Dr. Aaron Titus | Department of Physics, High Point University
PHY1050      Astronomy of Stars, Galaxies, and the Cosmos
home | WebAssign | textbook | Course Calendar course calendar


Some lessons have questions for you to think about, as indicated by the question mark icon. After answering each question, click the appropriate button that follows the question.

Any quantity that is measured has units.

Suppose that an employer wants to hire you for the same job you are doing now but offers to pay you 100,000 per year. Will you take the job?

Units are essential in science and everything else in life. It was failure to properly convert units that led to a multimillion dollar crash of a Mars orbiter in 1999.

Three of the seven base SI units are: 1. mass; 2. distance; 3. time.

In fact, in astronomy, we are principally interested in questions of mass, distance, and time. And using distance and time, we also want to know speed (which is distance / time).

For example, we want to know things like:

  • What is the mass of Sun?
  • What is the mass of the blackhole at the center of the Milky Way Galaxy?
  • What is the distance across the Milky Way Galaxy?
  • How far is Earth from Sun?
  • How old is Sun?
  • How old is the Universe?
  • How fast are various stars moving in a galaxy?
  • How fast is a galaxy moving?
  • How fast is the universe expanding?

All of the above questions are answered with units of mass, distance, time, or speed (distance / time).






Note: to keep spammers out, the feedback form requires you to type the class name, such as PHY1050, in order to submit feedback.


Class (enter PHY1050):



On a scale of 1 to 5 stars, with 5 stars
being the best, how do you rate this lesson?