Scholars Online Astronomy - Chapter 5: 5-8: Stellar Spectra
Reading: Astronomy, Chapter 5: Sections 5-8 Stellar Spectra, Composition, and the Doppler shift
Study Notes: notes on your assigned reading from the text
- Section 5.5 After spending a lot of time talking about light waves, you now have to learn a different theory: light as particles, or photon theory. The two models explain different light phenomena, and so both are useful. When light is being transmitted, it acts like a wave. When it interacts with matter, it acts like a particle of energy, shifting specific amounts (quanta) of energy to electrons. Every light wave has a corresponding energy—so light is simply the propagation of a certain form of energy through space. Keep that firmly in mind as you work through the different phenomena.
- Section 5.6 The amount of energy an electron in an atom can absorb or radiate depends on the element type of the atom and its excited state (temperature). Light is emitted by hot substances, and absorbed when passing through cool substances. Both phenomena create spectra that identify the source (emission spectra) and filtering substances (absorption spectra). So each element produces a unique spectral pattern that can be used to identify the element. Be sure that you understand the conditions that produce a continuous spectrum, and emission spectrum, and an absorption spectrum.
- Section 5.7 You are probably pretty familiar already with the atomic model: atoms consist of nuclei made of positively-charged protons and neutral neutrons, surround by constantly-moving, negatively-charged and very low-mass electrons.
- Section 5.8 When light of the correct frequency strikes an atom, its electrons absorb the light and shift to high-energy orbitals. If the electron cannot absorb the energy at that frequency, the light will pass through the atom unchanged. As an atom cools, electrons shift to lower-energy orbits closer to the nucleus; the energy difference is radiated as light of a particular frequency. So all elements have spectral "signatures" that depend on their type and temperature. By analyzing the spectral patterns given off by stars, we can determine their composition and temperature. Note the difference between Balmer's formula, which is empirical like Kepler's laws, and Bohr's explanation and more generalized version. The names of the series apply only to the hydrogen atom; other atoms have different and for the most part unnamed energy series.
- Section 5.9 For all waves (sound waves included), a non-zero relative motion of the source and observer will change the perceived wavelength and frequency. This is called the Doppler shift, and is readily heard in the change in pitch of a passing fire truck's siren. If the source and object move toward one another, the wavelength observed is shorter than if the source and object have a fixed relationship. If the source and object move away from one another, the perceived wavelength is longer than it would be if the source and object were fixed. By comparing the observed wavelengths for a given spectral pattern to lab values (where the relationship between observer and source is fixed), we can determine the object's radial velocity (speed along a line between us and the object).
Key Formulae to Know
Read the following weblecture before chat: Light and how it tells us what's out there (Part 2)
- Planetarium Program: Use your planetarium program to examine the Virgo cluster. (If your program does not include information in detail, you may look up the cluster in Wikipedia or another source.). The Virgo Cluster is a group of more than 2000 galaxies extending across 9 million light-years, located about 50 million LY from Earth. Use your program to examine galaxies in the cluster more closely. Are all of the galaxies the same color? Do all galaxies contain stars of the same surface temperature? Locate several spiral galaxies. One of the dominant colors of the inner and outer regions of these galaxies? What can you conclude about the surface temperatures of the brightest stars in the inner regions compared to the service temperatures of those in the outer spiral?
- Optional Websites: Astronomy Answers (On Light) has a number of questions (and answers) commonly asked about light by astronomy students.
Chat Preparation Activities
- Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
- Go over the list of Key Words and Key Ideas at the end of the chapter. If you don't remember the definition of the key word, review its use (the page number on which it is explained is given).
- Read through the Review Questions and be prepared to discuss them in class. If any of them confuses you, ask about it!
- Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.
- Required: Complete the Mastery exercise with a passing score of 85% or better.
- Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
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