Content Benchmark E.12.B.2
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Students know stars are powered by nuclear fusion of lighter elements into heavier elements, which results in the release of large amounts of energy. I/S

The nearest star to us, the Sun, provides the vast majority of energy received on Earth. The Sun shines with a tremendous amount of light, consequently releasing large amounts of energy. But how is this energy created?

The primary source of energy in our Sun and all stars is nuclear fusion. In the predominant mechanism, four hydrogen ions (simply protons) are combined through collisions to create one helium ion (containing two protons and two neutrons). This process takes several intermediate steps, and can take place only where the temperature and density of the gas are higher than critical values.

Figure 1. In nuclear fusion of hydrogen, the basic process involves the conversion of four individual protons into a helium atom, containing two protons and two neutrons. Energy is emitted in the form of light in the process.

In this process, a tiny amount of mass is converted into energy through Einstein’s famous equation, E=mc². However, when you consider that a star is extremely massive and contains more than a trillion trillion trillion trillion (that’s a 1 with 48 zeros after it) particles, it is no wonder that the star emits a huge amount of energy. As an example, the power (energy per unit time) emitted by the Sun is about 4 x 10²⁶ Watts. The Sun puts out in one second the same amount of energy as would be created by 2.5 billion large power plants in one year!

To learn more about the process of nuclear fusion in stars, go to http://www.windows.ucar.edu/tour/link=/sun/Solar_interior/Nuclear_Reactions/
Fusion/fusion_reactions.html&edu=high.

The high temperatures and densities required for nuclear fusion can be found in the cores of stars. The gravitational attraction between the gas particles pulls them into a very small region and causes them to move incredibly fast. In the core of the Sun, the temperature is more than 15 million Kelvin.

To learn more about the properties of the Sun, such as its core temperature and density, go to http://www.seds.org/nineplanets/nineplanets/sol.html.

The fusion of hydrogen into helium constitutes what astronomers call the star’s “main sequence” lifetime – roughly analogous to a person’s middle age. The length of this lifetime is related to the star’s mass, although not in the way one might imagine. The most massive stars undergo fusion at an extremely high rate, and so use up their fuel very quickly – thus existing on the main sequence for a shorter length of time than less massive stars. The Sun is anticipated to be a main sequence star for approximately 10 billion years (about 5 billion of which have already passed); a star ten times the mass of the Sun will be a main sequence star for a mere 30 million years.

To learn more about stars’ lifetimes and the main sequence, go to
http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html.

During the main sequence, hydrogen is converted into helium. However, after the main sequence phase, helium can also undergo nuclear fusion, as can other elements. In fact, the most massive stars will continue to fuse one element into another until iron is created. At this time, much more complicated fusion processes occur when the massive star undergoes a supernova event.

To learn more about nuclear fusion of heavier elements, go to
http://casswww.ucsd.edu/public/tutorial/StevII.html.

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Content Benchmark E.12.B.2

Students know stars are powered by nuclear fusion of lighter elements into heavier elements, which results in the release of large amounts of energy. I/S

Common misconceptions associated with this benchmark:

1. Students incorrectly think the Sun and other stars are burning and/or involve explosions

Simplistic definitions of stars such as “burning balls of gas” serve to perpetuate this misconception. For years, students hear that the Sun is far hotter than anything we know on Earth; as a result, students often misapply analogies with lava, fire, and explosions, sometimes adding that the Sun or stars are hotter than these things on Earth. In fact, nothing like burning – in particular, combustion, in which material is rapidly oxidized and releases heat and light – is taking place in stars. Rather, light is released in the process of nuclear fusion, in which lighter elements are combined into heavier elements.

To learn more about this and other student misconceptions, see
http://aer.noao.edu/cgi-bin/article.pl?id=95

2. Students incorrectly think the Sun and other stars are powered by chemical reactions

Students also incorrectly identify the reactions in stars as chemical, rather than nuclear, or do not realize that there is a difference between the two. In chemical reactions, energy is released when electromagnetic bonds between the protons and electrons are broken or rearranged. Thus the atom retains its identity, although the electron structure is different from its state prior to the reaction. In nuclear reactions, however, it is the bonds of the strong nuclear force between protons and neutrons that are affected, changing the nuclear structure itself and resulting in new types of atoms.

To learn more about the different types of nuclear reactions, go to
http://www.lbl.gov/abc/Basic.html.

3. Students confuse nuclear fusion and nuclear fission

When students do know that nuclear reactions are present in stars, they can sometimes confusion nuclear fusion with nuclear fission (or simply not know the difference between the two). The processes are basically opposite of one another. In nuclear fusion, lighter elements are combined to create new, heavier elements (the most common example is hydrogen being converted into helium). In nuclear fission, heavier elements are broken apart into lighter elements (one example is the breaking down of uranium into barium and krypton). Both processes release energy. Nuclear fission is the primary source of energy used in today’s nuclear reactors, and in nuclear weapons. Sustained nuclear fusion is the primary source of energy in stars, and has not yet been replicated on Earth in any practical manner – the conditions required are, as of now, too difficult to reproduce for productive use.

To learn more about the confusion of nuclear fission and nuclear fusion, go to
http://www.lbl.gov/abc/wallchart/chapters/appendix/appendixg.html

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Content Benchmark E.12.B.2

Students know stars are powered by nuclear fusion of lighter elements into heavier elements, which results in the release of large amounts of energy. I/S

Sample Test Questions

1st Item Specification: Know that stars are composed mostly of hydrogen and helium and ultimately produce all other elements through nuclear reactions.

Depth of Knowledge Level 1

1. The process of converting hydrogen into helium in stars is called
1. nuclear fusion.
2. nuclear fission.
3. chemical reaction.
4. physical change.

2. A star is a hot ball of gas that produces energy by
1. reflecting it from a nearby source.
2. burning and mixing different gases.
3. combining atoms into heavier atoms.
4. breaking apart atoms into lighter atoms.

3. When a star is first formed, it is made mostly of which element?
1. Oxygen
2. Hydrogen
3. Carbon
4. Nitrogen

4. The light from stars that we see on Earth results from
1. nuclear reactions inside the stars.
2. chemical reactions inside the stars.
3. light reflected from the stars’ surface.
4. burning of gases on the stars’ surface.

5. The elements that make up our body were all originally made in
1. comets.
2. stars.
3. nebulas.
4. black holes.

6. Nuclear fusion in the core of main sequence stars converts hydrogen into
1. carbon.
2. oxygen.
3. helium.
4. iron.

7. At the end of its life, a high-mass star has a(n)
1. A. hydrogen core.
2. helium core.
3. carbon core.
4. iron core.

Depth of Knowledge Level 2

8. Elements from helium through iron are created
1. through nuclear fusion in stars, while elements heavier than iron are made in supernova shockwaves.
2. in supernova shockwaves, while elements heavier than iron are created through nuclear fusion in stars.
3. by chemical reactions in stars, while elements heavier than iron are made through nuclear reactions.
4. by nuclear reactions in stars, while elements heavier than iron are made through chemical reactions.

9. What is the relationship between the amount of hydrogen, amount of helium, and energy as a low-mass star ages?
1. Hydrogen increases, helium decreases, and energy is released.
2. Hydrogen increases, helium increases, and energy is absorbed.
3. Hydrogen decreases, helium decreases, and energy is absorbed.
4. Hydrogen decreases, helium increases, and energy is released.

10. The rate of fusion of high-mass stars greatly exceeds that of low-mass stars. A result is that high-mass stars
1. consume hydrogen much more slowly than low-mass stars.
2. spend less time on the main sequence than low-mass stars.
3. convert less of their matter into energy than low-mass stars.
4. switch to fission reactions much sooner than low-mass stars.

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Content Benchmark E.12.B.2

Students know stars are powered by nuclear fusion of lighter elements into heavier elements, which results in the release of large amounts of energy. I/S

Answers to Sample Test Questions

1. A, DOK level 1
2. C, DOK level 1
3. B, DOK level 1
4. A, DOK level 1
5. B, DOK level 1
6. C, DOK level 1
7. D, DOK level 1
8. A, DOK level 2
9. D, DOK level 2
10. B, DOK level 2

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Content Benchmark E.12.B.2

Students know stars are powered by nuclear fusion of lighter elements into heavier elements, which results in the release of large amounts of energy. I/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. Discovery Channel’s Savage Sun
The “Savage Sun” video focuses on recent understanding of the Sun through space-based solar missions. The basic description can be found at http://school.discovery.com/lessonplans/programs/savagesun/q.html, and an associated lesson plan can be found at http://school.discovery.com/lessonplans/programs/savagesun/.

2. National Solar Observatory
The National Solar Observatory (NSO) invites selected teachers to participate in summer research experiences. From one former participant, Joey Rogers, comes this activity to model nuclear fusion in the Sun: http://eo.nso.edu/ret/rogers/lpfuse.htm.

3. Contemporary Physics Education Project (CPEP)
This website contains several pages of explanation, diagrams, and activities relating to fusion. The project was sponsored by the US Department of Energy and the University of California Lawrence Livermore National Laboratory and can be found at http://fusedweb.pppl.gov/CPEP/Chart.html.

4. NASA’s Imagine the Universe
This activity allows students to model the layers of a massive star that result from a series of nuclear fusion reactions: http://imagine.gsfc.nasa.gov/docs/teachers/lessons/xray_spectra/activity-fusion.html.

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