Sun at End of Its Life Will Turn into Faint Planetary Nebula, Astronomers Predict
Planetary nebulae — massive rings of luminous, interstellar gas and dust — mark the end of the active life of 90% of all stars. But, for years, astronomers weren’t sure if the Sun would follow the same fate — until now. University of Manchester’s Professor Albert Zijlstra and colleagues predict the Sun will form a faint planetary nebula.
Professor Zijlstra and co-authors developed a new model that predicts the lifecycle of stars.
The model was used to predict the brightness (or luminosity) of the ejected envelope, for stars of different masses and ages.
“When a star dies it ejects a mass of gas and dust — known as its envelope — into space,” Professor Zijlstra said.
“The envelope can be as much as half the star’s mass. This reveals the star’s core, which by this point in the star’s life is running out of fuel, eventually turning off and before finally dying.”
“It is only then the hot core makes the ejected envelope shine brightly for around 10,000 years — a brief period in astronomy. This is what makes the planetary nebula visible. Some are so bright that they can be seen from extremely large distances measuring tens of millions of light years, where the star itself would have been much too faint to see.”
The new model also solves another problem that has been perplexing astronomers for a quarter of a century.
Approximately 25 years ago astronomers discovered that if you look at planetary nebulae in another galaxy, the brightest ones always have the same brightness.
It was found that it was possible to see how far away a galaxy was just from the appearance of its brightest planetary nebulae.
In theory it worked in any of type galaxy. But whilst the data suggested this was correct, the scientific models claimed otherwise.
“Old, low mass stars should make much fainter planetary nebulae than young, more massive stars. This has become a source of conflict for the past for 25 years,” Professor Zijlstra noted.
“The data said you could get bright planetary nebulae from low mass stars like the Sun, the models said that was not possible, anything less than about twice the mass of the sun would give a planetary nebula too faint to see.”
The team’s model shows that after the ejection of the envelope, the stars heat up 3 times faster than found in older models. This makes it much easier for a low mass star, such as the Sun, to form a bright planetary nebula.
The astronomers found that in the new model, the Sun is almost exactly the lowest mass star that still produces a visible, though faint, planetary nebula. Stars even a few per cent smaller do not.
“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, but for the rest the predicted brightness is very close to what had been observed. Problem solved, after 25 years,” Professor Zijlstra said.
“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 findings were published this week online in the journal Nature Astronomy.
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K. Gesicki et al. The mysterious age invariance of the planetary nebula luminosity function bright cut-off. Nature Astronomy, published online May 7, 2018; doi: 10.1038/s41550-018-0453-9