Planets, Stars, Galaxies, and the Universe



About Lesson 4

We are now officially beginning Unit 2, which is all about stars.

Astronomers often talk about the transition from “astronomy” to “astrophysics” taking place in the mid-19th and early 20th centuries. In the first two lessons, we discussed the study of the motions in the night sky and models for these motions based on our evolving understanding of the force of gravity and the orbits of celestial bodies. For many centuries, the field of astronomy consisted almost entirely of making observations of the sky and refining models to predict celestial phenomena (eclipses, retrograde loops of Mars, etc.).

In Lesson 3, we studied the Bohr atom and the spectrum of light emitted by a blackbody. These topics required an understanding of atomic structure and quantum mechanics, a field of physics that was booming about a century ago. Advances in quantum mechanics led quickly to an increased understanding of stars. As our physical understanding of stars grew, astronomers began to publish more “astrophysical” papers dealing with, for example, the structure of stars, the generation and emission of starlight, and the evolution of stars. In this lesson, we are going to study the observations of stars, their classification into different types, and the physical differences between types of stars.

What will we learn in Lesson 4?

By the end of Lesson 4, you should be able to:

  • Classify stars into spectral types;
  • Describe the temperature and luminosity of the stars in a given classification;
  • Describe the method of trigonometric parallax for measuring the distance to a star (or any other object);
  • Construct a temperature luminosity (HR) diagram for stars;
  • Explain the information contained in a temperature luminosity (HR) diagram;
  • Use the Doppler effect to measure the velocity of a star moving towards or away from us.

What is due for Lesson 4?

Lesson 4 will take us one week to complete.

Please refer to the Calendar in Canvas for specific time frames and due dates.

There are a number of required activities in this lesson. The chart below provides an overview of those activities that must be submitted for Lesson 4. For assignment details, refer to the lesson page noted.

Lesson 4 Requirements


Submitting Your Work
Lesson 4 Quiz Your score on this quiz will count towards your overall quiz average.
Lesson 4 Practice Math Problems There is a second quiz for this lesson in Canvas. This one is all short math problems. You will be graded only on effort on this quiz, that is you will be graded for taking it and working on the problems, but not on your answers.


If you have any questions, please post them to the General Questions and Discussion forum (not email). I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.