Astronomers glean information about distant objects from four types of energetic signals: electromagnetic radiation (such as visible light, radio waves and gamma waves); gravitational waves (subtle “ripples’ in spacetime, first detected in 2015); neutrinos (electrically neutral elementary particles having almost no mass); and cosmic rays (high energy particles, mostly protons, that, like neutrinos, travel at nearly the speed of light).
Being able to combine information from two or more of these in what’s called “multi-messenger astronomy” offers rich opportunities to learn much more about the sources of these signals than traditional “single messenger” astronomy allows. Recently, the new kid on the block, gravitational wave detection, came into its own with the successful deployment of the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in the U.S. (in Washington and Louisiana) and Italy’s Virgo (“Listening to the Universe with LIGO,” Oct. 26, 2017).
Aug. 17, 2017 was a red-letter day for multi-messenger astronomy, when the LIGO-Virgo system detected the collision of two dense neutron stars in a distant galaxy. (How dense? A sugar-cube’s worth of neutron star material weighs as much as Mount Everest.) That collision, dubbed GW170817, not only sent “gravity ripples” across 130 million light years of space, these ripples were accompanied by a wildly powerful burst of energetic electromagnetic waves, a so-called kilonova event, that arrived 1.7 seconds later and was detected by more than 70 earth- and space-bound radio, optical, x-ray and gamma-ray telescopes.
These disparate single-source signals, gravity and electromagnetic, were a bonanza for astronomers, and dozens of papers were published the very day of the event on the arXiv website, where scientists can publish unreviewed papers. Of particular interest to theorists was information about so-called r-process elements, elements too heavy to be created by fusion in the cores of stars. For decades, astronomers have struggled to fully understand how most elements heavier than iron (including platinum in your car’s catalytic converter, gallium in your smartphone and iodine in your thyroid gland) were created. Up to iron, the process has been known with some certainty since English astronomer Fred Hoyle proposed, in the mid-1900s, that as stars age, their original hydrogen nuclei fuse to form progressively heavier elements due to extreme pressures and temperatures in the stars’ cores. Beyond iron, regular stellar fusion doesn’t cut it. It takes more extreme events to create heavier elements, in particular r-process elements (where “r” stands for “rapid,” as in milliseconds).
Now, thanks to the multi-messenger information obtained from the GW170817 event, astronomers have confirmed that neutron star collisions can synthesize r-process elements. Take a look at the spectrum above, detected one and a half days after the event. Note the dip where the expected (dashed line) light curve deviates: that deviation is the signature of the r-process element strontium, the salts of which give us the brilliant reds of fireworks.
Collisions between neutron stars are probably not the only violent events that create r-process elements. We have limited evidence that mergers between black holes and neutron stars can also do the trick, as (perhaps) can some supernovae. But at this point in our knowledge, it seems colliding neutron stars produce most of the heavy r-process elements in the universe.
So now, when you’re asked (as I’m sure happens daily) where the gold in the ring on your finger came from, you can answer confidently, thanks to multi-messenger astronomy: “It was probably synthesized in the collision of two neutron stars.”
Barry Evans (he/him, barryevans9@yahoo.com), who is made of star-stuff, has just published another Humboldt book (in color!): Humbook Two. It’s at Eureka Books, Booklegger, Northtown Books and elsewhere.
This article appears in Launching Nigilax̂.
One of the delights of this gig is my interactions with Real Scientists: I wrote to Dr. Darach Watson at the Niels Bohr Institute in Copenhagen after reading a couple of articles about his work, asking if I could adapt a graph that accompanied the paper on which he was the lead author. Two minutes later, “Sure! How can I help?” Scientists: kids who grew up without losing their sense of wonder.