Stellar EvolutionNate Childers

Hart Middle SchoolRochester, Michigan

1. Stellar Evolution E5.2xStars, including the Sun, transform matter into energy in nuclear reactions. When hydrogen nuclei fuse to form helium, a small amount of matter is converted to energy. These and other processes in stars have led to the formation of all the other chemical elements. There is a wide range of stellar objects of different sizes and temperatures. Stars have varying life histories based on these parameters.

2. Narrative:

Like humans, stars pass through different stages in their lives. They are born, they mature and, eventually, they die. However, unlike humans, the typical star may last for millions or billions of years. While we cannot witness the complete life cycle of any one star, the night sky does reveal stars in various stages of stellar development. In some ways we’ve got a time machine that enables us to look back and out into the future. In so doing we can glimpse aspects of our own star’s past and destiny.

Like all stars, our Sun was formed from a cloud of hydrogen gas and dust that almost certainly included the ashes from an earlier star gone supernova. In its death throes, it created elements heavier than iron that our solar system inherited. Gravity pulled the cloud together into a giant ball. When enough gas and dust had gathered, enormous pressures at the center forced hydrogen atoms to begin fusing into helium, thereby releasing energy and raising the temperature at the core to over 15 million degrees. Another star was born, and the Sun began to glow.

The birth of all stars is much like that of the Sun, but the mass of the gas and dust comprising the star will determine its precise destiny. Medium-sized stars like our Sun eventually use up their hydrogen fuel, cool and expand into red giants. Later they shed their outer layers and appear as a diffuse cloud called a planetary nebula (which, confusingly, has nothing to do with planets!), lose more gas, shrink down to become white dwarf stars, and eventually even smaller stellar corpses called black dwarfs. At this stage they may be only a few thousand miles in diameter.

In general, the smaller the mass of a star, the longer its life. Our Sun is now near the mid-point of its estimated 10 billion year life. Stars that are many times more massive than our Sun experience dramatic and sometimes explosive endings. Following the red giant stage they may continue to expand into supergiants. The core then shrinks and grows hotter and denser. Eventually internal forces erupt to cause the star to explode as a supernova. The stellar remnants become either extremely dense neutron stars or mysterious black holes, objects with such a strong gravitational pull that not even light can escape. Matter ejected from a supernova is blown out into interstellar space and may enter a kind of cosmic recycling program, seeding new solar systems of stars, planets, and moons with the gold and iron of the earlier generation of stars.

Stars come in several colors—red, orange, yellow, white and blue. The color of a star offers us a clue to its surface temperature. Red stars are the coolest and blue stars are the hottest. Our Sun is classified as a medium-sized, yellow dwarf star. Astronomers use star color and actual brightness to determine the stage in a star’s life cycle.

3. Activity Title: Comparing Star Color

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Content Expectation: E5.2f Explain how you can infer the temperature, life span, and mass of a star from its color. Use the H-R diagram to explain the life cycle of stars.

4. Question to be Investigated: Why do some stars appear reddish, some white, and some bluish white?

5. Activity Description:1)Have student groups look at pictures of various stars from

http://nix.ksc.nasa.gov/info;jsessionid=wrqwwqpiam3k?id=GL-2002-001195&orgid=6,and have them create a list of similarities and differences of the stars.

2) Show students the images from “The Size of Our World” to introduce the differences in size between stars.

3) Have students form a hypothesis about how the colors emitted from a glowing object relate to the object’s temperature.

4) Set up a lamp with an unfrosted light bulb plugged into a dimmer switch.5) Darken the room.6) Turn on the lamp and dim it until it is just barely on.7) Ask students to observe and note the color of the glowing filament in the bulb.8) Slowly turn up the bulb so it gets brighter and brighter.9) Have students observe and note the changing color of the glowing lamp filament.10)Next, give each group of students a candle in a small tin pan. Have students light

a candle and observe the flame.11) In groups, have the students discuss and note the different colors observed in the

flame.12)As a class, discuss the different colors observed.13)Have students look back over the star pictures and discuss the differences they

see now, and how the color of the star relates to the temperature.14)Have students create a graph, using the Star Comparison Chart, of the stars color

versus their temperature. They should observe a clear relationship.15) In groups, have students discuss the relationship that exists between stars color,

temperature, mass, and power.16)Have students write a reflection on the activity describing the relationship

between color and temperature, mass and power of stars.

6. Teacher NotesAdditional Information:

Many students think of something very hot as being “red hot.” They seldom think of blue as being a “hot” color. With the lamp turned down to a minimum, the filament glows a dim red color. The filament glows because it is hot, but it can get much hotter and glow much more brightly. As the lamp is turned up the filament receives more power. It gets hotter and glows brighter. Its color progresses from dim red to orange to yellow to white. As it gets hotter it glows brighter. There is a relationship between the temperature and the color of the glowing filament. If the filament could be made to glow even hotter, it would progress from white to blue in color.

Many students have seen the color progression from red through yellow to blue in the flame of a camp fire. The cool flame is red or orange and the hottest flame is blue. Explain that the color of the flame near the wick of the candle is bluish in color and hottest in temperature. The outside edge of the flame is yellow or red and is the coolest part of the flame. The same color

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progression holds true for stars. Scientists use color to help determine the temperature of a star. The coolest stars at 3,000 degrees Celsius glow red. The hottest stars have surface temperatures in the tens of thousands of degrees and glow with a fierce blue light. Such stars are extremely bright, powerful, and easy to see at great distances.

The white and blue stars burn their fuel at the highest temperatures. They produce the most energy and so they are the most powerful stars. These stars are also the biggest stars.

The bigger a star is, the more mass it has. More mass means the star presses in on itself more strongly. It gets hotter in the core than a smaller star, so it burns faster and at a much higher temperature. Because it burns hotter it also gives off more energy than a smaller, cooler star.

Safety ConcernsDo not touch the light bulb, the light bulb will become very hot.

Care should be taken when using matches. Students should wear safety goggles and be reminded of fire safety in the classroom.

7. Graphs, Charts, Illustrations:Star Comparison Chart

Star Color Temperature (˚C)

Mass in terms of Solar Mass

Power in Suns

Sun Yellow 5,700 1 1Proxima Centauri Red 2,300 0.1 UnknownBarnard’s Star Red 3,000 0.1 0.01Epsilon Eridani Orange 4,600 0.1 0.4Alpha Centauri Yellow 6,000 1 2Altair White 8,000 3 12Vega White 9,900 3 61Sirius White 10,000 3 27Rigel White 10,000 3 52,000Regulus White 11,000 8 221Hadar Blue 25,500 20 79,000Alnilam Blue 27,000 20 112,000

8. Credits and Sources Starry Night; The Stars; www.starrynight.com

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1. Title: Magnitude and the H-R DiagramContent Expectations:

E5.2e Explain how the Hertzsprung-Russell (H-R) diagram can be used to deduce other parameters (distance).

E5.2hCompare the evolution paths of low, moderate and high mass stars using the H-R diagram.

2. Question to be investigated: How are stars classified and placed on the Hertzsprung-Russell diagram?

3. Activity Description: 1) Have students form a hypothesis about why some stars appear brighter than others.2) Hand students the investigation called “Magnitude and the HR Diagram.”3) Review the concepts of apparent magnitude versus absolute magnitude.4) Have students complete the investigation.5) Discuss the resulting chart with the class.

Describe the general trend between temperature and brightness. What is the color and brightness of the most abundant stars? The

rarest stars? What are the characteristics of the stars that do not conform to the

graph’s trend? In terms of the graph’s trend, is our sun typical or exceptional? If you replaced the temperature scale on the graph’s x-axis with a color

scale, which color would be closest to the graph’s origin and which would farthest away?

In the stars that fit the general trend (these are often called Main Sequence stars), what relationship do you notice between color and expected lifetime?

6) Students should complete the Classified Stars activity.

4. Teacher NotesAdditional Information:

* Stellar Magnitude For historical reasons, astronomers still call the brightest stars magnitude 1 stars, the next brightest 2, then 3, 4, 5, and on down to the dimmest stars, magnitude 6 stars. This backwards scale is now defined as mathematically as:

brightness of star 1 = difference in magnitude brightness of star 2 (2.512)

Using this sort of scale, we can actually describe the brightness of stars in real numbers. Really bright objects, like the full moon have a negative magnitude.  

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Object Apparent Magnitude

Sun -26.5Full Moon -12.5Venus -4.1Sirius -1.4North Star 2.0limit of naked eye

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Pluto 15limit of HST 28

** Spectral Classes Stars are easily classified by the amount of hydrogen which they contain as revealed to us by the intensity of their spectrum. Stars with the most hydrogen are called A stars, the remaining letters designate varying amounts of different chemical stellar compositions. If we consider the temperature of stars, as revealed to us by their color, we find that the O stars, are hottest, followed by B stars, A stars, F, G, K, and M stars. A star which has an intermediate temperature between an O star and a B star is called an O5 star. This layout of temperature is given from hottest to coolest as:

O0, O1, O2, O3, O4, O5, O6, O7, O8, O9, B0, B2, B3, B4, B5, B6......

Astronomers sometimes humorously teach their students to remember this sequence from hottest to coolest as:

O h, b e a f ine g uy (or gal), k iss m e!

Our Sun is G2 star, which means that it is a rather intermediate temperature

The Hertzsprung - Russell diagram is a graphical tool that astronomers use to classify stars according to their luminosity, spectral type, color, temperature and evolutionary stage.

Stars in the stable phase of hydrogen burning lie along the Main Sequence according to their mass. After a star uses up all the hydrogen in its core, it leaves the main sequence and moves towards the red giant branch. The most massive stars may also become red super giants, in the upper right corner of the diagram. The lower left corner is reserved for the white dwarfs.

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Web-SitesStellar Encounters:http://amazing-space.stsci.edu/resources/explorations/light/stellarEncounters-frames.html

Pearson Tutorialshttp://media.pearsoncmg.com/bc/bc_bennett_cosmicpers_2/medialib/tutorials/index.html

Astrobiology an Integrated Approachhttp://astrobio.terc.edu/samples/chpt2_act3.html

Interpretting the H-R Diagramhttp://www.smv.org/jims/l6a.htm

Hertzsprung-Russell Diagram Interactive Labhttp://aspire.cosmic-ray.org/labs/star_life/hr_diagram.html

5. Credits and Sources

Magnitude and the HR Diagram: http://www.westosha.k12.wi.us/Departments/Science/Pollard/EarthScience/H-R%20diagram%20and%20magnitude%20activity.htmCornell

Astronomy; Hertzsprung – Russell diagram; http://www.astro.cornell.edu/academics/courses/astro201/hr_diagram.htm

Montana State University Solar Physics; Life Line of Stars; http://solar.physics.montana.edu/tslater/plunger/hr_diag2.htm

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Background: There are essentially two ways to describe the brightness of stars; apparent magnitude and absolute magnitude. Apparent magnitude is the brightness of a star as seen from Earth; absolute magnitude is how bright a star would be if it were 32.6 light years away (10 parsecs). For historical reasons, astronomers still call the brightest stars magnitude 1 stars, the next brightest 2, then 3, 4, 5, and on down to the dimmest naked-eye stars, magnitude 6 stars. Using this sort of scale, we can describe the brightness of stars in real numbers. Really bright objects, like the full moon have a negative magnitude, because they are brighter than a magnitude 1 star. In other words, the brighter an object is, the smaller its magnitude.

Object apparent magnitude Sun -26.5 Full Moon -12.5 Venus -4.1 Sirius -1.4 North Star 2.0 limit of naked eye 6 Pluto 15 limit of HST 28

Astronomers also classify stars by their type of spectrum. Stars are easily classified by the amount of hydrogen that they contain as revealed to us by the intensity of their spectrum. Stars with the most hydrogen are called “A” stars, the remaining letters designate varying amounts of different chemical stellar compositions.

If we consider the temperature of stars, as revealed to us by their color, we find that the O stars, are hottest, followed by B stars, A stars, F, G, K, and M stars. A star whose temperature is exactly between an O star and a B star is considered an O5 star. This layout of temperature is given from hottest to coolest as: O0, O1, O2, O3, O4, O5, O6, O7, O8, O9, B0, B2, B3, B4, B5, B6...... Our Sun is G2 star, which means that it is a rather intermediate temperature star.

Procedure:

1. Plot the top 20 Near Stars on the HR Diagram. 2. Plot the top 20 Bright Stars on the HR Diagram using a different color.

IF ANY OF THE STARS APPEAR IN THE SAME LOCATION ON THE GRAPH, DRAW A DOT IN THE COLOR FOR NEAR STARS, THEN DRAW A CIRCLE AROUND IT IN THE SECOND COLOR FOR BRIGHT STARS.

3. Complete the questions that follow.

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The brighter an object is, the smaller

its magnitude.

Name ___________________Date _______________Hour ______

Magnitude and the H-R Diagram

Star Name Spectral Class

Absolute Magnitude

Star NameSpectral

ClassAbsolute

Magnitude

1. Sirius A A1 +1.4 19. Aldebaran A K5 -0.2

2. Sirius B B8 +11.5 20. Aldebaran B M2 +12

3. Canopus F0 -3.1 21. Crucis A B1 -4.0

4. Centaurus A G2 +4.4 22. Crucis B B3 -3.5

5. Centaurus B K5 +5.8 23. Antares A M1 -4.5

6. Arcturus  K2 -0.3 24. Antares B B4 -0.3

7. Vega A0 +0.5 25. Spica B1 -3.6

8. Capella A G0 -0.7 26. Pollux K0 +.08

9. Capella B M0 +9.5 27. Fomalhaut A A3 +2.0

10. Capella C M5 +13.0 28. Fomalhaut B K4 +7.3

11. Rigel A B8 -6.8 29. Deneb A2 -6.9

12. Rigel B B9 -0.4 30. Beta Crucis B0 -4.6

13. Procyon A F5 +2.7 31. Regulus B7 -0.7

14. Procyon B F0 +13.0 32. Adhara B2 -5.0

15. Achernar  B5 -1.0 33. Castor A A1 +2.1

16. Beta Centari B1 -4.1 34. Castor B A5 +2.9

17. Betelgeuse M2 -5.5 35. Castor C K6 +8.8

18. Altair A7 +2.2 36. Shaula  B1 -3.3

- - - 37. Bellatrix  B2 -4.2

Table 2 Near Stars(Stars which are close to the Earth)

Star NameSpectral

ClassAbsolute

MagnitudeStar Name

Spectral Class

Absolute Magnitude

1. Sun G2 +4.8 16. Procyon A F5 +2.7

2. Centari A G2 +4.4 17. Procyon B F0 +13.0

3. Centari B K5 +5.8 18. Struve 2398 M4 +11.1

4. Centari C M5 +15.0 19. Struve 23948 M5 +11.9

5. Lalande 21185 M2 +10.5 20. Groom 34 A M1 +10.5

6. Sirius A A1 +1.4 21. Groom 34 B M6 +13.2

7. Sirius B B8 +11.5 22. Lacaille 9352 M2 +9.6

8. Ross 154 M4 +13.3 23. Tau Ceti  G8 +5.7

9. Ross 248 M5 +14.7 24. BD +5 1668 M4 +11.9

10 Epsilon Eridani K2 +6.1 25. Lacaille 8760 M0 +8.7

11. Luyten M5 +14.7 26. Kapteyn's Star M0 +8.7

12. Ross 128 M5 +13.8 27. Krueger 60 A M3 +11.8

13. 61 Cygnus A K5 +7.5 28. Krueger 60 B M4 +13.4

15 61 Cygnus B K7 +8.3 29. Ross 614 M5 +13.1

15. Epsilon Indi K5 +7.0 30. BD -12 4523 M4 +12.0

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Table 1 Bright Stars(Stars which appear very bright from

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Analysis Questions:1. Our star, the Sun, is a G2 spectral class star with an absolute magnitude of

4.8 . How do its temperature and absolute magnitude compare to the other Near Stars on the diagram?__________________________________________________________________________________________________________________________

2. How do the Sun’s temperature and absolute magnitude compare to the other Bright Stars on the diagram?__________________________________________________________________________________________________________________________

3. Which spectral class is most common? _________________________

4. Which spectral class is the least common? _________________________

5. In general, does there seem to be a relationship between the temperature of a star and its brightness? Explain your answer….__________________________________________________________________________________________________________________________

6. Most of the stars seem to be along a line from the upper left corner to the lower right corner of the HR Diagram. Stars which fall into this category of stars are called main sequence stars . Does our Sun fit into this category? If so, in what location compared to the other main sequence stars?__________________________________________________________________________________________________________________________

7. Consider the stars in the upper right hand corner and the lower left hand corner. What type of stars are the bright, cool stars? What type of stars are the hot, dim stars? __________________________________________________________________________________________________________________________

8. Stars which are "on the main sequence" are generally very stable stars which are combining their hydrogen atoms into larger helium atoms (this reaction is called fusion and gives off energy). Where in the star do you think that this fusion reaction is most likely occurring? __________________________________________________________________________________________________________________________

9. Why do you think black holes do not appear on the HR Diagram?__________________________________________________________________________________________________________________________

10.What do you think might occur when a star depletes its supply of hydrogen? What will it use as fuel instead?_____________________________________________________________

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Classified Stars

Background Information

At the beginning of the 1900's, two astronomers, Danish Ejnar Hertzsprung and American Henry Russell, determined a pattern in the life of stars. They arranged stars on a chart according to their color and brightness. The most amazing thing is that they did not even know one other, and did their experiments completely independent of each other. Therefore, this chart is called the Hertzsprung-Russell (HR) diagram.

The diagram shows you how the sizes and colors of stars change with brightness and temperatures. The largest stars in the galaxy are found near the top; the smallest stars near the bottom. The bluest stars appear on the left, and the reddest stars on the right. The stars that appear near the top of the chart are the brightest and those toward the bottom are the faintest. The hottest stars are plotted at the far left and the coolest stars appear at the far right. Of course, this diagram does not show how the stars would appear to you while gazing into the night sky. The absolute magnitude and luminosity are used for that.  They give you the relative brightness based on all of the stars being the same distance away from the earth.

On this diagram, you do not see all of the individual stars. Since there are so many stars, only a few were actually scattered around and along each of the areas that you see. The four major star types are white dwarf, main sequence, giant, and supergiant, but there are many groups of stars that fit within each type. Each star is also classified by its spectral class. Each star has a unique composition, and this can be seen in its spectral class. The different spectral classes are OBAFGKM, where O are the bright, hot, blue stars, and M are the dim, cold, red stars.

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Procedure

1. Below is an HR-diagram.

 

2. Answer the following questions: a. What color would an M class star be? b. If a star has a temperature of 3,700 Kelvin and a luminosity of

0.1, what is its spectral class? c. If a star has an absolute magnitude of +5, what is (are) the

possible spectral class (classes)? d. What range of absolute magnitudes would you expect a

supergiant star to be? e. If you were to look at the sky at night, what type of star would

you most likely see? To help answer this, draw a line through the pattern of the main sequence stars. Draw a circle around all of the white dwarf, supergiant and giant stars. When completed, you should have one wavy line and three ellipses (ovals).

f. Our sun has a spectral class of G. What are the temperature, luminosity and magnitude ranges for our sun?

g. If available, shade in the main sequence stars with the appropriate color. Otherwise, put an R (red), O (orange), Y (yellow), G (green), and B (blue) along the line you drew for question e.

http://www.nasaexplores.com/show_912_student_st.php?id=050201160919

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Title: Life Cycles of Stars

Content Expectations: E5.2e Explain how the Hertzsprung-Russell (H-R) diagram can be used to deduce other parameters (distance).

E5.2f Explain how you can infer the temperature, life span, and mass of a star from its color. Use the H-R diagram to explain the life cycle of stars.

E5.2h Compare the evolution paths of low, moderate and high mass stars using the H-R diagram.

1. Question to be investigated: How do stars change throughout their life and change position on the Hertzsprung - Russell diagram?

2. Activity Description:1) Place one wooden bead inside each of 12 red balloons, and 12 yellow

balloons.2) Place a marble inside each of 4 white balloons.3) Place one ball bearing inside each of 2 blue balloons.4) Begin by introducing the ways in which stars come into being and produce

energy.5) Ask if all stars are the same, and ask students to help make a list of things

that might vary between stars: mass, color, heat. Make sure to include "life cycle."

6) Ask if students know how black holes form (answer: they form when certain kinds of stars die). Ask how often students think that black holes form, and if they believe our Sun will form a black hole. Don't forget to ask them to explain the reasons behind their ideas! This information will be helpful to you in determining how best to structure your questions through the rest of the lesson.

7) State that the class will do an activity that illustrates how all of these differences in stars' characteristics are related, and will show when, and how often, black holes form.

8) Pass out balloons, distributing different colors, one balloon per student. You should have significantly more red and yellow balloons than blue and white, roughly 80% red and yellow, 15% white, and 5% blue. Explain that the property that causes the main differences between stars is mass. As you pass out balloons, tell students the approximate mass of their star. (Refer to the Life Cycle of Stars Information Chart)

9) Ask students which balloons they think represent the hottest stars. Point out that actually red stars are the coolest, and blue stars are the hottest. Ask what color our Sun is (yellow).

10)Ask which color star students believe will live longest, and why. 11)Guide students through the following series of steps. For each age, tell

students what to do for their color of balloon. To help students follow the progression, you might write different stages on a board or overhead as you move on, and note important events. Also, ask students to make predictions as you work.

12)As stars “die” have the students that represented those stars summarize their life cycle.

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13)After all stars are "dead," review the sequence you have just covered. Point out which stars died first, which last.

14)Point out the fate of the yellow stars like our Sun. Note that they live quite a long time and don't become either black holes or neutron stars.

15)Point out that black holes are the rarest type of stars in our group.16)Have students complete the interactive lab at

http://aspire.cosmic-ray.org/labs/star_life/starlife_main.html17)Finally, have students pretend they are a type of star (blue supergiant , sun-

like star, red dwarf) and write an autobiography. They should describe their life cycle from formation to death with illustrations and trace their path on the HR diagram.

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Life Cycle of Stars Information Chart

  Red Balloons Yellow Balloons White Balloons Blue Balloons

Mass of Star →Age of Star↓

0.4 Solar Mass (2/5 the mass of our Sun): Red stars

1 Solar Mass (the mass of our Sun): Yellow Stars

3 Solar Masses (3 times the mass of our Sun): White Stars

9 Solar Masses (9 times the mass of our Sun): Blue Stars

(start) Blow up the star to about 3" diameter

Blow up the star to about 3" diameter

Blow up the star to about 3" diameter

Blow up the star to about 3" diameter

5 Million Years

Wait. Do not change diameter of balloon.

Wait. Do not change diameter of balloon.

Wait. Do not change diameter of balloon.

Blow slightly more air into balloon.

10 Million Years

Wait Wait Blow up a little more Blow up star as fast and as much as you can. When star is fully inflated, teacher pops balloon--a supernova.

500 Million Years

Wait Wait (note that planets are forming)

Continue to slowly inflate star. As it gets bigger, star cools, so color it yellow and red (make squiggles on surface with markers).

This popped star has become a black hole; all of the super nova remnants can be thrown out into space.

1 Billion Years

Wait Blow up a little bit. Quickly blow up star until fully inflated; pop balloon. Make sure to catch marble

Still black hole!

8 Billion Years

Wait. Blow up more. The star is getting cooler, so color it red with marker. It is now a supergiant.

This star has exploded. Holding on to neutron star (marble), throw supernova remnants into space. Place remnants in a recycle bin to demonstrate stellar gas is recycled into new star matter.

Still black hole

10 Billion Years

Wait Blow up a little more. Outer envelope dissolves, so cut up balloon. The inside bead becomes a white dwarf, and the bits of balloon represent the planetary nebula.

Neutron star Still black hole

50 billion years

Blow up a little more Move "planetary nebula" farther away. Place remnants in a recycle bin.

Neutron star Still black hole

200 billion years

Deflate; star has shrunk and died. Color black. Wooden bead inside is a white dwarf.

Nebula is gone. Discuss that the white wooden bead turns black to show that it has burned out.

Neutron star Still black hole

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4. Teacher NotesAdditional Information:

Where are Stars Born?Astronomers believe that molecular clouds, dense clouds of gas located primarily in the spiral arms of galaxies are the birthplace of stars. Dense regions in the clouds collapse and form 'protostars' so a star begins its life as a large and comparatively cool mass of gas. The contraction of this gas and the subsequent rise of temperature continue until the interior temperature of the star reaches a value of about 1,000,000°C (about 1,800,000°F).

At this point a nuclear reaction takes place in which the nuclei of hydrogen atoms combine with heavy hydrogen deuterons (nuclei of so-called heavy hydrogen atoms) to form the nucleus of the inert gas helium. The latter reaction liberates large amounts of nuclear energy, and the further contraction of the star is halted. Once the star has started nuclear fusion, it becomes a 'main sequence' star.

Main Sequence StarsMain sequence stars are stars, like our Sun, that burn hydrogen to helium in their cores. For a given chemical composition and stellar age, a stars' luminosity, the total energy radiated by the star per unit time, depends only on its mass. Stars that are ten times more massive than the Sun are over a thousand times more luminous than the Sun. However, we should not be too embarrassed by the Sun's low luminosity: it is ten times brighter than a star half its mass. The more massive a main sequence star, the brighter and bluer it is.

For example, Sirius – the dog star, located to the lower left of the constellation Orion, is more massive than the Sun, and is noticeably bluer. On the other hand, Alpha Centauri, our nearest neighbour, is less massive than the Sun, and is thus redder and less luminous.

Since stars have a limited supply of hydrogen in their cores, they have a limited lifetime as main sequence stars. This lifetime is proportional to f M / L, where f is the fraction of the total mass of the star, M, available for nuclear fusion in the core and L is the average luminosity of the star during its main sequence lifetime. Because of the strong dependence of luminosity on mass, stellar lifetimes depend sensitively on mass. Thus, it is fortunate that our Sun is not more massive than it is since high mass stars rapidly exhaust their core hydrogen supply.

Once a star exhausts its core hydrogen supply, the star becomes redder, larger, and more luminous: it becomes a red giant star. This relationship between mass and lifetime enables astronomers to put a lower limit on the age of the universe.

Death of an "Ordinary" Star

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After a low mass star like the Sun exhausts the supply of hydrogen in its core, there is no longer any source of heat to support the core against gravity. The core of the star collapse under gravity's pull until it reaches a high enough density to start converting helium to carbon. Meanwhile, the stars' outer envelope expands and the star evolves into a red giant. When the Sun becomes a red giant, its atmosphere will envelope the Earth and our planet will be consumed in a fiery death. The Sun will eventually evolve into a red supergiant as it exhausts the helium in its core. At this stage, it will have an outer envelope extending out towards Jupiter. During this brief phase of its

existence, which last only a few tens of thousands of years, the Sun will lose mass in a powerful wind.

Eventually, the Sun will lose all of the mass in its envelope and leave behind a hot core of carbon imbedded in a nebula of expelled gas. Radiation from this hot core will ionise the nebula produces a striking 'planetary nebula', much like the nebulas seen around the remnants of other stars. The carbon core will eventually cool and become a white dwarf, the dense dim remnant of a once bright star. The final fate of low-mass dwarfs is unknown, except that they cease to radiate appreciably. Most likely they become burned-out cinders, or black dwarfs.

Death of a Massive StarMassive stars burn brighter and perish more dramatically than most. When a star ten times more massive then Sun exhaust the helium in the core, the nuclear fusion cycle continues. The carbon core contracts further and reaches high enough temperature to burn carbon to oxygen, neon, silicon, sulphur and finally to iron.

Iron is the most stable form of nuclear matter and there is no energy to be gained by converting it to any heavier element. Without any source of heat to balance the gravity, the iron core collapses until it reaches nuclear densities. This high density core resists further collapse causing the in-falling matter to 'bounce' off the core.

This sudden core bounce (which includes the release of energetic neutrinos from the core) produces a supernova explosion. For one brilliant month, a single star burns brighter than a whole galaxy of a billion stars. Supernova explosions inject carbon, oxygen, silicon and other heavy elements up to iron into interstellar space. They are also the site where most of the elements heavier than iron are produced.

Future generations of stars formed from this heavy element enriched gas will therefore start life with a richer supply of heavier elements than the earlier generations of stars. Without supernova, the fiery death of massive stars, there would be no carbon, oxygen or other elements that make life possible.

The fate of the hot neutron core depends upon the mass of the progenitor star. If the progenitor mass is around ten times the mass of the Sun, the neutron star core will cool to form a neutron star. Neutron stars are potentially detectable as ‘pulsars’, powerful beacons of radio emission. A limit exists for the size of neutron stars, however, beyond which such stars are gravitationally bound to keep contracting until they become a black hole, from which light radiation cannot escape.

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If the progenitor mass is larger, then the resultant core is so heavy that not even nuclear forces can resist the pull of gravity and the core collapses to form a black hole.

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Safety ConcernsEye protection should be worn during the investigation. Flying debris from the balloons could cause eye injuries.

Web-SitesLife Cycles of Starshttp://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/LC_title.html

Chandra X-Ray Observatory: Stellar Evolutionhttp://chandra.harvard.edu/xray_sources/stellar_evolution.html

Cosmic Evolution an Interdisciplinary Approachhttp://www.tufts.edu/as/wright_center/cosmic_evolution/docs/fr_1/fr_1_stel.html

Life Cycle of a Star: Interactive Labhttp://aspire.cosmic-ray.org/labs/star_life/starlife_main.html

SimulationsStellar Evolution on the H-R Diagramhttp://instruct1.cit.cornell.edu/courses/astro101/java/evolve/evolve.htm

6. Graphs, Charts, Illustrations:

7. Credits and SourcesAlder Planetarium; Milky Way Galaxy Gallery; http://www.adlerplanetarium.org/pub/MWGuide.pdf

Astronomy Today; Stellar Evolution – the lives of stars; http://www.astronomytoday.com/cosmology/evol.html

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