Stellar Evolution By Adithya Koundinya

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ACKNOWLEDGEMENT The satisfaction and euphoria that accompany the successful completion of any task would be but incomplete without mention of the people who made it possible, whose constant guidance and encouragement crowned our efforts with success.

I would like to thank Prof.Gururaj, Mr. Udaya, Mr. Sathya and Mr. Kiran for providing me with sufficient material and guidance regarding the concepts Nuclear and Astro physics

Last but not least, I owe a lot to my parents and my brother for their unconditional support and encouragement to carry out my work.

Adithya B

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ABSTRACT

Big- bang marked the beginning of the universe, the universe formed, numerous galaxies, the gases within it formed millions of stars, and the starts triggered the formation of planets. But among the lot, one that is very special is the planet earth, for the reason that it is the mother of life. Later was born the two legged creatures, which could now turn up their head towards the heavens and wonder the marvels of the universe. Due to his enormous thinking power, many of the pages of hidden universe are turned open.

But here, in this report, I try to give a life story of one such object formed due the action of fundamental forces - The star. Our own sun has long served as the laboratory to research on the formation, evolution and conclusion of a star. We have found that, the chemicals, elements and compounds, which are now found and extracted from earth’s core, are the remnants of dead stars. Which the advent of science and technology, man came forward and went beyond, what he could see through his naked eye. Neutron starts, Pulsars, Black holes are some of product of the advanced technology.

In this report, I try to give a brief description of the formation of a star from a mass of dense cloud, its transformation from a protostar to a star, the nuclear reactions involved with the tiniest particles, the after effect of the reaction, the power of the gravity, various phases of a star in its life cycle, the fates of a average, medium and small size stars, formation of pulsars, Black holes in space and so on…

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TABLE OF CONTENTS

ACKNOWLEDGMENT

2

ABSTRACT

3

TABLE OF CONTENTS

4

1:

INTRODUCTION 1.1

The celestial sphere

6

1.2

The universe

8

1.2.1 The big-bang cosmology

9

The content of the universe

10

1.3.1 Galaxies

10

1.3.2 Stars and planets

11

1.3.3 The rest of the universe

15

The forces the hold the universe

15

1.4.1 Dominant forces in the outer space

16

Summary

17

1.3

1.4

1.5

2.

NUCLEAR ENERGY SOURCES 2.1

Nuclear fission

18

2.2

Nuclear fusion

20

2.3

Fusion in stars

22

2.3.1 Carbon cycle

23

2.3.2 Proton-proton cycle

23

2.4

Conversion of mass into energy in nuclear reaction

24

2.5

Condition for the nuclear reaction in stars

26

2.6

Summary

28

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3.

STELLAR EVOLUTION 3.1

3.2

3.3

4.

Gravitational consideration

29

3.1.1 Protostar

30

3.1.2 Raising temperature of protostar

31

3.1.3 Ignition of hydrogen

33

Red giants

34

3.2.1 Increase in temperature of helium core

34

3.2.2 The structure of a red giant

36

3.2.3 Helium flash

36

3.2.4 After the helium flash

37

Summary

38

DEATH OF A STAR 4.1

4.2

4.3

4.4

4.5

Death of a small star

39

4.1.1 Planetary Nebula

39

4.1.2 The white Dwarf

40

Death of a massive star

41

4.2.1 The supernova explosion

41

Pulsars and neutron stars

42

4.3.1 Pulsars

43

4.3.2 Neutron stars

43

Black holes in space

44

4.4.1 Formation of a black hole

44

Summary

45

5.

EPILOGUE

46

6.

BIBLIOGRAPHY

47

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CHAPTER 1

INTRODUCTION “In the first half of the 20th century, the word universe was used to mean the whole space-time continuum in which we exist, together with all the energy and matter within it. Attempts to understand the universe in this sense, on the largest possible scales, are made in cosmology, a science that has grown from physics and astronomy.”

1.1

The Celestial Sphere

On the clear dark night we are dazzled by the sight of a large numbers of stars in the sky. We observe some of the bright stars over a period of 2-3 hours. You will find that they show a regular motion from east to west, keeping their positions with respect to each other constant. And it’s a well know fact that, its really due to the rotation of earth around its axis. The starts for themselves are so far away that their actual movements are too small to be noticed from the earth; only through some sensitive instruments the actual distance can be measured. Thus for all practical purpose, from on the surface of a transparent sphere centered on us, called the Celestial Sphere.

In astronomy and navigation, the celestial sphere is an imaginary rotating sphere of "gigantic radius", concentric and coaxial with the Earth. All objects in the sky can be thought of as lying upon the sphere. Projected from their corresponding geographic equivalents are the celestial equator and the celestial poles. The celestial sphere projection is a very practical tool for positional astronomy. The celestial sphere can be used geocentrically and topocentrically. The former means that it is centered upon an imaginary observer in the centre of the Earth, and no parallax effects need to be taken into account. In the latter case it is centered upon

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an observer on the surface of the Earth and then horizontal parallax cannot always be ignored; especially not for the Moon. In the Aristotelic and Ptolemaic models, the celestial sphere was imagined as a physical reality rather than a geometrical projection, the celestial sphere is divided by projecting the equator into space. This divides the sphere into the north celestial hemisphere and the south celestial hemisphere. Likewise, one can locate the Celestial Tropic of Cancer, Celestial Tropic of Capricorn, North Celestial Pole, and South Celestial Pole. The directions toward various objects in the sky can be quantified by constructing a celestial coordinate system.

Figure 1.1 The Celestial sphere

As the Earth rotates from west to east around its axis once every 23 hours 56 minutes, the celestial sphere and all objects on it appear to rotate from east to west around the celestial poles in the same time. This is the diurnal motion. Therefore stars will rise in the east, culminate on the north-south line (meridian) and set in the west, (unless a star is circumpolar). On the next night a particular star will rise

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again, but with our normal clocks running a 24 hour 0 minutes cycle, it will do so 4 minutes earlier. By the following night the difference will be 8 minutes, and so forth with every following night (or day).

The reason for this apparent mis-adjustment of our clocks is that the Sun is not standing still on the celestial sphere, as the stars do, but moves about 1° per day eastwards over a great circle known as the ecliptic (which is 360° or a full circle in one year, the annual motion of the Sun). As an angle of 1° corresponds to 4 minutes in time (360° = 24 hours), we need therefore 4 extra minutes of diurnal motion to see the Sun back on (for example) the meridian again, making the duration of one rotation just 24 hours exactly. Normal clocks therefore indicate solar time. Astronomers studying the movements of stars may want clocks indicating sidereal time, going around once in 23h 56m (solar time units). A celestial sphere can also refer to a physical model of the celestial sphere. Also known as a star globe, this sort of celestial sphere will indicate which constellations are visible at a given time and place.

1.2

The Universe

The term universe came from Old French univers, from Latin universa, from uni-, combining form of unus meaning one and versus meaning turned, literally "all turned into one" or "revolving as one".

The Universe is defined as the summation of all particles and energy that exist and the space-time in which all events occur. Based on observations of the portion of the Universe that is observable, physicists attempt to describe the whole of spacetime, including all matter and energy and events which occur, as a single system corresponding to a mathematical model. The generally accepted scientific theory which describes the origin and evolution of the Universe is Big Bang cosmology, which describes the expansion of space Adithya

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from an extremely hot and dense state of unknown characteristics. The Universe underwent a rapid period of cosmic inflation that flattened out nearly all initial irregularities in the energy density; thereafter the universe expanded and became steadily cooler and less dense. There are more than one hundred billion (1011) galaxies in the Universe, each containing hundreds of billions of stars, with each star containing about 1057 atoms of hydrogen.

1.2.1 The Big-Bang cosmology The most important result of physical cosmology is that the universe is expanding, this concept is derived from red-shift observations and quantified by Hubble's Law. That is, astronomers observe that there is a direct relationship between the distance to a remote object (such as a galaxy) and the velocity with which it is receding. Conversely, if this expansion has continued over the entire age of the universe, then in the past, these distant, receding objects must once have been closer together. By extrapolating this expansion back in time, one approaches a gravitational singularity where everything in the universe was compressed into an infinitesimal point; an abstract mathematical concept that may or may not correspond to reality. This idea gave rise to the Big Bang Theory, the dominant model in cosmology today.

Figure 1.2 The Big-Bang

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During the earliest era of the big bang theory, the universe is believed to have formed of a hot, dense plasma, as expansion proceeded the temperature steadily dropped until a point was reached when atoms could form. At about this time the background energy (in the form of photons) became decoupled from the matter, and was free to travel through space. The left-over energy continued to cool as the universe expanded, and today it forms the cosmic microwave background radiation. This background radiation is remarkably uniform in all directions, which cosmologists have attempted to explain by an early period of inflationary expansion following the Big Bang.

Others suggest that the universe had no beginning, because time goes in a loop. However, any such ideas are at best hypothetical and much more research is needed before anything can be concluded for certain.

1.3

The contents of the universe

We are evident that our observable universe consists of about 100 billion galaxies, each containing on the average, about 100 billion stars of different varieties! Number of estimated starts in the universe is therefore 1022. Since most of the space between galaxies, and between starts within a galaxy is empty, Perhaps we can get an idea of the vastness of the universe and our own insignificant place in it. Although there is no strong evidence yet, there is good reason to believe that life supporting planetary systems like ours must be plentiful in the universe, even within out galaxies.

1.3.1 Galaxies The Sun is just one out of more than 100 billion starts which are gravitationally bound to form our galaxy, the Milky Way. Since we are the part of this galaxy its difficult to make out the shape and size of the galaxy. However, astronomers have accumulated a wealth of evidence which indicates that our galaxy has approximate shape and dimensions schematically indicated in the Figure 1.3. Adithya

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Figure 1.3 Representation of Milky Way as seen from side

Galaxies are basically of three types, those are elliptical, spiral and irregular. Milky Way is a spiral galaxy. The spiral galaxy nearest to us is the well known Andromeda galaxy, designated as M31. it is about 2.2 million light years away from us. On a clear dark moonless night, it can bee seen with the naked eye as an oval shaped cloudy patch of light, looking very much like a nebula till its exact nature was established late in 19th century. Good telescope show large number of similar galaxies distributed throughout the constellation. In some constellation we can make out clusters of such galaxies. Super clusters of galaxies are also known to exist. The Magellan clouds are two ‘mini’ irregular galaxies, much smaller and closer than Andromeda. They can be seen in the southern skies. The supernova of 1987 took place in Large Magellan cloud (LMC).

1.3.2 Stars and Planets The Sun spews out huge amount of energy in the form of electromagnetic radiation in all directions in space. Only a small part of it is intercepted by earth, but this has been more than sufficient to meet the energy requirements on our planet. Despite its gigantic size in relation to the planets, the sun is just an average size star among the billions and billions of stars spread out in this vast universe. These stars are not distributed uniformly in the space, but are found in the clusters or groups. A galaxy is a huge collection of stars, numbering about a hundred

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billion (1011). There are more stars in an average sized galaxy than the number of grains of sand on all the beaches all over the earth. We discuss in more detail regarding the formation, life cycle of a star, in this document.

Figure 1.4 Shows some of the galaxies in the universe.

As the electron revolves around its dense central nucleus, in the space this act is reflected by the larger objects called planets. The planets are the objects which revolve around its star due to the gravitational influence of the star. The planets are not luminous objects, they reflect the light that fall on it, and therefore we won’t see them twinkle in the night sky. Sun is one such starts that holds planets in its orbit. Our own planet Earth is the unique planet among the eight of its companion planets revolving around the star SUN. Its considered unique because

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of the existence of life on the planet and the existences of intelligent two legged creatures who now wonder this marvelous universe.

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto are the 9 planets revolving around Sun. each of these planets have their own set of chemical and physical properties. These planets along with their satellites revolving around Sun form a system called the solar system. Sun covers 99% of the solar system, remaining 1% is left for the planets to occupy.

Mercury is the inner most planet of the solar system. It has an orbit more elliptical than that of any other planet, except Pluto. Its distance from the sun varies from 46 – 70 million Km. On mercury a day constitutes of 2 hours, therefore one have never been able to get a good view of this planet. Venus is similar to that of earth in its size and density. But the similarities and here. Its dense atmosphere, mostly carbon di-oxide, and surface temperature of over 4000C make it an furnace where even an unmanned spacecraft can’t survive. This orbit is closer to that of a circle and rotates with the period of 244.5 days, but in the direction opposite to that of any other planets. Thus Sun rises in the west and sets in the east as seen from this planet. Mars has long been regarded as most likely planed to harbour or support some form of life. Usually the surface temperature is less than 00C everywhere, but can rise as high as 250C at the equator. The polar icecaps are the most prominent feature of this planet. Mars has two very tiny satellites, called Phobos and Deimos. Planet Jupiter and other outer planets are called as gas giants.

Jupiter is one of the largest planet of the solar system and was discovered by Galileo. It has a period of rotation of just under 10 hours, resulting in bulging in the equator. The most prominent feature of the planet is the Great Red Spot in the southern hemisphere near the equator. Io, Europa, Ganimede, Callisto are some of

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the satellites of this planet, indeed they are many more. The most spectacular object in the solar system is the planet Saturn. Its easily and always recognized by the rings girdling the planet. Its density is less then than of water, and perhaps the lowest in the solar system. Its large satellite Titan is the only satellites known to have its own atmosphere. Saturn rotates with the speed slightly less then Jupiter.

Figure 1.5 A view of the solar system

Uranus is the third largest planet. Its axis of rotation has a tilt of 820, i.e. the planet is nearly in the plane of the earth’s orbit around Sun. Uranus has 15 satellites. A ring system has also been discovered around this planet in 1977 by Prof. J.C Bhattacharya and K. Kuppu swamy of IIA (Indian Institute Of Astrophysics, Bangalore). Neptune is the smallest among the gas Giants. Not more about the plane is still know. Pluto is the last and outermost planet of the solar system. Its distance from the Sun varies from 4.4 to 7.4 billion Km. the inclination of its orbital plane to the orbital plane of the earth is the highest for any planet, 170. Pluto has a satellite named Charon, which is found to be very close to the orbit of the planet.

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1.3.3 The rest of the universe The rest of the universe contains vacuum, exotic matter, dust, asteroids, meteorites, comets, nebulas and so on. Below is the estimated ratio of contents of universe

• 70% vacuum energy • 26% exotic dark matter (not made of protons and neutrons like ordinary matter) • 4% ordinary matter (our world, stars, galaxies) • 0.005% radiation (light)

1.4

The forces that hold the universe

All objects in the universe, form the smallest atomic nucleus to the largest galaxies, are held together by only three fundamental forces: The nuclear force, The electromagnetic force, and The Gravitational force.

The most powerful among these three forces is the Nuclear force which binds Protons and Neutrons in the atomic nuclei. This strong force of attraction pulls the particles of the nucleus together into a very compact body with a density of one billion tons per cubic inch. Although it is an exceedingly strong force, it has a very short range. The nuclear force will not attract if the particles are one ten-trillion of an inch apart. The next strongest force is the electrical force, which is approximately 100 times weaker than Nuclear force. This force binds electrons to nuclei to form atoms, and it binds atoms together into a solid matter. It grows weaker with increasing distance between two particles, although unlike the nuclear force, it does not disappear entirely at any point. The least of the fundamental forces is the Gravitational force. The gravitational force is exceedingly weak,

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about 1038 times weaker than the force of electricity. Gravity like electricity falls of in strength with increasing distance, but never disappears entirely.

1.4.1 Dominant force in outer space In spite of the strength of nuclear force, it has a negligible effect on the motion of objects in the outer space. In turn, the motion of stars, planets and other objects are controlled by the relatively weak force of gravity. The reason behind it is, nuclear force is confined to extremely small distance unlike force of gravity. Now the question is, Why is the force of electromagnetism not dominant over gravity in controlling the motion of stars and planets? This force can also extend to great distance, just as the force of gravity dose, and it is many times more powerful. The answer comes out by the fact that when both kind of charges, positive and negative, are present, their effect tend to cancel. If an object has precisely the same amount of positive and negative charges, it is electrically neutral and exerts no electrical force at all on the objects even relatively closer.

Many electrons and atomic nuclei move freely in the space around every star or planet. If the star or a planet picks up an excess of protons from the space around it, it becomes positively charged, it immediately exerts a force of attraction on all the negatively charge particle nearby. These electrons are migrated to the stars or planets and are captured by it, the stars or a planet continues to attract particles of opposite charge until it becomes electrically neutral. But the force of gravity cannot be canceled in this way because there appear to be only one sign of “gravitational charge.” Mass always pulls other mass, and two masses never repel one another. There may be second kind of gravity or Anti-Gravity, but thus far physicists have been unable to find evidence for it.

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1.5

Summary

In this chapter, we discuss some basic concepts of the universe. Try to define the universe and motivate the readers towards the study of Astronomy and Astrophysics. We hear briefly describe the concept of big-bang, the contents of this vast universe and also some ideas as to how the objects in space are bound to each other by three fundamental forces.

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Chapter 2

NUCLEAR ENERGY SOURCES The amount of energy that comes out of star staggers the imagination. By using Einstein’s law stating the equivalence of mass and energy, E=mc2, and converting the energy radiated by sun from units of mass with the aid of this formula, we find that almost five million tons of mass per second is radiated by sun in the form of electromagnetic energy.

Is the Sun wasting away to nothing as the result of this loss of energy? The fossil record indicates that life on this earth existed on earth for billions of years; hence, the earth and the sun also must have existed for as long a time. In a billion years the sun radiates into space 1 X 1029 grams, which is less then one part in 104 OR one hundredth of one percent of its mass. Thus the sun has lost only an insignificant amount of its total substance by radiating of energy from its surface. But the 1 X 2929 grams is still an enormous amount of matter. It is equal to 17 earth masses. What force can convert all the matter in 17 earths into energy? Asking this question is the same as asking, why the sun and stars shine so brightly. Surprisingly, this incredible power of sun and starts are from its tiniest particles. Laboratory research in nuclear physics revealed the existence of the only force strong enough to convert matter into energy at the extraordinary rate at which stars pour their energy into space. This force is the nuclear force - The strongest force known to the mankind.

2.1

Nuclear Fission

How do nuclear forces release energy? One way is by fission, that is, by splitting of nucleus into two or more pieces. This is the method used in atom bomb. The

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nucleus used in such a bomb is usually uranium or plutonium. The nucleus of a uranium atom or a plutonium atom is usually like a stick of dynamite; it is unstable, and can explode or can dis-integrate into several pieces, simultaneously releasing large amount of energy. The energy realized when one uranium or plutonium atom breaks up is roughly 10 million time greater than the energy realized during a TNT explosion. These nuclei will spontaneously disintegrate if they are left by themselves for a sufficient length of time; for example, U235, the isotope of uranium used in the first atom bomb test near Alamogordo, New Mexico, disintegrates spontaneously I time of 880 million years. Or these nuclei will explode if disturbed by an incoming slow moving neutron.

Figure 2.1 Release of energy by chain reaction in Uranium.

The explosion breaks U235 nucleus into two fragments, which turn out to be the nuclei of two medium weight substances such as Barium or Krypton. In addition to the two main fragments of the original uranium nucleus, a few free neutrons usually emerge at the same time. These neutrons can trigger the explosion of other uranium nuclei, releasing more neutrons and so on, setting in motion the chain

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reaction that can yield enormous amount of energy. This is the principle of atom bomb. However, uranium and other substance nuclei that yield energy by fission cannot be sources of energy within stars. For these heavy elements are present in minute traces in a star. In the Sun for example the amount of uranium is only one tenbillionth of one percent. If uranium is too scarce in the star then perhaps the energy comes from some other element that is present in abundant in the stars. The most abundant element in the stars is Hydrogen, but its impossible for fission to take place with hydrogen as it consist of a single proton. Thus fission cannot be the source of stellar energy.

2.2

Nuclear fusion

There is a second kind of reaction involving nuclei in which energy is released. It is called nuclear fusion. The reaction consists in fusing two nuclei together, instead of trying to break a single nucleus into two pieces. Two protons can be joined in this way to form a single larger nucleus, with a large amount of energy released in the process. Since protons- the nuclei of hydrogen are very abundant in start, it is assumed by all astronomers that the fusions of protons are the main source of energy in stars. One might ask how two protons can be joined together, since a nucleus consisting of two protons does not exist. The answer is that at the very moment of collision between the two protons, one of them sheds its positive charge of electricity in the form of a positive electron, also called a positron. The removal of the positive from the proton leaves behind a neutron which is locked to the other proton to form a deuteron. The positron, accompanied by a mass-less, electrically neutral ghost-like particles known as the neutrino, carries off most of the nuclear energy released in the fusion as shown in the Figure 2.2

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Figure 2.2 Fusion of two protons p (Proton)

A positively charged particle, relatively massive; it is the nucleus of the hydrogen and one of the two basic building blocks of heavier nuclei.

n (neutron)

Electrically neutral particle about the same mass as proton; it is the other basic component of atomic nuclei

d (deuteron)

A particle composed of a proton and neutron bound together, containing the same energy as that of protons but double its mass; it is nucleus of heavy hydrogen or deuterium.

-

e or e (electron)

A negatively charge, relatively light particle. About 1/1840 times the mass of proton.

+

e (positron)

Similar to electron with same mass but positively charged

ν

(neutrino)

A mass-less, charge-less particle; it is produced in some nuclear reaction, generally with a positron or electron, and carries off some of the energy released in the reaction; because it has no electric charge and no mass, the neutrino can pass through large amount of matter, such as the entire body of a star.

γ

(gamma)

A photon or packet of electromagnetic radiation similar to an ordinary photon, but having extremely high energy and corresponding short wavelength

Table 2.1 A list of particles appearing in nuclear reaction.

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Nuclear physicists have developed useful shorthand for nuclear reaction like the one shown above. Suppose we let the symbols d, n, e+, and v represent, respectively, the deuteron, proton, neutron, positive electron or positron, and neutrino. Then the reaction in which two protons fuse to from a deuterons can be written: P + P
d + e+ + ν In order to see the reason for the release of energy in nuclear fusion, let us consider what will happen when two protons approach one another on a collision course and come within range of a the nuclear force of attraction. Immediately they are seized by this very strong attractive force, and rush violently towards one another; when they collide, owing fuse together, forming a single, heavier nucleus in place of the two separate protons that existed before. At the same time, the energy of their violent collision is released to the surroundings in the form of heat and light. It turns out that the amount of energy released in this way, by the fusion of two protons, is nearly as great, per pound of protons, as the energy released per pound of uranium in the fission of uranium

2.3

Fusion in stars

The fusion of protons into deuterons is the first in a series of step in which heaver elements are built up from lighter ones in the interior of stars. The continuing succession of nuclear reaction manufactures all the other elements of the universe out of the basic ingredient hydrogen.

The conversion of hydrogen to helium can take place in two methods, one by carbon cycle in which carbon acts as a catalyst and the other is proton-proton cycle, which is believed to be more effective in releasing solar energy than that of carbon cycle

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2.3.1 Carbon cycle 6C

12

7N

+ 1H1

13


7N13 + Energy
6C13 + 1e0

13

+ 1H1


7N14 + Energy

7N

14

+ 1 H1


8O15 + Energy

8O

15

7N

15

6C


7N15 + 1e0 + Energy + 1 H1


6C12 +

2He

4

Summing up all the above equation, we have the carbon cycle as “4 1H1


2He4 + 2[1e0] + Energy”

These means, four protons combine together to yield a helium nucleus and two positrons together with the emission of energy of about 24.69 MeV. It may be noted that the initial carbon atom remains even at the end of the reaction. It thus acts as a catalyst.

2.3.2 Proton-proton cycle 2[1H1] + 2[1H1]


2[1H2] + 2[1e0] + Energy

2[1H1] + 2[1H2]


2[2He3] + Energy

2[2He3]


2He4 + 2[1H1] + Energy

Summing up all the above equation, we have the proton-proton cycle as “4[1H1]


2He4 +2[1e0] +Energy”

The electrical barriers acting in carbon cycle are much higher than those acting in the proton-proton cycle, the barrier is a basic carbon-proton reaction being six tuimes higher than proton-proton barrier. As a result, the proton-proton cycle Adithya

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dominates at low temperature, and carbon cycle does not become important until higher temperature is reached. It is evident that about 10% of sun’s energy is contributed by carbon cycle and the remainder by the proton-proton cycle. Around 2x1019 Kg of hydrogen is converted to helium annually; sun has to emit energy for 5 billion years.

Figure 2.3 Nuclear fusion

2.4

Conversion of mass into energy in nuclear reaction

The amount of energy released during nuclear fusion or nuclear fission is so great that it causes an appreciable reduction in the masses of the nuclei that are left behind. According to Einstein’s law on the equivalence of mass and energy. E = mc2 ; When an amount of energy E is given off in a nuclear reaction, the mass of the nucleus that is left behind must be smaller by the amount E/c2, where c is the speed of light. For example consider a reaction where two protons combine to form a deuteron: p + p → d + e+ + ν If we add the masses of two protons on the let hand side, and compare the total masses of the deuteron and positive electron (positron) on the right hand side, we

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find that the total mass after the reaction is smaller then the mass after the reaction:

2 × mass of proton

Mass of deuteron

= 2 × 1.67243 × 10-24 g

= 3.34486 × 10-24

3.34321 × 10-24 g

+ Mass of positron = + 0.00091 × 10-24 g

= 3.34412 × 10-24 g

Difference in mass = 0.00074 × 10-24 g The difference, 7.4 × 10-28 g, represents the mass that has disappeared in some way during the course of the reaction. This difference seems very small. However, if it is compared it with the masses of the fundamental particles, we see that the missing mass is nearly as great as the mass of an electron.

In some way, the equivalent of nearly an entire electron has disappeared during the reaction. This seems impossible. Matter simple cannot disappear into nothing; at least, it never dose in ordinary experience. In fact, the conclusion that matter is indestructible became so firmly established in the early years of science that by the beginning of the nineteenth century it had been adopted as one of the basic principles of physics. It was called the law of conservation of mass. Yet the law was wrong; and the fact it is wrong is proved simply looking at the Einstein’s formula for the equivalence of mass and energy. Einstein’s formula says that mass alone cannot be conserved; if anything, it must be the sum of mass and energy that can be conserved.

Many experiments have proven that this is correct law. The sum of mass and energy is indestructible. In any event occurring anywhere in the world, the total of Adithya

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the masses and the energies of the particle taking part in the event remains unchanged, regardless of the way in which the particles themselves are changed. In this way, an object can literally disappear from the universe, or another object can appear where there was none before; the law of physics do not place any limitations on such conjuring act; but they do require that whenever an object , large or small, vanishes, an amount of energy equal to mc2 must appear in its place.

According to Einstein’s equation, every reaction that releases energy must involve a corresponding loss of mass. This is true for a chemical reaction, such as the explosion of nitroglycerin or burning of coal, as much as it is for the explosion of nuclear bomb. In both the cases the masses do not balance before and after the reaction, because some mass is always carried out in the form of energy. In the case of chemical reactions the amount of mass that is carried out as the form of energy cannot be measure directly with that precision, thus Einstein’s formula was not discovered by laboratory experiments prior to the advent of his theory.

2.5

Condition for the nuclear reaction in stars

Since nuclear fusion yields an enormous amount of energy per pound of fuel, why is it not used as an energy source in the everyday life? Energy has been released through nuclear fusion for brief moments in the explosion of the hydrogen bomb, but no one has yet succeeded in fusing nuclei in such a way that the energy can be harnessed for constructive purpose. The difficulty is that enormous temperature, ranging up to tens of millions of degrees, are needed to produce a significant amount of energy by nuclear fusion. The need for a high temperature is connected with the electrical forces between nuclei. Two protons, for example, repel one another electrically because each proton carries a positive electric charge. But if the protons approach within a very close distance of each other, the electrical repulsion gives way to the even stronger Adithya

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force of nuclear attraction. However, the proton must be closer together than one 10-trillionth of an inch for the nuclear force to be effective. Under ordinary circumstances, the electrical repulsion serves as close an approach as this. In a collision of exceptional violence, however, the protons may pierce the barrier which separates them, and come within the range of their nuclear attraction. Collision of the required degree of violence begins to occur frequently in a gas when the temperature of gas reaches 10 million degrees Kelvin.

Once the electrical barrier between two protons is pierced in a collision, they pick up speed as the result of their nuclear attraction and rush towards each other, fusing together in the reaction described in earlier. The fusing of two protons into a single nucleus is the only first step in the series of reactions by which nuclear energy is released during the life time of the star. In subsequent collision, two additional protons are joined to the first two to forma nucleus containing four particles. Two of the protons shed their positive charges to become neutrons in the course of the process. The result is a nucleus with two protons and two neutrons. This is the nucleus of the helium atom. Thus, the sequence of reactions transforms protons, or hydrogen nuclei, into helium nuclei.

Helium dose not fuse into heaver nuclei at the ordinary stellar temperature of 10 million degrees because the helium nucleus, with two protons, carries a double charge of positive electricity, and, as a consequence, the electrical barrier between two helium nuclei is stronger than the repulsion between two protons. A temperature of 100 million degree Kelvin is required to produce collision which will pierce the helium barrier. If the temperature in the start will reach this level the helium nuclei will fuse in groups of three to form carbon nuclei, releasing more nuclear energy in the process.

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The fusion of hydrogen to form helium id the first and longest stage in the history of stars, occupying about 90% of its life time. In the second stage, which takes up most of the remaining 10% of star’s life, three nuclei of helium combines to form the nucleus of the carbon atom? Afterwards, the nuclei of oxygen and other still heavier elements are fabricated, at an increasingly rapid pace, until all elements have been built up. In this way the elements of the universe are manufactured out of hydrogen nuclei at the center of the star during the course of its life. These facts of nuclear physics complete the essential body of information needed for an understanding of the life of stars.

2.6

Summary

Here we try to explain the concept of energy, and the source of energy in the stars. The reactions involved in nuclear fusion and nuclear fission. Also we discuss the reason for why nuclear fission is not the source of stars energy. The conditions required to start with the series of reaction and thus formation of elements in the core of a star. The chapter gives a detailed description about the reactions taking place in a star, and it should help in understanding the life cycle of a star explained in the coming chapters in easy way.

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Chapter 3

STELLAR EVOLUTION The star’s existence begins with the tedious cloud of matter that fill all of space. If the atoms in such a cloud together by accident, the force of gravity pulls the atoms closer together, forming a condense pocket of gas. The continuing gravity will condense it further, as the result the temperature at the center rises, this raise of temperature as it reaches a critical value the hydrogen starts converting into helium, releasing vast amount of energy in the form of heat and light. The energy passes to the surface and is radiated into to the outer space.

3.1

Gravitational consideration

Gravitational force is the force that is responsible for all the activities happening in this universe. Even though it is the weakest force known to the man kind, its amazing property of extending for a great distance has caused life on this planet earth.

The life cycle of star begins with the accretion of hydrogen and helium in the galaxies wandering in the interstellar space. As the atoms pass one another, however, each atom exerts a small gravitational attraction on its neighbors, which counters the tendency of the atoms to separate, if the gravitational attraction were sufficiently strong, it would hold them together and prevent them from dispersing again into space. The effect of gravity would convert the pocket of gas from a temporary condensation to a permanent one. But suppose that there are a large number of atoms. Because the force of gravity extends over great distances, each atom feels the gravitational pull of all the other atoms in the pocket. If the number of atoms is sufficiently large, the combined effect of all these minute pull of gravity will be powerful enough to prevent any of the atoms in the pocket of gas to

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leave the pocket and fly away in to the space. The pocket now is a permanent entity, held by mutual attraction of all atoms within it upon one another.

This is the heart of the theoretical explanation of the birth of stars. According to the theory, a star is conceived when a condensed pocket of gas forms by accident in outer space, and when the number of atoms in the pocket of gas is so great that their own gravity holds them together permanently. The pocket is not yet a star, but it will become one a little later. This cluster of atoms, formed by accident and held in the grip of its own gravity, is called a protostar.

3.1.1 Protostar How large must a cluster of atoms be before its own gravity is strong enough to hold it together permanently? If three or four are not enough, will a million atoms or a trillion atoms suffice? Theoretical astronomers, using pencil and paper and the laws of physics, have calculated the number of atoms that are necessary. The result shows that the answer depends strongly on the temperature of the gas, which controls the speed at which the atoms move about. Clearly, the higher the temperature and the higher the speed of atoms, the more difficult it is for gravity to hold them together.

Under almost all conditions in space, however, the required number of atoms is much larger than any of the numbers we have mentioned. Under average conditions the theoretical results is about 1057 atoms are required. This is a staggeringly large number. It is found that a comparison of the number 1057 with the number of grains of sand in all the beaches of the world; but even that seemingly uncountable number is only a mere 1025. in fact, the number of neutrons and protons contained in the nuclei of all the atoms of the entire earth is only 1051. The trouble is that no number on earth can possibly match this number; it is

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number that responds to the building blocks of objects the size of stars, not objects the size of planets.

Figure 3.1 A protostar in the process of accretion

If we translate the 1057 atoms into grams, assuming that each atom is hydrogen, we find that the cloud weights approximately 1033 grams, which is roughly the mass of the sun. Thus, it turns out that the number of atoms theoretically required to hold a pocket of gas together in space has a mass equal to the mass of an average star such as the sun. This agreement provides evidence that the theory is correctthat the process of gravitational condensation we have discussed is, in fact, the way in which stars are born in space.

3.1.2 Raising temperature of protostar how does a protostar; a tenuous collection of hydrogen atoms drawn together out of the cold gas of space; become the dense, flaming sphere of gas we call a star?

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Figure 3.2 The view of an Orion nebula, where numerous starts are expected to take birth.

The answer depends again on the force of gravity which draws every atom in the proto star towards the center of the cloud. The continuing action of gravity, pulling all the atoms towards the center, causes the protostar to shrink in size. As it becomes smaller, its density increases. The force towards the center is not a mysterious or an unfamiliar phenomenon. Every object on the face of the earth is attracted towards the center of the planet by the force of planets gravity. In case of the earth, the solid surface prevents us form falling to the center. A protostar, however, is not solid; it is a globe of gaseous matter, and its atoms are unimpeded by the resistance of a solid surface. These atoms literally fall towards the center of the protostar. The protostar collapses under its own gravitation.

As the atoms in the protostar move towards its center they pick up speed. Because the average speed of the atoms in a gas determines the temperature of that gas, the contracting protostar with its accelerating atoms gets hotter at the beginning, the

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temperature of the protostar is same as the temperature of the interstellar gas out of which it formed. This temperature is roughly about 100 K. At this temperature the average speed of an atom of hydrogen is 1 mile per second. As the gas cloud contracts under its own gravity, the temperature at the center mounts steadily eventually reaching 50, 000 K. At the temperature of 50, 000 K, the hydrogen and the helium atoms at the center of the protostar collide with sufficient violence to dislodge all its electrons from its orbits around the nuclei. The original gas of atoms, each consisting of an electron circling around a nucleus, becomes a mixture of two gasses, one composed of electron and the other of the nuclei.

At this stage the globe of gas has contracted from its original size, which was trillions of mils in diameter, to a diameter of 100 million miles. When the temperature near the center of the protostar is 50000K, the protons in the interior of the globe of gas move at a speed of 20 miles per second. This velocity is still far from adequate to penetrate the electrical barrier and initiate a chain of nuclear fusion. In a sense the true birth of the star has not yet occurred but the protostar has already become a luminous object because of heat liberated due to collapse.

After still more time has passed, the protostar has shrunk to 50million miles, its internal temperature has risen to 150000K, and its surrounding temperature has risen to3500K. At this stage, protostar is a highly luminous object and makes its debut on the Hertzsprung-Russell diagram. Protostar should emit enough radiations in the visible region of the spectrum to be seen with a telescope. However the protostar stays in the visible region for relatively short time therefore, astronomers see them very rarely.

3.1.3 Ignition of Hydrogen Suppose we concentrate on the 10million mark in the evolutionary track. The protostar has been collapsing since its formation, rapidly at first, and then more Adithya

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slowly as the density increased while atoms moving towards the center have met with increasing resistance. After 10 million years the protostar has shrunk from its trillions of miles of diameter to 1.5 million miles. This diameter is close to the size of our sun. at the same time, the temperature at the center of the protostar has risen to 10 million K. 10 Million K marks a critical threshold in the life of a collapsing protostar. Why is this temperature critical? At this temperature, for the first time, the protons at the center of the proto star are finally moving and colliding at speeds great enough to penetrate the electrical barrier and come with in the reach of the Nuclear force of attraction. At this point the nuclear fusion sets in, at the center of the spear and the protostar has now become a star.

The surface temperature of the new born star is 4500K and its luminosity is half the luminosity of the sun. The release of the nuclear energy at its center the star becomes somewhat hotter and more luminous 27million years after he collapse the protostar began, it comes to a resting placer. Here it lives its most of the life in a balance between the invert pressure created by the force of gravity and the outward pressure generated by the force of nuclear energy.

3.2

Red giants

The helium produces by the fusion of hydrogen atom accumulates the center of a star, where most of the reactions take place. When an appreciable amount of the hydrogen with in the star has been converted into helium and its center is filled with a core of pure helium, the star begins to show pronounced signs of age.

3.2.1 Increase in temperature of helium core The first major change involves the condition in the helium core. At this point in the life time of the star, the temperature of the core is not high enough to fuse helium into heaver elements. Because there is no nuclear burning at the center, no

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energy is released there. The central region of the star, which had been supported against gravitation collapse by the release of nuclear energy, no longer possesses the means for sustaining itself from the inward force of gravity. Under the influence of gravity the helium core shrinks, and its temperature rises, just as temperature of the entire star rose when it was collapsing at the beginning of its life. The center of the star now consists of core of helium that is collapsing and steadily heating up. The rest of the star is a shell of hydrogen surrounding the helium core. As the core gets hotter, it heats the hydrogen immediately surrounding it; this hydrogen commences to burn vigorously to form helium.

The structure of the star is now different from that during the birth. Formerly hydrogen was burnt at the center, while now it consists of nonburning helium. As the helium core becomes hotter as it contracts, the nuclear reaction rate in the shell increases. However when the fire goes out at the center of the star, the helium in the core contracts and heats up. The higher the temperature in the core, in turn, raises the temperature of the immediately surrounding hydrogen. The end result is that the hydrogen in the shell around the core blazes more brightly than hydrogen in the center did before. As its size increases and its luminosity remains constant, the amount of energy radiated at each square centimeter of the surface drops, and the surface temperature drops to a value between 3000 K and 4000 K. A star a surface temperature in this range is distinctly red in color.

As the time proceeds the helium core continues to collapse and its temperature continues to rise. Eventually the burning of hydrogen in the shells, hence affecting the brightness of star. The envelope of star absorbs some of this radiation and expands even more rapidly. The rate of energy release within the star is now hundreds of greater than it was. The star, still red in surface color, has become brilliantly luminous; it has become a red giant.

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3.2.2 The structure of a red giant Red giants are very old stars. The core of helium at the center of the red giant is enormously compressed, with a density equal to one ton per cubic inch. Onequarter of the mass of the entire star is packed into the core, although its radius is only one one-thousand of the radius of the star. This core has a diameter of 20,000 miles – about twice the size of the earth. But it weighs nearly 100,000 times as much. Around the core lies a thin shell of burning hydrogen is an enormously distended and very tenuous envelope of hydrogen gas, 100 million miles across. The average density of the hydrogen gas, 100 million miles across. The average density of the hydrogen gas in the envelop is one millionth of an ounce per cubic inch. This density would be very good vacuum in a physics laboratory on the earth. To bring out particular structure of red giant, suppose that if reduce the size of a typical red giant by a factor of about one trillion. Then the star is a sphere the size of a basket ball, but the helium core, containing one-quarter of the mass of the star, is just a dot at the center.

3.2.3 Helium flash The helium core becomes more and more compressed with the passage of time, and temperature continues to rise. Therefore, the red giant becomes more and more luminous. Finally, the helium core reaches the critical threshold temperature of 100 million K. when the core reaches 100 million K, the helium nuclei begin to fuse, producing carbon and oxygen nuclei. At this point the helium at the core is packed in with a very high density, equal to many tons per cubic inch. Because the core is also very hot, the helium atoms are entirely stripped of their electrons. The core further contracts and becomes as dense as a steel ball. The electrons wont have any free space to move about. At this stage the as the temperature of the core raises. The rise in temperature should cause an expansion of the core. The expansion should cause a drop in the temperature and therefore, in the nuclear reaction rate. The drop of nuclear reaction rate should stop the expansion of the Adithya

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helium core. The star should then live on burning helium at the center at the rate sufficient to balance the inward attraction of gravity.

But nothing of that sort happens to the core of red giant. It has the properties of a solid steel sphere, like all solids it expands only a considerable amount. The helium nuclei burn raising the temperature in the core. The temperature stays high; at the high temperature, helium nuclei burns even faster. It becomes hotter and hotter. The core becomes so hotter that it literally explodes. This happens with in few hours after the onset of helium fusion. This explosion is termed as helium flash.

3.2.4 After the helium flash The explosion changes the internal structure of the star in a way that causes the luminosity to decline. Up to the point at which the explosion occurred, the luminosity of red giant is fueled by furious amount of hydrogen burning in the surrounding shell. As soon as the core explodes the temperature drops, and the temperature of the surrounding shell also drops. Therefore the rate of hydrogen burning in the shell decreases. The diameter of the star decreases at the same time, for, with its nuclear energy sources greatly decreased, the red giant lacks the source needed to keep its envelope distended. Relieved of the enormous outward pressure created by the intense hydrogen burning in the shell, the envelop commences to collapse under its own gravity.

When the starts to descend from the red giant region, the helium in the core is not burning vigorously, because its temperature has been greatly reduced by the expansion of the core that followed the helium flash. During the course of the star’s decent, however, the helium in the core is again slowly but steadily compressed of gravity, because there is no nuclear energy source within it to sustain it against ties inward force. As always, the compression heats the core, and Adithya

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its temperature rises. After several thousand years, the temperature gets high enough to start a very small amount of helium burning at the center of the core. By the end of 10, 000 years, the temperature of the core has risen enough. Helium burning has accordingly become great enough.

Eventually, the helium burning will consume all the helium in to carbon and oxygen, leading to the carbon and oxygen core. While the shell is now formed of helium not hydrogen. The process repeats in more rapid way. The entire move towards the red giant stage is completed in few million years. When the core of carbon accumulates at the center of a star, and it enters a red giant for the second time, the star is very close to the end of its life.

3.3

Summary

We discuss in detail the formation of protostar the formation of a star from the protostar. Further we deal with the life cycle of the star starting with ignition of hydrogen fusion to the formation of carbon core. The red giant phase of the star. The numerous phenomenon the star passes through.

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Chapter 4

DEATH OF A STAR 4.1

Death of small star

The dividing line between small and large star is about four solar masses. Consider a star whose mass is less than 4 solar masses, in the star’s previous approach to the red giant region, its ascent was terminated by the onset of helium flash. We would expect the second ascent to be terminated in a similar manner by the onset of carbon flash, that is, an explosive rapid onset of carbon burning; However a carbon flash cannot take place in a star which is not massive enough to attain the temperature required to ignite carbon. According to evidences the temperature required for the onset of carbon burning is 600,000 K. calculations show that the star to attain the temperature required for the onset of carbon burning, it must have the mass greater than 4 solar masses. For a small star, it continues in the red giant stage, hence increasing its diameter. Eventually, the outer layer of the star becomes so red – that is, so cool – that the nuclei in these layers begin to capture electrons to become neutral atoms. The formation of neutral atoms continues unchecked until a substantial part of star’s mass is in form of neutral atom rather than separate electron and nuclei.

4.1.1 Planetary Nebula Countless photons are created by the formation of neutral atom, and then absorbed shortly thereafter on their way out of star. Their absorption heats the layer outside. The heat generated due to the absorption of photons in the outer layer is modest compared to the heat developed due to nuclear reaction at the center. The envelope heated expands in to the outer space cooling the temperature of the outer shell even more. The cooling of the shell accelerates the process of formation of more neutral atoms. As the more neutral atoms form the more photons are liberated; and Adithya

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more heat, hence more expansion. The shell continues to expand unchecked. Finally it reaches the stage where the star completely blows its outer shell in to the outer space leaving behind the dense white-hot object - the core. Surrounded by the softy glowing diffused shell of gas – the blown of envelope. Such an object is called the planetary nebula, for the reason that it looked similar to a planet in the night sky. However the luminosity of a planetary nebula is unaffected due to the constant burning of helium in the core. Now the star consists of carbon- oxygen core surrounded by helium shell.

4.1.2 The white dwarf Now the star begins its contraction from planetary nebula to white dwarf. The star is now composing of a carbon -oxygen core surrounded by the helium burning shell. The temperature at the core at this point is still not sufficient enough for the onset of carbon burning. Therefore no source of nuclear energy at the center causing the attraction of gravity and keeps the star collapsing. If there were no electrons in the star the star would have contracted until the temperature of 600 million K, and hence carbon core would have fused but this is not the case, since the core consists of numerous amount of electrons the particular in compressibility comes into play, as it did in the earlier stage of the stars life just before helium flash. No one knows what happens to the star between this point and the whitedwarf stage.

Once the star reaches the white dwarf point the course becomes clear. The star is exceedingly dim in comparison to the earlier stage of its life. Medium sized star would be 100 times fainter at this point. The diameter of the star is very much small and the shrunken star is exceedingly dense. Into its relatively small volume, no more than that of a good sized planet, is packed an enormous mass, hundreds of thousands of times greater than the mass of earth. A match box filled with the material from this dense star would weigh 10 tones. Adithya

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Although the star is very faint its surface is quite hot, with the temperature ranging up to 30000 K. such star small, dense and exceedingly faint, but white hot at the surface are called white dwarf. A man attempting to land on a white dwarf would weigh 150 million pounds, he and his spacecraft will literally be flattened by the enormous force of the white dwarf’s gravity. The white dwarf shrinks very little in radius. Slowly the white dwarf radiates the last of its heat into space, moving downward in luminosity and temperature as it does so. Progressively the white dwarf changes its color from white to yellow and then to red, until it fades to a cold, dark lump of matter and enters the grave yard off the star.

4.2

Death of Massive Star

For the star whose mass is greater than the four solar masses a different awaits. Because the weight of the star is so great, its collapse generates an enormous amount of heat. It has been found that the temperature at the center of such a star can reach a critical value of 600Million K.

4.2.1 The Super Nova Explosion The massive star with a core of carbon and oxygen surrounded by helium burning shell begins to contract under its own weight, again as a small star. In the massive star; well before the core contracts to the size of a white dwarf, the temperature reaches 600 million K which is a critical value for the onset of carbon burning. In the burning process Neon, Magnesium and other elements are formed as per the reaction

C12 + C12
Ne20 + He4 C12 + C12
Mg24 +

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The supernovas occurred by the detonation of carbon core. For stars in the intermediate mass of about 4 to 8 solar masses a violent detonation occurs as soon as the carbon burning begins. This detonation is similar to the helium flash, but more violent. The pressure generated by the detonation of the carbon core causes the star to explode. The exploding star is called the supernova. In the after math of the super nova. The dense debris is spelled out to the space carrying the elements in the star a manufactured during its life cycle. At the original place of the star there remains a small compressed star.

A second kind of supernova. If the mass of the star is very great, greater than 8solar masses, the carbon flame ceases to occur, because the density at the center never reaches the point where incompressibility can cause the flame. Instead, carbon starts burning at the core and increasing the temperature in the core, eventually the burning of oxygen also sets in forming heaver and heaver elements until the core of the star becomes iron. At this point the process stops, for iron is a very special element. The nuclear reaction involving iron do not radiate energy, instead absorbs the most of the energy cooling the temperature of the star causing the gravity to contract the core more and more rapidly.

As the consequence of the collapse, the material of the collapsing star piles up, creating very high pressure and densities. When the density of the core is so high that the neighboring nuclei touch each other, the star collapses no further leading to a violent explosion occurs. The energy realized during such a supernova is far more then the collective energy realized by the galaxy as a hole.

4.3

Pulsars and Neutron stars

In either cases of supernova a cloud flies around from the scene of the event. We know that the gas mixes with primordial gases until its identity is lost. But what happens to the center of the star of the supernova. This was unknown until 1967. Adithya

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4.3.1 Pulsars Astronomers found unexpectedly that certain places in the skies were emitting short, rapid bursts of radio waves at regular intervals. Each burst lasted no more than one hundredth of a second. The rapid succession of bursts seemed like a speeded up, celestial Morse code. The interval between the successive bursts was about 1 second. In fact, it did not change by more than one part in 10million. No star or galaxy had ever before been observed to emit signals as bizarre these.

At first it was thought that intelligent beings from a different world were trying to contact through Morse code, later it was found that the signals were more natural than artificial. The first clue to answer was the sharpness of the pulses. When an object in space emits a burst of radio waves, the waves from different parts of the object arrive to the earth at different times, blurring the sharpness of the pulse. The smaller the object the less blurred the pulse and shorter the duration; from the fact that each pulse lasted for one hundredth of a second they calculated that the pulses were not more than 10 miles in radius.

This is the startling conclusion. Until then scientists thought that the white dwarfabout 5000 miles in radius- was the smallest, densest star in the universe. How could an object as massive as sun be only 10 miles in radius? The matter in this compressed object is one billion times denser than the matter in the white dwarf, i.e. a match box filled with the material of this object would weigh 10 billion tones.

4.3.2 Neutron Stars Several theoretical astronomers pointed out that when a large star collapses and explodes as a super nova, the pressure on the core of the star compresses it so severely that individual electrons and protons combine to form neutrons. A ball of pure neutrons hence forms at the center of the star, only 10 miles in radius, but Adithya

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with large part of the star’s original mass packed into it. The scientists called this ball of neutrons as the neutron star.

Explanation of pulses of neutron stars: Why do neutron stars emit the sharp, regularly repeated bursts of radiation from which they derive their other name of pulsar? Scientists believe that a neutron star, like sun and most of the stars, is subjected to violent surface storms that spray particles and radiations into space. Each storm occurs in a localized area on the surface of the neutron star and sprays radiations in a narrowly defined direction. When the earth lies in the path of one of these storms the radio telescopes on earth picks up the signals and indicate the presence of the pulsars which is in fact a neutron star. As the neutron stars spins the stream of radiations from its surface sweeps through the space like light from a revolving light house. This would prove the fact that the neutron stars spin numerous numbers of times a second.

4.4

Black Holes in Space

With the realization of the connection between the neutron stars, pulsars, and the supernovas, many astronomers felt that it would mark the end of a star. But recent evidence has generated an amazing fact that the neutron stars are not the ultimate state. The core of a star may be squeezed beyond the 10 mile limit to about 2 miles. At that point the theory of relativity predicts the sudden occurrence of an extraordinary phenomenon.

4.4.1 Formation of a black hole According to Einstein’s theory, a ray of light should possess mass. If Einstein is right, a ray of light emitted from a star is pulled back by its own gravity. When a star is normal in size; about 1 million miles in diameter, the force of gravity on its surface is not strong enough to keep the light rays from escaping and they leave

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the star although, with less energy. But if the matter of the star is squeezed into very small volume the force of gravity is very great. This can happen to the core of a star after supernova. Suppose the core several times massive as sun is squeezed into a radius of few miles, the gravity at the surface now is billions of times than the former, the tug of that enormous force prevents the light from leaving the surface of the star; in other words the escape velocity needed is far more greater than the speed of light (3x108 meters/sec). Thus the star is invisible and is called as a black Hole in space.

4.5

Summary

The chapter completely deals with the end of any stars life cycle. It was considered that the formation of a white dwarf and the supernova marks the end of stars life, but evidences proved that, this is the case with only the average and small size stars. Massive stars have a different fate. The chapter deals with the end of small, average and large size stars. It also introduces the reader with the concept of black hole in space.

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Chapter 5

EPILOGUE When a supernova explosion occurs and the outer layers of the star are sprayed out to space, they mingle with fresh hydrogen to form a gaseous mixture containing all the chemical elements. Later in the history of the galaxy, other stars are formed out of the clouds of hydrogen enriched by the product of many supernovae. The sun is one of such stars that contain debris of countless supernovae dating back to the earliest years of the galaxy. The planets thus formed around such stars also contain the debris; and the earth, in particular, is composed almost entirely of it. We owe our corporeal existence to events that took place billions and billions of years ago, in the stars that lived and died long before the solar system was born.

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BIBLIOGRAPHY [1].

Robert Jastrow, Malcolm Thompson, Astronomy: Fundamentals and

frontiers, third edition, January, 1974 [2].

Dr. S.N. Prasad, RCE, Mysore, A text book of science, 2003.

[3].

A text Book of Physics, V. Ranganayaki, M.Y. Viswanatha Sastry, M.

Gururaj. [4].

www.wikipedia.org

[5].

Britannica, Encyclopedia – 2004.

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