Andromeda, our galactic neighbor, has been discovered to be home to a recurrent nova – a star that regularly undergoes dramatic eruptions that shed its outer layers, resulting in a large shell of ejected material.
The object in question, known as M31N 2008-12a, has two striking characteristics: It has one of the largest shells of material ever seen in such a star and, due to that, it erupts more often than previously studied recurrent novae. The discovery is reported in Nature.
“When we first discovered that M31N 2008-12a erupted every year, we were very surprised,” co-author Allen Shafter, from San Diego State University, said in a statement.
Usually, these types of stars erupt less often. A more typical pattern is to have nova explosions about every 10 years. However, the yearly eruptions of M31N 2008-12a over millions of years of activity have led to the formation of its remarkable shell of material that now spans almost 400 light-years across.
Recurrent novae are believed to be caused by the interaction between a white dwarf, the remnant of a dead star, and another star. The white dwarf steals material from its companion and, as this gas precipitates on the dense white dwarf, it gets compressed and heated until it explodes. This is the nova, when the star can suddenly become a million times brighter and some of the material is thrown into space, forming a remnant shell.
Beyond the shell size and frequency of eruption, there’s another reason why the international team is interested in this object. The system has the hallmarks of possessing a massive white dwarf, potentially nearing the limit of how much material it can steal before going full supernova. If the team could catch this object in the act, it would be of tremendous importance.
A white dwarf going supernova is a moderately rare event, with very specific characteristics. So specific that astronomers have been able to use them as “measuring rods” to work out how far away galaxies are. Studying the physics of these supernovae help us understand the universe as a whole.
The team is now investigating whether what they have seen in this object is common or not in the universe. There could be an undiscovered population of these objects.