Helium (chemical symbol He) is the second most common element in the universe, the most common being Hydrogen. It is important in astronomy, as it is both a product and fuel burnt within stars, and in cosmology, as primeval helium was formed in the first few seconds after the Big Bang. Despite making up 24% of the observable matter in the universe, it is quite rare here on earth. Stable Helium-4 has 2 protons, and 2 neutrons and does not react easily with other atoms.
Helium can be produced by 2 main methods in astronomy, each of which has important implications in how we observe and interpret the universe.
The majority of the helium in the universe was produced in the first few seconds after the big bang. The big bang is the accepted scientific (if perhaps not religious) explanation as to how the universe was formed. This was a huge explosion, so big as to be beyond imagination. This explosion caused temperatures that were initially millions upon millions of degrees. So hot, that matter, as we know today, could not exist, but rather, there was only radiation and energy. As the universe expanded, it cooled, and the energy changed first into sub – atomic particles, and then into hydrogen and deuterium atoms. At this point, Helium could be formed, but only if a correct sequence of collisions occurred between these particles. For example, 3 deuterium atoms would need to collide (each with enough, but not too much energy) in sequence to form Helium – 4 atoms. Another way is for 2 deuterium atoms and a neutron to collide, resulting in Helium and a release of energy in the form of a gamma ray.
This fusion process could have continued to produce heavier atoms, however, as the universe expanded, it also cooled. This cooling and the decrease in chance of the right combination of collisions means most of the early matter (over 99%) remained as either hydrogen or helium. The amount of primeval helium can therefore tell us a great deal about the conditions in the big bang.
The second method of helium production is within stars. Stars are formed from giant clouds of interstellar gas, clumping together, and forming a ball under it’s own gravity. When it gets hot and dense enough, the hydrogen will start to ‘burn’, and it will form a star, much like our own sun. Don’t mistake this burning with the burning we have on earth. When astronomers talk about hydrogen burning, they are referring to hydrogen fusion reactions – i.e. a nuclear reaction. In the core of a star, the hydrogen will be at a great enough density and hot enough, that when atoms collide, they have a chance of fusing together. With the right collisions, helium can be formed, and energy in the form of heat, released. At this stage, the core will not be hot enough for helium burning to occur, because if the temperature increases, the core will expand, and so cool. However, over time, the concentration of hydrogen decreases (its being burnt) and helium increases (its being produced), until some point, the chance of colliding hydrogen atoms will decrease. This results in the core contracting, whilst the outer shell of gas expands, and so forms a red giant. The collapsing core heats up through the release of gravitational energy, until the concentration and temperature are high enough for helium burning to start. Lithium, carbon and other elements are produced this way. This continues, depending on the size of the star, until iron is produced. At which point reactions stop and the star will go super nova.
Our sun is currently burning hydrogen, and will do so for about another 4 billion years. When helium-burning starts, the sun will already have expanded so much as to engulf the earth in its upper atmosphere. Fortunately, I don’t think we need worry ourselves about it too much.
