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Performance Benchmark E.12.B.4
Students know the ongoing processes involved in star formation and destruction. W/L
Like virtually everything in nature, stars change over time. Although stars are not
biological organisms, this process of change is often called the “stellar life cycle.” Just
like living organisms, stars are born, develop, mature, and, eventually, die. This process
takes many millions to billions of years, so it is not possible for humans to observe the
complete life cycle of a single star. However, with rapid telescope technology advances
in the 20th Century, astronomers have been able to view many different stars, all in
various stages of maturity. With these observations and our basic understanding of
physical process, scientists have developed a robust model of how stars change with time.
Figure 1. A schematic of how stars of different mass change with time. (From:
http://chandra.harvard.edu/resources/illustrations/stellar_fate2.html)
The story of star formation, change, and destruction is really a story of the gravitational
force, and specifically, how the other fundamental forces (electromagnetism and the
strong nuclear force) balance the continual gravitational attraction. Because gravitation is
the underlying force involved in stellar change, it is no wonder that the star’s mass is the
primary factor which determines how a star forms, changes, and dies. In essence, the
mass of the star predetermines its fate.
Stellar Formation
Stars form from vast regions of gas and dust, which are sometimes called “stellar
nurseries.” These vast regions contain mostly hydrogen gas, but also trace amounts of
“dusty” molecular material, are very cold (just a few degrees Kelvin), and turbulent. Even
though these very low-mass particles of gas and dust are attracted to each other by
gravitation, the turbulent mixing keeps the cloud in a state of relative equilibrium.
However, energetic disturbances can change this equilibrium, causing the gravitational
force to collapse regions within the cloud. Passing supernovae shockwaves and spiral
galaxy density waves, as well as nearby star formation, can cause such energetic
disturbances.
To learn more about stellar nurseries, go to
http://archive.ncsa.uiuc.edu/Cyberia/Bima/StarForm.html
Figure 2. A Hubble Space Telescope image of the Orion Nebula,
which at 1500 light years from Earth, is the nearest stellar nursery.
(From: http://www.spacetelescope.org/images/html/heic0601a.html)
As the cloud collapses under the gravitational force, regions of higher density gas and
dust form. With greater density, the pressure and temperature of the cloud increases
causing much more rapid particle motions. Eventually, the central temperature will reach
hundred to thousands of degrees Kelvin and begin to “shine,” predominantly in infrared
light. The shining region is now called a protostar. The protostar temperature increases
and it continues to grow as it accretes additional mass from the cloud of gas and dust.
More details about protostars can be found at
http://aspire.cosmic-ray.org/labs/star_life/starlife_proto.html
Stellar Birth
Gravitational attraction still dominates in a protostar, allowing it to accrete more mass
and its internal temperature to rise. Particles (mostly protons) in the protostar gain more
and more kinetic energy as matter is compressed and electromagnetic forces repel these
like-charged particles in every direction. However, when core temperatures reach tens of
millions of degrees, the particles are so dense and moving so rapidly that electromagnetic
forces cannot repel them sufficiently far apart. The particles come so close together that
they “enter the halo” of the nuclear force, binding them together in a process called
nuclear fusion. In this process, some of the combined particles’ mass is converted directly
to energy, and with so many particles in the core, a tremendous amount of radiative
pressure is created that pushes directly against the gravitational attraction.
To learn more about stellar nuclear fusion, go to
http://www.astro.cornell.edu/academics/courses/astro201/fusion_control.htm
A star is born when nuclear fusion is sustained in its core. However, some protostars
never gain enough mass for sustained fusion. As a matter of convenience, stellar masses
are compared to that of the Sun, rather than using kilograms. The Sun is 1 MSun (“one
solar mass”), which is roughly equal to about 2 × 1030 kilograms. The smallest stars are a
little less than 10% of the mass of the Sun, or 0.08 MSun, while the most massive star yet
measured is about 150 MSun. At masses less than 0.08 MSun sustained fusion will not
occur. These failed stars are called brown dwarfs.
More details about brown dwarfs can be found at
http://astro.berkeley.edu/~stars/bdwarfs/formtnbd.html
Stellar Maturity – The Main Sequence
For most of a star’s existence, nuclear fusion is converting hydrogen (really protons) into
helium (really helium nuclei) in the star’s core. This phase is called the main sequence,
derived from a star’s properties when graphed on the Hertzsprung-Russell, or H-R,
Diagram (named after two astronomers who independently developed it). On this graph,
temperature (or sometimes spectral class, which is related) is plotted on the horizontal
axis, with the values decreasing to the right. Luminosity is plotted on the vertical axis,
using a logarithmic scale. When the corresponding values for a large number of stars are
then plotted on this graph, groupings of stars can be easily identified. Ninety percent of
the stars fall upon a band called the main sequence; in these stars, hydrogen is being
fused into helium in the stellar core.
Figure 3. The H-R Diagram shows different groupings of stars
that have been interpreted as different stages of a star's lifetime.
(From
http://cse.ssl.berkeley.edu/segwayed/lessons/startemp/l6.htm)
To learn more about the H-R Diagram, go to
http://aspire.cosmic-ray.org/labs/star_life/hr_diagram.html.
The length of time a star spends on the main sequence depends almost entirely on its
mass. Stars with greater mass have a greater gravitational attraction – causing the core
temperature to be greater, which in turn increases the rate of nuclear fusion and decreases
the star’s time on the main sequence. Likewise, lower mass stars have lesser rates of
fusion and greater amounts of time on the main sequence. Based on precise
measurements and computer modeling, our Sun is expected to have a main sequence
lifetime of 10 billion years. A star with a mass of 15 MSun has a main sequence lifetime of
only 15 million years, whereas a star with 0.5 MSun has a main sequence lifetime of 200
billion years. Note that these very low mass stars have lifetimes so long that they may be
on the main sequence for the rest of the universe’s existence. These are called red dwarf
stars and constitute about 70% of the stars in our galaxy, and likely, the universe.
To learn more about red dwarf stars, go to http://www.solstation.com/stars/pc10rds.htm
Stellar Seniority – Red Giants and Supergiants
When a main sequence star has fused much of its hydrogen to helium, radiative pressure
weakens and the gravitational force causes the now helium-rich core to collapse relatively
slowly. As the core collapses, the temperature increases from tens of millions to hundreds
of millions of degrees Kelvin, and at these temperatures, the helium begins to fuse into
heavier elements such as carbon. Radiative pressure from this higher temperature fusion
is greater and the star expands to maintain equilibrium. With expansion, the outer regions
cool and the star radiates red light from these outer shells.
For mid-mass stars, such as our Sun, this is called the red giant stage, where helium is
being fused into carbon. Scientists have gathered strong evidence that our Sun will
become, in about 5 billion years, a red giant where its outer layer may spread as wide as
Earth’s orbit. The temperature of our planet will increase greatly when this happens,
making the Earth uninhabitable.
More details on red giant stars can be found at
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/redgia.html
Stars of about 8 MSun or greater will have even higher core temperatures, where fusion of
helium into heavier elements (e.g., oxygen, silicon, and iron) occurs. These stars are
called red supergiants. Periods of nuclear fusion followed by slight changes in the star’s
size will continue, creating layers of different elements and fusions processes. In the most
massive stars, this will continue until iron is created in the star’s core.
To learn more about the fusion process in red supergiants, go to
http://imagine.gsfc.nasa.gov/docs/teachers/elements/imagine/05.html
Figure 4. An artist's conception of a red supergiant, with a schematic
of the nuclear fusion occurring in it core. (From
http://cse.ssl.berkeley.edu/bmendez/ay10/2000/cycle/massive.html)
The red giant stage lasts just a small fraction of the star’s total lifetime (a few million to
hundreds of million years, depending on the mass of the star).
Stellar Death and Remnants
The end of a star’s life is marked when its core nuclear fusion ceases. For mid-mass stars
like our Sun, nuclear fusion stops when most of the helium in its red giant core has been
fused to carbon. The ever-present gravitational attraction compresses the carbon core, but
there is not enough mass to initiate fusion of carbon. The carbon continues to compress
until the gravitational attraction is balanced by the repulsive pressure of the electrons in
the carbon atoms. The remainder of the star has already been expelled during its final
phase as a red giant, creating a beautiful cloud known as a planetary nebula (a misnomer
– in the telescopes of the time planetary nebula looked like planets, though they did not
exhibit the same kinds of motion as the planets).
Figure 5. A planetary nebula known as the Ant Nebula, with a white dwarf star in the center.
The image is a composite false color image using many wavelengths of light including X-ray,
visible, and infrared. (From: http://chandra.harvard.edu/photo/2006/pne/index.html)
Eventually, the planetary nebula is lost to the interstellar medium, and the solitary and
extremely hot carbon core remains to slowly cool over billions of years. This stellar core
remnant, which has been compressed to about the diameter of Earth (roughly 10,000
kilometers), is known as a white dwarf.
More details about white dwarf stars and planetary nebulae are found at
http://chandra.harvard.edu/xray_sources/white_dwarfs.html.
For massive stars (stars with a total mass of 8 MSun), nuclear fusion stops when the
quantity of iron in the core reaches about 1.4 MSun — a quantity known as the
Chandraskhkar Limit. Unlike the lighter elements created in the stellar core, iron requires
more energy for fusion than is released. When the Chandraskhkar Limit is reached, the
radiative pressure of fusion is no longer strong enough to balance the gravitational
attraction, and the star’s core collapses in a fraction of second. In that instant the diameter
of the stellar core is reduced to the size of a small city (about 20 kilometers) and a
tremendous amount of energy is released. The power output from this single event is
equal to more than a thousand billion stars at the instant of the collapse, resulting in an
explosion called a supernova.
Figure 6. Supernova remnant G11.2-03 located about 16,000
light years from Earth. (From:
http://chandra.harvard.edu/photo/2007/g11/index.html)
During a supernova, the entire star (with the exception of the iron core) is hurled through
the interstellar medium at a significant fraction of the speed of light in what is called a
shockwave. The shockwave is composed of nuclear fragments of such elements as
oxygen, silicon, and iron. As the shockwave particles interact intensely with the outer
atmosphere of the star and the interstellar medium (consisting mainly of hydrogen),
elements heavier than iron are created through nuclear fusion. Elements such as gold,
silver, and uranium are created in the shockwaves of supernova explosions.
To learn more about supernova explosions, go to
http://imagine.gsfc.nasa.gov/docs/science/know_l1/supernovae.html
Because of the rapid and drastic compression of the stellar core, electrons in the core
material are slammed into protons in the atomic nuclei, creating a very dense stellar
remnant composed predominantly of neutrons. The nuclear force that binds these
neutrons together is then strong enough to prevent further gravitational collapse. This
creates a very dense star aptly named a neutron star. Due to conservation of angular
momentum, most neutron stars rotate several times a second and can be called pulsars.
More information about neutron stars are found at
http://cassfos02.ucsd.edu/public/tutorial/SN.html
If the star is extremely massive, the nuclear force is not strong enough to balance
gravitational attraction in the core collapse. There is no force remaining to counter
gravity and the star collapses into an object of extremely small diameter; theoretically the
star collapses into a singularity with no dimensions at all. This stellar remnant is called a
black hole. Scientists have not yet discovered ways to observe black holes directly;
however, orbiting X-ray telescopes have observed high temperature emissions from
material falling into the areas immediately surrounding black holes.
Figure 7. Artist's conception of a stellar mass black hole accreting
matter from a companion star. (From:
http://chandra.harvard.edu/photo/2007/n4472/n4472_ill.jpg)
To learn more about stellar black holes, go to http://amazingspace.stsci.edu/resources/explorations/blackholes/teacher/sciencebackground.html
Performance Benchmark E.12.B.4
Students know the ongoing processes involved in star formation and destruction. W/L
Common misconceptions associated with this benchmark:
1. Students incorrectly think forces other than gravitation cause clouds of gas and
dust to collapse into stars.
Scientists have strong evidence that disturbances can change equilibrium within an
interstellar cloud of gas and dust. But when this disequilibrium occurs, the force that
causes the cloud to collapse is clearly gravitation.
In our universe, gravitation is an ever present force of attraction between all masses. The
greater an object’s mass, the greater the gravitational attraction. But, even very small
mass objects, such as particles of gas and dust, create a gravitational force field around
them, and under the right astronomical conditions, these small particles will move closer
and closer to each other over time.
Students believe that gravitation is an interaction associated only with very massive
objects, such as planets, stars, and galaxies. Many students also have a prior knowledge
that stars form from clouds of gas and dust. Therefore, many incorrectly construct ideas
that forces, such as magnetism, or false forces, such as centrifugal effects, result in the
collapse of these clouds and dust.
A very good site that discusses misconceptions associated with nebulae, including star
formation regions is found at
http://www.astrosociety.org/education/family/resources/deepspaceprint.html#2a
2. Students incorrectly believe that more massive stars live longer than less massive
stars.
Since more massive stars contain more hydrogen, the possibility that it will take longer
for that hydrogen to be fused into helium seems perfectly reasonable. However, this is not
the case. More massive stars have a larger gravitational attraction pulling the gas particles
together, resulting in higher temperatures and pressures in the core. This leads to a higher
rate of fusion (the transformation of hydrogen into helium), so the hydrogen is depleted
faster than in a less massive star. The more massive stars therefore have shorter lifetimes
than less massive stars.
To learn more about how a star’s mass relates to its lifetime, go to
http://www.astronomynotes.com/evolutn/s2.htm.
3. Students incorrectly think that our Sun will end as a supernova explosion.
All stars that are about 8 MSun or greater will end as a supernova, leaving some kind of
stellar remnant (e.g., a neutron star or black hole). Specifically, these massive stars will
end as a Type II supernova. In massive stars, their stellar core mass is about 1.4 MSun, a
value known as the Chandraskhkar Limit. At the end of the star’s life, when stellar fusion
ceases suddenly, core masses at or above this limit have so much gravitational attraction
that they collapse to very small size and this resulting rebound of energy results in a
supernova.
It is possible for a star less than 8 MSun to supernova, but its core mass must be greater
than the Chandraskhkar Limit. This can only happen if a mid-mass star is in a binary
system. In this case the mid-size star has ended fusion and reached the white dwarf stage.
When its companion star reaches the red giant stage and the companion’s outer
atmosphere comes closer to the white dwarf, material from the red giant can be accreted
onto the white dwarf star increasing its mass. If enough material is accreted onto the
white dwarf so that its mass exceed the Chandraskhkar Limit, the white dwarf will
supernova. This is called a Type Ia supernova and results in total annihilation of the star
(i.e., there is no left-over stellar remnant).
However, our Sun is not in binary system, so it cannot end as a Type Ia supernova. Nor is
its mass great enough to end as a Type II supernova. Therefore, our Sun will end its life
in the relatively benign white dwarf stage, not in an explosion.
To learn more about Type Ia supernovae, go to
http://csep10.phys.utk.edu/astr162/lect/supernovae/type1.html
4. Students incorrectly believe that black holes are giant “gravity” vacuums that
seek out and suck up all matter.
Stellar black holes do not have more gravitational attraction than the original star from
which the black hole came. However, the black hole’s mass is collapsed into a very small
space making the escape velocity (a property which is the function of the gravity and size
of an object) greater than the speed of light, the maximum velocity an object can have in
our universe. Objects, including beams of light, which come close to the black hole, will
not be able to escape its gravitational attraction. This boundary can be thought of as the
“surface of the black hole” because we cannot observe within this distance and is called
the event horizon. For a stellar black hole, the event horizon diameter is only a few
kilometers across. Therefore, this small distance is the extent to which objects would not
escape.
To learn more about how gravitational attraction around a stellar black hole, go to
http://ircamera.as.arizona.edu/NatSci102/lectures/blackhole.htm
Performance Benchmark E.12.B.4
Students know the ongoing processes involved in star formation and destruction. W/L
Sample Test Questions
1. Star C will have a lifetime of 10 million years, while star D will have a lifetime of
300 million years. What can you say about the masses of these stars?
a. Star C has the greater mass.
b. Star D has the greater mass.
c. Stars C and D have about the same mass.
d. You cannot determine star mass from lifetime.
2. Which of the following determines most characteristics and future events of a
star’s existence?
a. size (diameter)
b. temperature
c. color
d. mass
3. Stellar black holes are remnants from massive stars. These black holes will
a. Suck in all matter within several light years due to its tremendous
gravitational attraction.
b. Periodically explode and contract as matter is attracted into the black hole
by it strong gravity.
c. Only capture light that comes within its small (few kilometers wide) event
horizon.
d. Periodically form a wormhole that allows matter to travel great distances
in a short amount of time.
4. When all nuclear fusion ceases in our Sun’s core, our Sun will
a. Explode as a supernova.
b. Collapse into white dwarf star.
c. Contract into a black hole.
d. Continue burning as a red giant star.
5. The red and yellow line on the diagram below shows a how a single star is
changing with time. Note that the star is about the same mass as our Sun.
Figure 8. From http://chandra.harvard.edu/edu/formal/stellar_cycle/
Which of the following statements is most correct?
a. The red and yellow line with the arrow shows the star after it has fused all
of it hydrogen fuel and is expanding into the red giant stage.
b. The red and yellow line with the arrow indicates the protostar stage before
sustained nuclear fusion has begun in the star’s core.
c. The red and yellow line with the arrow indicates when the star becomes a
white dwarf stellar remnant.
d. The red and yellow line with the arrow shows how the star moves through
space after it has formed from a gas and dust cloud.
Performance Benchmark E.12.B.4
Students know the ongoing processes involved in star formation and destruction. W/L
Answers to Sample Test Questions
1.
2.
3.
4.
5.
(a)
(d)
(c)
(b)
(b)
Performance Benchmark E.12.B.4
Students know the ongoing processes involved in star formation and destruction. W/L
Intervention Strategies and Resources
The following list of intervention strategies and resources will facilitate student
understanding of this benchmark.
1. Stellar Evolution Unit
NASA’s Chandra X-ray Observatory has created a suite of activities that can be used for
a unit on stellar cycles. This unit was featured in the cover article for the February 2005
Issue of the Science Teacher. This cover article can be found at
http://www.nsta.org/main/news/stories/science_teacher.php?category_ID=88&news_stor
y_ID=50175.
The stellar evolution unit contains background information, hands-on activities that
reinforce the unit content, and an assessment activity. Many of these activities are in two
forms: (1) online/interactive and (2) pencil/paper.
To get the unit overview and download these activities, go to
http://chandra.harvard.edu/edu/formal/stellar_ev/
2. Imagine the Universe: Life Cycles of Stars Activities
NASA’s Goddard Space Flight Center has created a Web site called Imagine the
Universe! This site is dedicated to helping students and the public to deepen
understanding about astronomy and cosmology.
You can access the site at
http://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/LC_title.html
This site is heavily text-based, but many of the activities are appropriate for middle
school level students. The teacher’s corner of the site contains these activities and can be
accessed at http://imagine.gsfc.nasa.gov/docs/teachers/lesson_plans.html
3. Stars and Nebulae Web-based Activities
The Sloan Digital Sky Survey’s Education Program has created a Web site with many
activities that involve student use of actual data. These activities require classroom use of
Internet-capable computers, or if assigned as homework, students should have access to
the Internet.
Stars and Nebulae are a series of activities that lead students through understanding about
Hertzsprung-Russell (or H-R) diagram, stellar evolution, nebulae, and brown dwarf stars.
The activities are loaded with information and allow students to use data collected by the
Sloan Digital Sky Survey telescopes to construct understanding about stars.
The site can be found at http://cas.sdss.org/dr5/en/astro/stars/stars.asp#hrdiagram