Stellar dancing couple with a dark future

Artist’s impression of the binary star system SSN 7: The smaller and hotter star with 32 solar masses (left) loses material to its 55 solar mass partner. © UCL / J.daSilva

An astronomical elephant tango in sight: Researchers report on two closely connected giant stars that exchange material and, according to the calculations, are heading for an exciting fate. According to them, they will soon form black holes, which could then coalesce in a bombastic way in the distant future. The collision will be so violent that gravitational waves will shake the universe.

It was a bang in the history of astronomy: In 2016, scientists were able to detect gravitational waves for the first time, the existence of which until then had only been pure theory. These are distortions in space-time that can be caused by the collision of extremely massive celestial bodies such as black holes. Since then, astronomers have recorded several instances of the cosmic tremors, and the topic continues to be an intense focus of astronomy. A questioning look is directed at the systems that lead to the extreme collisions.

“Thanks to the Virgo and LIGO gravitational-wave detectors, dozens of merging black holes have been detected in recent years. So far, however, we have not observed any stars collapsing into black holes of this size,” says first author Matthew Rickard of University College London. Together with his colleague Daniel Pauli from the University of Potsdam, he therefore targeted a promising candidate system: The binary star system SSN 7 is located in a neighboring galaxy of our Milky Way - in the Small Magellanic Cloud, about 200,000 light years away.

Cosmic duo with a special relationship

To get more detailed information about the duo, which consists of two very massive stars, the astronomers used data from different telescopes on the ground and in space. They analyzed the spectroscopic characteristics of the radiation emanating from the binary system in different wavelength ranges. The results allowed conclusions to be drawn about the radial velocity of the stars, which indicates how fast they are moving towards us or away from us. In addition, the researchers were able to gain more precise information about the masses, brightness, temperatures and orbits of the two celestial bodies. They then implemented the information in an evolutionary model that sheds light on the future of the binary star system.

As Rickard and Pauli report, the system consists of two cosmic elephants: a star weighing 55 solar masses and its equally heavy partner. Although it is slightly smaller at 32 solar masses, it burns significantly hotter than its larger companion. They have a common center of gravity and orbit each other once every three days, the astronomers report. This leads to an exciting interaction: As can be seen from the spectroscopic analysis results, the larger star sucks material from the shell of the smaller one. "It is therefore a contact binary system and the most massive that has been observed so far," says Pauli.

Bombastic finale

As the researchers continue to report, the model calculations based on the data revealed an exciting fate for the duo. Based on its characteristics, the smaller partner will be the first to collapse into a black hole in just 700,000 years. This dark celestial body will then begin siphoning material from the companion that previously fed on it. "He's going to take revenge on his partner," Rickard said.

Pauli explains the further fate of the system: "After only 200,000 years - an astronomical moment - the companion star will also collapse into a black hole. After that, the two will continue to orbit each other for a few billion years. They will slowly lose energy through the emission of gravitational waves until they orbit each other faster and faster every second. Then, in 18 billion years, they will merge while releasing enormous amounts of energy via gravitational waves."

In conclusion, Rickard says: "The discovery of stars on this evolutionary path so close to our galaxy gives us an excellent opportunity to learn even more about the formation of these systems," says the astronomer.

Source: University of PotsdamArticle: Astronomy & Astrophysics, doi: 10.1051/0004-6361/202346055

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