SUPERNOVA1987A
The still unraveling remains of supernova 1987A are shown here in this image taken by Nasa's Hubble Space Telescope. The bright ring consists of material ejected from the dying star before it detonated. The ring is being lit up by the explosion's shock wave. NASA/ESA/Hubble

The absence of a neutron star at its core and a lopsided explosion continue to capture the interest of astronomers in the supernova 1987A that exploded in the Large Megallanic Cloud, and is one of the most-studied supernovas.

Light from the supernova blast first reached Earth in 1987, around 160,000 years after the explosion.

In continuing observations, Nasa's Nuclear Spectroscopic Telescope Array, or NuSTAR, has found ejected material flying in one direction and the core of the star in another, proving that core-collapse supernovae are inherently asymmetrical.

"Stars are spherical objects, but apparently the process by which they die causes their cores to be turbulent, boiling and sloshing around in the seconds before their demise," said Steve Boggs of the University of California, Berkeley, lead author of a new study on the findings, appearing in the journal Science. "We are learning that this sloshing leads to asymmetrical explosions."

Three glowing loops of stellar material formed when the fast expanding supernova collided with the dense, slower moving material in the stellar wind ejected by the former star about 20,000 years before it went supernova.

These collisions produced powerful optical and X-ray energy emissions.

Outer, ejected materials lit up first, followed by the innermost materials powered by radioactive isotopes, such as cobalt-56, which decayed into iron-56.

'Smoking gun'

Now, NuSTAR has found the "smoking gun" for the asymmetric explosion in the form of a radioisotope called titanium-44. It was first found by ESA in 2012.

"Titanium is produced in the very heart of the explosion, so it traces the shape of the engine driving the disassembly of the star," said Fiona Harrison, the principal investigator of NuSTAR at the California Institute of Technology in Pasadena.

"By looking at the shift of the energy of the X-rays coming from titanium, the NuSTAR data revealed that, surprisingly, most of the material is moving away from us."

The NuSTAR spectral data reveals that titanium-44 is moving away from us with a velocity of 2.6 million kilometres per hour.

The compact core of the supernova seems to have kicked off in the opposite direction.

"Radioactive titanium-44 glows in the X-rays no matter what and is only produced in the explosion," said Brian Grefenstette, a co-author of the study at Caltech.

"This means that we don't have to worry about how the environment influenced the observations. We are able to directly observe the material ejected in the explosion."

Another mystery is that of the missing neutron star at the heart of the supernova, 166,000 light years away.

The star that exploded to create SN1987A was a blue supergiant with typical surface temperatures of over 50,000°C, and luminosity a million times that of the Sun.

The violent death of such a high-mass star should leave behind a stellar remnant in the form of either a neutron star or a stellar mass black hole.

However, astronomers have been unable to find a neutron star in the remnants.

This could be because it is surrounded by an extremely dense cloud of thick dust. Or, so much material fell back onto the neutron star that it further collapsed into a stellar mass black hole.

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