Our Cosmic Home Study Guide (page 3)

Updated on Sep 25, 2011

How Elements Are Born

You, me, the other animals, all the trees, even the atmosphere and rocky Earth itself are made of chemical elements that were born (manufactured) in the nuclear furnaces of stars. Elements can be characterized by their atomic numbers, which is the number of protons in their nuclei. As we have seen, the elements that existed at 300,000 years after the Big Bang were only the ones with the lowest atomic numbers, the light elements of hydrogen, helium, and a trace of lithium. The key concept to the formation of all other elements is nuclear fusion.

Figure 1.6 shows a general diagram for how nuclear fusion works. Atomic nuclei from two or more elements are squeezed by hot temperatures and pressures in the center of a star to create a new fused nucleus. For atoms from hydrogen up to the atomic number of iron, energy is released when atoms are fused to make larger atoms. This is because the protons and neutrons inside the nuclei of the larger atoms (again, up to iron) contain less mass per subatomic particle and therefore less energy according to Einstein's equation E=Mc2 (where E is energy, M is mass, and c is the speed of light). The excess energy of fusion is released as heat and radiation. To us, this released energy is the warm sunlight we feel and all the light we see from stars.

Figure 1.6 Nuclear Fusion

Stars are hot and are able to emit vast quantities of radiation into space because of fusion reactions deep within their cores. Inside stars, the first element to be fused is hydrogen, the most abundant primordial element. Under intense temperature and pressure, two hydrogen atoms are fused into one atom of helium, releasing energy and making stars hot, thus sustaining further fusion reactions. When the hydrogen is used up, helium is fused into carbon, and then the carbon and some helium are fused into oxygen. All the elements up to iron can be made in this way. Note the sequence of how elements are made: Hydrogen (H) → Helium (He) → Carbon (C) → Oxygen (O). All these fusion reactions release energy.

Stars can run out of matter to fuel fusion, they can "die." Some stars die by throwing off gases then withering into small, smoldering white dwarfs. Don't worry; we still have billions of years to go before that!

Very massive stars, on the order of ten times the mass of our sun, can create supernova explosions at their deaths. One supernova, for example, occurred in our galaxy in A.D. 1066, which is now the Crab Nebula. Ancient people then observed this bright new star in the sky before it faded.

Supernovas are important parts of how our universe works. They do two special things. First, all elements heavier than iron (such as gold and uranium) are made in the intense heat and pressure of the supernova. Second, the supernovas disperse all the elements inside the former star out into space. We can see these elements in the emission and absorption spectra in the regions surrounding former sites of supernovas. Note that in the dispersal of elements by supernovas, two categories of elements can be found: those that were made earlier in fusion reactions during the long, ordinary lifetime of the star, as well as those that are new (that is made only in the supernova itself).

The elements dispersed into space can eventually gather into gas clouds and possibly contract, after mixing with remnants of other supernovas, into totally new stars and their planets. We are, literally, stardust!

Practice problems of this concept can be found at: Our Cosmic Home Practice Questions

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