Starstuff, neutron starstuff

In the words of the late American astronomer and science communicator Carl Sagan:

"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."

Or, to put it more lyrically in the words of American musician Richard Melville Hall, better known as Moby:

"We are all made of stars"

Never a truer annotated periodic table of the chemical elements begun in 2008 and updated in January 2017 shows the origin of each element. Jennifer Johnson of Ohio State University wanted to correct some of the deceived wisdom about element origins and points out that some elements have mixed origins, they are not created by one single process. The simplest, lightest elements hydrogen and helium of course, are the truly primordial elements. They were formed in the Big Bang along with some lithium, although some of that was also made through cosmic ray fission and in dying low mass stars. Helium, of course, is also still being created in unimaginable quantities by the nuclear fusion of hydrogen in stars. Although here on Earth, we are in danger of running out of this element for use in science and medicine as we allow too much to boil off into space (floating party balloons are probably only a tiny part of the problem).

Most of the carbon, nitrogen, barium, and several of the heavy metals including zirconium, niobium, and molybdenum were released into space from dying low mass stars. Lighter metals, such as sodium and magnesium and some proportion of the aforementioned elements were derived from exploding massive stars. Beryllium and boron are unique, if two things together can be unique, in that they were formed entirely by cosmic ray fission as far as we know. Most of the transition metals were formed mainly by exploding white dwarf stars. The likes of francium, radium, polonium, and neptunium, are generated by radioactive processes from other elements, and of course, there are a whole host of synthetic elements that last for tiny fractions of a second to which we have only recently accepted exist and assigned names.

So, where does that leave iridium, platinum, gold even, and holmium, uranium, plutonium, and others? Well, the answer lies in those gravitational waves that have been in the science news recently. On 17 August 2017, an astronomical collaboration used the combined talents of LIGO and Virgo (Laser Interferometer Gravitational-Wave Observatory and the Virgo interferometer, respectively) in the detection of a gravitational wave, a ripple in spacetime. This phenomenon was predicted by Albert Einstein in 1916 and emerges from his theory of general relativity. The gravitational wave named GW170817 was caused by the collision and merging of two neutron stars in the elliptical galaxy of NGC 4993, which lies in the constellation of Hydra. The gravitational wave was followed less than two seconds later by a short gamma ray burst, GRB 170817A, and 11 hours later by a visible light event SSS17a, now officially known as AT 2017gfo.

The technical astronomical details are of interest, of course, to astronomers. But, what does any of this have to do with the periodic table? Well, it seems that a good number of the chemical elements, including gold, europium, bismuth, iodine and xenon are formed by such neutron star collisions in what physicists refer to as the r-process (rapid neutron capture process) and ejected into space by the explosive nature of such an astronomical event. Indeed, the r-process mechanism of nucleosynthesis is now thought to account for the formation of about half of the atomic nuclei heavier than iron.

The lines drawn between elements and their origins are still slightly blurred, some sources have it wrong and even Johnson's latest data and interpretations have some degree of ambiguity. Notes on her blog post discuss many of the issues that remain.

Of Sagan's "starstuff" quote, Johnson points out that it hides all manner of mechanisms. "This is an evocative statement. It gets at the heart of the matter," she says, "However, it leaves out all the different ways that stars make the elements. It is not just collapsing stars, it is merging stars, burping stars, exploding stars, and the start of the Universe itself."

To quote Moby again though:

"I can see forever, to the deep wrapped in cold, late at night"

What that says about our place in the universe and our understanding of it remains to be seen, but while we wait for the next piece of evidence, the next gravitational ripple in spacetime you can remain on the edge of your seat or get up on the dance floor, as it were. And if you like to shake your bling, ponder this: the amount of gold generated by that one collision between neutron stars was 200 times the mass of the Earth.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is now available.