Free Astronomy Magazine September-October 2018

52 SEPTEMBER-OCTOBER 2018 SPACE CHRONICLES “We see these really high velocities in a star that seems to have had a pow- erful explosion, but somehow the star survived,” Smith explained. “The easiest way to do this is with a shock wave that exits the star and acceler- ates material to very high speeds.” Massive stars normally meet their final demise in shock-driven events when their cores collapse to make a neutron star or black hole. Astronomers see this phenomenon in supernova explosions where the star is obliterated. So how do you have a star explode with a shock-driven event, but it isn’t enough to com- pletely blow itself apart? Some vio- lent event must have dumped just the right amount of energy onto the star, causing it to eject its outer lay- ers. But the energy wasn’t enough to completely annihilate the star. One possibility for just such an event is a merger between two stars, but it has been hard to find a scenario that could work and match all the data on Eta Carinae. The researchers suggest that the most straightforward way to explain a wide range of observed facts surrounding the eruption is with an interaction of three stars, where the objects exchange mass. If that’s the case, then the present- day remnant binary system must have started out as a triple system. “The reason why we suggest that members of a crazy triple system in- teract with each other is because this The mass transfer alters the gravita- tional balance of the system, and the helium-core star moves farther away from its monster sibling. The star travels so far away that it gravitation- ally interacts with the outermost third star, kicking it inward. After making a few close passes, the star merges with its heavyweight partner, producing an outflow of material. In the merger’s initial stages, the ejecta is dense and expanding rela- tively slowly as the two stars spiral closer and closer. Later, an explosive event occurs when the two inner stars finally join together, blasting off material moving 100 times faster. This material eventually catches up with the slow ejecta and rams into it like a snowplow, heating the mate- rial and making it glow. This glowing material is the light source of the main historical erup- tion seen by astronomers a century and a half ago. Meanwhile, the smaller helium-core star settles into an elliptical orbit, passing through the giant star’s outer layers every 5.5 years. This interaction generates X- ray emitting shock waves. A better understanding of the physics of Eta Carinae’s eruption may help to shed light on the com- plicated interactions of binary and multiple stars, which are critical for understanding the evolution and death of massive stars. T he Milky Way viewed over the Magellan telescopes (a pair of 6.5-meter) at Las Campanas Observa- tory (LCO), Chile. LCO was built and is operated by Carnegie Observatories. is the best explanation for how the present-day companion quickly lost its outer layers before its more mas- sive sibling,” Smith said. In the team’s proposed scenario, two hefty stars are orbiting closely and a third companion is orbiting farther away. When the most massive of the close binary stars nears the end of its life, it begins to expand and dumps most of its material onto its slightly smaller sibling. The sibling has now bulked up to about 100 times the mass of our Sun and is extremely bright. The donor star, now only about 30 solar masses, has been stripped of its hydrogen lay- ers, exposing its hot helium core. Hot helium core stars are known to represent an advanced stage of evo- lution in the lives of massive stars. “From stellar evolution, there’s a pretty firm understanding that more massive stars live their lives more quickly and less massive stars have longer lifetimes,” Rest explained. “So the hot companion star seems to be further along in its evolution, even though it is now a much less massive star than the one it is orbiting. That doesn’t make sense without a transfer of mass.” T he sky rotates above the Gemini Observatory North. [Joy Pollard] !