When a dead star plunges into an active star, expect an explosion of astronomical proportions that isn't supposed to happen yet.
Astronomers were able to work out this stellar puzzle after tracing an unusual burst of bright radio waves in 2017. The radio waves were captured by the Very Large Array Sky Survey, a collaborative effort to observe the night sky and search for radio wavelengths.
Scientists determined that the flare of radio waves was caused when a dead star, either a black hole or a neutron star, catastrophically collided with a companion star. The study published in the journal Science last week.
"Massive stars usually explode as supernovae when they run out of nuclear fuel," said Gregg Hallinan, study author and professor of astronomy at the California Institute of Technology, in a statement. "But in this case, an invading black hole or neutron star has prematurely triggered its companion star to explode."
Astronomers have never been able to observe this process, so this marks the first time this type of supernova has been confirmed by scientists.
Spotting unusual astronomical events
But some astronomical events, especially unusual ones, leave temporary calling cards. Radio transients are short, bright bursts of radio waves that fizzle out quickly, but capturing a glimpse of them at the right time can reveal the explosive nature of merging stars.
Caltech graduate student Dillon Dong noticed a particularly bright radio wave source in data from the Very Large Array Sky Survey, which was dubbed VT 1210+4956.
Dong's analysis of the event revealed that the energetic outburst came from a star, which was enveloped by a thick shell of gas. The star had shed the gas hundreds of years before. The bright burst occurred when the star exploded in a supernova. The exploded material interacted with the gas shell, triggering a bright burst of radio waves.
A star-studded mystery
But Dong had more questions about when and why the star originally released its gas.
Another Caltech graduatte student, Anna Ho, suggested that the radio transient should be compared with known bright X-ray events, which only appear for a few seconds.
Dong was able to find X-rays that appeared in the same spot as the bright radio transient, which meant they were probably created by the same thing.
"The X-ray transient was an unusual event -- it signaled that a relativistic jet was launched at the time of the explosion," Dong said. Relativistic jets are beams of energized matter that reach close to the speed of light.
"And the luminous radio glow indicated that the material from that explosion later crashed into a massive torus of dense gas that had been ejected from the star centuries earlier. These two events have never been associated with each other, and on their own they're very rare."
What researchers think happened
To understand the rare occurrence, the research team modeled the most likely event.
They believe that a leftover remnant of a star that previously exploded -- either a black hole or dense neutron star -- was orbiting another star. If it was a black hole, it was likely drawing off the star's atmosphere and pulling it out into space to create a gas shell.
The gravitational pull of this process also brought the dead star and the active star together. Then, the black hole crashed into the active star, which prematurely caused it to collapse in on itself and explode.
When the star collapsed, its core release a jet of material, creating the X-ray burst. The radio waves wouldn't be created until years later when the exploded material finally reached the gas shell.
It's not unusual for two stars to exist in a stable orbit that slowly brings the two objects together over time. In fact, this collision is what causes gravitational waves, or ripples in space-time.
But this collision created distinct X-ray and radio wave bursts -- which had been predicted, and now scientists have evidence that it can actually occur.
"We had ideas of what we might find in the VLA (Very Large Array) survey, but we were open to the possibility of finding things we didn't expect," Dong said.
"We created the conditions to discover something interesting by conducting loosely constrained, open-minded searches of large data sets and then taking into account all of the contextual clues we could assemble about the objects that we found," Dong said.
"During this process you find yourself pulled in different directions by different explanations, and you simply let nature tell you what's out there."
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