Astronomy Reading Guide
Chapter 5: Light- The Cosmic Messenger
Scott Hildreth – Chabot College
ONE key idea for the chapter
The fifth chapter presents perhaps the most important physical concept crucial to the advancement of
astronomy since Galileo’s time, and – since the 1960’s – the single most important technique humans
have put to use to learn how stars and planets are born, what are they made of, how they change, and how
the universe itself has started. And while you might think I am referring to the telescope, the device we
use to gather light from space, I am not. That is technology we use to help us gather light, changing as
we develop better computers, detectors, optics, and electronics.
Instead, I believe the single most important scientific technique we use in Astronomy – and the single
most important concept presented in this chapter, is spectroscopy, and how it tells us so much. So the
key question from Chapter 5 is:
How can light from objects in space tell us about the composition, motion,
temperature, and other physical qualities of the objects – and of the space that
light travels through?
Consider all of the physical qualities listed below about a star like the sun:
i) Temperature
ii) Composition (what elements it is made of)
iii) Rotation (how fast it spins on its axis)
iv) Density (how its mass is spread out through its volume)
v) Radial Motion (velocity towards or away from observers on Earth)
vi) Magnetic Field Strength
vii) Presence of planets orbiting the star
viii) Age
What is amazing to me is that ALL of these are told using spectroscopy.
THREE key questions for the chapter we’ll discuss
You should be able to answer these questions after reading the chapter, and attending the lectures.
5.1 What is the Electromagnetic Spectrum of light? How is light collected in each area of the spectrum?
5.2 What are the three basic types of spectra, and how does light tell us what things are made of, their
temperatures, and speeds of distant objects?
5.3 How do telescopes help us learn about the universe – and what are their two most important
properties?
FIVE key passages, pages, pictures, and paragraphs you should review
1. Figure 5.2 – The electromagnetic spectrum, and the explanation about light from pages 111114. Note how many different “kinds” of light exist in nature, and that we humans can only
see a small, small slice. Note how those kinds of light differ in size and energy, but not in
speed – every kind of light moves at the same speed in a vacuum.
2. Figure 5.8 and pages 117-118 – the three basic kinds of spectra are continuous, emission,
and absorption. Be careful not to confuse these types of spectra – which can occur in every
kind of light from gamma rays to radio waves, with the “electromagnetic spectrum” even
though the words sound very familiar.
3. Figure 5.14 and Pages 124-125 – The Doppler Shift is a concept you’ll see mentioned in
many ways throughout our class. Don’t worry about the calculations, but do make sure you
understand the idea that the amount of red-shift or blue-shift in the spectral lines is what will
tell us about the radial velocity towards or away from us. Note that you need the emission or
absorption lines to see the shift – they are what we can use to see that something has changed.
4. Figure 5.15 – Pages 126-127 - This two-page image summarizes all three kinds of spectral
patterns (continuous, emission lines, absorption lines) AND multiple ranges of the
electromagnetic spectrum, from UV (ultraviolet) through visible and infrared light. And you
can even see mention of the Doppler Shift and radiation laws, too.
5. Page 133-134 – How technology is revolutionizing astronomy? The key concepts in this
section include adaptive optics, and interferometry. Both are used extensively today in
astronomy to improve resolution – the ability to see fine detail in an image.
Three key end-of chapter resources you should look at

Page 135 – 136 (Chapter Summary) – especially page 136, which does a nice job of providing
you small “bites” of information about each key concept. If you are having trouble reading the
book, START with this section first in the next chapters, and come back to it at the end. You
should understand all of the terms used, and the ideas in these small paragraphs.

Page 137 – Test your Understanding Questions.

Pages 137-138 – Quick Quiz Questions 27 - 36. Many of these are identical to those asked on the
reading quiz at Mastering Astronomy. If you have any questions about these, please ask in class
or in office hours!
Questions/Concepts YOU want to Review: