Astronomy quiz question: mass, densities, and event horizons

Astronomy 10 Quiz 10, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q10.1

[3.0 points.] Which one of the following choices best explains why a black hole is the only object that can have an event horizon?
(A) It has zero density.
(B) It has infinite mass.
(C) It has zero size.
(D) It has infinite energy.
(E) It can distort spacetime.

Correct answer: (C)

Section 4160
(A) : 5 students
(B) : 7 students
(C) : 15 students
(D) : 1 students
(E) : 2 students

Section 5166
(A) : 6 students
(B) : 16 students
(C) : 11 students
(D) : 3 students
(E) : 5 students

Previous posts:
Astronomy clicker question: masses, densities, and event horizons.
Astronomy in-class activity: masses, densities, and escape velocities.

Astronomy clicker question: masses, densities, and event horizons

Astronomy 10, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q10.1

Students were asked the following clicker question (Classroom Performance System, einstruction.com) near the end of their learning cycle (specifically, following the astronomy in-class activity: masses, densities, and escape velocities).

[0.3 points.] Why is a black hole the only object that has an event horizon?
(A) Because of its mass.
(B) Because of its density.
(C) Because of both its mass and density.
(D) (Neither its mass nor density matters.)

Correct answer: (B)

Student responses
Section 5166
(A) : 1 student
(B) : 21 students
(C) : 14 students
(D) : 3 students

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.

Astronomy clicker question: entering the event horizon

Get into vortex you say, by ?rico Lopez
icanhascheezburger.com
January 3, 2008

Astronomy 10, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q10.1

Students were asked the following clicker question (Classroom Performance System, einstruction.com) at the start of their learning cycle:

[0.3 points.] Why is entering the event horizon (the "Schwarzchild radius") surrounding a black hole considered a "point of no return?"
(A) All matter is crushed into nothingness.
(B) All matter is instantly converted into energy.
(C) The escape velocity is faster than the speed of light.
(D) Time runs backwards.

Correct answer: (C)

If students ask about what's inside the event horizon of a black hole, point out that they could certainly try to enter it and find out for themselves. However, in the very unlikely event that would be able to survive the journey, they would not be able to tell anyone in the outside universe about their discoveries, as nothing, not even light would be able to escape from within the event horizon. So they should bring a friend to share in the experience. Otherwise, it'd be lonely in there...

And yes, the expression on the middle kitten is priceless.

Student responses
Section 4160
(A) : 9 students
(B) : 4 students
(C) : 17 students
(D) : 0 students

Section 5166
(A) : 7 students
(B) : 7 students
(C) : 23 students
(D) : 0 students