Free Astronomy Magazine March-April 2016

STELLAR EVOLUTION times more energy than the Sun, Eta Carinae ap- pears today as a star of magn i tude 7.6, with var- iations that a few times a century al- lows it for short periods to be still visible to the naked eye under dark and clear skies. Astronomers suspect that the paroxysmal phenomena of this system are linked to the star’s duplici- ty, as, in fact, some of the most sig- nificant historical eruptions have oc- curred during the two stars’ perias- tron passage. Nevertheless, the cause of the erup- tive phenomena remains today es- sentially unknown. And this brings us back to the issue of the uniqueness of Eta Carinae, since with a sin- gle specimen it is impossible to pro- pose theoretical models capable of accurately describ- ing what hap- pened in the past and neither pre- dict the number and distribution of similar objects. In short, it would be desirable to dis- cover twin stars of Eta Carinae. But what to look for and where? The two blue giant stars that make up the pair under discus- sion produce immense flows of ultraviolet radiation that is absorbed by the surround- ing dust, which by warming, reradiates this energy mostly in the form of infrared rays. Thanks to the observations of the Spitzer Space Telescope, we now know that for Eta Carinae that kind of energy rises rapid- ly between 3.6 and 8 µm (microns), to then drop from 8 to 24 µm. This behaviour indi- cates the presence of circumstellar dust with a temperature between 400 and 600 Kelvin. From the apparent brightness in visible light of the pair of stars and from the comparison with the theoretical brigh- tness expected in the absence of dust, it has been possible to determine through A s shown in this video, the complex bilobed structure of the dust cloud sur- rounding Eta Ca- rinae is due to the star’s duplic- ity and to the tur- bulent periastron passages of the two components. [GSFC/NASA] On the side, a suggestive pic- ture of the south- ern hemisphere with Mount Kili- manjaro on the horizon, just up on the left the Southern Cross and at the top centre the Great Carina Nebula, within which hides Eta Carinae. [Babak Tafreshi]