Mon. Not. R. Astron. Soc. (in the press)

ESA/NASA

Sometimes, an apparent supernova fades quickly and the unexploded progenitor star is downgraded to a 'supernova impostor' — such as Eta Carinae (pictured) in our own galaxy (which could erupt at any time, and appear as bright in the sky as the Moon). Another impostor is the luminous blue variable (LBV) star, SN 2009ip, located in Pisces Austrinus. However, Jon Mauerhan and co-workers argue that it is, in fact, a true supernova.

SN 2009ip first erupted in 2009, but its brightness faded in days. A second event a year later was very similar. The latest outburst occurred in July 2012, but this time after fading, it suddenly brightened again. The accompanying ultraviolet emission was even more pronounced, reaching a peak luminosity of ∼ 1.3 × 109 times that of the Sun. The authors present spectroscopic evidence for broad emission and high ejecta speeds of up to 13,000 km s−1, which are unprecedented for any known impostors. Instead, the large velocities are more comparable to those observed for SN 2005cs, a type-II supernova.

As a supernova, SN 2009ip raises a number of questions. Neither the multiple outbursts nor the direct transition from LBV to supernova — without first becoming a Wolf–Rayet star — are accounted for in stellar-evolution models.