Supernovae are one of the ways stars end their life. Incredible fiery explosions rip the stars apart, leaving in their wake a neutron star or a black hole surrounded by a nebula of ejected gas. They are defining moments, but it turns out that some stars might be faking it.
On May 16, 2015, a luminous blue variable star was seen releasing a huge amount of energy that looked like a supernova. The event, known as SN 2015bh, took place in NGC 2770 – a galaxy 80 million light-years away. Even though the detections seem straightforward, astronomers are still not sure the star actually exploded.
According to a paper published in Astronomy & Astrophysics, this could be a “supernova impostor”. The variable star has produced several violent eruptions since 1994. It also had several eruptions in 2015 that were seen as precursors to the May explosion.
“Luminous blue variable stars like this one show two different kinds of eruptions: regular outbursts after which the star returns to its original state and giant eruptions which alter the star permanently,” lead author Christina Thöne, from the Institute for Astrophysics of Andalucia, said in a statement. “A prominent example is Eta Carinae, a star which has already lost mass equivalent to 40 times the mass of our Sun though winds and eruptions.”
The team observed the star from the beginning of 2015 (when it had a minor eruption) and 200 days after the alleged supernova. They know that the star lost luminosity after the event and shows bluer colors than before, but they cannot be certain the star actually exploded.
The team suggests that SN 2015bh might become a Wolf-Rayet star, a type of older star rich in heavy elements. The eruption could be the right mechanism to take away hydrogen, and if it survived the explosion, it could have lost a significant chunk of its main element.
“SN 2015bh is not an isolated case and there are possibly many more similar objects out there that have gone unnoticed, but it seems we have encountered a new type of stellar event," Thöne added. "Now we need to uncover the mechanism driving those events and find out why the observed cases show such very similar behavior."
While SN 2015bh exhibits some unusual patterns, it is not the only supernova impostor – SN 2009ip, for example, erupted in 2012. The researchers think it is too soon to understand what happened and suggest that these objects should be revisited decades after their eruptions to see what they are becoming.