Step back 1000 years, look up at the night sky and you might notice some extra dots of light compared with today. Back then, Chinese astronomers called these “guest stars” and believed them to be harbingers of great change.
We now know these were likely to have been supernovae – explosions borne from dying stars – and they are one of many happy accidents caught when astronomers were looking at the right spot at the right time.
But at the turn of this century, looking for these “transient” events became a tactic in its own right, and it is changing the way we do astronomy altogether. We have since found a myriad of intermittent events throughout the cosmos, lasting from nanoseconds to longer than a human lifetime.
“You think the universe has a different range of spatial scales, but it also has these ranges of time scales, and they’re incredibly poorly explored in astronomy,” says Jason Hessels at the University of Amsterdam in the Netherlands.
Relying on chance to capture these events risks missing much of the action, so astronomers have now automated the process of serendipity, with surveys like the Palomar Transient Factory, which ran from 2009 to 2012, coordinating telescopes as one well-oiled machine. The main telescope in San Diego, California, would see an interesting flash and another would investigate further. “It was really set up like a conveyor belt,” says Hessels.
Many more telescopes whose purpose is to search in time, rather than space, have followed. These include the Zwicky Transient Facility, Palomar’s successor, and the Pan-STARRS survey, which has collected the largest volume of astronomical data of all time, at 1.6 petabytes, from its perch in Hawaii.
These telescopes and others have produced a torrent of data that has unveiled the universe’s blinks and flashes: gamma-ray bursts, fast radio bursts, gravitational waves and stars exploding either of their own accord or because they are being torn apart by black holes.
Transient astronomy is transforming the way we depict the universe. “We started with drawings, and then we had photographs, and then we had something like stop-motion film,” says Hessels. Now, we are getting closer to a full movie, he says. “It seems like every time we tweak the way we look at the sky, we fill in more and more of the movie.”
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