to explain about blackbodies. We see colors because various colors of light are reflected from pigments in whatever object we are observing. For example, a plant leaf looks green because it is reflecting green light. An object that reflects no light looks black. If you look at a plant leaf in the absence of green light, it would look black. Light that is not reflected by an opaque object is absorbed by it. As I mentioned earlier, the plant uses the blue and red light it absorbs to make various carbohydrates; it reflects the green light back towards the viewer. The idea of a blackbody extends the idea of blackness to the whole electromagnetic spectrum. So, a blackbody absorbs all electromagnetic energy and reflects none of it. However, when heated, it does emit electromagnetic energy. It emits electromagnetic energy at all wavelengths; although not all of the wavelengths are emitted at equal intensity. The amount of energy emitted by the blackbody, and the frequency at which it is emitted most intensely, depends on the temperature of the blackbody. Stars can generally be described as blackbodies. The concept of blackbodies is discussed in both Universe and Stars, galaxies, and cosmology.
Using starlight to determine material composition
Electromagnetic energy emissions from a substance are used to determine its composition. Different types of matter absorb light at different characteristic wavelengths. In the laboratory, this property of matter can be used to identify unknown substances using a spectrophotometer. In the lab an unknown substance is placed in a glass tube and light is passed through it. The light that comes through the tube is collected and analyzed. The difference between the light that was sent into the tube and the light that comes out is the absorption spectrum of the substance. These photons get absorbed and re-emitted with lower energies (i.e., lower frequencies) by the outer layers. The atoms in the relatively cold gas in the outermost layers of the Sun completely absorb some of the photons. Because stars emit light at all wavelengths the material composition of the star can be determined by the light that does not pass through the outer layers of the star.
Laboratory Activity: Electromagnetic spectrum
When I teach this unit, the students will do an experiment using their spectrophotometers at this point in the unit. The students will use their spectrophotometers to observe various light sources. If I have the resources available, I will purchase specially made tubes containing different gases. The tubes fit into a florescent light socket stand and when energized, the gases glow with their characteristic spectra. I would like to have the students observe the different gases and attempt to identify the gases by their spectra. The characteristic spectra for gasses are available in many places. I have found two interactive webpages contain emission spectra for most of the elements in the periodic table. The links to those pages are listed below in the resources section of the unit.
If the fluorescing tubes are beyond my budget, I plan to go shopping at a local hardware store and buy several different bulbs. A tungsten filament bulb acts as a blackbody, but it has a different emission curve from that of the sun. There are colored bulbs available that are basically a tungsten filament bulb that is painted to allow only certain wave lengths through. They should provide an interesting contrast to the ordinary tungsten filament bulbs. A halogen bulb has a different spectrum than those of the florescent tubes we have in our ceiling lights; and both have different spectra from the incandescent bulb and sunlight. Having investigated this possibility, I am assured that there are many other lighting choices beyond those just mentioned. I urge you to be creative.
In either case, my students will be expected to record the approximate wavelengths of light that they see, or at least the colors of light that they are able to see. The students will then be asked to try to figure out: first, if the light source is a blackbody; and second, if it is not a blackbody, what substance is emitting or absorbing the light.
Using starlight to determine temperature
As I stated earlier, blackbodies emit electromagnetic energy in all wavelengths, but not all wavelengths of light are emitted at the same intensity. A blackbody curve shows the intensity of electromagnetic energy at all the wave lengths. Illustrations of blackbody radiation curves are found by the thousands on the Internet. A blackbody curve will show a peak at the highest intensity wavelength, called Λ
m a x
. Using a relationship discovered by Wilhelm Wien, found below in the equations section of the Appendix, the temperature of the blackbody can be determined from Λ
m a x
Using starlight to determine position and movement
The universe appears to be expanding. The degree and direction of the expansion are still debated, but most scientists agree that the universe is expanding. This means that objects in the cosmos are moving apart. The movement causes the apparent wavelength of light emanating from objects to change. These changes in wavelength yield information about the movement of the object and the observer.
Figure 3: The Doppler Effect
(image available in print form)
The Doppler effect causes apparent changes in frequency of light emanating from moving light sources. As seen in figure 3, an object that is moving towards the observer continues to emit light. As the object moves, the newly emanated light waves pile on top of the previously emitted light waves. The two sets of waves are seen by the observer at the same time, causing the apparent frequency of the light to increase. In astronomy this shift causes visible light to look more like colors toward the blue end of the spectrum. When the object retreats, the waves spread apart, causing the apparent frequency to decrease. This shift causes visible light to look more like colors towards the red end of the spectrum. Although light in the non-visible portions of the spectrum are not red or blue, the same terminology is applied to similar shifts.
Distance from us
There are two methods that are commonly used to determine the position of stars. The first method is parallax. Parallax is a form of triangulation. This method is useful for objects that are reasonable close to us. Two observations of an object are made from two different positions. The object will appear to have moved against the background by a certain angle. The angle of movement of the object is determined by the distance of the object from the observer. Parallax is a fascinating subject for discussion, but it is only tangentially related to the topic in question in this unit. The other method of measuring distance is more applicable to this unit.
The second method of measuring distance to object is to use Hubble's law. This method is useful for objects that are far away from us. As stated earlier, the universe is expanding. This means that observed objects are moving apart from the observer. Strictly speaking, some objects, like orbiting pairs of binary stars, are sometimes moving towards the observer, but as a whole, objects in the universe are moving apart. This movement of objects, particularly in aggregate, like galaxies, causes the light to shift red. This red shift is used with Hubble's law to determine the distance between the observer and the moving object.