Differences in Stellar Composition
In the late 1950’s astronomers began to distinguish stars by their chemical composition. The older stars (those that formed when the universe was new) were deficient in the heavier elements. The older stars are Population II, because astronomers discovered them later than Population I, the stars nearby. Their lack of heavy elements requires Population II stars to reach high temperatures before they can begin burning hydrogen. Their light at the moment of emission is bluish-white. However, the stars look red if they are far away and receding from us rapidly. Newer stars like our Sun are not deficient in the heavier elements. These stars burn their hydrogen at a lower temperature than Population II stars. They can do that because the heavier elements catalyse the nuclear reactions. This is why their light at the moment of emission is yellow or red.
NASA scientists recount this discovery as follows:
NASA scientists recount this discovery as follows:
However, a little more than a decade ago the astrophysicists started to measure the abundances of the elements in stars with good quantitative accuracy, and this changed the picture considerably. No longer was it a tenable assumption that the universe was chemically homogeneous. Two general types of abundance anomaly were found. On the one hand, it was found that the oldest stars in our galaxy, particularly those likely to be considerably older than the Sun, had a deficiency of heavier elements relative to hydrogen as compared to the Sun itself. This deficiency ranged from a factor of a hundred to a factor of a thousand in some extreme cases. The second type of anomaly concerned stars that in their overall composition might be quite similar to that of the Sun, but in which specific elements could be seen to be overabundant. Thus certain stars were observed to be enriched in carbon and other stars to be enriched in certain heavy elements.[i]
[i] Truran, J. W. and A. G. W. Cameron, Chapter 23, “Nucleosynthesis,” op. cit., p. 985.
The discovery of differences between stars led to understanding the processes that made the rest of the elements, those that were not present at the end of the first morning.
Nuclear Binding Energy
Stars release energy from nuclear reactions, not chemical reactions. When hydrogen and oxygen burn and become water vapor, chemical energy is released. This is the electromagnetic energy that attracts the negative electrons to the positive nucleus. The energy of weak nuclear reactions is about a million times greater than the energy of chemical reactions. The energy of strong nuclear reactions is about a thousand times greater that the energy of weak nuclear reactions.
When we say that stars burn hydrogen into helium, we are using the word “burn” to describe a nuclear reaction. The two kinds of burning, chemical and nuclear, are alike because both consume fuel and produce heat. They are different because nuclear reactions produce much more energy than chemical reactions. Chemical reactions release the electronic binding energy of molecules. Nuclear reactions release the binding energy of nuclei. Chemical burning can raise the temperature a few thousand degrees. Nuclear burning starts when the temperature is millions of degrees.
Nuclear reactions are much more dangerous for people than chemical reactions, because nuclear reactions emit nuclear radiation. Nuclear radiation consists of fast-moving electrons, helium nuclei, heavier nuclear particles, and gamma rays. These particles and rays can permanently damage the cells of our bodies. Chemical reactions produce thermal radiation or heat. Too much heat can cause burns, but lesser amounts are agreeable when we feel cold. To enjoy the heat of a nuclear fire safely, we need something to stop the nuclear radiation. We bask in the warmth of the Sun, unafraid of nuclear radiation, because the atmosphere of the Earth protects us.
Energy from Fusion and Fission
Nuclear binding energy comes from both the strong force and the weak force. When two lightweight nuclei combine with one another and become one heavier nucleus, they mainly release the potential energy of the strong force, the energy of fusion. When a heavy nucleus breaks up and becomes two lighter nuclei, it usually releases the energy of the weak force, the energy of fission. Midway between lightweight and heavy nuclei is iron, the most stable element of all. The most abundant form of iron has 26 protons and 30 neutrons. It takes energy either to break up an iron nucleus or to add to it. Neither fission nor fusion releases any energy from the most abundant iron nuclei.
Of all the nuclear reactions, the ones that release the most energy per particle are fusion reactions that turn hydrogen into helium. This is what powers the Sun and most of the stars.
Nuclear Binding Energy
Stars release energy from nuclear reactions, not chemical reactions. When hydrogen and oxygen burn and become water vapor, chemical energy is released. This is the electromagnetic energy that attracts the negative electrons to the positive nucleus. The energy of weak nuclear reactions is about a million times greater than the energy of chemical reactions. The energy of strong nuclear reactions is about a thousand times greater that the energy of weak nuclear reactions.
When we say that stars burn hydrogen into helium, we are using the word “burn” to describe a nuclear reaction. The two kinds of burning, chemical and nuclear, are alike because both consume fuel and produce heat. They are different because nuclear reactions produce much more energy than chemical reactions. Chemical reactions release the electronic binding energy of molecules. Nuclear reactions release the binding energy of nuclei. Chemical burning can raise the temperature a few thousand degrees. Nuclear burning starts when the temperature is millions of degrees.
Nuclear reactions are much more dangerous for people than chemical reactions, because nuclear reactions emit nuclear radiation. Nuclear radiation consists of fast-moving electrons, helium nuclei, heavier nuclear particles, and gamma rays. These particles and rays can permanently damage the cells of our bodies. Chemical reactions produce thermal radiation or heat. Too much heat can cause burns, but lesser amounts are agreeable when we feel cold. To enjoy the heat of a nuclear fire safely, we need something to stop the nuclear radiation. We bask in the warmth of the Sun, unafraid of nuclear radiation, because the atmosphere of the Earth protects us.
Energy from Fusion and Fission
Nuclear binding energy comes from both the strong force and the weak force. When two lightweight nuclei combine with one another and become one heavier nucleus, they mainly release the potential energy of the strong force, the energy of fusion. When a heavy nucleus breaks up and becomes two lighter nuclei, it usually releases the energy of the weak force, the energy of fission. Midway between lightweight and heavy nuclei is iron, the most stable element of all. The most abundant form of iron has 26 protons and 30 neutrons. It takes energy either to break up an iron nucleus or to add to it. Neither fission nor fusion releases any energy from the most abundant iron nuclei.
Of all the nuclear reactions, the ones that release the most energy per particle are fusion reactions that turn hydrogen into helium. This is what powers the Sun and most of the stars.