Strange Death of a Star: Astronomers Witness Never-before-seen Type of Supernova Explosion

A team led by researchers from the California Institute of Technology have observed the unusual death of a massive star—one quite unlike anything that's ever been seen before. According to a paper published in the journal Science, the event is the first recorded "ultra-stripped supernova"—a surprisingly faint and rapidly fading supernova.

Supernovae are titanic explosions that occur when massive stars—at least eight times the mass of the Sun—exhaust all of their nuclear fuel, causing the core to collapse before rapidly expanding outwards. Usually a lot of material—many more times the mass of the Sun—is seen being blasted away from a supernova, but the researchers saw that the event in question, dubbed "iPTF 14gqr," only ejected matter equivalent to about one-fifth of the Sun's mass.

"We saw an usual explosion of a star that is unlike anything we've seen before," Anthony Piro, a theoretical astrophysicist at Carnegie Observatories and author of the study, told Newsweek. "Typically these supernovae will rise in brightness for about 2 weeks, and then decline for many months after. This one reached it's maximum brightness in less than a week!"

"For a star to explode like this but have so little mass, it must have been massive in the past but had it's material siphoned away by a companion star," he said. "And since this mass stripping was so extreme, that companion had to have been a compact, exotic star like a black hole, white dwarf or neutron star. Out of these possibilities, we favor a neutron star companion due to the details of the explosion."

Neutron stars are incredibly dense, compact objects that are left behind when massive stars shed most of their outer layers in a supernova (provided that the exploding star's mass is insufficient to produce a black hole).

In the case described in the latest study, the astronomers propose that a previous supernova had already left behind a neutron star before iPTF 14gqr produced another, creating a twin or "binary" neutron star system. While scientists already knew about neutron star binary systems, this could be the first time they have witnessed the birth of one.

These twin systems—which must have started out as binary systems of two massive stars—have long posed a problem for scientists. This is because it was believed that the explosion of the second star would expel most of the remaining mass, making the system unable to form a pair in the first place.

But now this team has proposed a new hypothesis, where the gravitational force of the neutron star formed during the first supernova strips most of the outer layer of the remaining star. When this "ultra-stripped" star then explodes, the supernova has much less material to eject and the system can remain stable.

iPTF 14gqr was first spotted at the Palomar Observatory in San Diego as part of a nightly survey—known as the intermediate Palomar Transient Factory (iPTF)—which searches for transient, short-lived cosmic events like supernovae.

Left: Red and green composite image from the Sloan Digital Sky Survey (SDSS) taken before supernova iPTF14gqr. Right: Red/green/blue composite image from the Palomar 60-inch telescope taken on October 19, 2014, during supernova iPTF14gqr. The circles indicate the position of the supernova. SDSS/Caltech

"Looking into the future, this discovery really opens up a lot of possibilities for finding more of these unique events," Kishalay De, lead author of the study from the Cahill Center for Astrophysics at Caltech, told Newsweek. "While iPTF did a really good job of finding many supernovae within hours of the explosion, we are stepping up our search with its successor, the Zwicky Transient Facility [ZTF]."

"ZTF is nearly 10 times faster than iPTF in scanning the skies, so we hope to find many more of these events right after their explosion and follow them up with our global network of telescopes called GROWTH," he said. "Finding more of these events will tell us how these binaries form, where they form and how frequently do they form, all of which are crucial for our theoretical understanding of these systems."

According to Piro, the latest findings come at a time of increasing interest in neutron stars.

"There has been a lot of excitement about neutron star binaries recently," he said. "Last August a neutron star binary was seen to merge for the first time with gravitational waves by LIGO—[this] was named the Science Breakthrough of the Year. Nevertheless, the events that make these important binaries have been a mystery to us until now. With iPTF 14gqr, we finally have our first example of what this should look like."

"Neutron stars are really crazy stars—one sugar cube-sized piece of a neutron star has as much mass as all the humans on Earth put together. Because these are such extreme systems, they are excellent probes of Einstein's Theory of General Relativity," he said.

Neutron stars usually measure between just 10 and 20 miles in diameter, which is very small compared to most stars. Despite this, they tend to have a mass greater than that of our Sun as a result of their incredibly high densities. This density generates powerful gravitational fields. In fact, the gravitational field at a neutron star's surface is around 200 billion times that of the Earth's. The stars can also spin incredibly fast, rotating up to several hundred times per second.

This article has been updated to include additional comments from Kishalay De and Anthony Piro.