Content Benchmark E.12.B.4
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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://amazing-space.stsci.edu/
resources/explorations/blackholes/teacher/sciencebackground.html

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Content 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

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Content Benchmark E.12.B.4

Students know the on-going processes involved in star formation and destruction. W/L

 Sample Test Questions

1st Item Specification: Identify the processes of stellar evolution.

Depth of Knowledge Level 1

1. Which of the following determines most characteristics and future events of a star’s existence?
1. Color
2. Mass
3. Size
4. Temperature

2. In a main sequence star, equilibrium is reached when matter pulled inward by the gravitational force is pushed outward by
1. immense pressure created by energy in the core.
2. convection of stellar material from the core.
3. coronal mass ejections originating from the core.
4. endothermic cooling of hydrogen fuel in the core.

3. When a star is on the main sequence, it is
1. burning hydrogen and oxygen to make heavier elements.
2. burning radioactive elements to create lighter elements.
3. converting hydrogen into helium through nuclear fusion.
4. converting uranium into helium through nuclear fission.

4. What is the name given to a star as it is initially forming?
1. Protostar
2. Nebula
3. Star cluster
4. White dwarf

5. The force that dominates the formation of a star is
1. pressure.
2. magnetism.
3. gravity.
4. electricity.

Depth of Knowledge Level 2

6. As a star leaves its main sequence stage, it gets
1. cooler, larger, redder, and brighter.
2. cooler, smaller, bluer, and dimmer.
3. hotter, smaller, bluer, and dimmer.
4. hotter, larger, redder, and brighter.

7. What happens to cause a star to leave the main sequence? The gravitational pull inward becomes
1. less than the outward pressure, so the core and the outer layers of the star both expand, creating a black hole.
2. less than the outward pressure, so the core and the outer layers of the star both expand, creating a red giant.
3. greater than the outward pressure, so the core and the outer layers of the star both shrink, creating a black hole.
4. greater than the outward pressure, so the core of the star shrinks while the outer layers expand, creating a red giant.

2nd Item Specification: Recognize the life-cycles of mid-size and massive stars and their stellar remnants.

Depth of Knowledge Level 1

8. When fusion of hydrogen ceases in our Sun’s core, the Sun will
1. explode as a supernova.
2. collapse into white dwarf star.
3. contract into a black hole.
4. expand into a red giant star.

9. Stellar black holes are remnants of very high-mass stars. These black holes will
1. pull in matter from distant star systems with their huge gravitational attraction.
2. periodically explode and contract as matter is pulled into the black hole.
3. capture light that comes within its small (few kilometers-wide) event horizon.
4. form wormholes that allow matter to travel great distances in a short time.

10. Which of the following lists the stages of life of a high-mass star in correct order?
1. Red giant, white dwarf, black hole, main sequence
2. Red giant, main sequence, planetary nebula, neutron star
3. Main sequence, black hole, supernova, planetary nebula
4. Main sequence, red giant, supernova, neutron star

Depth of Knowledge Level 2

11. Star 1 will have a lifetime of 10 million years, while star 2 will have a lifetime of 300 million years. What can you say about the masses of these stars?
1. Star 1 is more massive.
2. Star 2 is more massive.
3. Stars 1 and 2 have about the same mass.
4. A star’s lifetime is unaffected by its mass.

12. The red and yellow line (also indicated by an arrow) 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.

(From http://chandra.harvard.edu/edu/formal/stellar_cycle/)

Which of the following statements is most correct? The red and yellow line shows

1. the star after it has fused all of it hydrogen and is expanding into a red giant.
2. the protostar stage before sustained nuclear fusion has begun in the star’s core.
3. when the star becomes a white dwarf stellar remnant, near the end of its life.
4. how the star moves through space after it has formed from a cloud of gas.

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Content Benchmark E.12.B.4

Students know the on-going processes involved in star formation and destruction. W/L

Answers to Sample Test Questions

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

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Content Benchmark E.12.B.4

Students know the ongoing processes involved in star formation and destruction. W/L

Intervention Strategies and Resources

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

1. Stellar Evolution Unit

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

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

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

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