Tutorial: Star Formation and Lifetimes
(Adapted from Lecture-Tutorials for Introductory Astronomy, © CAPER Team, Preliminary Edition, 2002)
Stars like our sun begin life as a cloud of gas and dust. The birth of a star begins when a disturbance, such as the
shockwave from a supernova, triggers the cloud of gas and dust to collapse inward.
Imagine that you are observing the region of space where a cloud of gas and dust is beginning to collapse inward to
form a star (this region is called a protostar).
1.
Will the material in the cloud become more dense, less dense, or stay about the same density? Explain
your answer.
2.
What physical interaction of force causes the cloud to collapse once it gets started?
3.
Would you expect the temperature of the protostar to increase or decrease with time? Why?
4.
Would you expect the pressure in the middle of the collapsing cloud to increase, decrease, or stay just
about the same? Explain.
The collapse of the cloud into a protostar causes enormous changes within the center. Density, pressure, and
temperature all increase. Once the density and temperature reach critical levels, nuclear fusion begins. During this
fusion, hydrogen nuclei fuse to form a helium nucleus. During this process (which involves a number of steps), energy
is released as gamma-ray photons (light). As you might imagine, the onset of fusion is a violent event. Once the
nuclear reactions are sustained, the star settles down on the main sequence. The Sun is a main sequence star, currently
about half-way through its 10-billion lifetime in this stage. As the protostar was collapsing, gravity was in control
(halted periodically so that the star could radiate its thermal energy created during the collapse). Once fusion starts, the
star no longer collapses. The outward pressure caused by the energy released by the fusion reactions pushes back
against gravity and the star reaches a state of balance called hydrostatic equilibrium.
Mass of the Star (expressed as multiples of the Sun’s mass)
0.67 Msun
1.0 Msun
1.3 Msun
2.0 Msun
6 Msun
60 Msun
5.
2/16/2016
Main Sequence Lifetime of the Star
45 billion years [4.5 x 1010]
10 billion years [1.0 x 1010]
800 million years [8.0 x 108]
500 million years [5.0 x 108]
70 million years [7.0 x 107]
800 thousand years [8.0 x 105]
This is quite obvious, but which live longer: high mass or low mass stars?
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6.
Based on your answer to question 5, would you think that the nuclear fusion rate for a high-mass star is
greater than, equal to, or less than the nuclear fusion rate of a low-mass star? Why do the lifetimes imply
this?
7.
The nuclear fusion rate of a star depends on both the temperature and the density in the star’s core. How
would the temperature and density of the core for a high-mass star compare to that of a low-mass star?
8.
Which of the following statements best describes how the lifetimes compare between a star that has a mass
equal to the Sun and a star with 6 times the mass of the Sun?
A star with a mass the same as the Sun will:
a. live less than 6 times as long as a star with a mass 6 times the mass of the Sun.
b. live 6 times shorter than a star with a mass 6 times the mass of the Sun.
c. have the same lifetime as a star with a mass 6 times the mass of the Sun.
d. live 6 times longer than a star with a mass 6 times the mass of the Sun.
e. live more than 6 times longer than a star with a mass 6 times the mass of the Sun.
Explain your reasoning for the choice you made.
9.
The Sun has a main sequence lifetime of approximately 10 billion years. If you could determine the rate
of nuclear fusion for a star with twice the mass of the Sun, which of the following would it be?
a. less than the nuclear fusion rate of the Sun.
b. a little more than the nuclear fusion rate of the Sun.
c. twice the nuclear fusion rate of the Sun.
d. more than twice the nuclear fusion rate of the Sun.
Explain your reasoning for the choice you made.
10. Here is a figure from your text showing the “mass
luminosity” relationship. How does this graph relate
to the mass-lifetime relationship we’ve been discussing
here?
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