Most people are probably not aware that every known element in the universe was created inside stars like our sun. For a long time the idea that everything in the universe was made from stardust has been a key component of physicist’s understanding of the universe; years of understanding were proven wrong in the blink of an eye.
On June 3rd, 2013 at 3:49pm UTC, NASA’s Swift Burst Alert Telescope picked up a gamma ray burst that barely lasted a fifth of a second and catalogued it as GRB 130603B. Since the burst fell well below the required two seconds, it was classified as a short-duration gamma ray burst and drew the attention of several deep space observatories. Following the burst, a bright glow was spotted in the suspected host galaxy that faded significantly after 10 hours with large amounts of near-infrared radiation following for weeks; this was what several scientists had been searching for: a kilonova.
A kilonova is when two high density bodies, like two neutron stars or a neutron star and a black hole, collide. The discovery of this kilonova is big news because it’s being cited in papers led by Nial Tanvir and Edo Berger to show support for two theories: short-duration gamma ray bursts are caused by the collisions between super dense objects and kilonovae may actually be where the majority of the heavy elements in the universe originate. The latter theory is a big deal, because it’s a shift away from the long standing theory that pretty much every known element was made in stars or during their death. To explain why this new theory might be a better explanation for the origin of some of our most important elements, like gold or uranium, it’s best to start small and talk about elements and their atomic level creation.
The creation of small, lighter elements in stars follows two basic pathways: the main sequence and the triple alpha process. A star spends most of its life in the main sequence where it fuses hydrogen and its various isotopes into helium. After billions of years, the star runs low on hydrogen to fuse, causing it to swell into a red giant and start fusing the helium that it made through the triple alpha process. The process is called the triple alpha process, because it has three fusion reactions and because helium nuclei are called alpha particles in physics. In the process, two helium atoms combine to form beryllium, then another fuses to create carbon, and then another to create oxygen. A star will start to have stray neutrons flying around inside of it as a result of fusing some nuclei together, which it can then use to form larger elements through the “R-process” and “S-process.”
S-process stands for slow neutron capture, and this is where an atom’s nucleus absorbs neutrons one by one until it becomes unstable. To remain stable the atom undergoes beta decay, turning some of the neutrons into protons, forming a new, larger element. If a star is a second generation or higher star, then it will have some larger elements in it from previous stars like calcium or iron; these elements can grab neutrons in the star and beta decay into heavier elements like zinc. The s-process takes a very long time to create sizable elements and the upper limit to this is bismuth, if the star last long enough to process elements that heavy.
When a star runs low on both hydrogen and helium to fuse, it collapses in on itself and violently explodes out in a supernova; this allows for the creation of a few elements via the r-process. The r-process is like the s-process on steroids; instead of absorbing neutrons one by one, lots of neutrons are absorbed all at once. These neutron heavy isotopes can undergo beta decay fast enough that they can “jump” past bismuth and create heavy, radioactive elements, like the ones of the Actinide series of the periodic table. (If you’d like a great video explanation of nuclear fusion click [here])
Jonas Lippuner is one of many scientist that see a problem with the idea of the r-process creating heavy elements when a star supernovas. According to Lippuner, most stars have a proton-to-neutron ratio at or above 0.4, which is not enough to go through the full r-process and create all the elements of the Actinide period. A paper published by Freiburghaus, Rosswog, and Thielemann back in 2000, actually showed that supernova can barely produce elements with an atomic mass greater than 120, let alone the elements of the Actinide series, where the atomic masses can reach twice that at 240. To create many of the elements we rely on, like gold or uranium, we need something with a lower proton-to-neutron ratio. Ironically enough, the answer to this problem actually comes from the supernovae that aren’t neutron rich enough to create the elements.
Neutron stars are the cores from stars four to eight times the size of the sun after they have been extremely compacted in a supernova. Neutron stars weigh one to two times as much as our sun, yet are only between 10-20 kilometers in diameter. With a mass to size ratio like this, neutron stars are one of the densest objects in the universe, with one cubic centimeter weighing approximately 1 billion tons. To put that density into perspective, that’s the same as 10,000 U.S. aircraft carriers.
Neutron stars are, unsurprisingly, composed mainly of neutrons, because the stars extreme density forces protons and electrons together creating neutrons. The proton-to-neutron ratio inside the neutron star can vary greatly with the ratio at the surface being 0.5, while at the core it can be lower than 0.1. These values for an average neutron star falls well within the needed proportions for the r-process to create heavy elements; now the matter just has to be freed. (If you want to read more about neutron stars you can here [1][2])
This neutron rich matter is freed when one neutron star collides with another in a kilonova. In the kilonova, the stars pass close enough to one another that they become trapped in each other’s gravity fields. This causes the stars to become tidally locked and slowly “swirl” closer and closer in a pattern that looks similar to a hurricane. When the stars finally collide, a lot of matter is slung out into space. Freiburghaus’s study, estimated that between 0.01% and 1% the mass of our sun would be ejected at speeds ranging from 10 to 30 percent the speed of light, depending on collision conditions. Edo Berger’s estimate of the mass ejected from the collision that caused GRB-130603B, showed that much more could actually have been ejected with him stating between 3% and 8% the mass of our sun in his paper.
Traveling at these speeds in such neutron rich material, it is easy for ejected nuclei to gather enough neutrons to actually reach atomic masses up to 300, then undergo several beta decay cycles in just a few minutes according to Lippuner. With atomic masses ranging up to 300, all possible elements can be created like silver, tin, barium, gold, and uranium. A notable portion of elements being created will come from the Lanthanide and Actinide series.
Tanvir notes in his paper that the presents of large amounts of Lanthanide and Actinide period elements will create an environment that is very “opaque” to electromagnetic radiation. These elements will absorb a lot of the present electromagnetic radiation and re-emit it back in the near-infrared range. As mentioned in the beginning, this pattern was exactly what was observed following GRB 130603B, a gamma ray burst followed by an optical glow that faded greatly within about 10 hours in this case, followed by near-infrared radiation for weeks following. Not only does the idea that most of the elements in the universe with an amount mass greater than 120 have great support from the analysis of GRB 130603B, but the observed patterns of elemental distribution in the galaxy also, according to both Luppiner and Freiburghaus.
All three studies and their authors, do acknowledge that there are some problems with the kilonova creation theory. One critical aspect of the theory is how often kilonova occur, which is currently unknown, but estimated to be very rare with the paper by Freiburghaus, Rosswog, and Thielemann stating that it’s probably in the ballpark of one every 100,000 years in a galaxy. Lippuner also stated, “[the other major] issue is that we see very old stars in our galaxy that formed very early on, when the galaxy first formed, with r-process material inside of them.” Lippuner would also later explain though, that this issue may not actually be a major concern if it is reexamined later.
Although the kilonova theory still has quite a way to go before it can be confirmed, it shows great promise for becoming the new model of heavy element creation in the universe. Next time you look at your phone, or flip a light switch, just think that the gold inside its circuits, or the uranium fueling the far away power plant, was born in the clash between two cosmic titans.
By, R. Pierce
References
Gideon Boulton. “r capture and s capture.” Youtube, uploaded by Gideon Boulton, 18 Feb. 2016, https://www.youtube.com/watch?v=OVS4I7a0mKM, Accessed 30 Aug. 2017
Patruno, Alessandro. “Neutron Stars” Astrosplash, WordPress.com, https://apatruno.wordpress.com/neutron-stars/, Accessed 30 Aug. 2017.
Tanvir, N. R., Levan, A. J., et al. “A ‘kilonova’ associated with the short-duration γ-ray burst GRB 130603B.” Nature. 500-7464, 29 August 2013. Accessed 28 Aug. 2017
Berger, E. Fong, W. and Chornock R. “An r-Process Kilonova Associated with the Short-Hard GRB 130603B.” The Astrophysical Journal Letters, 774-2. 26 August 2013. Accessed 28 Aug. 2017
Freiburghaus, C., Rosswog S., and Thielemann F.-K. “r-Process in Neutron Star Mergers.” The Astrophysical Journal Letters, 525-2, 6 October 1999. Accessed 28 Aug. 2017
Jonas Lippuner. “Jonas Lippuner JINA-CEE online seminar ‘r-Process in neutron star mergers.’” Youtube, uploaded by JINA-CEE, 18 Feb. 2016, https://www.youtube.com/watch?v=DbgHP0QUsJU, Accessed 31 Aug. 2017