1. The Evolution of Atoms

1.7 The Formation of Heavy Atoms

Thirteen billion years ago, the Milky Way was a rotating mass of turbulent hydrogen gas. Under the influence of gravitational forces, the gas in regions of low turbulence and high density condensed into 100 billion stars. As the stellar material contracted, gravitational potential energy converted into thermal kinetic energy and the interior of the star became very dense and hot. When the temperature reached 107 degrees Kelvin, and the density reached about 100 grams of hydrogen per cubic centimeter, this hydrogen began to interact. The fusion reaction produced helium and released even more heat. Contrary to man-made hydrogen bombs, the hydrogen found in the Universe is of a common type that will fuse peacefully without explosions. Its product of fusion will only be helium. This fact is very important; it is hard to imagine how further evolution would have been possible if the hydrogen had been more reactive, so that the Universe would have been only a succession of firecracker explosions.

The conversion of hydrogen "fuel" into helium "ash" occurs preferentially in the core of stars because the temperature and density are highest there. There is however no mixing of the helium "ash" with the hydrogen still present in the outer membrane of a burning star. As long as the hydrogen will undergo fusion, further collapse is not possible because the energy released by the fusion reaction opposes it. In the long run, the depleted central region stops releasing energy, while the outer shell reaches temperatures of 3 x 107 degrees K. Once this stage is reached, the helium in the central core begins to contract. Again, gravitational energy is transformed into heat. The sudden rise in temperature of the core of a contracting star heats up the envelope. This envelope will expand tremendously, and a Red Giant will be formed (fig. 1.8). In the Red Giant, the temperature of helium reaches about 108 K°, and the density approaches 105 gm (100 kilograms) of helium per cubic centimeter.

Figure 1.8. Comparative sizes of various stellar objects. All objects have the same mass, i.e. that of the sun. The Black Hole is only 6.4 km in diameter. The diameter of the Neutron Star is about 20 km. The diameter of the White Dwarf is the same as that of the earth and the diameter of the Red Giant is 320 million km.

These are conditions that will allow fusion reactions of helium nuclei. Carbon and oxygen will be formed by this fusion. This process is accompanied by a release of energy that may provoke the explosion of the star, causing hydrogen, helium, carbon and oxygen to be blown into space, where these elements may condense, later on, into secondary stars.

Fusion reactions started in the sun about 5.5 billion years ago. These will continue for about 5 billion years more, before the conditions required for the next contraction will be met. In five billion years, the sun will have consumed most of its fuel and have grown, in the process, to a diameter of 320 million kilometers, swallowing up Mercury, Venus and the Earth. At this stage of its evolution, the sun will be a Red Giant, whose substance will be only a tenth as dense as air. The density of the sun is now about a fifth of that of the Earth. If the star survives the Red Giant stage, the process of fusion will continue until all its helium has been transformed into carbon and oxygen. At the point of helium exhaustion, the star will again contract, and thus heat up, so that the heavier elements now present will begin in their turn to undergo fusion. The result of this fusion reaction is the formation of neon, magnesium, silicon and sulfur. The bloated Red Giant sun will then reverse its expansion and start contracting. This process will stop only after the sun has reached a mere one-hundredth of its present diameter. It will have thus contracted then to the size of the earth. At that stage of its evolution, it will be a White Dwarf (fig. 1.9).

Figure.1.9. JPLCalltech/ESA/NASA. White dwarf, at 650 light-years. The dwarf is the barely visible tiny white point in the center of the picture. In blue, we see a hot medium that cools down toward the exterior, taking a red color. More Pictures can be obtained at www.spitzer.caltech.edu.

The density of the sun will then be enormous. All that remains thereafter for the sun to do is to cool off and pass from the stage of a White Dwarf to that of a Black Dwarf, i.e. a dead star.

This normal fate for stars that have the size of the sun is well documented. Many stars exist, whose diameter is up to 50 times that of the sun. In this case, the gravitational pull originating from the center of the star at the White Dwarf stage will become extremely strong. If the star remains stable and does not explode, the evolutionary process continues. At temperatures near 3 x 109 degrees and densities of 3,000 kilograms per cm3, with various atoms now available, other nuclear reactions more complicated than simple fusions do occur, with the production of cobalt, nickel and iron. These elements are very stable.

Fusion cannot create elements heavier than iron. Elements as gold, lead, and uranium had to be forged in some other way. Their synthesis depends on rare events. One possibility is the slow formation of such elements in secondary stars that have reached the iron-group stage. The burning of helium for hundred of thousands of years produces copious amounts of neutrons. These bombard the light elements produced by fusion in the aging star. Under the neutron assault, these atoms capture more and more of the neutrons and become heavier and heavier. This slow process accounts for roughly half of the elements beyond iron. The other half of elements heavier than iron are due to a rapid process, that requires a million billion times as many neutrons as a dying star can produce: the light atoms must be bombarded with an immense number of neutrons in a matter of seconds. After the bombardment ceases, the products decay into stable and semi stable elements (such as uranium) that dot our solar system. The rapid process is the gravitational collapse of a primary star that succeeded in reaching the iron-group stage. If the star is very big, the gravitational pull may be so great that the star will collapse through the White Dwarf dimension. Owing to the extreme densities and temperatures involved, the White Dwarf will explode into a supernova. A supernova is the final uncontrolled explosion and death of a star. Supernovae come in two flavors: the types Ia are thermonuclear explosions of white dwarfs. The brightest type Ia supernovae are used to estimate the age and expansion of the universe. The types Ib are produced by the core collapse in massive stars. A supernova will outshine for a few days an entire galaxy, while as much as 90% of the star’s mass could be ejected into space. The collapsed core of the supernova will eventually remain and find equilibrium as a Neutron Star.

On earth, ordinary matter can be compressed to a density of about 2.7 x 1014 grams per cubic centimeter. This is 2.7 x 1011 kilos per cubic centimeter and represents the saturation density. In supernovae, the density is about 4 times the saturation density and, in neutron stars, it is nine times the saturation density. These Neutron Stars have fantastic densities: gravitational pull has pushed the electrons into the protons and, in this way, formed neutrons. Pulsars are very probably rotating Neutron Stars. A star like the sun, upon reaching the stage of the Neutron Star, would have only about 10 to 20 kilometers in radius; the sun has now a diameter about 70,000 times greater than the diameter of the corresponding neutron star.

Once a total mass equal to 3 solar masses is involved in the formation of a Neutron Star, the densities produced during the contraction would be too great to reach equilibrium and the Neutron Star will explode. An alternative to this fate would be further contraction of the Neutron Star into a Black Hole. Two stars have been observed that do not fit within the process here described: too small, too cool and too dim to be neutron stars. These strange objects may be composed of quarks.

Conventional “collapsar” theory, first proposed by Stan Woosley of the University of California, Santa Cruz, holds that gamma ray bursts5 occur when a star at least 10 times as massive as the sun collapses into a black hole. At that moment, it spews jets of matter into space at close to the speed of light and, in the mean time, blows away its outer layers in an enormous supernova explosion. Supernovae are powered by the slow decay of radioactive nickel, and will take one to several weeks to reach their maximum brightness. The collapsar model predicts that the gamma ray burst occurs before the supernova reaches its peak brightness. The Hubble Space Telescope allowed verifying that the gamma ray burst followed the supernova instead of preceding it. It indicates that the core of the exploded star first collapses into a neutron star, which triggers a supernova. During the supernova explosion, there are produced silicon, sulfur, argon, magnesium and calcium. Heavy elements that are produced by nuclear fusion are ejected into space during the supernova explosion. This is followed by the collapse of the neutron star into a black hole, which provokes the gamma ray burst.

The earth is provided with a relative abundance of heavy elements. It has long been thought that a Supernova explosion occurred near our part of the galaxy before the formation of the earth and fertilized the solar system’s embryonic cloud with rare isotopes. Superior life needs such elements to thrive and progress. In this hypothesis, the chance for superior life to appear somewhere in the universe becomes very hazardous. At a meeting held in 20016, an alternative source of relatively abundant quantities of heavy isotopes on earth was proposed, based on the observation of the evolution of 43 stars with a size similar to that of the sun, at ages 300,000 to 10 million years. Ninety five percent of them actively emitted x-rays almost continuously with astonishing ferocity. The flares were about 30 times more powerful and 300 times more frequent than the most intense flares unleashed today by the sun. This study indicates that the sun was unexpectedly and outrageously energetic in its first million years and was an intense accelerator of solar cosmic rays, boosting protons and other particles to near the speed of light. These, in turn, would have created radioactive isotopes readily. However, cosmic rays alone are unable to account for all the types of heavy isotopes now recorded on earth. The story is probably more complicated than it appears, even if one assumes that both solar cosmic rays and a supernova explosion nearby contributed to the supply of the earth in heavy isotopes.

The halo of our galaxy contains the oldest stars. They were formed over 10 billion years ago, little later than the galaxy itself. These stars are very deficient in heavy elements. Some have over 1000 times less heavy elements than the sun. The enigma is that, up till today, not one single old star has been found that was totally devoid of heavy elements. Since we have assumed that matter consisted initially only of hydrogen, helium, lithium and monopoles, old stars should be composed of only these elements. Yet all of them are contaminated with heavy elements.

Hydrogen in the proportion of 75%, helium at 25%, deuterium at 0.01 % and traces of lithium were formed during the three minutes that followed the Big Bang. The cooling and the expansion thereafter effectively stopped the process of synthesis of these elements. Other elements could not be created, unless in stellar nuclear reactors. Hydrogen transmutes into helium at a few dozen millions degrees. Helium needs 100 million degrees to form carbon and 500 million degrees are needed for carbon to produce neon, oxygen, magnesium and silicon. At higher temperatures, chromium, manganese, nickel and iron appear. Beyond that stage, it is only through an implosion that the star is able to produce the fantastic temperatures needed to create elements heavier than iron (e.g. lead, silver, gold, platinum, uranium). With stars ten to sixty times more voluminous than the sun, this whole process, ending up with the supernova implosion, occurs in a few million years. On the contrary, smaller stars need tens of billions of years to accomplish the same evolution, after which they fail to implode but simply cool off.

Immediately after the Big Bang, the first stars that accreted from the primeval hot nebula were very massive, i.e. about 60 solar masses. They very rapidly went through all the stages of heavy elements formation and, just as rapidly, exploded. The rapid formation and dissemination of these elements was an occasion for younger, smaller stars that appeared later to incorporate these elements so that all existing stars possess at least a minimum of these heavy elements.

The sun is a primary star that peacefully and all by itself burns its hydrogen fuel. When the protostar that formed the sun imploded, a large proportion of the released matter condensed separately. This matter formed Jupiter, which alone contains 75% of the mass of all the planets. Jupiter and Saturn together form 90% of this mass. The remaining matter composes the 7 other planets. Note that Jupiter influences the fate of the earth : his gravitational attraction on asteroids compresses their trajectory in a more ellipsoidal form to such an extent that the ellipse of some of them may cross the path of the earth. A chance encounter of earth with a big asteroid is scheduled in the years 2035.

How long did the formation of planets take?

Early estimates asserted that it had taken about 100 million years for Earth and the other planets to form. Today, it is estimated that the accretion was accomplished in a few tens of millions of years. How were the measurements done? They are based on the radioactive decay of hafnium-182 to stable tungsten-182, with a half-life of 9 million years. As a planet accretes, molten iron separates from the rock and falls inward to form the metallic core. The iron carries tungsten with it but leaves hafnium in the rocks of the mantle, whose “decay clock” is effectively reset to zero. A comparison was made between isotopic abundance in meteorites and in the mantle of the earth, wherewith it was found that Earth’s accretion was complete by 30 million years after the solar nebula formed.

References

5. The wavelength of gamma rays can be as short as 0.001 angströms.

6. Two years of Science with Chandra, 5-7 September 2001, Washington, DC.

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