A new bright star appears in the sky in June 1670. It is seen by the Carthusian monk Father Dom Anthelme in Dijon, France and by the astronomer Johannes Hevelius in Gdansk, Poland. Over the next few months, invisibility has gradually disappeared.
But in March 1671 he reappeared – even brighter and among the 100 brightest stars in the sky. Again, he faded and by the end of the summer he was gone.
Then, in 1672, he made a third apparition, barely visible to the naked eye. After a few months, he was gone and has not been seen since.
It always seemed like a strange event. For centuries, astronomers have considered it the oldest known nova – a type of star explosion. But this explanation became untenable in the 20th century.
A nova is a fairly common event, when hydrogen ignites in an otherwise extinct star, causing a thermonuclear runaway reaction. Stars can also explode as supernovae, following an implosion of their nucleus. However, we now know that neither one nor the other would give the kind of repeated appearance seen in this event.
So what was it? Our new research, published in Monthly Notices of the Royal Astronomical Society offers a whole new explanation.
In 1982, the American astronomer Mike Shara found a nebula – an interstellar cloud of dust, hydrogen, helium and other gases – at the position of the missing star, which had since acquired its name CK Vul between.
This proves that it's actually happened here. Astronomers later noticed that the nebula was expanding and that this expansion began about 300 years ago. But the star itself could not be seen.
Things got even stranger when the astronomer Tomasz Kamiński discovered that the nebula contained a very unusual mixture of elements, very abundant in two isotopes (elements with a different number of neutrons in their nucleus compared to the "normal" atom): a type of nitrogen (15N) and radioactive aluminum (26Al). These require very high temperatures to form. Whatever the case may be, the event had been very energetic.
We observed the location of the star with ALMA observatory in Chile . This spectacular looking telescope uses 64 separate plates and observes in the region of microwave light. It is particularly effective for detecting radiation emitted by molecules in the space.
What we have discovered is that the debris of the event is visible in the form of two dust rings resembling an hourglass. This hourglass is integrated into a larger hourglass than previously observed, and contains itself other structures, nested like a Russian doll.
Such hourglass lobes indicate the presence of jets coming from the center, which blow the opposite bubbles. But the hourglasses have slightly different angles.
This suggests that the original structure was spinning, which requires a lengthy process. In any case, it was not just a blast. The ejection had to take a while.
But if it was not an explosion, what was happening? The alternative to a stellar explosion is a collision between two stars. These are rare events that have attracted a lot of attention in recent years.
In 2008, a collision was captured near the center of our galaxy . The colliding stars were closely encircled before merging definitively.
During the event, the stars became 100 times brighter than before and, over the next two years, they disappeared again. A similar event could occur in 2000, when a star called V838 Mon was suddenly lit, then slowly fainted.
CK Vul could be the result of a merger between two normal stars. But that did not seem to fit. Fortunately, there is a zoo full of possible collisions because the stars come in many types. We have now determined that two stars on the opposite side of the star spectrum could have produced the pattern observed in the sky.
The main actor would have been a white dwarf a dense remainder after a star as the sun reached the end of his life.
The supporting actor would have been a brown dwarf an object located in the twilight zone between stars and planets: too light to produce the fusion of hydrogen that normally occurs at center of the stars, but also heavy to be a planet.
They are 10 to 80 times heavier than Jupiter. Brown dwarfs are probably very common, but they are hard to find because they are so weak.
A collision between a white dwarf and a brown dwarf would be spectacular. The brown dwarf would be shredded by the white dwarf, much heavier and denser.
Part of the shredded dwarf rained down on the white dwarf and fueled a thermonuclear reaction. The rest of the brown dwarf would be swept away by the debris of the explosion.
Unlike a normal star, white dwarfs may be extremely weak and, after fusion and thermonuclear explosion, would eventually return to this brightness.
The remaining shells may have also contributed, making it opaque to visible light. A fusion of normal stars would have left a star of normal brightness, and even if it were darkened, it could have always been seen in the infrared.
Has this really happened? We have developed a plausible model, but additional tests would be needed to produce conclusive evidence.
For example, would this collision create the right conditions to form radioactive aluminum? Future observations could examine the details of the deepest region of the hourglass-shaped structure to discover it.
Our discovery represents the very first detection of a collision between a white dwarf and a brown dwarf. Once confirmed, we can use it to search for other similar events. Astronomy is an adventure: a beautiful mix of physics and discovery. We are still learning.