The Sciences

The Star That Rose From the Dead

A star 500 million lightyears away appears to be a Matryoshka doll of puzzles.

There’s another mystery out there.

Giant stars between 10- and 80-times as massive as our Sun have the privilege of dying in a spectacular explosion called a supernova. The star expels its outer layers in a cataclysm of quakes, upheavals and radiation while whatever is left behind collapses in on itself.

This display typically lasts for about a 100 days. In this period, the star glows very brightly, often outshining the light of all the other stars in its home galaxy combined. And after releasing such tremendous energy, the supernova begins to dim; in time, it disappears from our view as it collapses either into a neutron star or a black hole.

We have observed at least 20 supernovae in recorded history, including three big ones within the Milky Way galaxy. But we have never seen anything like the one we spotted in 2014.

In September that year, astronomers saw a dim dot of light from a star dying 500 million light years away, in the direction of the Ursa Major constellation. They saw that the amount of light the star was emitting out was falling. “It was getting fainter, so we all figured it was an old supernova that had passed its peak luminosity,” Iair Arcavi, a NASA Einstein postdoctoral fellow at the Las Cumbres Observatory, California, and lead author of the paper his team published in November 2017, told The Wire.

They christened the phenomenon iPTF14hls. The ‘iPTF’ stood for the Intermediate Palomar Transient Factory, a collaboration of observatories around the world led by Caltech. The star was expected to flicker out in 100 days.

But in the first week of February 2015, Arcavi’s student Andrew Wong noticed that the star was glowing bright as ever. When he shared his data with astronomers at Las Cumbres, they were puzzled. Somehow, the star was becoming brighter.

They promptly studied the star’s optical spectrum, examining the light coming from it and determining which elements were in the supernova from the various frequencies we know they emit light at. The supernova was of type II-P: it was rich in hydrogen and its core had collapsed in on itself, blowing all its outer layers away and burning bright.

From February until May, telescopes of the Las Cumbres Observatory continuously monitored iPTF14hls. They watched its brightness go up and down, but at no point did this emission dance stop. In May 2015, the supernova disappeared behind our Sun (as seen from Earth). In September, a full year after it was first observed, the star emerged into view once again. This time, astronomers expected it to have dimmed. It was even brighter than before.

Scientists maintained a watchful eye or dozen on iPTF14hls for the next two years. They needed more data as they began trying to piece together the physics of whatever was happening.

Blast from the past

When a star blows up as a supernova, the speed of the ejected material can be measured. The outer layers are typically the lightest elements, like hydrogen. After the explosion, these speed away first, followed by heavier elements from deeper within the star. The inner elements move more slowly than the lighter, outer ones do.

However, iPTF14hls seemed to be ejecting matter at a consistently high speed. Iron – one of the heaviest elements ever found in a star – was measured to have been moving at a constant speed for a stunning 18 months.

“The spectrum of iPTF14hls at 600 days looks like that of a normal type II-P supernova at 60 days,” the scientists wrote in their paper announcing the findings.

The supernova also continued to dim and brighten five times for nearly three years, with its light output varying by as much as 50%. Though it hasn’t vanished out of view to this day, it’s been dimming steadily. “It is continuing to slowly dim, and we’re using larger and larger telescopes to keep track of it,” Arcavi explained. “The dimming is still slower than in normal supernovae, so it’s still much brighter now than it ‘should’ be.”

But the bigger surprise came later. While some astronomers were busy with observations, others were combing through the data archives to see if the star had been studied by others in the past. They found that the star likely had a similar eruption in 1954 before it dimmed down.

“If it isn’t the same supernova, this would be the first time we’ve seen two unrelated supernovae in the same position on the sky,” Arcavi noted.

Credit: DOI:10.1038/nature24030

Credit: DOI:10.1038/nature24030

The birth of iron

We don’t know what this star’s deal is – but we have a few guesses.

Even the smallest stars are so heavy that they’d collapse inwards due to their gravity. That they don’t is because of the millions of nuclear fusion reactions happening in their cores. The energy from these reactions produces an outward pressure that balances the inward gravitational pressure.

When the Sun is mostly out of hydrogen, it will start fusing helium atoms to produce carbon. The leftover lighter hydrogen will float out and forms a shell around the core. At this point, a star more massive than our Sun will have the sufficient temperature (~500 million K) and pressure in its core to fuse carbon as well. At the same time, any leftover helium will float out and form a shell within the shell of hydrogen.

This process of fusing atoms to form heavier elements, with the lighter ones exiting the core to form an onion-like layer of shells, happens with each massive star. With each step, the stellar core becomes smaller and denser.

The whole thing grinds to a halt when the core gives birth to iron. The fusion of iron needs more energy than it will release, which means the core will have to start suckling on the star’s energy and fuel. So now the outward pressure of the core starts to fade, gravity gets the upper hand, and the star’s outer layers crash into the core at millions of kilometers per second.

This action briefly gives the core the energy it needs to fuse iron. However, star-matter that has bounced off the core produces a shockwave, causing the star to eject material out into space. This is what we see as a supernova – a star blowing up, in its wake a remnant that is either a neutron star or a black hole.

Stars that are 100-130-times as a massive as our Sun undergo what’s essentially a multi-step supernova. The fusion reactions in a star’s core produce pairs of electrons and anti-electrons, a.k.a. positrons. In such large stars, carbon fusion produces these particles in large quantities.

As they heat up inside the star, they prompt stray atoms of oxygen to fuse, causing a small surge in the energy radiated from the core. This energy bursts out as a shockwave and temporarily overpowers the gravitational pressure. This is called a pulse.

A star’s outer layers experience lower gravitational pressure than those near the core. When a shockwave comes out, the star blows away its outer layers and decreases in mass without fully collapsing. Several such pulses can occur till the star’s mass becomes stable – i.e. when the pressure due to gravity is balanced once again by fusion pressure from the core, until, later, it goes normal-supernova.

Each pulse ejects material at such high speeds that it collides into the predecessor ejecta, causing bright flashes of light. This phenomenon is called a pulsational pair instability (PPI). To humans looking at this star from 500 million lightyears away, the flashes resemble those of a supernova, but it’s only a supernova impostor.

We aren’t sure that this was definitely a PPI. We still haven’t been able to see what’s behind the light: a stable massive star after a pulse or a remnant after a supernova.

But while a PPI makes most sense at this point, astronomers looking at the light spectra of iPTF14hls have inferred that’s way more hydrogen being ejected than there should be. Such quantities are usually found only in a type II supernova.

There have been two papers published since the original by Arcavi et al trying to provide alternative explanations.

Jennifer Andrews and Nathan Smith, both of the University of Arizona, Tucson, uploaded a paper to arXiv earlier this year saying that the supernova’s strength can be understood if we assume we’re looking at ejected material rushing out and colliding with a disc of matter surrounding the star.

Luc Dessart, from the University of Chile, Santiago, uploaded a paper around the same time arguing that the brightness can be explained if the supernova remnant is transferring energy to the supernova itself. Sometimes, a neutron star will rotate rapidly to produce a very powerful magnetic field and emit bursts of gamma rays, which Dessart says could be feeding the supernova.

However, Arcavi thinks none of these ideas can individually explain everything they’ve been seeing. He also pointed out that they don’t touch upon the 1954 observation, focusing instead on data taken in the last three years. “Maybe by combining a few such new ideas together we’ll eventually get to the bottom of what happened.”

Sandhya Ramesh is a science journalist in Bengaluru.