Star’s coughing fit cools it down

Astronomers have solved the mystery of a rapidly-fading star at the centre of an inside-out nebula called HuBi 1, which has edges hotter than its middle.

An international team, led by Martin Guerrero from the Instituto de Astrofísica de Andalucía, in Spain, believes the cloud of glowing dust was turned inside out when the ageing star at its centre had a late burst of energy that sent a shockwave through the nebula.

HuBi 1 comprises a cloud of gas and dust thrown off by its star as its fuel ran out and it shrank from a red giant to a white dwarf. What attracted the astronomers’ attention was the highly ionised gas in its outer shell compared with its cooler inner region. A cool inside is the opposite of other planetary nebulas, which are powered by 100,000 degree Celsius white dwarfs at their centres.{%recommended 4089%}

“It’s like a bonfire that as you move closer to it, it gets colder,” says Guerrero. “It’s completely against any expectation.” 

Even stranger, the star at the centre of the nebula has become around 10,000 times fainter in the last 50 years.

Despite the dramatic loss of brightness, the team believe HuBi 1’s central star is a typical hot white dwarf, but think that late in its life it had a burst of energy – a sort of cosmic coughing fit that sent a shockwave through the surrounding cloud.

Bursts of energy are not uncommon in white dwarfs. They shrink after using up the hydrogen fuel in their core, having fused it first into helium and then to carbon and nitrogen. 

Without the fusion energy pushing the star apart, it collapses and the pressure increases. As it does so remnants of hydrogen or helium on the outer regions of the star begin to fuse and the star briefly lights up again.

The team’s paper, published in journal Nature Astronomy, shows that HuBi 1’s inside-out structure can be explained by just such a cough. The burst of energy threw material off the star and formed a shockwave travelling through the nebula at around 500 kilometres per second. The fast-moving gas is ionising the nebula it encounters.  After the shockwave passes, the gas cools down and the carbon from the star condenses into a cloud of sooty dust.

It is this that the astronomers believe has obscured the star, making it appear to fade. The cloud is also preventing the energy from the star penetrating and heating the nebula around it, which explains why its centre has cooled.

The team is working to pin down the timing of the star’s cough. Guerrero suggests it occurred in the past 200 years, perhaps as little as 50 years ago.

“We’ve had theories about this kind of born-again event, but we had never witnessed one,” he says.

“At the beginning we thought it was a very funny object, but by the end we understood it was the key to understanding a mystery.”

After the shockwave has passed, the whole nebula will cool down. Because HuBi 1 has a similar mass to the sun, being engulfed by a planetary nebula is a possible future for Earth. But, adds Guerrero, the Earth is only middle-aged, so we have another five billion years to wait before we find out.

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