An article published in the journal “Nature” describes the first direct observation of granulation on the surface of a star outside the solar system. An international team of Astronomers led by ESO’s Clauda Paladini used the PIONIER instrument installed on ESO’s VLT (Very Large Telescope) to conduct that observation on the star Pi1 Gruis. It was possible to observe its granules, convective cells each about 120 million kilometers across, because the dust that usually hinders these studies was far from the star.
About 530 light years away from Earth, the star Pi1 Gruis is a red giant with a mass that is about 1.5 times the Sun’s but is in the final phase of its life and even the red giant phase is probably ending to move to that of planetary nebula. It’s an obsolete expression in the sense that today we know that it has nothing to do with planets but it remained in use.
In the case of Pi1 Gruis, the evidence of the beginning of the planetary nebula phase was found in previous studies. Shells of materials were observed almost a year light from the star and it’s estimated that they were ejected about 20,000 years ago. It’s a short period from the astronomical point of view so it’s not easy to find a star at that stage and for astronomers it’s a very interesting object of study.
In its red giant phase, the star Pi1 Gruis expanded so much that it’s about 350 times larger than the Sun and thousands of times brighter, with a variability in its brightness. On its surface, convection phenomena are occurring, as normally happens not only on stars but also in other astrophysical processes. So far, stellar convection has been studied above all on the Sun while on the surface of the most evolved giant and supergiant stars the surface is often obscured by dust.
On the surface of the Sun there are about two million convective cells typically around 2,000 kilometers across, a phenomenon called granulation. The knowledge accumulated above all by observing this phenomenon on the Sun allowed to create models that predict that on the surface of giant and supergiant stars there are only a few large convective cells due to the low gravity on the surface of stars whose mass is strongly expanded.
Thanks to its ability to capture details, the PIONIER (Precision Integrated-Optics Near-infrared Imaging ExpeRiment) instrument allowed to study the surface of the star Pi1 Gruis, not obscured by dust. The result was a confirmation of the theoretical model with the discovery of a few large convective cells, so large as to be about a quarter the diameter of a star that at the moment is really huge.
The star Pi1 Gruis will eventually collapse on itself, probably leaving a white dwarf, without a supernova because it’s not massive enough. Instead, it represents an example of what will happen to the Sun in a few billion years, one more reason to study its agony.