When regular stars like our Sun run out of hydrogen to burn, they turn into red giants. The stars swell up thousands of times and their outer layers begin to spread into interstellar space under the push of stellar winds. This leads to the formation of vast structures with complex shapes known as planetary nebulae.
Researchers report today in Science some new insight in these stellar winds and their connection to the planetary nebulae. The high-resolution observations employed in this study tell a different story from what was expected. The winds are not spherical, released equally from every part of the star. Instead, they have complex structures suggesting that something is shaping them.
"The Sun – which will ultimately become a red giant – is as round as a billiard ball, so we wondered: how can such a star produce all these different shapes?" lead author Professor Leen Decin, KU Leuven, said in a statement.
"We noticed these winds are anything but symmetrical or round. Some of them are actually quite similar in shape to planetary nebulae. Some stellar winds were disk-shaped, others contained spirals, and in a third group, we identified cones."
The team has a simple explanation that allows for the creation of these dramatically different shapes: smaller companions. If a dying star is orbited by a smaller, much dimmer companion, like a brown dwarf or a planet, then these celestial bodies will stir the gas around, creating the remarkable structures astronomers have observed.
"Just like how a spoon that you stir in a cup of coffee with some milk can create a spiral pattern, the companion sucks material towards it as it revolves around the star and shapes the stellar wind," Decin explained.
Up until now, calculations of how stars evolved were based on the idea aging Sun-like stars have spherical stellar winds. "Our findings change a lot. Since the complexity of stellar winds was not accounted for in the past, any previous mass-loss rate estimate of old stars could be wrong by up to a factor of 10," Decin said.
The team used models to test their idea and it confirmed that the shape of stellar winds can be explained by their companions. The incredible observations were conducted with the Atacama Large Millimeter/submillimeter Array (ALMA), one of the finest radio observatories in the world.
"We were very excited when we explored the first images," added co-author Dr Miguel Montargès also at KU Leuven. "Each star, which was only a number before, became an individual by itself. Now, to us, they have their own identity. This is the magic of having high-precision observations: stars are no longer just points anymore."
Astronomers believe that this is the fate of our Sun in about 5-7 billion years. Once our star turns into a red giant, under the influence of Jupiter and Saturn, solar winds will create beautiful shapes. But we'll never get to see it. The Earth will be long gone by then.