Study material

Introduction

The story of stellar evolution is long and fascinating. From birth to death, a star advances through a myriad of stages determined by intricate interplay of all the four fundamental forces ofnature-gravitation, electromagnetic, weak and strong. At the present level of understanding, it appears that after a star has exhausted most its fuel, it is fated to end its life as a compact star in the form of white dwarf, neutron star or a black hole, depending on its mass. What are compact stars? How do they differ from normal stars and do the neutron stars convert themselves into stars made of constituents of neutrons, namely quarks? To understand these questions let us breifly recuont the story of evoultion.The stars are composed of gas and are in hydrostatic equilibrium, which means that in every point in the interior, pressure must be sufficient to bear the weight of the underlying layers. This pressure has to be obtained by maintaining either high density and relatively low temperature or by having a lower density and higher temperature at the centre. To maintain this pressure, the star has to burn fuel. How does it all start? The

Where are Stars Born?

Astronomers believe that molecular clouds, dense clouds of gas located primarily in the spiral arms of galaxies are the birthplace of stars. Dense regions in the clouds collapse and form 'protostars' so a star begins its life as a large and comparatively cool mass of gas. The contraction of this gas and the subsequent rise of temperature continue until the interior temperature of the star reaches a value of about 1,000,000°C (about 1,800,000°F). At this point a nuclear reaction takes place in which the nuclei of hydrogen atoms combine with heavy hydrogen deuterons (nuclei of so-called heavy hydrogen atoms) to form the nucleus of the inert gas helium. The latter reaction liberates large amounts of nuclear energy, and the further contraction of the star is halted. Once the star has started nuclear fusion, it becomes a 'main sequence' star.

The Star that Dies: The White Dwarf and the Supernova

At some point all of the nuclear fuel in the star has been exhausted. A greater part of the hydrogen has been converted into helium and most of helium has been converted into carbon and oxygen. What happens now to the star depends on the mass of the star.

• small stars

Smaller stars, about 4 masses of our Sun or less, cool down. Given time, the outer layers of the star become cool enough to leave the plasma state. The atoms reverse to their neutral state and capture electrons. The capture of electrons accelerates the expansion of the outer layers, which causes more atoms to leave the plasma state. The envelope of the star becomes finally a transparent and extensive shell of atoms; this shell can only be seen from the side from very long distances, thus giving the surroundings of the star a peculiar appearance of a luminous ring. Once upon a time, astronomers believed those rings were the first stage of formation of planetary systems; because of that the rings were called "planetary nebulae" . We know today that there is no connection between the planetary nebulae and planetary systems, but the name remained. The only remnant of the star is now the core of the star and it is a tiny and not very bright object in the middle of the nebula. In the beginning the core of the star is still glowing with a white glow, dissipating the heat from the nuclear fires, now extinct. It is called a"white dwarf" . A white dwarf weighs much less that the original star, for example a star four times heavier than the mass of our Sun gives origin to a white dwarf having 1½ of the mass of our Sun.

• Medium sized stars

Larger stars, between 4 and 8 masses of our Sun, encounter a more violent fate. The stopping of nuclear reactions makes them collapse more rapidly and more violently than the small stars. The core of the star consists now of solid carbon, which is not burning. However, the contraction of the star generates enormous amounts of heat. At the point when the temperature of the core reaches 600 000 000 K, the carbon starts a nuclear reaction, generating neon, helium, magnesium and some other elements. But again, the core of the star is solid and cannot expand to release the internal pressure that builds up because of the nuclear reaction. The core becomes an uncontrollable nuclear reactor in alike manner as in the case of helium flash. But now, the temperature is higher, the pressure is higher and the result more violent. The star explodes in a SUPER-NOVA. The explosion is so bright that a super-nova can even be seen on the Earth in daylight. The explosion shatters the star and may be so violent that it disperses all of the material of the star in outer space. If there are any remnants of the star, it will be a small compressed partt of the core of the star.

• Large stars

Really large stars, with masses greater than 8 masses of Sun, do not become a super-nova at this point of time. The stopping of nuclear reactions causes contraction, like for the smaller stars. But the core of the largest stars does never become as dense as the core of medium sized stars. This is probably caused by the intense radiation inside the core of the largest stars, giving an abundance of highly energetic photons that drive out matter from the centre of the star. At some point the temperature of the core reaches the 600 000 000 K that ignite the carbon. The nuclear reaction will not be as violent as in medium sized stars, because the core of the largest stars is less dense. The carbon core burns at moderate pace; the temperature eventually increases, putting also oxygen on fire. when carbon and oxygen are exhausted, the star cools down and shrinks again, which again heats the core of the star to higher temperatures. Those higher temperatures ignite the heavier elements produced from coal and oxygen, giving still heavier elements. After a while the star is a series of envelopes contained within each other; each of the envelopes burns different chemical elements. The heavier elements are in the inner envelopes, while the helium and hydrogen are in the outer envelopes.

A star at this stage of life can produce elements no heavier than iron. The nuclear reaction comes to an end with iron. Iron does not engage in nuclear fusion. Fusion of elements less heavy than iron releases energy, while fusion of iron and elements heavier than iron consumes energy.

The creation of iron extinguishes the nuclear fire inside the star. The star shrinks for the last time. The iron core of the star absorbs most of the heat generated by the contraction of the star, which accelerates the contraction even more. When the temperature inside the core reaches trillions of degrees and the neighbouring atomic nuclei touch each other, there can be no more contraction. Instead, the star rebounds in a great explosion.

This explosion is also called a SUPER-NOVA and may be as spectacular as for the medium sized stars. The star is billions times brighter than any time previously and it may even be as bright as an entire galaxy.

The explosion of the heavy super-nova shatters even the atomic nuclei to pieces; those pieces get captured by other atomic nuclei, forming elements beyond iron, like silver, gold and uranium. Elements beyond iron do not abound in nature - and that is attributed to their creation during the short super-nova blast. Those heavier elements can later be captured into other clouds and become part of new stars and new planets. Because of that, heavy elements like uranium should also exist in stars. For many years it was only a theory; in the beginning of year 2001 the "European Southern Observatory" (ESO) in Chile discovered that the star called CS 31082-001 indeed has uranium in it. This was the first ever measurement of uranium outside of our planet.

The Remnants of the Stars: Black Dwarfs, Pulsars, Neutron Stars and Black Holes

• The Black Dwarfs

A white dwarf cools down slowly. The colour of the glow of the surface changes from white to yellow, to orange and red. Finally the remnant of the star becomes a cold dark lump of matter - the black dwarf. The black dwarf has the size of our planet and a gravity that is millions of times higher than the gravity we experience on the Earth. The black dwarf is simply a quiet, desolate and dead remnant of a star, moving forever through the cold Universe.

• Pulsars

Some time ago it has been discovered that there are celestial objects that emit extremely regular radio signals, no longer than 1/100 of a second. In the beginning the scientists thought that it was a signal from an alien civilisation. But the signals were emitted over a very large band of radio frequencies, thus requiring tremendous amounts of energy. By measuring signal distortion the scientists came to conclusion that the object emitting the signals was around 10-20 kilometres in radius, and yet as massive as the Sun. The signal's interval and duration came from the object's rotation. It resembles of a lighthouse with the light beam sweeping around. But what are those objects?

• Neutron Star

The existence of neutron stars has been predicted by theoretical astronomers. It has been pointed out that during the supernova explosion the star core (or the remaining part of it) can become so compressed that protons and electrons may be forced to merge. Merging protons and neutrons form together neutrons. The neutrons of the star would form a very compact ball with a radius of maybe 10-20 kilometres and with most of the star's mass packed inside it. The matter in a neutron star would be so dense that a cubic centimetre filled with it weighs billions of tons. There have been no direct observations of neutron stars. In the places where scientists predicted one would find neutron stars, pulsars have been found instead. Nowadays scientists are certain that pulsars and neutron stars are the same thing.

• Black Holes

A very massive star core, remnant of a super-nova explosion, can exert such a tremendous gravitational force that not only solid objects, atoms cannot escape from the star's surface. Also light "falls down" to the surface of the star. That kind of object is called a "black hole" . The matter within the black hole probably shrinks to smaller and smaller volumes all the time. The star shrinks to a few kilometres, then a few centimetres and - finally - to a "singularity", which is one single point in space. Even though the matter inside the black hole collapses into sizes smaller than anything mankind has ever measured, the black hole itself does not change in size. After all, the name "black hole" applies to the radius around the degenerated core of the star, determining the line between place where we still can see into and the place where we can't see anything. This radius determines the size of "event horizon".

How are compact stars observed ?

White dwarfs, because of their small radii, are characterised by much higher effective temperatures and are luminous and radiate away their residual thermal energy. Conversely no light can escape from a black hole. Isolated black holes appear absolutely 'black' except when quantum mechanical effects are taken into consideration. Neutron stars lose thir energy through neutrino emission. Strange stars cool more rapidly through neutrino emission and have interesting surface properties. Thus

1. White dwarfs are observed directly by optical telescope.
2. Neutron star are pulsating radio sources(pulsars) or as Xray pulsars.
3. Black holes indirectly through their influence on environment, for example gas -accreting a periodic x-ray sources.
4. Strange stars through their ability to circumvent eddignton limit and as rapidly rotating submillisecond pulsars.