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An out-of-step waltz

With its slow cosmic dance the star, Eta Carinae, recovers its brightness earlier than expected and surprises observers

Illustration Lynette Cook/Gemini ObservatoryEta Carinae, the biggest, brightest and most studied star in the Milky Way, after the Sun, began to surprise astronomers again at the beginning of the year. Observed since December by ten telescopes on earth and four in space it was going through one of its typical dimmer periods – a reduction in its brightness, comparable to the loss of luminosity in thousands of stars like the Sun, that lasts three months and is repeated precisely every five and a half years, as determined in 1993 by the astrophysicist from Parana, Augusto Damineli, from the University of São Paulo (USP). Like someone who is getting their strength back after a bout of flu, Eta Carinae should recover its brightness slowly and progressively as from the end of March until it reaches its full strength some months later. This time, however, that is not the way it was. In the last week of February, a month before it was expected, the star began to come out of the partial dimness in which it found itself and started to shine again.

Accustomed to the inconstancy of Eta Carinae, which he has been studying for the last 20 years, Damineli accompanied the fading and return of the star to brightness through one of the most modern telescopes on earth – the Southern Observatory for Astrophysical Research (Soar), built in the Chilean Andes with Brazilian and North American funding. He was not upset when he saw that part of what he forecast did not materialize. “Eta Carinae always was a very peculiar star”, says the professor from the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) at USP, who even celebrated the unexpected happening. “With the early return to brightness astronomers the world over began to ask for more time on the main telescopes on the planet to observe it for the next six months.”

Observation time is everything that Damineli has always battled for since he started studying Eta Carinae in 1989. Even with a telescope of modest proportions – with a 1.6 meter diameter mirror, installed on the Pico dos Dias, in Minas Gerais and called the jungle telescope by its competitors -, Damineli recorded the dimming in 1992 and established the period in which it was likely to be repeated. He also proposed the model which so far best explains the star’s cyclical loss of brightness, which made him internationally recognized. Eta Carinae is 7500 light-years away from the Solar System and is not a single star, but two stars. The bigger has a mass that is almost 90 times that of the Sun and is colder – the surface temperature does not exceed 15,000 degrees. With just a third of the mass of the main star the smaller one is hotter (its atmosphere reaches almost 50,000 degrees) and ten times less bright than the bigger one. “This model imposed a degree of regularity on the behavior of Eta Carinae, which is not such an exotic star as had been imagined”, explains Damineli.

Illustration Lynette Cook/Gemini ObservatoryExplosion of 1843: Eta Carinae launches into space a mass equivalent to 30 stars like the SunIllustration Lynette Cook/Gemini Observatory

With movements that recall a couple dancing a waltz, the stars move apart and back together again over a five and a half year period. At the moment of maximum proximity – called perisastron – the bigger star covers part of the smaller one. But this eclipse does not completely explain the loss of brightness detected by the telescopes, which see different bands in the electromagnetic spectrum (radio, infrared and X-ray) progressively disappear. If the eclipse was the only mechanism behind the dimming all these energy lines would disappear at the same time. Three-dimensional simulations of the behavior of the stars presented last year by Atsuo Okazaki’s team from the Tokkai-Gakuen University in Japan and by Michael Corcoran, from the North American space agency (Nasa), indicate that the dimming is caused by disturbances in the wind of particles that emanate from stars and collide at extremely high speeds, emitting X-rays. In the period in which they are closest the smaller star is engulfed by the wind from the bigger one, which is denser and conceals the secondary star’s brightness.

According to Damineli, the reduction by a month in the dimness this year has added a complexity to an already complicated scenario. “The stars behaved like two ballerinas until the moment they came together, when they shimmied around and sped up their steps a little as they moved away from each other”, comments the astrophysicist from USP. He already has a possible explanation for Eta Carinae being out of step, too. Because it has a huge mass (today corresponding to 90 times the mass of the Sun, but which was 120 solar masses before the explosion that occurred in 1843) the bigger star is less dense and its diameter may oscillate, like a party balloon that is blown up a little and then deflates. “In periods when it is more compact it loses less material and its winds become less dense”, explains Damineli. If approximation occurs in this phase the light from the secondary star can escape more easily from the winds that embrace it and, therefore, be observed from the Earth.

All this instability is not caused by the eccentric behavior of a star that is used to attracting attention. Measures of the mass that the main star has already thrown into space indicate that its end is near, and at the age of 2.5 million years, Eta Carinae is an old lady whose days are numbered. If the forecasts of Nathan Smith, an astrophysicist from the University of California and someone who studies Eta Carinae, are correct the star may at any moment suffer a super-explosion that will be much more intense than the one in 1843 and will be capable of reducing it to dust and bringing to an end once and for all its spatial ballet. In the explosion 90% of its mass would be pulverized and the rest would compact giving rise to a black hole, an emitter of gamma rays, which are the most intense radiation that exists. “It would be a fantastic event to observe”, says Damineli. The death of Eta Carinae will allow for an understanding of another stage in the life cycle of blue super-giants, the today-rare stars that dominated the primitive Universe between 7-10 billion years ago.