WHAT I REALLY WANT TO KNOW IS… Was our Sun born in a bubble?
Vikram Dwarkadas suggests that the Solar System could have formed in a shell blown out from a hot massive star
Stars form inside great clouds of gas and dust, like stellar nurseries. This is well known. We clearly see it happening in the Orion Nebula, for example, as well as many others.
I have been leading a team trying to understand the high level of aluminium-26 and low level of iron-60 in the early Solar System. Our conclusion is that it happened because the Solar System was born inside a bubble blown out by a massive star nearing the end of its life.
For several decades, astronomers have believed that a supernova was located near the Sun and the Solar System when they formed. However, evidence found in meteorites tells us that this is not the case. Scientists have studied many meteorites that are pristine remnants left over from the formation of the Solar System four-and-a-half billion years ago. They show that at that time there were high levels of the element aluminium-26 compared to what was in the galactic background. Something nearby must have supplied it.
A supernova would produce aluminium-26. However, it would have also produced the isotope iron-60, yet studies of the meteorites show much less of that isotope than we would expect to find.
I predict a Wolf-Rayet
So if not a supernova, then what else could have provided the building materials for the Solar System? We looked at two alternatives: a Wolf-Rayet star and an AGB star.
A Wolf-Rayet star is an extremely hot, luminous star, 40 to 50 times more massive than the Sun, that is nearing the end of its short life of four to five million years. It has lost its hydrogen and may eventually end its life as a supernova.
An AGB star is a less massive star in the final stages of its evolution, similar to a red giant. It is named after the Asymptotic Giant Branch of the Hertzsprung-Russell diagram, which charts stellar evolution. It is thought that the Sun will become an AGB star. We had to look at which of these types of star could release the material in just the right amounts and get it into the cloud that formed the Solar System. AGB stars have long lives of around a few to 10 billion years before they grow into giant stars and throw off a shell of gas as a planetary nebula. The chance that there would have been such a star in the neighbourhood is pretty small. Wolf-Rayet stars, on the other hand, don’t last long. It is sometimes said that massive stars are like rock stars: they live fast and die young. Stellar evolution models reveal that they shed vast amounts of many different elements in their later lives. More importantly, Wolf-Rayet stars release a lot of aluminium-26, but no iron-60. These stars also have dense, supersonic winds that sweep up material around them to form a thin, dense shell. These bubbles then continue to expand until they reach pressure equilibrium. We calculated that the amount of aluminium-26 coming from the star, combined with material in the dense shell around it, could be enough to form the Solar System. One puzzle was how the material gets injected into the shell. If you have a fan blowing in your room, air will hit a wall but not necessarily inject stuff into it. The winds from Wolf-Rayet stars contain dust – we can see it as infrared emission from mostly carbon grains around one micron in size. Because the winds are hot, we hypothesise that the aluminium-26 condenses onto the dust. The dust can be carried out with the wind, reaches the dense shell and penetrates it. And since there is observational evidence which suggests that stars do form in the dense shells of these bubbles – why not our star? Next we plan to refine our modelling by checking other isotopes which existed in the early Solar System, and whose abundances we can measure in meteorites. We will see whether a Wolf-Rayet star can provide them in the right quantities.