In this part of the lab, we will be looking at a simulation of the evolution of stars as they leave the Main Sequence. It is recommended that you use 500 stars.  Be aware that it only shows the evolution of a star through the middle of its Red Giant phase.  Also ignore the gap in the Main Sequence that forms at late times near a temperature of about 6300 K.  While you are viewing, pay attention to the following questions:
  • Which stars leave the main sequence first?
  • What does the H-R diagram look like when a few stars evolve off it? What does it look like when many stars evolve off it?
  • How does the shape of the points on the H-R diagram tell us about age?

You can measure the properties of any star by clicking on it. You'll be using these properties when answering questions about stellar temperature, luminosity, mass and cluster age.  The star's Main Sequence lifetime is one number reported.  For a star just leaving the Main Sequence, this is a good estimate of its age.    Such ages are much more accurate than the time ticker you will see, which is 2-3 times too slow (for the first 7 billion years or so), so do not rely on the time ticker when answering questions. 

Stellar Evolution Simulation    (courtesy of R. Scharein at the University of British Columbia)
Look at the Stellar Evolution simulation of an H-R diagram of a star cluster.  First, give the cluster 500 stars.
Clicking on an individual star allows you to see the properties of that star. Click on several stars at different locations in the diagram.

7. Qualitatively, how do the luminosity, temperature and Main Sequence lifetime of a Main Sequence star change with its mass, that is, which change dramatically with mass and which change more slowly with mass?
 
 

Now click on evolve.  Stop the simulation at three times: when stars of temperature 10,000 K, 7000 K and 6000 K are just leaving the Main Sequence.  In each case, click on such a star and record its mass, luminosity and Main Sequence lifetime.

8. 10,000 K star just leaving Main Sequence:

9. 7000 K star just leaving Main Sequence:

10.  6000 K star just leaving the Main Sequence.

11. In light of your answer to Question 7, why does the simulation seem to evolve more slowly as time goes on?
Main Sequence Turn-Off Point

As you just saw in the simulation, the H-R diagram looked different at different times. When astronomers look at the H-R diagrams of star clusters, they also see different patterns. The H-R diagram of a cluster can tell astronomers how old it is.

Look at the H-R Diagram below of the Hyades open cluster and identify the Main Sequence and the Red Giants:



This should look similar to something you saw in the simulation. Notice the top end of the Main Sequence. The point on the graph where the Main Sequence ends is called the "Main Sequence Turn-off Point." This is where a star is no longer fusing hydrogen in its core. The Main Sequence turn-off point is indicative of the age of the star cluster. The older a cluster is, the farther down the Main Sequence the turn off point will be. By matching the turn-off point in the simulation with the turn-off point you see in this graph, you can estimate the age of the Hyades.  Don't forget to do this by pausing the simulation and clicking on stars that are just turning off, rather than relying on the ticker.

12. Now use the simulation to estimate the age of the Pleiades cluster (its H-R diagram is on the "Estimating Distances to Clusters" page and shows its entire Main Sequence.  Take the temperature of the hottest star to be 13,000 K).