October 30, 2003

The First Stars

Stars play a fundamental role in origin of the elements of the universe. The atoms in the universe can be organized into a table, known as the periodic table of the elements. Each element has its own type of atom, built up of neutrons, protons and electrons. The more neutrons and protons an atom has, the heavier it is. Some atoms are very stable and long lived. Others, like uranium, decay very rapidly. The periodic table lists all the atoms in order of increasing weight, starting with the lightest element, hydrogen and ending up with some remarkably heavy elements that have been synthesized in the laboratory. Each element can be identified by the number of protons and neutrons it contains in its nucleus. For example, hydrogen only contains one proton and no neutrons. The next element is helium which has two protons and two neutrons. Carbon has six protons and six neutrons while oxygen has eight protons and eight neutrons. A very stable element is iron which has twenty six protons and thirty neutrons. The lightest two elements are synthesized in the first three minutes of the life of the universe. At that time, the universe was very dense and had a temperature of about a billion degrees Kelvin. Individual protons, which are nothing more than the nuclei of hydrogen, came together with neutrons to form the nuclei of helium atoms. Things happened far too quickly for any of the heavier elements to form.

The heavier elements of the periodic table were formed when the universe was already hundreds of millions years old. At this time the universe had cooled sufficiently for clouds of hydrogen and helium to start collapsing under their own gravitational pull. Gravity is attractive and the bigger, or more dense an object is, the stronger the pull. As these clouds collapsed, they became denser and hotter. Sufficiently big clouds collapsed with enough force that they compressed the cores at their centres. In these cores, the hydrogen atoms would be pushed together with such strength that they would fuse into helium atoms, releasing a tremendous amount of energy in the process. It is through this process of hydrogen burning that the stars that we see shine. As time goes by, most of the hydrogen burns out and, if the star is sufficiently heavy, the helium inside the stars continues to burn to form carbon, oxygen and even heavier elements. Gradually the cores of these stars accumulate a stockpile of heavy elements. This soup of atomic nuclei is ejected when, at the end of its life, these massive stars die in a supernova, a massive explosion that rips it apart and can emit as much energy as a whole galaxy. The heavy elements are then distributed throughout the interstellar medium to be reprocessed by other stars or even land on planets such as ours.

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