A mysterious explosion observed by astronomers in the 17th century was not a nova, but a far more rare and violent type of stellar collision, according to a new study published Monday in the journal Nature and based on observations made with the ESO’s APEX telescope.
Back in the good ol’ days
The event, witnessed by European researchers in 1670, was spectacular enough to be observed with the naked eye during its first outburst, the observatory said in a statement. However, it also left behind faint traces that had to be analyzed with powerful sub-millimeter telescopes over 340 years later in order to discover the true source of these unusual explosions, they added.
Johannes Hevelius, who discovered seven still-recognized constellations and has been dubbed the father of lunar topography, was among the astronomers that documented the appearance of a new star in the skies in 1670. Originally described as Hevelius as nova sub capite Cygni (a new star below the head of the swan), it is now known as Nova Vulpeculae 1670.
“For many years this object was thought to be a nova, but the more it was studied the less it looked like an ordinary nova – or indeed any other kind of exploding star,” explained lead author Tomasz Kamiński of the ESO and the Max Planck Institute for Radio Astronomy in Germany.
At first, Nova Vul 1670 could easily be seen without the need for telescopes, and over a span of two years, it varied greatly in brightness. It actually vanished and returned twice before it finally disappeared for good. While it was well documented for the era, the early astronomers alive at the time lacked the equipment required to solve the riddle of its unusual activity.
Astronomers came to realize that most novae could be explained by the explosive behavior of close binary stars during the 1900s, but the model still did not seem to apply to Nova Vul 1670, so it remained a mystery. It wasn’t until the 1980s that astronomers first realized that Nova Vul 1670 did not disappear completely, but that a faint nebula remained around its location.
It wasn’t until Kamiński and his colleagues investigated that area using the APEX telescope, the Submillimeter Array (SMA), and the Effelsberg radio telescope that they found a cool gas that had an unusual chemical composition surrounding the remnant. Thanks to their efforts, they were able to compile a detailed account of the region’s makeup.
What they discovered was that the mass of the cool material was too great to have originated from a nova explosion, and that the isotope ratios surrounding Nova Vul 1670 were different to those expected from a nova.
So if it wasn’t a nova, what was it?
As it turns out, the phenomenon is the result of a collision between two stars that is more brilliant than a nova but not as bright as a supernova. The collision produces what is known as a red transient, in which a star explodes due to a merger with another star and ejects material into space.
These events are rare, the researchers explained, and ultimately they leave behind just a faint remnant embedded in a cool environment, rich in molecules and dust. This new class of stellar eruptions precisely fits the profile of Nova Vul 1670, leading to a discovery that co-author and Max Planck Institute researcher Karl Menten called “completely unexpected.”