Free Astronomy Magazine March-April 2015

50 STELLAR EVOLUTION centre of gravity in 5.5 years, reaching at the closest approach (periastron) a mutual distance of 225 million kilometres, more or less similar to that separating Mars from the Sun. The various attempts to characterize Eta Ca- rinae A and Eta Carinae B through the dusty curtain surrounding them has not so far led to univocal results, so much so that even their type is not quite certain. The more massive star is of spectral type F (photo- spheric temperature around 10-12,000 kel- vin), with properties similar to those of the luminous blue variables (LBVs), despite being more brilliant than any other known rinae visible again to the naked eye. At the present day, although still overshadowed by the haziness created by the eruptions, the star has almost regained its former bright- ness, shining at magnitude 4.6. In the mean- time, about ten years ago, astronomers discovered that Eta Carinae is not, as pre- viously thought, a single giant star, but a bi- nary system consisting of two very massive stars: one of about 90 solar masses, which shines like 5 million suns, and the other of about 30 solar masses, bright as 1 million suns. Located in the constellation Carina, about 7,500 light-years away from Earth, the two stars revolve around the common T he orbits of the two big stars forming the Eta Carinae sys- tem. At the point of closest ap- proach the stellar winds interact intensely. The diagram below shows how the peaks of the X- rays emission coming from the centre of the sys- tem always occur at the time of the periastron. [GSFC/NASA]