Free Astronomy Magazine March-April 2018

35 MARCH-APRIL 2018 SPACE CHRONICLES π 1 Gruis is one of the brightest members of the rare S class of stars that was first defined by the Ameri- can astronomer Paul W. Merrill to group together stars with similarly unusual spectra. π 1 Gruis, R An- dromedae and R Cygni became pro- totypes of this type. Their unusual spectra is now known to be the re- sult of the “s-process” or “slow neu- tron capture process” — responsible for the creation of half the elements heavier than iron. When π 1 Gruis ran out of hydrogen to burn long ago, this ancient star ceased the first stage of its nuclear fusion programme. It shrank as it ran out of energy, causing it to heat up to over 100 million degrees. These extreme temperatures fueled the star’s next phase as it began to fuse helium into heavier atoms such as carbon and oxygen. This intensely hot core then expelled the star’s outer layers, causing it to balloon to hundreds of times larger than its original size. The star we see today is a variable red giant. Until now, the surface of one of these stars has A stronomers using ESO’s Very Large Telescope have directly observed granulation patterns on the surface of a star outside the Solar System — the ageing red giant π 1 Gruis. This remarkable new image from the PIONIER instrument reveals the convective cells that make up the surface of this huge star. Each cell covers more than a quarter of the star’s diameter and measures about 120 million kilometres across. [ESO] never before been imaged in detail. By comparison, the Sun’s photo- sphere contains about two million convective cells, with typical diame- ters of just 1500 kilometres. The vast size differ- ences in the c o n v e c t i v e cells of these two stars can be explained in part by their varying surface gravi- ties. π 1 Gruis is just 1.5 times the mass of the Sun but much larger, result- ing in a much lower surface gravity and just a few, ex- tremely large, granules. While stars more massive T his sequence takes the viewer towards the southern constel- lation of Grus (The Crane). We zoom in on the pair of stars π 1 Gruis (red) and π 2 Gruis (bluish-white), and the bright spiral galaxy IC 5201 is also visible. The final shot shows a very detailed view of the surface of the red giant star π 1 Gruis from the PIO- NIER instrument on the VLT Interferometer. [ESO/Digitized Sky Survey 2/N. Risinger (skysurvey.org ) Music: Astral Electronic] than eight solar masses end their lives in dramatic supernovae explo- sions, less massive stars like this one gradually expel their outer layers, resulting in beautiful planetary neb- ulae. Previous studies of π 1 Gruis found a shell of material 0.9 light- years away from the central star, thought to have been ejected around 20,000 years ago. This rela- tively short period in a star's life lasts just a few tens of thousands of years – compared to the overall lifetime of several billion – and these observa- tions reveal a new method for prob- ing this fleeting red giant phase. !