New observations including the APEX radio telescope show that the star that European astronomers saw in the sky in 1670 was not a nova, but a much rarer and more violent phenomenon:a stellar collision.
The event was spectacular enough to be visible to the naked eye, but the traces it left were very faint. Only after careful analysis using submillimetre telescopes could the 340-year-old riddle be solved. The result was published Monday in the journal Nature.
Some of the most famous astronomers of the seventeenth century, including Johannes Hevelius – the father of lunar cartography – and Giovanni Cassini, gave detailed accounts of a new star appearing in the sky in 1670. Hevelius described him as nova sub capite Cygni — a new star under the Swan's head — but today's astronomers know it as Nova Vulpeculae 1670. Historical records of novae are rare and of great interest to modern astronomers. Nova Vul 1670 is the oldest recorded nova appearance and the faintest nova subsequently rediscovered.
The lead author of the new study, Tomasz Kamiński of ESO and the Max-Planck-Institut für Radioastronomie in Bonn explains:“For many years this object was considered a nova, but the more it was examined, the less it resembled a regular nova. In fact, it didn't look like any kind of exploding star.”
With the naked eye
When it first appeared, Nova Vul 1670 was easily detectable to the naked eye and showed variable brightness for two years. Then it died out, only to disappear for good after two revivals. Although the phenomenon is well documented for the time, astronomers at the time lacked the equipment needed to explain the nova's curious behavior.
During the twentieth century, astronomers realized that most novae can be explained by the explosive behavior of close binaries. But Nova Vul 1670 did not fit well into this model and remained a mystery.
Even the ever-expanding telescopes were unable to find any trace of the alleged nova at first. It wasn't until the 1980s that a team of astronomers detected a faint nebula around the likely site of the star's remnant. While there seemed to be an intriguing connection to the 1670 sighting, the sightings failed to shed new light on the true nature of the phenomenon seen in the European sky more than three hundred years earlier.
Tomasz Kamiński continues:“We now surveyed the area at submillimetre and radio wavelengths. In doing so, we found that the remainder is surrounded by cool gas that is rich in molecules with a very unusual chemical composition.”
In addition to the APEX radio telescope, the team also used the Submillimeter Array (SMA) and the Effelsberg radio telescope to determine the chemical composition and measure the mutual ratios of the different isotopes in the gas. Taken together, this provided an extremely detailed picture of the nature of matter, and of its possible origin.
Spectacular collision
The team found that the mass of the cool material is too great to have come from a nova explosion. In addition, the isotope ratios measured around Nova Vul 1670 are not consistent with those of a nova. But if it wasn't a nova, what was it?
It appears to have been a so-called red nova — a spectacular collision between two stars, brighter than a regular nova, but less bright than a supernova. Red novas are very rare events in which stars explode by merging with another star. In the process, matter from the interiors of the two stars is blown into space, ultimately leaving only a faint remnant embedded in a cool environment rich in molecules and dust. Nova Vul 1670 almost exactly matches the profile of this recently recognized class of explosive stars.
“Discoveries like these are the most fun:a completely unexpected result!” concludes co-author Karl Menten, also of the Max-Planck-Institut.