Astronomers Find a Supernova Runt — and Discover an Unexpected Origin

Sometimes, even the biggest explosions in the Universe are duds, and that’s a good thing for anyone planning to get married this summer.

Strange, but true, and it will make sense once you understand the results described in a new paper published Wednesday in the journal Nature.

The researchers studied the binary star system CPD-29 2176, which contains a neutron star and a larger, Be-type star in orbit around each other. But in a nearly circular, rather than an elliptical orbit. This hints that the neutron star, the extremely dense remnant of a giant star that exploded, didn’t explode with enough of a kick to push away its companion star.

This is what’s known as an ultra-stripped supernova, according to the lead author of the study and Embry-Riddle Aeronautical University Astronomer Noel Richardson. An ultra stripped supernova occurs when a star has already lost much of its outer mass before collapsing and exploding near the end of its life.

And because of the strange dynamics of the CPD-29 2176 system, the Be-type star might one day explode in an ultra-stripped supernova, leading to a neutron star (a kind of dense zombie star the size of a city but the mass of the Sun) binary, and eventually, a neutron star merger, “which is a source of major elements in the universe like gold and other heavy elements,” Richardson tells Inverse.

What’s new? — The new paper describes how neutron star binaries might come about based on the interactions between two massive stars that generate ultra-stripped supernovae.

When stars about 10 or more times the mass of the Sun exhaust their fuel, they are unable to fight their own gravity and collapse in on themselves. This sudden and titanic compression generates one of the largest explosions in the cosmos, a supernova, which ejects much of the star’s mass out into space. What remains collapses again, either into an incredibly dense neutron star or a black hole.

What Richardson and his colleagues believe happened in CPD-29 2176 is that the two original, massive stars orbited too close to one another. The star that is today a supernova began to shed much of its outer mass, which was pulled onto the present-day Be-type star.

When the first star exploded in a supernova, it no longer had sufficient mass to push its companion away, allowing the two bodies to remain in a circular orbit. Normally, Richardson says, astronomers find black holes or neutron stars orbiting companion stars in highly elliptical orbits, the two bodies having been pushed apart by a supernova blast.

Today, the neutron star and the Be-type star orbit each other around every 50 days, Richardson says, with the Be-type star spinning rapidly on its axis. Eons of material falling onto the Be-type star not only increased its size but spun it like a top or playground merry-go-round so that it is now spinning so fast it can barely hold itself together with its gravity.

And that has led to the Be-type star losing its own material to space, which is collecting in a disk around its equator, “and we may end up with another one of these ultra-stripped supernovae in the future,” Richardson says.

How they did it — In 2016, NASA’s Neil Gehrels Swift Observatory, a space telescope looking for high energy radiation bursts, spotted a burst of X-rays scientists thought might be the signature of a magnetar, a bizarre type of neutron star with an extremely powerful magnetic field with trillions of times more powerful than Earth’s geomagnetic field.

“What was unusual about this was it was actually at the same point on the sky as a very bright star,” Richardson says. That star would turn out to be the Be-type star in the CPD-29 2176 binary.

This kept Richardson interested over several years and post-doctoral research positions, so that by the time he took on his current faculty position, “I was looking for a student to take this project on,” he says.

Undergraduate student Clarissa Pavao stepped up to the plate, working with Richardson remotely during the beginning days of the Covid-19 pandemic. They realized they would need further data, specifically high precision spectroscopy, which breaks down the wavelengths of light emanating from their target. The team booked time at the Cerro Tololo Inter-American Observatory in Chile.

Those observations helped Richardson and Pavao understand they were dealing with a binary system with a circular orbit, while further computer modeling by University of Auckland Astrophysicist and study co-author Jan Eldridge showed the only way the system could have wound up with the tight circular orbit was if the neutron star was created through an ultra stripped supernova.

Why it matters — In 2017, for the first time, scientists observed gravitational waves generated by the merging of two neutron stars. These two massive objects had followed ever-tightening orbits before finally smashing into each other in a massive explosion known as a kilonova, an event so violent it shook space itself.

“One of the main findings out of this was that a lot of the heavier elements that are out in the universe were produced in this one kilonova explosion,” Richardson says. “A lot of heavy elements like gold and other things we use every day on planet Earth as part of our lives.”

What astronomers still hadn’t observed, is a mechanism by which a neutron star binary might form, one day leading to a kilonova as was observed in 2017.

“I think the cool thing about these findings is we can now see that these ultra-stripped supernovae can play a very important role in how these systems form,” Richardson says. “

So we may have cosmic whimpers, rather than big bangs, to thank for everything from wedding bands to nuclear reactors.

What’s next? — The immediate next step for Richardson and Pavao is a forthcoming paper on the Be-star and its disk of ejected material.

“This disc is sent out from the equator of the star, it actually gets a little more or less material with time,” he says, “so she's actually trying to piece together if there are any timescales with that.”

But to really understand what is going on with the Be-type star, Richardson and other astronomers need a different instrument, since optical telescopes like the one they used for the current paper are blinded in many wavelengths due to the material thrown off by the star.

“We can do that if we can go into the ultraviolet with Hubble,” he says. “Hubble has these ultraviolet instruments, and we can hopefully characterize how much the star is spinning and what's going on with it in the current day so that we can place better constraints on the evolution of the system.”