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A star full of mysteries

Observatories to monitor Eta Carinae's blackout

HST/NasaHomunculus Nebula: formed by gas ejected from the star Eta CarinaeHST/Nasa

As from the last week of December – and for three subsequent months – astronomers in Brazil, Argentina and Chile will point their telescopes at the same part of the sky to the right of the Southern Cross Constellation, which, at this time of year, is only visible at dawn. They plan to observe in the greatest possible detail a phenomenon that repeats itself every 5.5 years in the Eta Carinae nebula: a brutal reduction in the amount of light coming from the largest and brightest star in the Milky Way, our galaxy.

The blackout of the star Eta Carinae that the astronomers expect to record took place 7,500 years ago. It will only be seen now because it takes that long for the light to reach the Solar System. First identified 16 years ago by astronomer Augusto Damineli, from the University of São Paulo, the drop in the level of luminosity of Eta Carinae, which is usually 5 million times brighter than the Sun, continues to intrigue astronomers. It gradually reaches different radiation bands (radio, infra-red and X-ray) and lasts for three months. Located 7.5 thousand light-years from Earth, the star can be observed with binoculars, but the blackout is only detectable with special equipment.

The regularity with which this occurs plus the fact that the star?s brightness lessens supports Damineli’s explanation for the phenomenon: Eta Carinae might be two stars, rather than just one:  a smaller, hotter star and another larger, colder, brighter one. In a type of cosmic ballet, the smaller star, with a mass 30 times greater than that of the Sun, gravitates around the main star, which in turn is three times larger than the first. The result of this mutual attraction is that the smaller star travels around the large one in an elliptical orbit that takes 2,023 days. By Damineli’s calculations, the blackout takes place when the two stars are at their very closest, at which point the smaller star is partially hidden by the main one.

Until recently it was believed that this eclipse explained the decreased light detected by telescopes both on earth and in space. However, the reality is not so simple. If the eclipse were the only cause, all the energy bands should become undetectable simultaneously. But this is not the case, as some of them disappear before the others.

Based on three-dimensional simulations of the behavior of the stars, Atsuo Okazaki, from Japan’s Tokkai-Gakuen University, interprets the blackout differently: besides the eclipses, the decreased luminosity could be due to disturbances caused by the particle winds discharged by the two stars; these particles collide at high speeds – when the two stars are at their closest the smaller star is surrounded by winds from the larger star, which enshroud it and interfere in the radiation that it emits.

Over the course of the current observations, the astronomers hope to be able to see through the winds and record when and in what order the radiation bands stop shining. “These are questions that could help us understand the obscure characteristics of the secondary star, such as its mass and surface temperature,” says Damineli says. “This information will make it possible to predict the future of these stars,” explains the astronomer, who will be monitoring the blackout from the telescope at the Southern Observatory for Astrophysical Research (Soar), in the Chilean part of the Andes.