Follow this link to skip to the main content

The Hidden Lives of Galaxies - Answer Key for Activities

Activity #4 — Identifying Unusual Galaxies
1. c. {Sombrero Galaxy} With a bright halo of stars and a large central bulge of stars, it looks like a hat.

2. d. {Whirlpool Galaxy} It looks like a whirlpool in the ocean or water going down a drain.

3. a. {Polar Ring Galaxy} It contains an inner central disk of old stars and an outer ring of younger stars giving it the appearance of a ring on a ringer.

4. b. {Siamese Twins Galaxy} It shows how gravitational pull sometimes causes two galaxies to collide or brush against each other, giving the appearance of two joined bodies.

Activity #5 - Open Clusters versus Globular Clusters
Answers should include the following similarities: both contain numerous red stars, and colored stars. Globular clusters contain older stars. Stars near the center tend to be brighter. The stars appear to be gravitating towards the center of the cluster, like bees swarming around the hive. The cluster contains hundreds to thousands of stars. The stars appear closer together. Open cluster stars appear scattered. The blue stars are more visible. Open clusters contain hundreds of stars, many which are bright, young, and blue.

Activity #6a - Evidence for Hidden Mass
1. There are 9 Solar System planets presented on the graph. The planets from the closest to Sun to the furthest from the Sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

Using the graph, the velocities of the solar system planets, from the lowest value to the highest value, are approximately 48, 35, 30, 24, 13, 10, 7, 5, 4 km s-1.

Using the graph, the distances of the planets from the Sun are, from least to greatest, 0, 110, 150, 250, 800, 1500, 2800, 4500, 6000 million km.

In general, the further the planet is away from the Sun the slower the velocity. The closer the planet is to the Sun the faster the velocity.

2. Answers should include fact that velocities first increase with increasing distance. At distances larger than 10 kpc, the velocity becomes constant with increasing distance.

Activty #6b - Extension: Weighing a Galaxy
1. The calculation for each of the planets should result in a value for M of 2.0 x 1030 kg, which is the mass of the Sun. It is the same for each because the central mass for the solar system is concentrated in the Sun.

2. The mass increases as the distance from the center of the galaxy increases. This is because stars move under the gravitational influence of all the matter within their orbit.

Distance (kpc) Velocity Mass (kg)
5.0 95.0 2.1 x 1040
10.0 110.0 5.6 x 1040
15.0 110.0 8.4 x 1040

So stars at greater distances move under the influence of more mass than stars closer to the center.

The best estimate for the mass of the galaxy is the one which includes the most amount of mass. From this calculation, the largest value is 8.4 x 1040 kg. The actual mass of the galaxy is likely to be more than this. From Part 1, we know the mass of the Sun is 2.0 x 1030 kg. So this galaxy is at least 4.2 x 1010 times more massive than the sun.


A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC

NASA Logo, National Aeronautics and Space Administration