Where Do Chemical Elements Come From?

1 Chemical Where Do Elements come From? By Carolyn Ruth In 1054, Chinese astronomers recorded what they called a guest star . in the constellation of Taurus, the Bull. This star had never been seen before, and it became brighter than any star in the sky. In the American Southwest, a culture rich in astronomical tradition called the Anasazi also witnessed this brilliant new star. Easily visible in broad daylight, the observers could read by it at night. Today, we know the Chinese and Anasazi were witnessing a huge star explosion, called a supernova. nasa,ESA, HEIC, and the hubble heritage team (stsci/aura). W hat these observers did not know is that during the explosion, the star not only emitted huge amounts of light more the links between the Chemical Elements has brought and keeps bringing many sur- prises and interesting discoveries.

2 In stars, these fusion reactions cannot form Elements heavier than iron. Up until the formation of iron nuclei, these reactions light than a billion suns but also released release energy, keeping the star alive. But chemicals in space. Inside the star were most nuclear reactions that form Elements heavier of the first 26 Elements in the periodic table, Stellar ovens than iron do not release energy; instead, they from simple Elements , such as helium and car- A young star is composed primarily of consume energy. If such reactions happened, bon, to more complex ones, such as manga- hydrogen, the simplest Chemical element. they would basically use the star's energy, nese and iron; and the giant explosion sprayed This hydrogen ultimately leads to all known which would cause it to collapse.

3 Them in space. During the explosion, other Elements . First, the two constituents of each Not all stars form iron, though. Some Elements were created as well, and after the hydrogen atom its proton and electron stars explode before creating that many ele- explosion, the chemicals in space combined are separated. The high pressure inside the with each other to form ions and molecules. star can literally squeeze together two pro- These Elements travel in space and ulti- tons, and sometimes, a proton will capture an mately end up in planets like Earth, being part electron to become a neutron. of everything we see around us and ourselves. When two protons and two neutrons The carbon in our cells, the oxygen in the band together, they form the nucleus of air, the silicon in rocks, and just about every helium, which is the second element in the element, were all forged inside ancient stars periodic table.

4 Then, when two nuclei of before being strewn across the universe when helium fuse with each other, they form the lawrence livermore national laboratory the stars exploded. nucleus of another element, beryllium. In turn, During the past century, scientists have the fusion of beryllium with helium produces been studying how Chemical Elements form in a carbon nucleus; the fusion of carbon and stars and in outer space. Like genealogists helium nuclei leads to an oxygen nucleus, and experts who study the origins of people and so on. This way, through successive fusion families these scientists can track down reactions, the nuclei of most Elements lighter Where most Chemical Elements came from than iron can be formed (Fig. 1). Scientists and how they descended from each other. call this process nucleosynthesis (for synthe- Figure 1.)

5 The Chemical composition of a star And, similar to forming a family tree, studying sis of nuclei ). before it explodes into a supernova. 6 Chemmatters, OCTOBER 2009 ments. In stars less massive than the sun, the and astrophysics at the University of Califor- reaction converting hydrogen into helium is nia at Santa Cruz. For example, SN1987A, a the only one that takes place. In stars more supernova seen in 1987, ejected 25,000 Earth nasa, esa, j. hester and a. loll (arizona state university). massive than the sun but less massive than masses of iron alone.. about eight solar masses, further reactions that convert helium to carbon and oxygen take How stars make place in successive stages before such stars explode. Only in very massive stars (that are Elements heavier more massive than eight solar masses), the than iron chain reaction continues to produce Elements Elements that are heavier than iron can up to iron.

6 Be assembled within stars through the capture A star is a balancing act between two of neutrons a mechanism called the s pro- huge forces. On the one hand, there is the cess. The process starts when an iron nucleus crushing force of the star's own gravity trying captures neutrons, thus creating new nuclei. The Crab Nebula is a six-light-year-wide expanding to squeeze the stellar material into the small- remnant of a star's supernova explosion. These nuclei can be either stable, that is, they est and tightest ball possible. On the other do not change, or radioactive, meaning that hand, there is tremendous heat and pressure they transform, or decay, into another element from the nuclear reactions at the star's center after a certain amount of time, which can be Ni-65 (28 protons, 37 neutrons).

7 Trying to push all of that material outward. as short as a fraction of a second and as long electron + Cu-65 (29 protons, 36 neutrons). The iron nucleus is the most stable as a few million years. nucleus in nature, and it resists fusing into Also, the newly formed nuclei can be Copper-65 is stable, so nothing happens after any heavier nuclei. When the central core of a different versions of a given element. These that. very massive star becomes pure iron nuclei, different versions of an element are called This neutron capture mechanism, called the core can no longer support the crushing isotopes. They all contain the same number the s process, is extremely slow. Hundreds force of gravity resulting from all of the matter of protons in their nucleus but have differ- or thousands of years might elapse between above the core, and the core collapses under ent numbers of neutrons.

8 Some isotopes are neutron strikes. But another process, called its own weight. radioactive, while others are stable and never the r process, which stands for rapid, . change. allows for the rapid capture of neutrons. For example, nickel can appear in Unlike the s process, which occurs inside a the form of 23 different isotopes. They all star before it explodes, the r process hap- have 28 protons, but each isotope contains pens only during the explosion of a star. between 20 and 50 neutrons. Of these 23. isotopes, only five are stable, while the others Exploding and are radioactive. If a nucleus produced through the s . cooking Elements process is stable, it may capture another at the same time neutron. If it is radioactive, it transforms into When a star explodes into a supernova, it nasa/cxc/asu/j.

9 Hester et al. another nucleus. This other nucleus can, in produces a huge amount of light and releases turn, absorb another neutron, leading to a an extremely high number of neutrons (on the heavier nucleus. order of 10 thousand billion billion neutrons For example, nickel-64, which contains per square inch per second). These neutrons 28 protons and 36 neutrons, can absorb a are then rapidly captured by the various nuclei Combined X-ray and optical images of the Crab neutron, leading to nickel-65, which contains that are also released by the exploding star, Nebula. 28 protons and 37 neutrons: producing new nuclei through the r process. The collapse of the core happens so fast In this process, even though that it makes enormous shock waves that many neutrons are available, only blow the outer part of the star into space a Ni-64 (28 protons, 36 neutrons) + neutron.

10 A limited number can be added supernova. It is during the few seconds of Ni-65 (28 protons, 37 neutrons). to a given nucleus; otherwise, the collapse that the very special conditions of a nucleus becomes radioactive pressure and temperature exist in the super- Nickel-65 is radioactive. It exists for only 2 and breaks up. Neutrons in a nucleus are nova that allow for the formation of Elements and a-half hours, and then transforms into thought to occupy shells similar to succes- heavier than iron. The newly created Elements copper-65 the next element in the periodic sive shells on a hard candy. When a nucleus are ejected into the interstellar dust and gas table, which contains 29 protons and 36 neu- gets saturated with neutrons, that is, when surrounding the star. trons. This is a process called beta decay, in its shells are filled up, it undergoes a beta The amount of Elements released which a neutron transforms into a proton and decay process to become the nucleus of the through a supernova is truly phenomenal, an electron: next element on the periodic table.