A team of astronomers from Liverpool John Moores University and the University of Montpellier have created an ‘early warning system’, which sounds an alert when huge stars are about to end their lives in a supernova explosion.
The researchers discovered that massive stars – typically between eight and 20 solar masses – in the last phase of their lives suddenly become one hundred times fainter in visible light before they die.
The work was published in Monthly Notices of the Royal Astronomical Society.
What causes supernova explosions?
The majority of supernovas are the last step in the death of a star. These explosions are caused by stars with a mass five times the size of the Sun.
Massive stars burn huge amounts of nuclear fuel at their cores. The material causes a huge amount of energy within the star, causing its centre to get extremely hot. The star’s gravity attempts to squeeze it into a small, tight ball, but the nuclear fuel creates strong outward pressure.
When the fuel runs out, the star cools off and this causes the pressure to drop. The star collapses in around 15 seconds, which creates enormous shock waves. The outer part of the star explodes during this process, known as a supernova explosion.
The largest explosions, caused by stars around ten times the size of the Sun, may leave black holes in the Universe.
Pre-explosion cocoons
The dimming around stars in their ‘red supergiant phase’ is caused by a sudden accumulation of material around the star, including oxygen, silicon, magnesium, and iron.
This process causes the heating of iron, making the star tightly packed, and shrinks the iron core to a neutron core. This new core is only around ten kilometres, obscuring the visibility of the star. The process was most famously observed by the Hubble Space Telescope in 1987.
Until now, it was unknown how long it took the star to accelerate this material. For the first time, this new study has observed how red explosions may look just before a supernova explosion.
Observing real-time explosions
Old telescope archives contain images of stars that went on to explode around a year after the image was taken. In these images, the stars appear as normal, so they cannot yet have built up the material that is gathered just before a supernova explosion.
From this information, the researchers concluded that the material must be assembled in less than a year, which is considered to be extremely fast.
Benjamin Davies, from Liverpool John Moores University and lead author of the study, explained: “The dense material almost completely obscures the star, making it 100 times fainter in the visible part of the spectrum. This means that, the day before the star explodes, you likely wouldn’t be able to see it was there.”
He added: “Until now, we’ve only been able to get detailed observations of supernova explosions hours after they’ve already happened. With this early-warning system, we can get ready to observe them in real-time to point the world’s best telescopes at the precursor stars, and watch them getting literally ripped apart in front of our eyes.”
