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Stars and solar physics

Stars and solar physics

Supernova 1987A enters a new phase

01 Mar 2017
Composite image of the SN 1987A inner ring
Moving on: SN 1987A is entering a new era. (Courtesy: NASA / ESA / A Angelich (NRAO / AUI / NSF) / R Kirshner (Harvard–Smithsonian CfA / Gordon and Betty Moore Foundation) / ALMA (ESO / NAOJ / NRAO) / R Indebetouw (NRAO /AUI / NSF) / CXC / K Frank et al.)

Thirty years after it exploded, supernova SN 1987A is starting a new phase in its development as the shock wave from the stellar explosion is finally passing beyond a ring of gas encircling the dead star.

On 23 February 1987, a blue supergiant star named Sanduleak –69° 202 exploded in the Large Magellanic Cloud, which is a dwarf-galaxy neighbour to the Milky Way 169,000 light-years away. Named SN 1987A, it was the first supernova since 1604 to be visible to the naked eye.

Robert Kirshner, an astrophysicist at the Harvard–Smithsonian Center for Astrophysics in the US, told that “SN 1987A is unique” in terms of the unprecedented scrutiny it has received as the nearest supernova in the modern age. Observations made over the past 30 years are teaching us what happened to Sanduleak –69° 202. And by comparing the light given off by SN 1987A to that from more distant supernovae we can learn about these objects and their progenitor stars.

Glowing rings

The flash of light from SN 1987A was preceded by a burst of neutrinos that arrived at Earth 3 h before the visible light. Light from the explosion illuminated three rings that surround the supernova. The two outer rings are faint and distant, but the inner ring is dense and clumpy, with a diameter of about a light-year. They are all made from material emitted from the star as it underwent pulsations in its outer layers tens of thousands of years before it exploded, therefore providing us with a window through time to show how the star was behaving in the run-up to its destruction.

After the initial flash of light from the supernova the rings faded, only for the inner ring to brighten once again when the supernova shock wave caught up with it in 2001. The shock caused the inner ring to heat up and emit X-rays, detected by NASA’s Chandra X-ray Observatory, with energies mostly in the realm of 0.5–2 keV, but at its peak as high as 8 keV. The inner ring continued to brighten until 2013, at which point it began to fade in uneven fashion as a result of being shredded by the expanding blast wave moving at 1800 km/s.

Intriguingly, the inner ring is fading unevenly, but is the ring itself lopsided “or does the uneven interaction of the shock with the ring indicate that the supernova explosion itself was asymmetric”? This is a question asked by Kari Frank of Penn State University, who led the most recent Chandra observations of SN 1987A. If the ring is lopsided then it could be the result of Sanduleak –69° 202 being part of a binary system, with the gravity of an unseen companion star influencing how the ring material was thrown into space.

Missing pulsar

There’s also the question of what the supernova left behind. Sanduleak –69° 202 had a mass estimated to be 20 times greater than the Sun and should have created a spinning neutron star – a pulsar – when its core collapsed and emitted the neutrino burst. Yet so far there has been no evidence that a pulsar is present. It could be that its beams are pointing away from us, but a pulsar should also produce thermal X-rays from its hot surface as well as a wind of radiation, neither of which has been observed.

“The most likely reason we have not seen any of this yet is because there is a lot of cold gas and dust still hanging out near the centre of the ring,” says Frank. Like a thick fog, this cold gas and dust could be blocking the pulsar’s emissions but, as that fog expands along with the rest of the supernova remnant, it will thin and eventually dissipate, revealing the pulsar within.

The reveal of the pulsar is one event that could transpire in the next 30 years, a time during which SN 1987A will “transition from a supernova to a supernova remnant, shaped and powered by the collision of the shredded star with the surrounding gas,” says Kirshner.

Cooling dust

Supernovae remnants are characterized by the cooling of dust spewed out into space by the stellar explosion. This dust contains elements such as carbon, oxygen, nitrogen, silicon and iron, all forged within the dead star. The cooling dust is visible to the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, which will monitor how the supernova is dispersing the dust into space to be recycled in the next generation of stars, planets and possibly even life.

Consequently, the next 30 years, like the previous three decades, will be a process of learning as astronomers begin to join the dots between SN 1987A’s remnant and other young supernova remnants in the Milky Way. There will undoubtedly also be plenty more surprises as the shock wave moves into new territory beyond the inner ring.

What will we find there? “We’re about to find out,” says Frank.

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