The Making of Elements

 

We may think that supernovas (exploding stars) are remote and uninteresting, but without them we would not exist. There would be almost no chemistry and absolutely no life anywhere in the universe.

With the exception of hydrogen, which was created in the big bang that started the universe off, all the atoms of all the chemical elements on which biology depends.were cooked up in large stars.

The supernova in which a large star explodes at the end of its life releases all the atoms it has made into the dust of interstellar space. The dust then gets incorporated into other solar systems. Each atom in our bodies, apart from the hydrogen, went through a supernova billions of years ago before our solar system was formed..

The different elements are defined by the number of protons in the nucleus of each atom. Hydrogen, with a single proton, is both the simplest element and the building block for all the others. Nuclei with more than one proton may also need one or more of another elementary particle called neutrons to keep them stable. Either too many or too few neutrons can make the nucleus break down.

Nuclear physicists have been working for the last few decades on reaction by which the sun, cooks hydrogen, which has one proton and can have one or two neutrons, into helium, which has two protons and two neutrons. The reaction, which also occurred in the big bang, generates huge quantities of energy.

In an unsustainable form the hydrogen fusion reaction is the basis of the H-bomb. Last week in this column Clive Cookson described attempts to control and sustain the reaction so that the energy it releases can be used to generate electricity.

Astronomers and theoretical physicists have worked out and checked the recipes by which fusion reactions produce the heavier elements. “It’s all starting to come together now” says Gerry Gilmore of the Institute of Astronomy in Cambridge. “The physics is very tricky because it happens in such a violent environment” he says.

One of the early breakthroughs was made by another Cambridge scientist, Fred Hoyle, who received the Crafwoord prize, the astronomers’ equivalent of a Nobel prize, last year for proving that most elements could not have been made in the big bang.

There are two features of the universe that prevented heavy elements being formed at the start. The first is that only very high speed nuclear cookery could be completed. The universe cooled and expanded rapidly as it formed. By the time the it was three minutes old it was too cool for atomic nuclei to fuse together. “Anything that takes more than three minutes to make couldn’t come from the big bang.” says Gilmore.

The second feature that prevented heavy elements forming is that beryllium, which is made by fusing two helium atoms, decays with a half-life of less than a trillionth of a second. Consequently the only new elements cooked up by the big bang were helium, which has two protons and lithium, which has three.

Astronomers and physicists are very confident in their assertions about what went on 15 billion years ago in the big bang. One reason is that the physics involved is very well developed. “Big bang models are just H-bomb models; we understand them” says Gilmore.

A more interesting reason for confidence is that models of the big bang, predict that hydrogen and helium should exist in the same proportions everwhere in the universe. The prediction holds up very well. Astronomers can tell the mixture of elements in a star from the colour of the light it emits. Individual stars vary, but the universe as a whole consists of about 75 per cent hydrogen and 24 per cent helium

Fortunately for us the primordial mix of elements produced by the big bang gets cooked further in the stars. The type of cookery depends on the size of the star. The heating is produced by gravity squeezing the material together, so bigger stars get hotter and burn faster.

Our sun, which is only 15 million degrees celsius inside can only make helium. The crucial reaction for generating heavier elements is the fusion of three helium nuclei to make carbon. This reaction occurs in stars about 10 times the mass of the sun when they have burned all their hydrogen into helium.

Another piece of luck is that the bigger hotter stars don’t just produce the heavier elements, they also explode at the end of their short lives to blow these elements back into space so that they can be part of smaller stars and more hospitable solar systems.

By the time our sun formed 4.5 billion years ago there had been enough cycles of star formation to generate a respectable mix of heavy atoms in the interstellar mix. The universe is still 99 per cent hydrogen and helium. The commonest elements in the other 1 per cent are carbon, oxygen and nitrogen. “It’s not chance that we are made of this stuff” says Gilmore.

 Posted by at 19:26