Astronomers at the Catholic University of Leuven (KU Leuven) have provided an explanation for the shape of planetary nebulae. The discovery is based on a series of observations of the stellar wind around aging stars. Contrary to popular belief, the team found that stellar winds are not spherical, but rather shaped like planetary nebulae. The team of scientists concluded that interaction with an accompanying star or exoplanet forms both stellar winds and planetary nebulae. The results are published in the journal Science.
Dying stars grow and cool, eventually becoming red giants. They create stellar winds, streams of particles that the star throws out, causing it to lose mass. Astronomers have always assumed that these winds were spherical, like the stars they surround. As it develops further, the star heats up again, and the stellar radiation causes the ejected layers of stellar material to glow, forming a planetary nebula.
The team of scientists observed stellar winds around the cold red giants using the ALMA Observatory in Chile, the largest radio telescope in the world. For the first time, they compiled a large, detailed collection of observations, each of which was made using the same method.
What the astronomers saw surprised them. “We noticed that these winds are not symmetrical or circular,” says Professor Decin. “Some of them are actually very similar in shape to planetary nebulae.”
Astronomers have even been able to identify different categories of shapes. “Some stellar winds were disk-shaped, others spirals, and in the third group we defined cones.” This is a clear sign that the shapes were not randomly generated. The team realized that other, low-mass stars or even heavy planets in the vicinity of the dying star were causing these patterns. These satellites are too small and faint to be detected directly. “Just like a spoon that you stir in a cup of coffee with milk can create a spiral pattern, a moon sucks in material to it as it orbits a star and forms a stellar wind,” explains Decin.
The team has put this theory into a model, and indeed: the shape of the stellar winds can be explained by their surrounding satellites, and the rate at which a cold, evolved star loses its mass due to the stellar wind is an important parameter. Decin: “All our observations can be explained by the fact that stars have a satellite.”
So far, stellar evolution calculations have been based on the assumption that stellar winds in aging sun-like stars are spherical. “Our results are changing a lot. Since the complexity of stellar winds has not been taken into account in the past, any previous estimate of the rate of mass loss of old stars could be wrong up to 10 times. ” The team is currently conducting further research to see how this might affect calculations of other important characteristics of stellar and galactic evolution.
The study is also helping to imagine what the Sun might look like when it dies in 7000 million years. “Jupiter or even Saturn – because they have such a large mass – will influence whether the sun spends its last millennia at the center of a spiral, butterfly, or whatever shape we see in planetary nebulae today,” Decin notes. “Our calculations show that a faint spiral is forming in the stellar wind of the old dying sun.”