If they don’t crash into one another and explode in a supernova, hot stars in the Milky Way galaxy might merge into a rare type of star.

Scientists reported in The Astrophysical Journal that they discovered two sets of white dwarf binary systems — in which two of the dense stars orbit the same point in space — where the stars are extremely close to one another. And the white dwarf stars in each of those pairs are drawing closer and closer, so much so that they might “merge into single massive [white dwarfs],” according to the study.

The companions are so close already that the orbital periods are only 40 minutes for one of the pairs and 46 minutes for the other.

According to the University of Oklahoma, there were previously only a handful of these tight white dwarf binary systems, in which orbital periods are less than one hour, known to exist in the Milky Way. They are significant because of what they do to the space around them.

“Short-period white dwarf binaries are interesting because they generate gravitational waves,” astrophysicist Mukremin Kilic said in the university statement.

He is referring to what are basically ripples in the fabric of space — similar to what you would see when you disturb the calm surface of a body of water — that stem from its biggest events, like two black holes orbiting one another.

merging-white-dwarfs Researchers have found white dwarf binary star systems where the stars are orbiting super close to one another and may one day merge. Photo: University of Oklahoma

One of the binary systems is eclipsing, meaning that as the two white dwarfs orbit the same point in space, they pass in front of one another and affect how much light is coming toward Earth. Astronomers can observe fluctuations in the light during their orbit.

According to the university, the research team will continue watching this eclipsing binary to measure what could be their crash course into one another. There might be an enormous explosion at the end, or the two might join forces.

“What occurs when the white dwarfs make contact continues to be a mystery at this point,” the university explained. “One possibility is an explosion — a phenomenon known as a supernova. Kilic predicts these two stars will come together and create an ‘exotic star,’ known as R Coronae Borealis.”

An R Coronae Borealis is a mysterious type of star that is characterized by changes in its brightness. It will go through irregular periods of stable brightness and of brightness levels that rise and fall dramatically.

There are only a few dozen of these stars known in the Milky Way.

One of the key pieces of the discovery is the amount of time it would take the white dwarfs in these two binary systems to merge: Between about 20 million and 34 million years, whereas systems with longer orbital periods have lasted as long as the galaxy. The researchers’ study notes that such a short period implies that these circumstances have happened many times before.

“The existence of double white dwarfs that merge in 20 to 35 million years is remarkable,” researcher Warren R. Brown, from the Smithsonian Astrophysical Observatory, said in the university statement. “It implies that many more such systems must have formed and merged over the age of the Milky Way.”