On August 17th, 2017, the collision of two neutron stars 300 million light years away was observed, initially detected by the arrival of gravity waves. It was a watershed moment in both observational astronomy and astrophysics.
By last fall, you may have become somewhat blasé about gravitational waves: four occurrences had been reported to the public beginning in 2015, all of them involving the merging of black holes in enormously distant galaxies, to little effect on Earth—any tremor you might possibly have felt could equally have been produced by a FedEx van five blocks away going over a speed bump.
Ah—but what occurred last August,* and released publicly to mainstream media on October 16, was an extra special event, arguably more interesting than any of the black hole stories. Not in the magnitude of gravity waves (again, something we animals would never have felt). . . but in the practice of astronomy, what came out of it was a game-changer, heralded by many as the advent of “multi-messenger astronomy.”
It was an astonishing event in (at least) three important respects:
(1) It was the first ever verifiable observation of a neutron star collision. Among other things, it revealed that neutron star collisions are a source of gamma ray bursts that have puzzled astronomers for decades. Also seen was the outpouring of heavy elements such as gold, platinum, and uranium, in large quantities.
(2) It was observed in the form of gravity waves, gamma rays, X-rays, radio waves, and visible light, all with different, specialized instruments.
(3) it brought to bear the capabilities of some of the most sophisticated and sensitive instruments ever built, and the collective brainpower of several institutes and research teams in both observational astronomy and theoretical astrophysics. The collaboration gave gravity wave detectors solid footing as useful tools of practical, observational astronomy rather than exotic physics experiments—raised to the status of true observatories. Without the detection of the gravity waves, the neutron star collision might have gone unnoticed, or mistaken for another phenomenon.
Neutron star basics: basically bizarre
Neutron stars are among the very strangest, most extreme objects in a universe full of strangeness. They are remnants of large stars that have blown off most of their material in supernovae. The insanely cataclysmic explosions called supernovae result in either neutron stars or Black Holes, depending on the mass of the parent star–huge gets you a neutron star, and huger still gets you a Black Hole.**
In a neutron star, gravity smushes together all the remaining protons and electrons so powerfully that they fuse into neutrons—leaving a macroscopic object with the density of an atomic nucleus.
(These stars represent the densest concentration of matter possible, with the exception of what’s inside Black Holes. Since we cannot see inside Black Holes, we don’t actually know what’s in there. . . but the physics implies that the matter falling in gets compressed so tightly we cannot even speak of it as matter anymore—it’s a Thing of infinite density with no spatial dimensions, called a Singularity.)***
A neutron star the diameter of Orlando, Florida, contains about 1.5 times the mass of the Sun, and a teaspoonful of it would “weigh” half a billion tons on Earth. They are way denser than white dwarfs, which are the relics of modestly sized stars like our own sun. Furthermore, they spin at fantastic speeds, from once every ten seconds up to 1,000 times per second—meaning that a dot on the surface of an Orlando-sized star could be traveling as much as 20% the speed of light!
Neutron stars spew huge amounts of electromagnetic radiation from their magnetic poles. In fact, it was the accidental detection of radio waves from a neutron star that led to their “discovery” in 1967. (Theory had determined the existence of neutron stars back in the 1930s, but no one confirmed their reality for another 30 years.) This brings us to the discussion of pulsars.
Pulsars are spinning neutron stars—with, as we have said, widely varying rotation rates. As with Earth, their magnetic poles are offset from the axis of rotation, so their output sweeps around the axis at regular intervals, like an old-fashioned lighthouse. Try twirling the index finger of one hand in circles around the end of a pencil held in the other hand to get the effect—the pencil is the axis of rotation (extended), and your finger is the stream of radiation emitting from one of the magnetic poles.
The effect viewed from afar is a series of “pulses” as the stream of radiation strikes the observer intermittently in lighthouse fashion. These stars do not actually “pulse” like a beating heart—it’s a trick of geometry. Because they were originally observed as apparent pulses of radio waves, the name stuck.
Ergo: all pulsars are neutron stars, but not all neutron stars are pulsars.
What you have just read is a generic simplification of neutron stars—there are no two that are exactly alike, with wide deviations. For those with a physics background, there’s a fascinating lecture, dense with formulae, on neutron stars to be found at Pierre Pizzochero on neutron stars
(The link gets you an abstract. For the full lecture, click on “Download PDF” in the righthand column. )
How about the collision itself?
As for the collision—it’s not, as many of us non-astronomers might picture it, a collision like a huge billiard ball zooming straight out of interstellar space to smack into another huge billiard ball, and it’s not like a gigantic asteroid plowing its way into another object, such as the one that ended the reign of the dinosaurs on Earth. Instead, it is more like a “merger”—but possibly as violent as a hostile corporate takeover.
You start with two neutron stars in a binary (two stars revolving around each other) system (multiple star systems are more common than single star systems). The two stars have been circling each other for eons in a seductive pas de deux, slowly drawn closer and closer by their mutual gravity, until they start ripping sheets of stuff off of each other (“stuff” is the technical term for what you get when you tear apart some of the densest material in the Universe). They keep ripping and merging in a dance like Orlando Florida-sized lovers in the throes of an all-consuming passion. . . until they are One.
For a general idea of the process, see colorful computer simulation, with dramatic music not heard in space:
The simulation shows the merger ending in a black hole. This will only happen if the combined masses are greater than 3.2 solar masses. Otherwise it will just end in a larger neutron star. The location of the recently observed merger is being observed intently, but as of this moment (Jan. 15, 2018), if they’ve figured out what’s there, it hasn’t been released to the public. If it was a pulsar, we should have heard by now. Black holes are elusive—they can only be “seen” by their effects on things around them. So it might take some time to confirm that’s what the merger produced.
(A link to the YouTube video so you can view the simulation in full screen is at the end of this post.)
Neutron star smashup triggers information explosion on Earth
Fortunately, I don’t have to say much about the ferment that the neutron star collision brought about in the fields of observational astronomy and astrophysics, because there are several accounts which are smarter and better than I could write up. I provide links below.
I recommend three accounts:
(1) An easy-to-read summary of the high points, concluding with quotes from some super-psyched scientists, enthusing about a “eureka moment” and a “new chapter in astrophysics:”
Astronomers spot two neutron stars colliding to form vast amounts of GOLD
(2) Good overview video (with another simulation) by Laura Cadonati, Physics professor at Georgia Tech:
Cadonati summary of detected neutron star collision
(3) For a blow-by-blow account by an insider at the gravity wave observatory, see this long but fascinating article written by a technical writer: Neutron star collision and multi-messenger astronomy
Finally, if you’re curious about the gravity wave observatories that are really gigantic interferometers, check out:
https://www.ligo.caltech.edu/page/ligo-technology
and https://www.ligo.caltech.edu/page/ligos-ifo
These will enlighten you as to how it is that instruments built by human hands can detect a difference in length between two 4-kilometer long tunnels to the precision of half the width of a proton. (Gasp!)
================ footnotes follow =====================
* Four days before the total eclipse, providing more grist for the mill of never-say-die Intelligent Design fans
** There is some disagreement about just what the threshold is between neutron-star-parent mass and black-hole-parent mass. Neutron stars are created by the collapse of stars of at least 8 solar masses. Black holes are definitely created by the collapse of stars of 25 solar masses. The different estimates for minimum mass for black hole formation fall in the 20-25 solar mass range (I’ve also heard one statement that 10 solar masses would do the trick—definitely a lowball). I’m pretty sure that 20 solar masses will get you a black hole, but if you’re looking around for an ideal candidate, call an astrophysicist.
(A supernova of a single enormous star is only one way of getting a Black Hole, and perhaps not the most common. Mergers of existing massive stars and especially massive-enough neutron stars is another way. There’s recent evidence that some single giant stars may “quietly” collapse into Black Holes without a supernova segue.)
*** For a really cool video on Black Holes, watch this episode of:
Nova: Black Hole Apocalypse) It’s close to an hour long, so you might want to find a stretch of relaxation. (You may have to open this in a new window (right-click etc.) to get it to run.)
LINK TO COMPUTER SIMULATION OF NEUTRON STAR MERGER:
A really well-written, humorous, impassioned explanation of neutron stars colliding. Even I was intrigued, and I thoroughly enjoyed reading this!