Excited conversations have been happening in the halls of an astronomy meeting this week, as scientists await reports about a potential collision of exotic dead stars. If the chatter is true, scientists have detected the first ripples in spacetime produced by a cataclysmic merger of two neutron stars.
These ripples, known as gravitational waves, were directly detected for the first time in 2015 by the LIGO Science Collaboration. That event, along with two more confirmed detections, occurred when two black holes collided.
Now, it’s possible astronomers have detected gravitational waves generated as two stellar corpses whipped around one another and eventually merged. And it seems the merger may have left a lingering cosmic fingerprint that teams are racing to observe with some of Earth’s sharpest eyes in the sky. (Find out why these spacetime ripples are so important to astronomy.)
Today, the LIGO team and the European Virgo team announced the end of the latest observing run in their hunt for these ripples, saying only that they have found some “promising gravitational-wave candidates.” We’ll have to wait for confirmation of the find once the teams have reviewed and verified their data.
In the meantime, the possibility that LIGO may have spotted stellar corpses in the final throes of a lethal dance got us wondering: How much do we know about neutron stars, and why should we care about them?
What exactly is a neutron star?
As their name implies, neutron stars are made almost entirely of neutrons, or uncharged subatomic particles. They form when a star much bigger and brighter than the sun exhausts its thermonuclear fuel supply and explodes into a violent supernova. Though the outer layers of the star get blasted into the space, its core collapses inward and forms a sphere about the size of San Francisco but with at least the mass of our sun. These stars, which spin rapidly, are the most compact objects outside of black holes—a sugar cube of the stuff would weigh a billion tons.
Whoa, that sounds nuts.
Yup. And it gets even more exotic.
Neutron stars are dead in the sense that they’re no longer fusing elements in their cores, so they don’t shine like the sun. But that doesn’t mean they’re placid. A neutron star’s magnetic field can be more than a quadrillion times stronger than Earth’s, and its gravitational field can be about a hundred billion times stronger. In other words, you do not want to get anywhere near one of these things if you appreciate being in one piece.
So if they’re not shiny, how can we see them?
Several ways, actually. Neutrons stars were the foundation for one of the most famous discoveries in astronomy. Fifty years ago this month, then-graduate student Jocelyn Bell Burnell observed pulses of radio waves coming from an otherwise unremarkable spot on the sky. The constant pulses were evenly spaced, prompting early speculation that perhaps aliens were somehow involved. (Also find out how a NASA spacecraft may help aliens find Earth.)
Turns out, a neutron star was the culprit. As some of these stars spin, they emit focused beams of electromagnetic radiation. As those beams wash over Earth, they appear as pulses of radio waves. Eventually, this flavor of neutron star became known as a pulsar. In addition to radio waves, neutron stars emit x-rays because their surfaces are extremely hot, in the neighborhood of a million degrees.
Why should we care about these dense zombie stars?
Astronomers study these intensely weird astrophysical objects to help test some very fundamental ideas about physics. More tangibly, though, colliding neutron stars could be the cosmic jewelers responsible for crafting precious metals, such as gold.
According to Enrico Ramirez-Ruiz of the University of California, Santa Cruz, the process goes like this: Colliding neutron stars unleash a flood of neutrons that quickly pile onto any surrounding heavy nuclei, eventually forming elements heavier than iron, such as gold, silver, and platinum. It’s called the r-process, for rapid neutron capture, and it occurs in both neutron star mergers and supernovas, although the whirling dance of undead stars is now thought to be the primary source of the galaxy’s gold. A single collision can generate enough gold to equal Jupiter’s mass.
How did those metals get to Earth and into my watch?
Precious star stuff made during these mergers ends up distributed throughout the cosmos like the chocolate chips in a cookie, says Ramirez-Ruiz. If another star happens to be born near one of those chips, then that gold or platinum will be tucked into the cloud of gas that birthed the new star and therefore be incorporated into the star and its planets.
Because the very young Earth was a quivering molten mass, its original gold atoms sank into its core. Later, crashing asteroids delivered additional gold, sprinkling Earth’s crust with all the precious, shiny stuff they had in their clutches.
OK. So why do we care about actually seeing neutron stars colliding?
One reason is that this story about them being cosmic jewelers is a bit controversial. Though the physics works well, it upends a long-standing theory that the majority of the universe’s gold is crafted by supernovas.
If we could actually see neutron stars in the act of colliding, it might help resolve the debate, because the r-process should be observable. Scientists could do that by aiming an infrared telescope, say NASA’s Spitzer Space Telescope, at the cataclysm and looking for the signs of element formation. There’s also the chance that seeing a neutron star merger could solve a mystery about what type of object such an event creates. It could yield a black hole, or even an ephemeral type of star made of some strange form of matter.
Why do we think LIGO has seen one of these mergers?
Without spending too much time amplifying the rumor mill, publicly available (yet circumstantial) evidence suggests that the LIGO team detected a gravitational wave signal that could also be observable at different electromagnetic wavelengths—which is exactly what should happen for a neutron star merger.
Starting late last week, telescopes on and off the planet quickly swiveled to observe the aftermath of a short gamma-ray burst that had gone off on August 17 in a galaxy called NGC 4993. That gamma-ray burst, now called GRB170817A, is the kind of thing that’s predicted to occur when two neutron stars collide.
The entry logs for observations by the Hubble Space Telescope and Chandra X-Ray Observatory clearly indicate that they are chasing a gravitational wave detection and GRB 170817A. Several of the European Southern Observatory’s telescopes are looking at the same spot as well.
But before making an official announcement, the LIGO team would want to be as sure as possible that the gravitational wave signal is real, and that it and the gamma-ray burst come from the exact same object. That takes time.
“We really want to have a chance to understand the data we have been collecting and ensure that we are confident in what we make public,” says LIGO spokesperson David Shoemaker of MIT. “Incremental releases of information at this point could easily need retraction or modification in the weeks to come. We are working as hard and as fast as we can!”