How is Gold Made? - Neutron Star Mergers!

The accuracy of the theorists' predictions compared to the observed merger is remarkable! Here is an article talking about it...

https://www.sciencedaily.com/releases/2017/10/171016102822.htm

Where and how this process of heavy element production occurs has been one of the longest-standing questions in astrophysics. Recent attention has turned to neutron star mergers, where the collision of the two stars flings out clouds of neutron-rich matter into space, where they could assemble into heavy elements.

Speculation that astronomers might see light from such heavy elements traces back to the 1990s, but the idea had mostly been gathering dust until 2010, when Brian Metzger, then a freshly minted graduate student at UC Berkeley, now a professor of astrophysics at Columbia University, co-authored a paper with Quataert and Kasen in which they calculated the radioactivity of the neutron star debris and estimated its brightness for the first time.

"As the debris cloud expands into space," Metzger said, "the decay of radioactive elements keeps it hot, causing it to glow."

Metzger, Quataert, Kasen and collaborators showed that this light from neutron star mergers was roughly one thousand times brighter than normal nova explosions in our galaxy, motivating them to name these exotic flashes "kilonovae."

Still, basic questions remained as to what a kilonova would actually look like.

"Neutron star merger debris is weird stuff -- a mixture of precious metals and radioactive waste," Kasen said.

Astronomers know of no comparable phenomena, so Kasen and collaborators had to turn to fundamental physics and solve mathematical equations describing how the quantum structure of heavy atoms determines how they emit and absorb light.

Jennifer Barnes, an Einstein postdoctoral fellow at Columbia, worked as a Berkeley graduate student with Kasen to make some of the first detailed predictions of what a kilonova should look like.

"When we calculated the opacities of the elements formed in a neutron star merger, we found a lot of variation. The lighter elements were optically similar to elements found in supernovae, but the heavier atoms were more than a hundred times more opaque than what we're used to seeing in astrophysical explosions," said Barnes. "If heavy elements are present in the debris from the merger, their high opacity should give kilonovae a reddish hue."

"I think we bummed out the entire astrophysics community when we first announced that," Kasen said. "We were predicting that a kilonova should be relatively faint and redder than red, meaning it would be an incredibly difficult thing to find. On the plus side, we had defined a smoking-gun -- you can tell that you are seeing freshly produced heavy elements by their distinctive red color."

That is just what astronomers observed.