Stars ‘need a partner to create universe’s brightest explosion’

When it comes to creating the biggest and brightest explosions in the universe, it takes two stars to tango.

Researchers have solved the mystery of how stars spin fast enough to create conditions to launch a jet of highly energetic material into space to make a gamma-ray burst.

Scientists at the University of Warwick discovered that tidal effects like those between the moon and the Earth are the answer.

The discovery, published in Monthly Notices Of The Royal Astronomical Society, was made using simulated models of thousands of binary star systems – solar systems that have two stars orbiting one another.

The majority of stars are located in these binary systems which are needed for massive explosions to be created.

A long gamma-ray burst (GRB), the type analysed in this study, occurs when a massive star about 10 times the size of the sun goes supernova – collapses into a neutron star or black hole and fires a relativistic jet of material into space.

Instead of the star collapsing radially inwards, it flattens into a disc to conserve angular momentum.

As the material falls inwards, that angular momentum launches it in the form of a jet along the polar axis.

However, in order to form that jet, the star has to be spinning fast enough to send the material out into space.

But this presents a problem, scientists say, as stars usually lose any spin they acquire very quickly.

According to the study, the effects of tides from a close neighbour, the same effect that has the moon and the Earth locked together in their spin, could be responsible for spinning these stars at the rate needed to create a gamma-ray burst.

Lead author Ashley Chrimes, a PhD student at the university’s department of physics, said: “We found that the effect of a star’s tides on its partner is stopping them from slowing down and, in some cases, it is spinning them up.

“They are stealing rotational energy from their companion, a consequence of which is that they then drift further away.

“What we have determined is that the majority of stars are spinning fast precisely because they’re in a binary system.”

GRBs are the most luminous events in the universe.

They are observable from Earth when their jet of material is pointed directly at us, meaning we only see around 10-20% of the GRBs in our skies.

The study used a collection of binary stellar evolution models created by researchers from the University of Warwick and Dr J J Eldridge from the University of Auckland.

Dr Elizabeth Stanway, from the University of Warwick department of physics, said: “This model allows us to predict what these systems should look like observationally in terms of their temperature and luminosity, and what the properties of the companion are likely to be.

“We are now interested in applying this analysis to explore different astrophysical transients, such as fast radio bursts, and can potentially model rarer events such as black holes spiralling into stars.”

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