Free Astronomy Magazine March-April 2016

STELLAR EVOLUTION twins candidates, during 2015 Khan and colleagues focused their attention on the M83, NGC 6946, M101 and M51 galaxies, located respectively 15, 18, 21 and 26 mil- lion light-years away. In the last century, in these four galaxies were observed not less than 20 supernovae, thus making them an ideal target. That choice was rewarded by some impor- tant results, presented at a press confer- ence of the 227 th American Astronomical So- T he 5 Eta twins viewed by Spit- zer (top row, mid- infrared at 3.6 µm) and by Hub- ble (near-infrared at 800 nanome- ters). Even though Spitzer’s resolu- tion is lower, it is however suffi- cient to show the radiation emitted by the dust sur- rounding the Eta twins. [NASA, ESA, and R. Khan (GSFC and ORAU)] On the side, the light curve of eta Carinae in visible light, obtained with various meth- ods and instru- ments from 1822 to date. [Fernán- dez-Lajús et al.] for compact sources that are bright in the mid-infrared, but at the same time dim in the visible spectrum. Having established how to look for them, it only remained to figure out where. For the aforesaid reasons it was necessary to look beyond the Milky Way, towards oth- er galaxies. Since stars of great mass are believed to be the progenitor of Type II su- pernovae (core-collapse supernovae) and hypernovae (Type II superluminous super- novae), the gal- axies most likely to host Eta twins are those with a high rate of star forma- tion (mainly spirals rich of hydrogen and dust clouds) and a greater num- ber of observed su- pernovae. But as these gal- axies must also be sufficiently close to allow the observa- tion of objects of very small angular size, the choice is rather limited. After having exam- ined dozens of Eta