Frontpage image: Illustration of spherical explosion (kilonova) of two neutron stars (AT2017gfo/GW170817) made by Albert Sneppen.

STRONTIUM is one of those elements that get less coverage in chemistry courses all the way up to undergraduate level.  These days one is most likely to hear of it in the context of archaeology, for example looking to see if the strontium to calcium ratio in the teeth of Neolithic person X was different from that characteristic of the site of burial, suggesting that they might have grown up in a distant place before arriving at their final earthy destination.

However these days, with science seemingly expanding faster than the universe, the element is again in the news, as the product of a really massive hadron collision, namely the merger of two neutron stars resulting in a KILONOVA, which gets its name GW170817/AT2017gfo from having been simultaneously detected in 2017 both by the resulting gamma ray burst and gravitational waves.

This one, though, does not seem to be “playing by the rules”.  Here is an information-rich video from Dr Becky relating how the material thrown out from the explosion appears to be going in all directions, rather than simply in the plane of the collapsing orbit.  The indication that something unexpected is here came from the contribution of strontium in its spectrum.

It is understood that the majority of the strontium in the universe, largely consisting of the stable isotope 88Sr, is produced by the rapid neutron capture process, especially the recently discovered Type III supernovae first postulated in 1980 by Ken’ichi Nomoto at the University of Tokyo.

Originally supernovae were classified into Type I, later divided into three subclasses, showing little to no hydrogen in their spectrum, and Type II with a lot of hydrogen.  According to this 2012 article The different types of supernovae | Type II supernovae result from stars with initial masses greater than 8 times solar.

Nomoto, however, predicted that a third type of supernova must also be allowed, for progenitor stars falling between 8–10 solar masses. In his theory, core collapse is again initiated as electrons are captured by neon and magnesium – but this time, the core is gravitationally supported by the resulting thermonuclear explosion. This would leave behind a neutron star in equilibrium, neither expanding nor contracting.  A suitable candidate for this type of supernova is SN 2018zd, detected in March 2018 by an amateur astronomer in Japan. [1,2]

A kilonova, though, is something entirely different.  They can result from the collision of two neutron stars, or the neutron star and a black hole.  One system has been observed containing a neutron star orbiting a massive companion star which is expected to have gone supernova and have produced a neutron star in millions of years’ time, whose orbits will decay to produce a kilonova [3,4].  They are now understood to be the source of the heaviest elements in the universe.  Gold and platinum are often mentioned in this context, but that would also include mercury, lead, and uranium.

The radiation from the kilonova is said to come from radioactive decay.  For decades it has been established that much of the brightness of a Type II supernova is produced by β+ decay of 56Ni (half-life 6.02 days) which decays to 56Co (half-life 77.3 days) which decays to the stable 56Fe.  But (I am surmising here) many of the superheavy nuclei would undergo fission.

Caesium-137, Strontium-90, and Iodine-131 are common fission products in a nuclear reactor: [5] Iodine-131 was particular significant in connection with the Chernobyl disaster, and people was taking iodine tablet to swamp it out and minimize the amount taken up into the thyroid gland.  But radioactive Strontium-90 from nuclear testing was a particular worry in the 1950s and 1960s, because it could substitute for calcium in the bones.  Its concentration in dairy products was particularly worrying because it would be present in milk consumed by infants [6].

Which has got me wondering: is the abundance of strontium seen in the spectrum of GW170817/AT2017gfo actually Strontium-90?  With its half-life of 28.78 years, only about 20% of the initial production would have decayed by now.

[1] Electron capture supernova spotted four decades after first predicted – Physics World

[2] Elusive new type of supernova, long sought by scientists, actually exists | Space 

[3] '1-in-10-billion' star system is doomed to explode in a fiery kilonova | Live Science 

[4] A high-mass X-ray binary descended from an ultra-stripped supernova  
[5] Why are cesium-137, strontium-90, and iodine-131 the fission products that get most talked about when many more fission products are produced in reactors?

[6] Washington State Department of Health – Office of Radiation Protection - Strontium-90 (Sr-90) Fact Sheet