Evolution of Stars


1. From what do stars form?
A)..Globular clusters.
B)..Open clusters.
C).. Black holes.
D)..Planetary nebulas.
E)..Interstellar clouds.

2. What makes a star shine?
A)...Reflected light from the galaxy.
B)...Radioactive decay of elements in its core.
C)...Nuclear Fusion of light elements.
D)... Nuclear Fission of heavy elements.
E)...None of the above.

3. What makes a gas cloud contract to form stars?
A)... Its magnetic field.
B)...The dust in it is drawn together by static electricity.
C)...Its gravity draws it inward.
D)...Pressure from meteor collisions squeezes it inward.
E)... Uranium atoms attract lead atoms by nuclear fusion.

4. What makes a star stop contracting?
A)... Its magnetic field.
B)... The dust in it gets packed so tightly.
C)...Its gravity gets too strong.
D)...It gets hot enough that its pressure builds up.
E)... It gets cool enough that it freezes.

5. When a star has stopped contracting and settled down, where is it in the HR diagram?
A)... On the main sequence.
B)... Below the main sequence in the lower left.
C)... In the upper right as a red giant.
D)...It is not on the H-R diagram at that time.
E)... In the instability strip.

6. What fuel do stars on the main sequence burn?
A)...Hydrogen
B)... Helium
C)...Carbon
D)...Iron
E)...Uranium

7.Why does fuel "burn" only in a star's core?
A)...The fuel settles to the core.
B)... Only the core is hot enough for fusion.
C)...Only the core spins fast enough.
D)...The statement is incorrect. Fuels "burn" just below the surface.
E)...The statement is true but none of the above are the correct answer.

8. What happens to the star's core as the hydrogen there is used up?
A)... Nothing. It just turns to helium.
B)...It expands and cools.
C)...It contracts and cools.
D)...It contracts and heats.
E)...As the helium accumulates the core rises to the surface of the star and escapes into space.

9. What happens to the star's outer layers as the fuel in its core is used up?
A)...They shrink and cool.
B)... They shrink and heat upl.
C)...They expand and cool.
D)...They expand and heat up.
E)...Nothing. Only the interior changes.

10. Why does the star's core get hotter as the core shrinks?
A)...The fuels settle there to liberate their heat.
B)... The core is compressed and compression heats a gas.
C)...The core spins faster and friction heats it.
D)...The statement is false. A shrinking core cools.
E)...The statement is true but none of the above are correct explanations of why.

11. Why does a high mass star evolve differently than a low mass star?
A)... It can burn more fuels because its core can get hotter.
B)... It has more material to burn so can last longer.
C)...It is so bright that it drives material in space away from it so it can't gather up more fuel.
D)...It has a lower gravity so it can't pull in more fuel from space.
E)...The statement is false. High and low mass stars evolve the same way.

12. What allows a high mass star to burn different fuels than a low mass star?
A)... Its greater mass means that it contains more fuels.
B)... More mass means a stronger magnetic field that helps heavier fuels burn.
C)...More mass means more compression and thus a hotter core that allows heavier fuels to burn.
D)...High mass stars spin slower and thus do not mix their fuels as well.
E)...The statement is false. Low and high mass stars can burn the same fuels.

13. What determines how rapidly a star burns up its fuel?
A)...Its mass.
B)... Its luminosity.
C)... Its location in the Milky Way.
D)...Both A) and B)
E)...None of the above.

14. What of the following most directly determines how much fuel a star has?
A)...Its density.
B)...Its mass.
C)...Its temperature.
D)...Its luminosity.
E)...Its speed of rotation.

15. What kind of object does a main sequence star become on first using up its core hydrogen?
A)...A white dwarf.
B)...A protostar.
C)... A supernova.
D)...A planetary nebula.
E)...A red giant.

16. What quantities are plotted in an H-R diagram?
A)...Density and distance.
B)... Temperature and luminosity.
C)...Radius and luminosity.
D)...Mass and volume.
E)...None of the above.


The following section looks at how to figure out the lifetime of a star. It will lead you step-by-step through the basic ideas.

Suppose your car burns 2 gallons of gasoline per hour and your fuel tank holds 10 gallons.

17. How many hours can you drive your car before it uses up a full tank?
A)...20 hours.
B)...2 hours.
C)...1/5 hour.
D)...5 hours.
E)...50 hours.


You can see from the above that the time to use up fuel is simply the amount of fuel divided by the burning rate.

Stars work the same way. They remain on the main sequence for a time equal to the amount of hydrogen fuel they have available divided by the rate at which they burn the hydrogen. Thus, to figure out the star's main sequence lifetime, we need to know the amount of fuel it has and the fuel burning rate.

The amount of fuel available to a star turns out to be about 10% of its mass. The rate at which it burns fuel is set by the star's luminosity. That is, the star burns fuel at whatever rate it needs to supply its luminosity.

For example, the Sun has a total mass of 2x1030 kilograms. About 10% of this is available for burning, so that the Sun has an amount of fuel equal to 2x1030/10 = 2x1029 kilograms.

The Sun's luminosity in metric units is 4x1026 joules per second. It turns out that one kilogram of hydrogen releases 6.3x1014 joules when it is converted into helium. Thus, to supply 4x1026 joules per second, the Sun must convert 4x1026 joules per second/6.3x1014 joules per kilogram to get (4/6.3)x1012 kilograms per second.

That is, to supply 1 solar luminosity of power, the Sun must burn 6.3x1011 kilograms of hydrogen into helium per second.

We can now figure out how long the Sun will be able to power itself by burning hydrogen.

If the Sun burns 6.3x1011 kilograms of hydrogen into helium per second, then, because it has a mass of fuel equal to 2x1029 kilograms, it can burn for a time equal to 2x1029 kilograms/6.3x1011 kilograms/second = (2/6.3)x1018 seconds = 3.2x1017 seconds.

Our answer above is in seconds. To convert that to years, divide by the number of seconds in 1 year: about 3.16x107. Thus, the Sun can burn for 3.2x1017 seconds /3.16x107 seconds = about 1010 years.

We can extend this result to other stars by scaling. That is, if a star has 10 solar masses, multiply the above result by 10. If it has 100 solar luminosities, divided the above result by 100. In other words the life time of a star is about 1010 M/L years.

18. Can the Sun burn all of the hydrogen it contains before leaving the main sequence?
A)... Yes.
B)...No.


Only the material in the core is hot enough to fuse and there is no very good way to mix the hydrogen from the surface inward. In fact, by the time a little over 10% of the mass is burnt, the star will begin to turn into a red giant and leave the main sequence. This is why we use 10% of a star's mass in estimating its age.




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