Solved: The Riddle of the Nova of 1670

This chart of the position of a nova (marked in red) that appeared in the year 1670 recorded by the astronomer Hevelius and was published by the Royal Society in England in their journal Philosophical Transactions. Image credit: The Royal Society

It is a 17th century astronomical enigma that has persisted right up until modern times.

On June 20, 1670, a new star appeared in the evening sky that gave 17th century astronomers pause. Eventually peaking out at +3rd magnitude, the ruddy new star in the modern day constellation of Vulpecula the Fox was visible for almost two years before vanishing from sight.

The exact nature of Nova Vulpeculae 1670 has always remained a mystery. The event has often been described as a classic nova… but if it was indeed a garden variety recurrent nova in our own Milky Way galaxy, then why haven’t we seen further outbursts? And why did it stay so bright, for so long?

Now, recent findings from the European Southern Observatory announced in the journal Nature this past March reveal something even more profound: the Nova of 1670 may have actually been the result of a rare stellar collision.

The remnant of the nova of 1670 seen with modern instruments
The remnant of the nova of 1670 seen with modern instruments and created from a combination of visible-light images from the Gemini telescope (blue), a submillimetre map showing the dust from the SMA (yellow) and finally a map of the molecular emission from APEX and the SMA (red). Image credit: ESO/T. Kaminski

“For many years, this object was thought to be a nova,” said ESO researcher Tomasz Kaminski of the Max Planck Institute for Radio Astronomy in Bonn Germany in a recent press release. “But the more it was studied, the less it looked like an ordinary nova—or indeed any other kind of exploding star.”

A typical nova occurs when material being siphoned off a companion star onto a white dwarf star during a process known as accretion builds up to a point where a runaway fusion reaction occurs.

ESO researchers used an instrument known as the Atacama Pathfinder EXperiment telescope (APEX) based on the high Chajnantor plateau in Chile to probe the remnant nebula from the 1670 event at submillimeter wavelengths. They found that the mass and isotopic composition of the resulting nebula was very uncharacteristic of a standard nova event.

So what was it?

A best fit model for the 1670 event is a rare stellar merger, with two main sequence stars smashing together and exploding in a grand head on collision, leaving the resulting nebula we see today. This event also resulted in a newly recognized category of star known as a “red transient” or luminous red nova.

Universe Today caught up with Mr. Kaminski recently on the subject of red transients and the amazing find:

“In our galaxy we are quite confident that four other objects were observed in outburst owing to a stellar merger: V838 Mon (famous for its spectacular light echo, eruption 2002), V4332 Sgr (eruption 1994), V1309 Sco (observed as an eclipsing binary before its outburst in 2008), OGLE-2002-BLG-360 (recent, but most similar to CK Vul eruption, 2002).Red transients are bright enough to be observed in nearby galaxies. Among them are M31 RV (first recognized “red variable”, eruption 1989), M85 OT2006 (eruption 2006), NGC300 OT2008, etc. Very recently, a few months ago, another one went off in the Andromeda Galaxy. With the increasing number of sky surveys we surely will discover many more.”

Though astronomers such as Voituret Anthelme, Johannes Hevelius and Giovanni Cassini all noted the 1670 nova, the nebula and suspected progenitor star wasn’t successfully recovered until 1981.  Often cited as the oldest and faintest observation of a nova, Hevelius referred to the 1670 apparition as ‘nova sub capite Cygni,’ or a new star located below the head of the Swan near the star Albireo the constellation of Cygnus. Astronomers of the day also noted the crimson color of the new star, also fitting with the modern red transient hypothesis of two main sequence stars merging.

This map includes most of the stars that can be seen on a dark clear night with the naked eye. It shows the small constellation of Vulpecula (The Fox), which lies close to the more prominent constellation of Cygnus (The Swan) in the northern Milky Way. The location of the exploding star Nova Vul 1670 is marked with a red circle.
This chart shows the small constellation of Vulpecula (The Fox), and the location of the exploding star Nova Vul 1670 (red circle). Image credit: ESO/IAU/Sky & Telescope

“We observed CK Vul with the hope to find some submillimeter emission, but were completely surprised by how intense the emission was and how abundant in molecules the gas surrounding CK Vul is,” Kaminski told Universe Today. “Also, we have ongoing observational programs to search for objects similar to CK Vul.”

Follow up observations of the region were also carried out by the Submillimeter Array (SMA) and the Effelsberg radio telescope in Germany. The Nova of 1670 occurred about 1,800 light years distant along the galactic plane in the Orion-Cygnus arm of our Milky Way galaxy, of which the Sun and our solar system is a member. We actually had a naked eye classical nova just last year in roughly the same direction, which was visible in the adjacent constellation of Delphinus the Dolphin.

Of course, these garden variety novae are in a distinctly different class of events from supernovae, the likes of which have not been seen in our galaxy with the unaided eye in modern times since Kepler’s supernova in 1604.

The Atacama Pathfinder Experiment (APEX) telescope on the hunt. Image credit: ESO/ Babak Tafreshi
The Atacama Pathfinder Experiment (APEX) telescope on the hunt. Image credit: ESO/ Babak Tafreshi

How often do stars collide? While rogue collisions of passing stars are extremely rare—remember, space is mostly nothing—the odds go up for closely orbiting binary pairs. What would really be amazing is to witness a modern day nearby red transient in the act of formation, though for now, we’ll have to console ourselves with studying the aftermath of the 1670 event as the next best thing.

Recent estimates give one (merger) event per 2 years in the Milky Way galaxy,” Kaminski told Universe Today. “But we currently know so little about violent merger events that this number is very uncertain.”

Previously cited as a recurrent nova, the story of the 1670 event is a wonderful example of how new methods, combined with old observations, can be utilized to solve some of the lingering mysteries of modern astronomy.