Name_________________________

STUDENT INSTRUCTION AND ANSWER SHEET

Activity 1: How Old, How Bright, and How Massive Should the Star Be to Support Complex Life?

Read the Introduction to Rare Earth. Now examine Graph 1 below:

graphs of stellar lifetime and luminosity versus stellar mass

Graph 1 shows how the main sequence lifetime of a star is related to the star's mass (dotted line). The graph also shows how the star's luminosity is related to its mass (solid line). Refer to Graph 1 to answer the following questions:

A. Does stellar mass increase to the right or to the left?

 

B. Does main sequence lifetime increase upward or downward?

 

C. Does stellar luminosity increase upward or downward?

 

D. Which feature on the graph shows how main sequence lifetime is related to stellar mass?

 

E. Which feature on the graph shows how stellar luminosity is related to stellar mass?

 

F. How does main sequence lifetime change as stellar mass increases? Does it increase or decrease? Does it change by the same amount for small values of M as it does for large values of M?

 

 

G. How does stellar luminosity change as stellar mass increases? Does it increase or decrease? Does it change by the same amount for small values of M as it does for large values of M?

 

 

H. Mark the position on the stellar mass axis where the Sun lies. Mark the position on the main sequence lifetime axis where the Sun lies. Mark the position on the stellar luminosity axis where the Sun lies. Place a mark within the graph where these values intersect.

I. What star is represented by the intersection of the two curves?

 

J. Do stars live longer or shorter, and are they brighter or dimmer, when they have a stellar mass that is less than the Sun's?

 

 

K. Do stars live longer or shorter, and are they brighter or dimmer, when they have a stellar mass that is greater than the Sun's?

 

 

L. How long did it take before complex life developed on Earth? What fraction of the Sun's main sequence lifetime is this?

 

 

 

M. Based on your answer to question L, estimate the minimum amount of time that a main sequence star can exist and still have a planet that has time to develop Earth-like complex life. Explain your reasoning.

 

 

 

N. Mark on the main sequence lifetime axis the minimum lifetime you estimated in question M. Label this point tmin. Find the stellar mass that corresponds to tmin. Label it on the stellar mass axis as either Mmin or Mmax. How did you decide which it should be?

 

 

 

O. Find the stellar luminosity that corresponds to the stellar mass that you labeled in question N. Label it on the stellar luminosity axis as either Lmin1 or Lmax1. How did you decide which it should be?

 

 

P. Place a mark on the stellar luminosity axis for a star that is brighter than the star you marked in question O. Find the position on the main sequence lifetime axis that corresponds to this star. Is it possible for complex life to exist on a planet around this star? Why or why not?

 

 

Q. Place a mark on the stellar luminosity axis for a star that is dimmer than the star you marked in question O. Find the position on the main sequence lifetime axis that corresponds to this star. Is it possible for complex life to exist on a planet around this star? Why or why not?

 

 

R. Based on your answers to questions P and Q, was the mark you labeled Lmin1 or Lmax1 in question O the correct limit? (If you wrote Lmin1, does it represent the minimum stellar luminosity for complex life? If you wrote Lmax1, does it represent the maximum stellar luminosity for complex life?) Explain your reasoning.