Performance 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
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.
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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/fu
sion_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.
Performance 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/cgibin/article.pl?id=95 and <Bailey, forthcoming>.
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
Performance 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
1. How does the Sun produce the energy that heats our planet?
a. The gases inside the Sun are burning and producing energy.
b. Atoms are combined into heavier atoms, giving off energy.
c. Atoms are broken apart into lighter atoms, giving off energy.
d. The core of the Sun has radioactive atoms that give off energy as they decay.
2. What is a star?
a. a ball of gas that reflects light from another energy source that is nearby
b. a hot ball of gas that produces energy by burning and mixing different gases
c. a hot ball of gas that produces energy by combining atoms into heavier atoms
d. a hot ball of gas that produces energy by breaking apart atoms into lighter
atoms
3. The light from stars that we see on Earth results from
a. nuclear reactions inside the stars.
b. reflection of sunlight.
c. chemical reactions inside the stars.
d. burning of gases inside the stars.
4. During the majority of a star’s existence, in which part of a star is its energy
produced?
a. radiative layer
b. nucleosphere
c. throughout the star
d. core
5. The amount of energy created during a nuclear fusion reaction can be calculated
using which of the following equations?
a. F = ma
b. E = mc2
c. a2 + b2 = c2
d. E = ½ kv2
Performance 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. (b)
2. (c)
3. (a)
4. (d)
5. (b)
Performance 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 list of intervention strategies and resources 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/activityfusion.html.