Activity 3: How Many Stars is That?

Complex life requires a minimum amount of energy to develop. For instance, we might establish that stars with a luminosity less than 0.3LSun are unable to provide enough energy to support complex life on a nearby planet.

A. Place a mark (Lmin) on Graph 1 that represents the lower limit on stellar luminosity for complex life.

B. Find the stellar mass that corresponds to Lmin and label it as either Mmin or Mmax. How did you decide which it should be?



C. Can stars with a stellar mass less than this have planets with complex life? Explain your reasoning.



D. Write down the possible range of stellar masses (in units of MSun) for stars that can support complex life.



E. Predict whether you think there are more stars, fewer stars or the same amount of stars in our galaxy that lie in the range between Mmin and MSun or between MSun and Mmax.



F. If you knew how many stars exist for each value of stellar mass, could you better answer the previous question? Provide a detailed description of how you would use this information to answer this question more accurately.



graph of number of stars versus stellar mass, for main sequence stars

Consider the information shown in Graph 3. The horizontal axis represents the range of stellar masses for main sequence stars. This type of graph, in which information is sorted into bins, is called a histogram. Since stars with a mass less than 0.5 MSun are quite dim there is not a large sample of data for these small mass and dim stars. As a result, Graph 3 starts with data representing stars that have a mass of 0.5 MSun and extends to stars with a mass of 10 MSun. Each bin of data in Graph 3 represents a range of 0.1 MSun. Notice that the first bin on the left represents stellar mass between 0.5 and 0.6 MSun, the second bin represents stellar mass between 0.6 MSun and 0.7 MSun, the third bin represents stellar mass between 0.7 MSun and 0.8 MSun, and so on. The last bin on the right is stellar mass between 9.9 MSun and 10 MSun. The height of the bar in each bin represents how many stars exist within a specific range of stellar masses. Multiply the number on the y-axis by 100,000,000 to get the number of stars in that mass range in the galaxy. The total number of stars represented in this histogram is approximately 40,000,000,000.

G. How many stars exist with masses between 0.5 M Sun and 0.9 M Sun? How many of these stars could support complex life?



H. According to this histogram, how many stars in the galaxy can support complex life?


I. What fraction of all main sequence stars can support complex life? Express your answer as a fraction, a decimal and a percentage.



The histogram shown in Graph 3 provides an estimate of the total number of stars with main-sequence lifetimes. These main sequence stars make up approximately 70% of all stars in the galaxy. The remaining stars fall into four broad classes: those with too little mass (e.g., brown dwarfs); those with too much mass (e.g., blue super giants); those that are too young (e.g., T Tauri stars); and those that are too old (e.g., red giants, white dwarfs, and neutron stars.)

J. Out of all the stars in our galaxy, how rare (what percentage) are stars that could support complex Earth-like life?





Challenge Questions:

A. If you were studying a star that has a stellar mass that is a little larger than the Sun, how will the luminosity and main-sequence lifetime compare to that of the Sun? Explain your reasoning.



B. Consider the following debate between three students.

    Student #1: If a star is too bright the star is too hot and there is no chance life could be found on the orbiting planet.

    Student #2: I disagree, if the luminosity is too high there is not enough UV radiation for life to be found on the orbiting planet.

    Student #3: I disagree, if L is too high then the lifetime of the star is too short for complex life to develop on the star.

    For each student response, state whether you agree or disagree with the student statement. State explicitly which ideas you think are correct and which are incorrect.







C. Is it possible for complex life to exist on a planet that is near a star that has a main sequence lifetime that is shorter than tSun? If not, why not? If so, what is the smallest main sequence lifetime the star could have? Explain your reasoning.