Cosmic Alchemy – How the elements are forged
We have already discussed some aspects of the nuclear fusion processes in stars during their lifetime and in the final stage. In this chapter we will review the formation of all the 92 elements that occur naturally in the Universe (and in our Solar system) in a systematic way. We will refer to the periodic Table shown here that explains the cosmic origin of the elements (click image for larger version in new tab).
|The Periodic Table lists the chemical elements according to increasing number of protons in the nucleus (atomic number). The table should be read like a book, from top to bottom and left to right. Hence the order of the elements is H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, Sc, Ti, V, Cr, Mn, Fe, etc.|
Big Bang NucleoSynthesis
In the early Universe soon after the Big Bang, matter in the form of fundamental particles, condensed out of the primordial energy. After about 380,000 years, atomic nuclei consisting of single protons or to some extend protons and neutrons, could bind electrons, thus forming the first atoms. Hydrogen (single proton and one electron) was by far the most common element (at about 75% by mass), with some Helium (up to 25%) and very little Lithium. This is why astronomers have the peculiar notion of calling all elements above Lithium “metals” as these were not formed in the Big Bang nucleosynthesis but were forged in early stars.
At the present day, the most common element in the Universe is still Hydrogen at about 74%. See Big Bang Fusion in the Periodic Table.
Dying low mass stars
When Sun-like stars end nuclear fusion in the Red Giant phase, the core collapse results in the throwing out of much material in the form of a planetary nebula. This nebula contains most of the material that was formed during the nucleosynthesis in the star (Helium, Carbon, Nitrogen and Oxygen), but the core collapse must also have triggered the instantaneous formation of many of the heavier elements that make it into the planetary nebula. That material is then available for future star formation.
Exploding massive stars
We discussed above that a massive star fuses up to Fe (Iron) up to its Super Giant stage. Then when the Super Giant’s core collapses it produces a core-collapse supernova, and a neutron star or even a Black Hole forms. This forges many of the heavier elements above Fe, although the modern view is that a large percentage of those elements is either destroyed during the actual explosion, or collapses with the core remnant (neutron star / black hole). So in the PT shown here we don’t see any heavier elements above zirconium (40) resulting from exploding massive stars.
Exploding White Dwarfs
We discussed the Type IA-supernovae explosions above, when a White Dwarf in a binary accretes material from the companion Red Giant, triggering runaway Carbon fusion and ultimately brings the star above the Chandrasekhar limit. Some of the medium heavy elements (up to atomic number 30) are expected to be formed during these events.
Merging Neutron stars
When two neutron stars merge from a binary couple, it must result in some of the most catastrophic (energetic) events in the Universe. (Astronomers are beginning to observe gravitational waves originating from such events). A large fraction of all heavier elements (above atomic number 40) is expected to be formed during these extreme events. It requires a very high concentration of neutrons in an extremely energetic environment to form these higher elements into stable nuclei (rapid neutron capture or r-process). Research on these processes is ongoing and there is no full consensus among astrophysicists. The more traditional view is that these heavier elements primarily result from core-collapse supernova explosions as discussed above.
Cosmic Ray fission (Spallation)
Cosmic Rays are not “rays” but streams of high energy particles in space. They consist of protons, proton-neutron combinations (nuclei of heavier elements) and also single electrons. When they collide with other matter and also mutually, they can form nuclei of some of the lighter elements such as Lithium, Beryllium and Boron. This fusion process called cosmogenic nucleosynthesis is held responsible for the observed abundances of these lighter elements. Some Lithium isotopes however are formed in dying low mass stars and also are primordial, originating from the Big Bang nucleosynthesis.
Read more on spallation here
|The Periodic Table above is produced by Professor Jennifer Johnson and others (see Credit and further discussion of the shown Periodic Table here). Jennifer, at Ohio State University, is one of the leading experts on stellar evolution and the origin of the elements. Both aspects of astrophysics are actively researched and debated, and in some details the origin of the elements is still uncertain.|
We are made of the stars
|“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 star stuff.” — Carl Sagan
Without nucleosynthesis in stars and during explosive events discussed above, there would be no elements heavier than Lithium. Our discussion above explained the occurrence of the heavier elements in the Universe. The abundance of elements in our Sun and the gas planets, generally reflects those of the Universe at large. Astronomers know that our Sun has been formed from material that has gone through previous stellar lifecycles, including all of the processes discussed above. Because of the abundances of the heavier elements astronomers conclude that our Sun is a third generation star, made of cosmic material that must have gone through at least two previous stellar life cycles of massive stars.
However in the rocky planets like Earth, the abundance of elements has changed. Solar radiation has removed part of the volatile elements such as hydrogen, helium, neon, nitrogen and carbon (in methane). In the early stage when the Earth was liquid, heavier elements segregated towards the mantle and core, and many chemical processes, including biological ones on Earth, have changed chemical abundances as well.
If we take this personal and look at the human body, 24 of the naturally occurring elements are found in the human body. Eleven of those are significant and the other thirteen are trace elements. The latter does not imply that they are less important for life!
All of these elements have a cosmic origin. We are indeed made of stardust.