Scaling the Spectrum (PDF file format—requires Adobe Acrobat Reader)
We are all familiar with the model of the electromagnetic spectrum that appears in textbooks and on posters. It is a useful model because it helps us understand the relationship among frequency, wavelength, temperature and energy for the different bands of the spectrum. However, like all models, this model presents a severely distorted view of what it represents. Because it is an exponential representation, we look at the model and think that radio waves occupy a large percentage of the total spectrum, and gamma rays a small percentage of the total. This activity presents the opportunity to construct a linear model of the electromagnetic spectrum. This model is impossible to portray in textbooks or on a poster, however it will show you the actual width of the individual bandwidths of radiation.
Stellar Spectral Classifications (PDF file format—requires Adobe Acrobat Reader)
Although the spectra from different stars reveal a great deal of variety, it has long been recognized that each spectrum fits into one of a limited number of categories of spectral classes. The spectral classes of stars are O,B,A,F,G,K,M - with O being the hottest, and M the coolest. A star's surface temperature can be deduced from its color, which is indicated by the peak in its broad spectrum. Because stars belonging to a particular spectral class are at the same temperature, the surface temperature of other stars can be deduced directly from their absorption spectra. However, because of the variety of the spectra within each spectral class, determining the spectral classification from a star's spectrum is not simple. This activity will lead you through a sequence of events that will allow you to classify a set of spectra into similar classes and visualize the relationship between spectra and temperature.
Questions for discussion or short essays:
Students will find these exercises akin to a pre-test, largely evaluating one's understanding either based on prior knowledge or on the brief presentation in the Prologue. They are therefore meant to provoke critical thinking. Much more information regarding each of these questions will be developed in greater detail in the principal epochs of this Web site.
1. When do scientists think that the Universe first came into being? What do they call this event?
2. How did the very first elementary particles materialize in the early Universe? Why were the more massive particles (hadrons) created before the less massive particles (leptons)?
3. Why were the elementary particles initially unable to combine into atoms and molecules?
4. Describe the cosmic event known as "decoupling." When did it occur? What evidence do we have that it actually happened? How is this event related to the transition from the Radiation Era to the Matter Era?
5. Imagine you are able to create a time machine that takes you back to the lepton era in the very early Universe. Describe what you would observe all around you.
6. What is light? How is it related to radiation?
7. Name the seven principal bands of the electromagnetic spectrum, in order of increasing energy. In what ways do we encounter each band in our everyday lives?
8. In what regions of the electromagnetic spectrum is the atmosphere transparent enough to make observations from the ground?
9. What is meant by the "grand unified theory"? Do you think physicists will ever be able to formalize such a unified theory, or even a "Theory of Everything"? Why or why not?
10. What are some of the difficulties encountered while trying to test grand unification and super-grand unification?
11. Compare and contrast the extent to which we can probe the past (a) by observing distant galaxies, (b) by studying the cosmic background radiation, and (c) by working with tested theoretical models of the early Universe.
12. What is the relationship between Hubble's constant and the present age of the Universe? What are the uncertainties in this age?
13. Explain why the evident order of the Universe around us is not a violation of the laws of thermodynamics.