by Kim Malville
The skies of November are going to be relatively quiet. All bright planets are clustered around dusk or dawn and will not be especially easy to see. I had initially written an article about the total eclipse of August and the awarding of the Nobel Prize in Physics to the discoverers of gravity waves produced by black holes. But, then on October 16 a bombshell arrived.
Rumors swell over new kind of gravitational-wave discovery
It reads like the start of a spy thriller or Orson Wells discussing the invasion from Mars. Gossip over potential detection of colliding neutron stars has astronomers in a tizzy as was reported by Nature, on 24 August 2017. The journal reported that rumors to that effect were spreading quickly throughout the astronomical community, much to researchers’ excitement. Such a detection could mark a new era of astronomy: one in which phenomena are both seen by conventional telescopes and “heard” as vibrations in the structure of space-time. Scientists who work with gravitational-wave detectors wouldn’t comment on the gossip because they were still analyzing their data. Public records showed that telescopes, at night, of course, around the world were turned toward the same galaxy in Hydra. Something special was happening. But astronomers were still reluctant to comment and any paper describing the results were embargoed. Major scientific journal agreed to refrain from publication until all the results could be carefully checked and presented evenly.
Gravity waves revealed on October 16
Gravitational waves had arrived on Earth on Aug. 17 (four days before our Great American Eclipse). Two months later, at 8:00 MST, Oct. 16, the embargo on the results was lifted and the director of the National Science Foundation, France Cordova, announced the extraordinary results, which had been methodically collected over the previous two months. Suddenly the world learned of the colliding neutron stars, reported in more than a dozen papers in the journals Nature, Science and the Astrophysical Journal Letters. The lead paper, which appeared in Astrophysical Journal Letters, lists roughly 3,500 authors, approaching the record set in 2015 by 5,154 Large Hadron Collider physicists who estimated the mass of the Higgs boson. The cost of LIGO has been a whopping 1.1 billion US dollars, but, by comparison, the Large Hadron Collider cost over 13 billion, and the International Space Station cost 150 billion dollars. By comparison, this bombshell is a bargain.
Some 130 million years ago in a galaxy 130 million light years away in the constellation of Hydra, two neutron stars orbited each, speeding up as they lost energy due to gravity waves. They came as close to each other as Denver is to Colorado Springs, travelling 60% the speed of light. When they collided, they caused vibrations of space-time to spread throughout the nearby universe and, for those who were watching, produced some wonderful fireworks.
Those fireworks are not at all surprising, considering the awesome nature of these neutron stars. They are the remnants of supernova explosions, pressed together by the immense pressure of the explosion. They are very dense—a matchbox of their neutrons on earth would weigh 3 billion metric tons. Because of conservation of angular momentum (like an ice skater spinning faster as she brings her arms in) they rotate at enormous rates. The fastest measured spinning neutron star spins 716 times per second. Because of their rapid spinning and small size, they have truly monstrous magnetic fields, some 100 million to a quadrillion stronger than that of the earth. The gravity on the surface of a neutron star is 200 billion that of the earth. It is no surprise that when two of these behemoths come together all hell breaks loose.
On Aug. 17, signals of that hell reached Earth—and sparked an astronomical revolution. At 8:41am ET a gravitational wave hit the Virgo detector in Italy and, 22 milliseconds later, set off the Laser Interferometer Gravitational Wave Observatory (LIGO) detector in Livingston, La. Three milliseconds after that, the distortion rippled through Hanford, WA. The signal generated by the gravitational waves lasted 100 seconds, vibrating faster and faster as the stars moved closer to each other. In comparison, the gravitational wave produced by colliding black holes since 2015 lasted only .02 seconds.
Just 1.7 seconds after the initial gravitational wave detection, NASA’s Fermi Space Telescope registered a brief flash of gamma radiation coming from the constellation Hydra, and the word went out to observatories around the world suggesting they turn their telescopes to a faint galaxy in Hydra. This also was the moment when Einstein’s prediction that gravity waves would travel at the speed of light was dramatically confirmed. More than 70 observatories in the world turned their telescopes on that explosion, observing practically the whole range of “light waves” from radio waves, infrared light, visible light, extreme ultraviolet light, up to energetic gamma rays.
This collaboration across national boundaries opened up a new era in space research known as “multimessenger astrophysics.” Collisions between neutron stars are some 1,000 times brighter than a typical nova, hence they are called “kilonovas” (or if you prefer kilonovae). They were thought to be the universe’s primary source of such elements as silver, platinum and gold, which has indeed been confirmed by spectroscopic analysis of the remnants of the collision.
In the early hours the explosion appeared blue and featureless—the light signature of a very young, very hot new celestial body. But unlike supernovas, which can linger in the sky for months, the explosion turned red and faded. The initial phase of this kilonova was so bright that it was detected in a 16-inch telescope, similar to those possessed by amateur astronomers. It is going to be a very exciting time for many amateur astronomers who have the right kind of telescopes and detectors and can join in the search.
The LIGO detectors shut down on August 25 for repairs and fine tuning. When they start up again next year the detectors should be twice as sensitive, reaching twice as far into space, which means they can explore a volume of space eight times larger. We suspect that there are some 5 million pairs of neutron stars orbiting each other in our own galaxy. Beyond our galaxy, there are 2500 galaxies like ours within 100 million light years. The future looks very bright for more fireworks sparked by neutron stars.