Astronomy in-class activity: masses, densities, and escape velocities

Astronomy 10 In-class activity 23 v.07.04.25, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q10.1

Students find their assigned groups of three to four students, and work cooperatively on an in-class activity worksheet to determine the relationship between the escape velocity with mass and/or density.

Start off with filling some of the entries for the students, before they start working in their groups. The main sequence star on the left is the smallest and least massive of the main sequence stars: "G2," "1.0 M_Sun." Point out the progression in increasing size and mass of the main sequence stars, from right-to-left. Also point out the progression in decreasing size from left-to-right for the compact objects, and also the increasing escape velocities. Undoubtably the compact object with zero size and an infinite escape velocity is the black hole.
There will be several ties for the list of increasing masses:

G2 = white dwarf; A3 = neutron star; B3 = black hole.

However, there is no trend in escape velocities for this list, such that there is no relationship between the mass and escape velocity.

There are also several (approximate) ties for the list of increasing densities:

G2 = A3 = B3; white dwarf, neutron star; black hole.

Since the escape velocities also increase from left-to-right along this list, there is a direct relationship between the density of an object and its escape velocity.

Follow-up post: Astronomy clicker question: masses, densities, and event horizons.