Two colliding stars

Two stars may collide and light up night sky in 2022

Two stars may collide into a stunning “red nova” in 2022 (or a few months earlier, most probably between September 2021 and September 2022), and for a few weeks become one of the brightest objects in Earth’s night sky, scientists predict. When two stars merge, they increase in brightness 10 thousandfold.

Two colliding stars
An illustration of what two colliding stars might look like from an artist’s eye.

The pair located about 1800 light-years away and known as KIC 9832227, a contact binary star system in the constellation of Cygnus. Their merger will produce a luminous red nova (a stellar explosion caused by the merging of two stars), is predicted to be visible to the naked eye, reaching magnitude 2.

Scientists have been observing the period of the variations which has growing shorter since 2013. Now they are so close that they even sharing the same atmosphere. It is predicted that, as the stars’ outer atmospheres interact with each other, the period will grow shorter still, and end in the merging of the two cores. The merger of the cores will release a very large amount of energy, a process which has been observed before in the star V1309 Sco, whose merger was observed in 2008.

Where to look at?

Cygnus in the Earth's sky
The constellation Cygnus as it can be seen by the naked eye, with the Northern Cross in the middle. KIC 9832227 is shown with a red circle (it is almost in line with the three stars of the cross bar). If it reaches 2nd magnitude in outburst as predicted, it will be as bright as they are.

What is the risk for Earth?

There’s no risk for Earth after the merger of KIC 9832227.

But, if the colliding stars are massive enough, after the event, they are expected to either merge to become one single ginormous star or to form a binary black hole. The latter would produce a rapidly rotating and possibly magnetic star. In a paper published in The Astronomical Journal of The American Astronomical Society, lead scientist Hugues Sana explains: “If it keeps spinning rapidly it might end its life in one of the most energetic explosions in the universe, known as a long-duration gamma-ray burst.”

The gamma-ray bursts (GRB) are extremely dangerous events. A GRB within a few parsecs, with its energy directed towards Earth, will mostly damage life by raising the UV levels; during the burst itself and for a few years thereafter. The major Ordovician–Silurian extinction events 450 million years ago (the second-or-third largest of the five major extinction events in Earth’s history in terms of percentage of genera that became extinct) may have been caused by a GRB.

But, fortunately for us, we live in out the “boring” suburbs of the Milky Way. Out here, distances between stars are so vast that collisions are incredibly rare. There are places in the Milky Way where stars are crowded more densely, like globular clusters, and we can observe the aftermath of these collisions.