2 Aug 2016

Astronomers have managed to peer into the past of a nearby star millions of years before its famous explosion, using a telescope in remote outback Australia at a site free from FM radio interference. Research led by a student at the University of Sydney, and including an international team of astronomers observing the region at the lowest-ever radio frequencies, has helped fine-tune our understanding of stellar explosions. The findings are published today in the journal Monthly Notices of the Royal Astronomical Society.
The research paints a picture of the star's life long before its death in what was the closest and brightest supernova seen from Earth, now known as supernova remnant 1987A (SN 1987A), which collapsed spectacularly almost 30 years ago.

Much had been known about the immediate past of this star through studying the cosmic ruins resulting from the star's collapse in 1987, which occurred in a neighbouring galaxy, the Large Magellanic Cloud. However it was the detection of the very faintest of hisses through low-frequency radio astronomy that has provided the latest insights. Previously, only the final fraction of the dead star's multi-million-year-long life, about 0.1%, or 20,000 years had been observable.
This latest research – which has enabled astrophysicists to probe the supernova's past life millions of years further back than was previously possible – was led byJoseph Callingham, a PhD candidate with the University of Sydney and the ARC Centre of Excellence for All-Sky Astrophysics(CAASTRO), under supervision from former Young Australian of the Year and former CAASTRO Director Prof Bryan Gaensler, now at the University of Toronto.

Operating the Murchison Widefield Array in the West Australian desert, the radio astronomers were able to 'see' right back to when the star was in its long-lasting red supergiant phase.
Mr Callingham explained previous studies focused on material that was ejected into space when the star was in its final blue supergiant phase. "Just like excavating and studying ancient ruins that teach us about the life of a past civilisation, my colleagues and I have used low-frequency radio observations as a window into the star's life," he said.

Researchers found the red supergiant lost its matter at a slower rate and generated slower winds that pushed into its surrounding environment than was previously assumed.
"Our new data improves our knowledge of the composition of space in the region of SN 1987A; we can now go back to our simulations and tweak them, to better reconstruct the physics of supernova explosions," said Mr Callingham.

Professor Gaensler explained that key to gaining these new insights was the quiet environment in which the radio telescope is located. "Nobody knew what was happening at low radio frequencies, because the signals from our own earthbound FM radio drown out the faint signals from space. Now, by studying the strength of the radio signal, astronomers for the first time can calculate how dense the surrounding gas is, and thus understand the environment of the star before it died."

Image:
(A) Artist’s impression of the supernova flare seen in the Large Magellanic Cloud on 23 February 1987.
(B) Artist’s impression of the star in its multi-million year long and previously unobservable phase as a large, red supergiant.