Black widow binary stars consist of a neutron star consuming a nearby companion star. Find out how scientists used a new technique to find one of these rare systems, and why their discovery has two other unique characteristics.
Like most people in the northern hemisphere, the first constellation I learned to recognize was the Big Dipper. Forgive me, the Big Dipper is technically an asterism, being only part of Ursa Major, the Great Bear.
Anyway, when I started using a telescope for stargazing, I started looking for double stars to try to split, and the first place I looked was the familiar Big Dipper in the northern sky. There’s a pair of stars in its handle called Mizar and Alcor.
We now know that Mizar and Alcor are part of a larger array of stars called the Ursa Major Moving Group. It’s not yet clear to astronomy if gravity holds the group together.
Distinction – Double Stars Versus Binary Stars
That’s an important distinction astronomers make. There are double stars that just happen to appear close together in the sky, and binary stars that revolve around a shared centre of mass.
Sir William Herschel explained this in 1802. He wrote about binary stars that they will, “remain united by the bond of their own mutual gravitation towards each other. This should be called a real double star; and any two stars that are thus mutually connected, form the binary sidereal system.”
This week, the journal Nature announced that scientists have found a new star system. It’s an example of a rare kind of celestial couple called a “black widow binary star.”
Neutron Star Orbiting Smaller Companion Star
A black widow consists of a neutron star circling and absorbing a smaller companion star. The fanciful name comes from the black widow spider.
The female of the species has a reputation for eating the male after mating. However this trait seems to be misunderstood, with females merely protecting their young from males who might eat them.
The Milky Way contains a couple of dozen black widows that we know about. These systems are powered by a pulsar, which is a quickly rotating neutron star.
Neutron Stars Form When Massive Star Collapses
Neutron stars form when a massive star collapses. Those that become pulsars rotate on their axis in just a few milliseconds, emitting the pulse of gamma and X-rays from which they take their name.
The team calls this new black widow binary star ZTF J1506+1222. The extraordinary thing about this system is its orbital period.
According to the researchers’ calculation, the pulsar and its companion circle one another every 62 minutes. That’s the shortest orbital period ever seen in a binary system.
Distant, Third Companion Orbiting Inner Two Stars
This binary system has yet another interesting feature. It seems to have a distant, third companion orbiting the inner two stars on a 10,000-year cycle.
The astronomers have come up with an origin story to explain this unique group. They believe that all three stars were part of an earlier globular cluster.
The researchers speculate that the cluster wandered too close to the massive black hole at the centre of the Milky Way. The black hole’s immense gravity would have ripped the original star cluster apart, leaving this triple star system as its remnant.
“Floating Around Longer than the Sun”
The study’s author is Keven Burdge, a Pappalardo Postdoctoral Fellow in MIT’s Department of Physics. As he explains, “This system has probably been floating around in the Milky Way for longer than the sun has been around.”
Professor Burdge and his team used an unusual technique to find this new black widow binary star. Until now, astronomers used the gamma and X-ray radiation from the central pulsar to find these systems.
This team located their new system by observing the visible light flashes from the companion star instead. Professor Burdge explains, “I thought, instead of looking directly for the pulsar, try looking for the star that it’s cooking.”
Changed Brightness by Factor of Ten in One Hour
The researchers sifted through data from a California observatory called the Zwicky Transient Facility. They narrowed their search down to stars that changed brightness within an hour by at least a factor of ten.
This method singled out the black widow binary stars astronomers had already catalogued. This result verified they were on the right track.
Then, the scientists identified an overlooked star that got 13 times brighter every 62 minutes. They determined this was a binary system and that the pulsar wasn’t emitting any gamma or X-rays. That’s the telltale sign of a black widow.
Telltale Sign of a Black Widow
As we unravel the mysteries underlying the origin of the Universe and our place in it, understanding the formation of stars and galaxies is crucial.
Beyond the macabre fascination we feel about one star cannibalizing another in deep space, this new discovery helps us understand how all stars form, and why gravity coalesces them into immense galaxies.
The team’s next step is to continue observing their new system to confirm their theories. They also plan to use their new, optical technique to locate other black widow binary stars and more common kinds of neutron stars in our galaxy.
“New Way of Looking for These Systems in the Sky”
Professor Burdge wrapped up the discussion saying, “There’s still a lot we don’t understand about it. But we have a new way of looking for these systems in the sky.”
We always have more to learn if we dare to know.
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