GS5: Evolution of the Elements: Factory which has not stopped yet!
Updated: Jun 27, 2020
When I was introduced with the several elements during my early classes in school, there was one question that popped up in my mind, Where did the elements come from? How was it formed? Do we only have a few elements? Are there elements yet to be discovered?
The first generation stars which formed right after the Big Bang constituted only of the light elements, which were hydrogen, deuterium, helium (2 isotopes), lithium and up to boron, as they were the only products of the Big Bang nucleosynthesis. This happened because of the temperature constrain, as for the formation of larger atoms with higher atomic number require greater temperature to overcome the threshold barrier for its formation. In contrast, the Universe of today contains 92 naturally occurring elements. Remember the only element with atomic numbers up to 5 were formed in the Big Bang nucleosynthesis. Then, where did the other 87 elements come from?
Supernova Remnant W49B, NASA
Physicists have shown that the elements such as carbon, silicon, gold, silver, uranium and others, are formed during the life cycle of stars, by the process of Stellar Nucleosynthesis. And, because of this process, we can consider stars to be 'Element Factory', constantly yielding larger atoms out of smaller ones. Not all the atoms formed in the stars during its lifetime are used in the production of new, larger atoms. Otherwise, how do we get lighter elements?
Some of them escape from the gravitational pull of the star into space. This stream of atoms emitted from a star during its lifetime is a stellar wind. Some escape only when a star dies. A small or medium star (like our Sun) releases a large shell of gas as it dies, forming a red giant, whereas the large star blasts matter into space during a supernova explosion. Most very heavy atoms, with atomic number greater than that of iron form during supernova explosions. Once ejected into space, atoms from stars and supernova explosions form new nebulae or mix with the existing nebulae.
When the first generation of stars died, they left a legacy of new, heavier elements that mixed with residual gas from the Big Bang. The second generation of stars and associated planets formed out of these more diverse compositions of nebulae. Then, Second-generation stars lived and died, and contributed heavier elements to third-generation stars. Succeeding generations contain a greater proportion of heavier elements. Since, not all stars live for the same duration of time, at any given moment the Universe contains many different generations of stars.
Summary of Stellar Nucleosynthesis (Burbridge, 1957):
Hydrogen Burning: The quiescent burning of the star forms Helium by the fusion of Hydrogen atoms by the process of a proton-proton chain reaction increases the core temperature to about 1,00,00,000 K (Atkinson and Houtermans, 1929). This process covers much of the life of the star and is a slow process. For the Sun this stage will last for 10 Ga but for most big stars (15 times the mass of the Sun) this stage may remain for 10 Ma.
When most of the hydrogen in the star is used up, the star contracts and its temperature rises greater than 1,50,00,000 K in the stellar core. However, the resulting rise in core temperature causes expansion of the outer hydrogen-rich layer of the star. This forms a huge low-density envelope whose surface temperature may fall to about 4000 K, observed as Red Giant. At this stage, a nuclear reaction occurs which permits the formation of Carbon, Oxygen and Nitrogen from Helium and other preexisting nuclei i.e. the leftover hydrogen.
When fusion of helium to carbon is ignited, further core contraction succumbs a temperature of 1,00,00,00,000 K, following the He exhaustion. At this point an alpha- particle emission occurs, allowing the building of heavier nuclei up to mass 40. However, the star quickly runs out of fuel and reaches a cools to a stage of White Dwarf.
There is a limit on the amount of mass a white dwarf can have. S. Chandrasekhar discovered this limit to be 1.4 times the mass of the Sun. This is appropriately known as the "Chandrasekhar limit".
However, a massive star of several times the mass of our sun has a different history of sequences. The fusion of Helium begins too early in red-giant stage. This is followed by further contraction and heating, allowing the fusion of Carbon and successively heavier elements. However, as the lighter elements are used up, contraction and heating occurs at an increased pace, until the contraction or implosion is stopped by the attainment of neutron-star density, resulting into a shockwave causing a Supernova explosion, which ends the star's life (Burrows, 2000).
However, minutes before explosion, temperatures exceed 3,00,00,00,000 K and very rapid nuclear interactions occur. This nuclear interaction facilitates the formation of heavier- elements up to Cf (Californium) within few seconds as the Supernova explosion itself lasts for few seconds only and is characterized by colossal neutron fluxes. The products of the supernova explosion is distributed throughout the space and is incorporated, later, in a new generation of stars.
Our Sun may be a third-, fourth-, or fifth-generation star. Thus the elements that we find on the Earth includes relicts of primordial gas from the Big Bang as well as the last remains of the dead stars.
Elements that make up your body once resided inside a star!
The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff. ~Carl Sagan
1. Stephen Marshak-Essentials of Geology.
2. Arthur Holmes-Principles of Physical Geology.
3. Atkinson, R. and Hourtermans, F. G. (1929).Zur Frage der Aufbaumöglichkeit der Elemente in Sternen.
4. Burbidge, E. M. et. al. (1957). Synthesis of the elements in stars.
5. Burrows, A. (2000). Supernova explosions in the Universe.