Our Sun is too small to explode in a supernova when it dies, but new findings suggest that our nearby star is in for a dramatic demise, creating a vast ring of gas and dust called a planetary nebula. Though that was always a leading theory, there's some small consolation for those that thought differently. Better yet, the research also clears up a problem that has been troubling astronomers for 25 years.
Size matters
The planetary nebula was perhaps the leading theory as to the fate of the Sun, which in some ways comes as no major surprise – 90 percent of all stars end this way having become red giants. Yet the new research reveals that the Sun is about as small a star as will produce a visible planetary nebula. Stars just a fraction smaller will not. This could be seen as a moral victory for those who thought the Sun was too small to create one. It is big enough, but only just.
To reach these findings, the researchers built a new model that predicts the lifecycle of stars of various masses. It also predicts the brightness of the envelope of matter that the star throws out at the end of its life. Remarkably, this envelope can contain up to half the star's mass.
These envelopes can shine brilliantly for some 10,000 years, making them visible tens of millions of light years away – further than the original star itself would have been visible. But it's the energy of the remaining stellar core that allows this to happen as the ultraviolet radiation it emits ionizes the ejected material.
Why are they called planetary nebula if they're created by dying stars? The term is thought to have been coined by astronomer William Herschel who, in Philosophical Transactions in 1789, referred to "the nebulae I have called Planetary," perhaps due to the visible similarity between these nebulae and planets.
A mystery solved
The research clears up another puzzle about planetary nebulae that has been troubling scientists for some 25 years. In that time, astronomers have observed that the brightest planetary nebulae in other galaxies have the same brightness – which allows us to work out the distance between us and that galaxy, regardless of the type of galaxy it's in.
But the scientific models contradicted these observations, predicting that older smaller stars should create fainter planetary nebulae than younger bigger ones.
The new research indicates that, having thrown out their envelope, stars heat up three times more quickly than previously thought. It's this rapid heating that allows smaller stars, our Sun included, to create a brilliant planetary nebula.
"We found that stars with mass less than 1.1 times the mass of the Sun produce fainter nebula, and stars more massive than 3 solar masses brighter nebulae," explains Albert Zijlstra of Jodrell Bank. "But for the rest the predicted brightness is very close to what had been observed. Problem solved, after 25 years.
"This is a nice result. Not only do we now have a way to measure the presence of stars of ages a few billion years in distant galaxies, which is a range that is remarkably difficult to measure, we even have found out what the Sun will do when it dies."
The team's research has been published in Nature Astronomy.
Source: University of Manchester