Supernova conception: Chemical evidence from meteorites suggests that the explosion of a truly massive star triggered the formation of our Solar System.
Credit: NASA
SYDNEY: New evidence backs up the idea that a shockwave from the explosion of massive star triggered the collapse of a dense, dusty gas cloud to form our Sun and its retinue of planets.
For many decades astronomers have postulated that the after effects of this violent supernova led to the birth of the Solar System.
But detailed models of this formation process have only produced the right results under the simplifying – and likely wrong – assumption that the temperatures during the violent events remained constant.
Devil in the details
Now, U.S. astrophysicists at the Carnegie Institution, in Washington DC, have shown for the first time that a supernova could indeed have triggered the Solar System's formation under the more likely conditions of rapid heating and cooling.
They argue that their results, published last month in the Astrophysical Journal, resolve a long-standing debate.
"We've had chemical evidence from meteorites that points to a supernova triggering our Solar System's formation since the 1970s," said Alan Boss, lead author of the study.
"But the devil has been in the details," he said. "Until this study, scientists have not been able to work out a self-consistent scenario, where collapse is triggered at the same time that newly created isotopes from the supernova are injected into the collapsing cloud."
Short-lived radioactive isotopes – versions of elements with the same number of protons, but a different number of neutrons – found in very old meteorites decay on time scales of millions of years and turn into different 'daughter' elements.
Hewn in the furnace of massive stars
Finding these daughter elements in primitive meteorites is helpful, in that it implies that the short-lived radioisotopes they were formed from must have been created only a million or so years before the meteorites themselves were formed.
"One of these parent isotopes, iron-60, can be made in significant amounts only in the potent nuclear furnaces of massive or evolved stars," explained Boss. "Iron-60 decays into nickel-60, and nickel-60 has been found in primitive meteorites. So we've known where and when the parent isotope was made, but not how it got here."
Previous models by Boss and former Carnegie colleague Prudence Foster showed that the isotopes could be deposited into a 'pre-solar cloud' if a shock wave from a supernova explosion was simplistically slow and cool.
"Those models didn't work if the material was [more realistically] heated by compression and cooled by radiation, and this conundrum has left serious doubts in the community about whether a supernova shock started these events over four billion years ago or not," said co-author Harri Vanhala, of the National Centre for Earth and Space Science Education in Columbia, Maryland.
Vanhala found the negative result in his doctoral thesis work at the Harvard-Smithsonian Centre for Astrophysics in 1997.
"We started with a Little Bang"
However, with a much more detailed and refined model, the Carnegie team have now succeeded in finding a way that a realistic simulation of a supernova aftershock could have triggered the pre-solar cloud of dust to form our Solar System.
With their new model they found that after 100,000 years the pre-solar cloud was 1,000 times denser than before. After 160,000 years, the cloud centre had collapsed to become a million times denser, forming the proto-Sun.
The researchers found that in the model, the isotopes from the shock front were mixed into the proto-Sun in a manner consistent with their origin in a supernova.
"This is the first time a detailed model for a supernova triggering the formation of our Solar System has been shown to work," said Boss. "We started with a 'Little Bang' nine billion years after the Big Bang."
With the Carnegie Institution.
