Free Astronomy Magazine March-April 2016

STELLAR EVOLUTION a distance of 10,000 light-years. The two stars that make it up, η Car A and η Car B, are no more than 3 million years old and their masses are at least 90 and 30 times that of the Sun. Astronomers estimate that the mass of the larger star was initially close to 150 solar masses, a large part of which were lost in the turbulent eruptions that characterize the unstable and short existence of this kind of giant stars. One of those eruptions occurred rather recently (in astronomical terms), encompassing a period of 18 years, from 1837 to 1856. In that time frame, Eta Carinae, catalogued until then of magnitude 4 (with a moder- ate variability), exceeded Canopus ( α Car) in brightness, thus becoming the brightest star in the southern hemisphere (mag ≈ -1) and the second brightest star of the entire night sky, outshined only by Sirius ( α CMa, mag -1.46). During what was entered in the annals of astronomy with the term “Great Erup- tion”, η Car A expelled a quantity of gas estimated at several solar masses. By mov- ing away from the pair of stars, and thus cooling down, in just a few years that gas facilitated the formation of a huge bipolar dusty envelope in expansion, that in mid- last century began to be called Homuncu- lus Nebula. As the dust envelope became increasingly thicker, the apparent brightness of Eta Ca- rinae dimmed down as far as falling in 1886 below the threshold of naked-eye visibility (mag > 6). While producing about 5 million A t the height of the Great Eruption, Eta Carinae (yellow arrow) rivalled Canopus in brightness, as shown in this sky map portraying the Carina con- stellation in March 1843. [Ce- lestia planetari- um software]