Energy source of stars Research and script: Dr. Arvind Dube Translation: BimanBasu (Signature tune...fades out) (Theme song fades out) (Opening music..fades out) Atom: I m an atom, the main constituent of all matter. You must have heard about atomic energy. But do you know how the Sun produces the stupendous amounts of energy that has been supporting life on Earth for ages? How the countless stars that we see in the night sky shine? Do you know, some of the stars are so far away that light from them takes thousands or millions of years to reach us? After all, what is the source of energy in them? Don t know? It is me, I mean the atom. But how? How has the Sun been shining for billions of years? How do stars produce temperatures of millions of degrees? Today we re going to tell you all about this. Place: Romi s room Scene 1 (Opening song.fades out) (Romi has been in the bathroom for some time. His mother has been callinghim. Ensure appropriate difference in the two voices) Mother: How long will you take, Romi? Today is Sunday, but that doesn t mean you should spend the whole day in bathroom! Romi: I have yet to take bath, mother. There is no power and the water is very cold. Mother: So, would you wait till the power comes? Romi: The water is very cold, mother. Mother: Oh, just finish with a few mugs of water, you won t feel cold. Finish quickly; others are waiting for their turn. Romi: Ok, mother. Romi(shivering): Mother, get me the towel. Mother: Oh, you re shivering! Was the water very cold? (Sound of Romi pouring water. When he comes out he is shivering. Use appropriate effect.)
Romi: Yes, very cold! Mother: Go upstairs and sit in the Sun. You ll feel better. Go now. Scene 2 Place: Terrace of Romi s house Time: Morning (Opening music..fades out) (Romi sitting in the Sun on the terrace. Romi s mothercomes calling him.) Mother: Romi, Romi how long will you sit in the Sun? Haven t you warmed upby now? Romi: No mother, I m just enjoying the Sun. Mother: Don t sit bare-bodied. Take this and put it on. Romi: Mother, it s such a pleasure sitting in the Sun in winter! How does the Sun produce so much heat? Mother: Must be some furnace within the Sun. I don t know. Romi: What kind of furnace that has been burning for billions of years and the heat from which reach us from a distance of millions of kilometres? Mother: I don t know; must be something. Why don t you ask your Papa? He teaches in the school. He ll be able to tell you. (Entry of father) Father (approaching voice): You mother is right, Romi. Only a physicist like me can answer your question. Mother: Oh, you re already up. Usually you get up late on Sundays! Father: Yes, I woke up, but couldn t find any arrangement for tea downstairs. So came up to the terrace to look for you. Mother (smiling): Did you really come looking for me or looking for tea? (after a brief pause) Ok, you enjoy the Sun and I ll bring your tea. (A female voice): Mother, also bring my tea here. Mother: Oh, Divya, you also got up so early on a Sunday? I think the Sun will rise in the west tomorrow! (leaves smiling)
Divya: Good morning, Papa. Father: Very good morning Divya. Come, sit here. Well, Romi, you were talking about some furnace in the Sun! Romi: Oh, I was just asking how the Sun produces so much heat. Mother told me that there must be some furnace producing the heat. Nonsense! Father: Your mother is right. Romi: Is there really a furnace in the Sun? Father: Yes, but not of the kind you re familiar with that burns coal or other common fuel. Divya: Papa, if it burned coal then it must have exhausted its fuel long ago and the Sun must have cooled down by now. Father: You re right, Divya. If the Sun were built of carbon and oxygen then it would have burnt off coal equivalent to its mass in just 1,500 years! Romi: But the Sun has been radiating heat for much longer! Father: Yes, and it s because the furnace in the Sun is not a coal-burning furnace but a nuclear furnace. Romi: What? A nuclear reactor? Divya: Yes, Romi. The Sun is a huge nuclear fusion reactor. Romi: But you told us the other day that building a fusion reactor is not an easy matter. Father: Not an easy matter on Earth, but conditions are different in stars. Divya: In stars, the temperature at the core is of the order of several million degrees Celsius and matter can only exist as plasma. Father: You re absolutely right. In the 19th century, physicists Lord Kelvin and Hermann Helmholtz said that in stars energy is generated by gravitational contraction. Romi: But how can energy be generated by contraction of a star? Father: These two scientists said that the outer layers of a star press down upon the inner layers and the core of a star. Divya: And the core of most stars is made up of hydrogen and helium. Father: Yes, Divya, the temperature of these gases keep rising steadily because of increasing pressure. Romi: And a time comes when the gases become so hot that the star starts radiating like the Sun.
Father: Yes, the two scientists had thought so, but.. Romi: Why but, Papa? Father: When other scientists looked deeper they found that simple gravitational contraction cannot be the source of the Sun s enormous heat. Romi: Why, Papa? Father: Because if gravitational contraction were the only source then to produces the amount of energy it does today, then 2.5 billion years agothe diameter of the Sun should have been as large as the Earth s orbit. Romi: So? Divya: So it s obvious that the energy in the Sun is produced by a different process, which is nuclear fusion. All other theories are sheer nonsense. Father: No, Divya, it s not so. The theory of gravitational contraction also is important. Divya: How, Papa? Father: Before a star is born its core isn t hot enough to start fusion of hydrogen. Romi: Then what happens, Papa? Father: At this stage, as a result of the shrinking of the star due to gravitational contraction, the temperature of its core rises steadily. Divya: Papa, isn t it known as a proto-star at this stage? Father: Very intelligent, Divya. You re absolutely right. It s indeed called a proto-star. At this stage the temperature of the star is not very high. Romi: Proto-star? Father: Yes Romi. As the temperature of the core rises the pressure of the gas inside increases steadily to slow down the rate of contraction due to gravity. Romi: Then what happens? Father: In about a hundred thousand or a billion years the temperature of the core rises so much that fusion of hydrogen in the core begins. Divya: And the star begins to produce enormous amounts of energy. Father: Yes Divya, it no more remains a proto-star; it becomes a real star and starts radiating heat and light. Divya: Papa, now the pressure of gas in the core must be increasing rapidly.
Father: Yes Divya, now the pressure of gases in the core becomes large enough to stop the shrinking of the star. Romi: That means now the entire energy in the star is produced by nuclear fusion. Father: Yes, you re right, Romi. Divya: Our Sun is also at this stage now. Father: Yes, Divya. But over a period of billions of years, the hydrogen in the core of the star gets depleted slowly. Romi: Then it must be the end of the star. Father: No, Romi. In later stages, heavier nuclei such as helium start fusing and producing energy, but. Divya: But what, Papa? Father: At this stage the temperature of the core is not high enough for fusion of nuclei heavier than hydrogen. Romi: Then? Father: Romi, after hydrogen in the core gets depleted, the pressure of gas in the core decreases. As a result, the star starts to shrink again and the temperature at the core rises again. Divya: And the temperature rises to such a level that fusion of nuclei heavier than hydrogen becomes possible. Father: You re right Divya. A star reaches this stage after shrinking for a few billion years. Once again the gas pressure inside the core rises so much that the star stops shrinking. Divya: And once again the entire energy of the star comes from the fusion reaction going on in the core. Father: Exactly! Romi: This means that when the star shrinks the temperature at the core rises and when the temperature becomes hot enough to start the fusion reaction fusion starts and the star stops shrinking. Divya: Yes, Romi, when the fuel gets depleted again, the core gas pressure falls and the star begins to shrink again. Romi: Which means that first shrinking, then fusion, again shrinking followed by fusion, and so on till the entire content of the star undergoes fusion. Father: No, Romi. It s not like that.
Romi: Then, what? Divya: Have you seen Mendeleev s Periodic Table? Romi: Of course, I have. Divya: All elements up to atomic mass of 60 in the Periodic Table; that is, iron, are such that their fusion leads to production of energy. Romi: And beyond iron? Father: Roni, the fusion of nuclei heavier than iron does not produce, but absorbs energy. Divya: And fusion of heavier nuclei requires much higher temperatures. Romi: Is that so? (Mother enters with tea. Sound of placing cups on the table.) Mother: I thought you must have run out of energy after a long discussion. That s why I have brought tea for you. Divya: Oh Mother, you re great! Father: No, she s the greatest! Mother: So, you ve also. Place: Romi s room Scene 3 (Opening music..fades out) Romi: Papa, have you seen today s newspaper? Father: No, Romi. Is there anything special? (Romi, Divya and their father talking) Romi: Yes Papa. It says, the science centre of our city will screen a 3D science film today. Can we go there to watch he film? Father: Sure! We ll certainly go. A male voice: I m the atom. Divya and Romi have gone to watch the 3D film on the Sun at the science centre. The Sun is actually the nearest star from Earth and is responsible for sustaining your life on Earth. I would say, if life is flourishing on Earth, it is only
because of the energy received from the Sun. Do you know what is behind the Sun s energy? Of course, it s me my nucleus, my protons.sun gets 92 per cent of its energy from fusion of protons. How? If I tell you everything then nothing else would remain for you to see in the film. But first we have to find Romi and Divya. There s such a big crowd at the science centre today. How do we find them? Oh, they are there! Place: Inside the science centre Scene 4 (Opening music..fades out) Romi: There is a big crowd here. Divya: Must be for the film on Sun. (Roni and Divya along with their father have come to the science centre to watch the film. There are many others there. Appropriate sound effects.) Father: Where is Romi? He could have got the tickets. The queue is quite long. Romi: Here I am. I bought the tickets when you were looking at the exhibits. Divya: Good. It s time now. Now let s go inside. Scene 5 (Opening music.fades out) Place: Inside the science centre auditorium. (Romi, Divya and their father are in the auditorium. Appropriate sound effects) Divya: Hey Romi, come here. Papa, you also come here. There are three empty seats here. Romi: Papa, isn t our Sun a star? Father: Yes, the star nearest to us. (The three occupy seats. Appropriate sound effects) Divya: Romi, now stop chatting. The lights are off. The show is starting. (Appropriate sound effects)
Voice of male narrator in the film: The Sun the star of our solar system is at a distance of 149.6 million kilometres from us. It takes light from the Sun 8 minutes 19 seconds to cover the distance to reach us. Voice of female narrator: It is said that the Sun was born out of a solar cloud or nebula some 4.6 billion years ago. A major part of the cloud almost 99.86 per cent collected in the middle from which the Sun was born. Romi(in low voice): Divyadi, that means the Sun makes up most of our universe! Divya: I ll tell you later. Now be quiet and watch the film. Female narrator: The planets were made out of the less than one per cent of the remaining matter in the cloud. Male narrator: It is the energy received from the Sun that sustains life on Earth. Female narrator: Every second the Sun radiates almost 385 yottawatts of power. Romi(in low voice): Papa, what s this yottawatt? Father: Yotta means 10 to the power 24. That is, 24 zeros after 1. Romi: So much energy per second? Fantastic! Divya(angrily): Romi, just shut up and watch the film. Male narrator: To produce this much energy the Sun uses up almost 6 billion metric tons of hydrogen every second. Female narrator: Almost three-fourths of the Sun is made up of hydrogen, about one-fourth helium and about one-and-a-half per cent is made up of heavier elements. Male narrator: Even after using up 6 billion metric tons of hydrogen every second, the Sun will continue to shine and give us heat and light for another 5 billion years. Female narrator: This huge amount of energy in the Sun is produced mainly from protonproton fusion reaction. Male narrator: A small amount of energy in the Sun is produced by another type of fusion reaction known as the CNO fusion cycle. Scene 6 (Opening music.fades out) Place: Romi s room (Romi, Divya and their father sitting together)
Father: How did you like the film? Divya: I really enjoyed it. Learnt many new things. Romi: Enjoyed, my foot! Much of the film went over my head! Divya: You need brain to understand it that you don t have! Father: Divya, don t tease him. Romi, tell me what you didn t understand. Romi: There are so many.i didn t understand proton-proton fusion, carbon fusion, neon fusion, oxygen fusion, and of course, what they call the CNO cycle. Understanding simple fusion was so difficult for me; leave alone so many types of fusion.it s more of confusion than fusion! Divya: Where is the confusion? In the stars all energy comes from fusion. You just remember this and don t bother about different types of fusion. Father: Romi, let s go to my friend Professor Puri who can tell us a lot about these things. Romi: Papa, let s go to him some day. Father: I ll speak to him tomorrow and fix an appointment. Divya: Why tomorrow? Why not call him today? Scene 7 (Opening music...fades out) Place: Prof.Puri s office Father: Hello Professor Puri. Puri: Oh Sagar, you ve come! Well in time. Where are the children? Romi and Divya(together): We are here, Puri Uncle. Divya: I m Divya and he s my brother. Romi: Romi. Puri: Come.Come here. Puri: Now, Sagar tell me what you want to know. (Romi, Divya and their father in Prof.Puri s office) (Sound effect of everyone sitting down)
Father: My friend, we had been to the science centre the other day. They were showing a film on the Sun. Since then the children have been asking me about how the Sun and the stars produce energy and shine. Puri: Good. Father: I had told them about the energy produced due to shrinking of stars and about hydrogen fusion, but they want to know more about it from you. Divya: Puri Uncle, Papa had told us that before a star is born, in initial stages; that is, in the proto-star stage, the gravitational pressure of the outer layers heats up the core to such high temperatures that fusion of hydrogen becomes possible. Puri: Divya, what your Papa had told you is absolutely correct. Long ago, scientists Jean Perrin and Arthur Eddington had postulated the fusion of four atoms of hydrogen into helium to be the source of energy in the Sun. Romi: Why? Puri: Because these two gases make up the major part of the Sun. Romi: Ok. Puri: After the development of quantum mechanics and nuclear physics, scientists proposed that the fusion of four protons into an alpha particle is the source of the stupendous energy of the Sun. Romi: Alpha particle? Puri: Yes, it is the nucleus of a helium atom. According to the two scientists, the temperature at the core of the Sun would not be high enough to cause fusion of four protons to form a helium nucleus. Romi(Surprised): Is that so? Father: Puri, wasn t it a scientist named Hans Bethe who gave the correct explanation? Puri: Yes, Sagar, it was indeed Hans Bethe who came out with the correct reaction that answered all the questions about energy production in the Sun. He was awarded the Nobel Prize for this work. Atom: I m an atom. It was not easy to understand my role in the stars and the Sun. All the reactions occur under extreme conditions that cannot be created here on Earth. We can only imagine them. The higher the temperature of the core of a star the easier it would be to fuse nuclei of heavier elements. Let me tell you something. When two nuclei fuse to produce a third nucleus, the mass of the third nucleus is always slightly less than the total mass of the two fusing nuclei. This is because a small amount of
mass is entirely converted into a huge amount of energy. According to Einstein s theory, E=mc 2,where E is the amount of energy produced by conversion of m amount of mass and c is the velocity of light. Scene 8 (Opening music fades out) Place: Prof.Puri s laboratory (Father, Romi, Divya and Prof.Puri in the lab) Divya: Romi, look here. It s a chart showing the proton-proton cycle-1. Romi: Yes, Divyadi, here is proton-proton cycle-2 and in that chart is the proton-proton cycle-3. Divya: And in this chart we can see the CNO cycle. Papa, what are these? Father: Divya, these charts show the different nuclear reaction cycles that are responsible for producing energy in stars. Puri: One of these is the proton-proton energy cycle. I hope you know about protons. Romi: The particles found in the nucleus of atoms? Puri: But here we are talking about the hydrogen atom from which the electron has been removed. Romi: But why? Puri: What do you find in a hydrogen atom? Romi: A proton and an electron. Puri: If we remove the electron only the proton remains. Romi: You mean hydrogen ion? Puri: You may say that. In stars as big as or smaller than our Sun, the main source of energy is the proton-proton cycle. Father: And in stars a hundred times bigger than our Sun, energy is produced mainly through the CNO cycle. Divya: But in these charts three types of proton-proton cycles are shown.
Puri: You re right, Divya. In general, in stars like our Sun all the three cycles work to generate energy. But in any star, only one of the cycles predominates in energy production. Father: How s that, Puri? Puri: Sagar, in stars that have a core temperature of 10 to 15 million degrees, the first protonproton cycle; that is, p-p cycle-1, is the main reaction generating energy. Divya: How s that, Uncle? Puri: At this temperature two protons; that is, two hydrogen nuclei fuse to form a deuterium nucleus. Do you know what deuterium is? Diviya: Yes, it s an isotope of hydrogen containing one proton and one neutron in the nucleus. Puri: The mass of a neutron is slightly less than that of a proton. So the mass of as deuterium nucleus is slightly less than the mass of two protons by the fusion of which it is produced. Romi: So? Puri: That is why this reaction also releases a neutrino and a positron. Do you know what positron is? Romi: Yes. It has the same mass as the electron but a positive charge. Puri: Right, this positron instantly reacts with an electron to destroy each other and generates energy. This reaction takes almost one billion years to happen. Divya: One billion years? Puri: Yes, but immediately after it is formed deuterium nucleus combines with a proton to form a helium-3 nucleus. Romi: Helium-3? Puri: Yes, it is an isotope of helium containing two protons and a neutron in the nucleus. Divya: Quite interesting! Then what happens? Puri: After about one million years,two helium-3 nuclei fuse together to form a helium-4 nucleus and release two protons. All the energy in a star is produced in this stage. Divya: That means hydrogen nuclei ultimately turn into helium nuclei. Puri: Yes. Romi, did you understand? Romi: Yes. Two hydrogen nuclei take one billion years to combine and form a deuterium nucleus, which combines with a hydrogen nucleus within a second to form helium-3.
Two helium-3 nuclei take about one million years to fuse to forma helium-4 nucleus and release two protons, which releases enormous amounts of energy. Puri: Very good. Energy is produced in our Sun by this series of reactions. Father: You mean through proton-proton cycle-1? Puri: That s it, Sagar. Divya: What about proton-proton cycles-2 and 3? Puri: Divya, in stars with core temperatures of 14 to 23 million degrees the main energyproducing reaction is proton-proton cycle-2. Father: Puri, I think in this reaction elements like helium and beryllium also fuse together after forming. Puri: True, but ultimately nuclei of helium-4 are formed. Divya: And what about proton-proton cycle-3, Prof.Puri? Puri: Divya, p-p cycle-3 is the main source of energy in stars with core temperature higher than 23 million degrees. In this cycle heavier elements like lithium, beryllium and boron also fuse. Father: But in the end the same helium-4 is produced. Isn t it, Puri? Puri: You re right, Sagar. All proton-proton chain reactions ultimately result in formation of helium-4 and most of the energy is generated in the last step in each case. Romi: That means energy is generated during conversion of hydrogen into helium. Puri: That s right, Romi. Divya: What s this CNO cycle, Puri Uncle? Puri: Divya: C stands for carbon, N stands for nitrogen, and O stands for. Romi: Oxygen. Puri: Absolutely right, Romi. In this reaction too, hydrogen nuclei fuse to form helium-4. This reaction is the main source of energy in stars hundred times or more massive than the Sun. Father: This chain of reactions begins with a carbon capturing a proton. In between, nitrogen and oxygen are formed, but ultimately again carbon is formed and the cycle begins again. Divya: You mean to say, energy is not produced through the CNO cycle in our Sun? Puri: No, Divya. Only a very small fraction of the total energy of the Sun comes from the CNO cycle. Divya: Puri Uncle, there must be a time when the entire fuel reserve of hydrogen and helium in a star gets exhausted.
Father: Yes, indeed. Such a stage does come, but it takes billions of years. And every star has to pass through this stage. Puri: Yes, Sagar. When a star exhausts its hydrogen reserve and if the star is very massive, then it starts shrinking due its large mass. Romi: And the increasing pressure on the core makes its temperature rise steadily. Puri: Absolutely right, Romi. How do you know this? Romi: Papa had told me. Puri: When the temperature of the core reaches 600 to 700 million degrees, fusion of heavier nuclei with helium starts. Father: And new elements are formed. Many of the elements found in the universe were formed in stars in this way. Isn t that so, Puri? Puri: Yes, Sagar. This process is known as nucleosynthesis. Divya: Nucleosynthesis, meaning synthesis of new elements? Puri: Yes. When the core temperature of a star reaches 600 million to 700 million degrees, the process of carbon fusion starts. Romi: Does it mean fusion of carbon and helium nuclei start? Puri: No, no. Carbon fusion means formation of carbon nuclei by fusion of helium nuclei with other heavy nuclei. Divya: That means, a fusion reaction is named after the final product formed. Puri: Yes, when the temperature of the core of a star reaches 100 million degrees, oxygen fusion starts. Romi: That means oxygen is formed as a result of fusion of heavy nuclei. Puri: You re absolutely right, Romi. Further, nuclei of elements up to silicon and iron start forming when the temperature of the core rises to 200 million degrees Celsius. Father: Perhaps this is called the advanced burning stage of fusion. Isn t it, Puri? Puri: Yes, Sagar, The stage between oxygen and silicon fusion up to iron fusion is known as advanced burning stage. Firstly, these reactions require extremely high temperatures and secondly, these reactions produce relatively little energy. Divya: Puri Uncle, it means that elements beyond iron cannot be created by fusion in nucleosynthesis. Puri: Well done Divya! I must say you really know quite a lot! Divya: Papa had told us about it the other day. Puri: Divya, in fusion of iron nuclei to form heavier elements, energy is not produced,rather is absorbed.
Father: This makes the temperature of the core and the gas pressure start falling. Puri: And also makes the star start shrinking. Father: Does it not mark the end of a star? Puri: No, Sagar. What happens before a star dies depends on the mass of the star. If the star is of the size of our Sun or a little bigger, the star shines steadily till the hydrogen in the coreis used up. Romi: Then what happens? Puri: At this stage energy production stops in the core, which starts shrinking. Divya: Then fusion starts again? Puri: Yes, but not in the core. Now heat generated in the shrinking core restarts hydrogen fusion outside it, the heat from which expands the outer gas envelop, turning the star into a red giant. Divya: Then what happens? Father: In the end the outer shell of gas is blown away, leaving behind a small dense object called white dwarf, which cools down slowly. Puri: Do you know? When our Sun becomes a red giant it will become so big that it will almost touch the Earth s orbit! Divya and Romi(together): Wow! Romi: Puri Uncle, what happens if the star is much bigger than the Sun? Puri: If a star has a mass 9 to 30 times that of the Sun then it dies as a supernova. Romi: Supernova? What is it? Puri: The massive star continues to shine till iron fusion starts in its core. Then something remarkable happens. Divya: Remarkable! What s it, Puri Uncle? Puri: Because iron fusion absorbs energy, energy production suddenly stops in the core and the stars collapses within a matter of seconds like a giant roof crashing when the pillars supporting it are suddenly removed. Divya: Does it mark the death of the star? Puri: Not really. But it marks the beginning. After collapse, the compressed core rebounds in a fraction of a second,sending a powerful shockwave, which blasts through the outer layers of the star in a brilliant flash of light we see as a supernova. The blast causes heavy nuclei such as calcium, lead and uranium to form by fusion and get scattered into space. Divya: Does it mean elements heavier than iron are formed only in a supernova? Puri: Yes, you re absolutely right.
Father: You didn t tell them what remains after a supernova. Puri: I was coming to that, Sagar. A supernova usually leaves behind a remnant that, again, depends on the mass of the star. A star 10 to 20 times as massive as our Sun usually leaves behind a neutron star, also known as a pulsar. Romi: And what happens for still bigger stars? Puri: Stars with a mass 30 to 50 times that of the Sun leaves behind a small but extremely dense object known as a black hole. Romi: Is it black in colour? Puri: Not really. But it is so dense and its gravitational pull is so strong that it does not allow even light to come out of it and so it cannot be seen. Divya: Then how do we know about them? Puri: Astronomers can locate black holes by the gravitational and other effects they produce on nearby stars. Father: So you know now how stars die becoming a red giant and leaving behind a white dwarf or becoming a supernova and leaving behind a neutron star or a black hole. Divya: Puri Uncle, from what you ve told us it appears energy in the Sun is produced mainly in the core. How does the energy come to the Sun s surface? Puri: Very good question. Can you tell me in how many ways energy can travel? Romi: Let me tell you. Conduction, convection, and radiation! Puri: Very good. Actually, in the Sun, the energy produced in the core has to travel through many layers to the outer surface of the Sun before it escapes into space as sunlight. Divya: What layers? Puri: There are basically three layers. The layer just outside the core is the radiative zone. Here solar material is hot and dense enough that thermal radiation is all that is needed to transfer the intense heat outward. There is no thermal convection here. Romi: What are the other layers? Puri: Next is the convective zone where energy is carried by convection to the outermost layer of the Sun called the photosphere, the visible surface of the Sun, which sends out the light and heat that we receive on Earth as photons. Romi: That s how get to enjoy the Sun to beat cold in winter! (Everyone laughs) Atom: I m an atom. By now you must have understood where stars get their energy from. Yes, it is nuclear fusion. You ve also learnt that all stars ultimately die. But I am immortal; I live forever.