An international team of astronomers has successfully imaged the surface of the geriatric star π1 Gruis, revealing enormous convection cells that cycle material between the interior and surface of the star – essentially acting as a massive stellar lava lamp. In about five billion years, our own Sun will make the transition from a main-sequence yellow dwarf star, to a red giant, with dramatic implications for Earth.
When a star less than eight times the mass of our Sun runs out of the supply of hydrogen fueling the thermonuclear reaction raging in its stellar core, it may transform into a red giant instead of ending its life in a dramatic supernova explosion.
At first, the hydrogen depleted star would shrink in on itself, but not for long. The shrinking process super heats the star to around 100 million degrees, which in turn causes it to fuse helium reserves into heavier atoms such as oxygen. The outer layers of the star are then expelled from the super-hot core, resulting in a stellar body that is hundreds of times its original size.
In the case of π1 Gruis, we are left with an enormous variable red giant star, roughly 350 times the size of our Sun, but with only 1.5 times its mass.
A team of astronomers has used the Precision Integrated-Optice Near-infrared Imaging ExpeRiment (PIONIER) instrument mounted aboard the ESO's Very Large Telescope (VLT) to image the massive red giant star.
The infrared capabilities of PIONIER in conjunction with the VLT's advanced interferometer allowed the astronomers to pierce through a cloud of stellar material thrown off by π1 Gruis some 20,000 years earlier.
The new PIONIER image of π1 Gruis reveals enormous granules, or convection cells that each span roughly 120 million km (75 million miles) across. In these cells, plasma (the stuff of stars) is heated in the interior of a star, causing it to rise to the surface. Material that has been on or near the surface of the star for some time gradually cools relative to the upcoming material, causing it to slide down closer to the core, where it is once again heated, with the process beginning anew.
To put the sheer size of π1 Gruis's convection cells in context, our Sun has roughly two million cells, each of which is a (relatively) measly 1,500 km (932 miles) across. It is thought that the cells on π1 Gruis are able to grow as large as they have because the surface gravity of the red giant is very weak compared to that of the Sun.
A similar mechanism is thought to be at work recycling material in the cell-like structures found in the Sputnik Planum region on Pluto, but on a much, much smaller scale.
The convection process is thought to be a driving factor in the creation of stellar winds, and energy transportation throughout a star.
Perhaps the most interesting aspect of these new observations is their implications for the future of our own solar system. Roughly five billion years in the future, the Sun will make its own transformation, and become a red giant much like π1 Gruis. At this point, it is possible that the innermost planets of our solar system – Mercury, Venus, and Earth – could be swallowed by our encroaching star. Alternatively, Earth's orbit could shift outward, allowing the planet to survive.
These stars end their lives as stunning planetary nebula, which enrich the next generation of stars with heavier elements, encouraging stellar evolution in our Milky Way.
A paper detailing the convection cell observations has been published in the journal Nature. The video below zooms in on π1 Gruis from the perspective of Earth.
Source: ESO