The Survival Story of a Star near Black hole

Life is a glide of stories, be it hard or easy, every nook and corner have one of their own. What about one in science? Cool to say but tough to go on!! Just as we move on every day, buried deep in thoughts of past, present and future, our companions in the sky too struggle hard to survive, not in the perks of future but something extremely scientific to ponder.

NASA’S Chandra X-ray observatory and ESA’s XMM Newton revealed out their so far observations on a star, near a supermassive black hole in a galaxy at a distance of about 250 million light-years from Earth. Sketching it from the red giant stage onwards, the data of its wandering was collected for more than a fair timescale. The black hole is located in the GSN 069 galaxy and is about 400,000 times the mass of the Sun.

The red giant entered the black hole cavity and due to the strong gravitational force, the outer layers of the star, composed of hydrogen was stripped off, leaving behind the white dwarf (core) alone.

Andrew King of the University of Leicester, UK said, “In my interpretation of the X-ray data the white dwarf survived, but did not escape”, as a part of his study on the same. He also added that the white dwarf was caught in an elliptical orbit around the black hole and moved around once every 9hrs. During its motion, it loses more mass to the black hole at its closest point of approach and also X-rays are emitted from the stellar material entering the black hole. This was the main source that XMM and Chandra detected. King also predicted that there are chances of gravitational wave emission by the pair, especially at the closest point of approach.

The survival story proceeds on and what astronomer’s are worried, is regarding the future and fate of the star. The elliptical path which the dwarf makes gradually grows in size and becomes more circular, due to the strong effect of gravitation and an increment in the size of the star. As a result, the white dwarf struggles hard to escape, but the black hole will bind it more vigorously in its field as its prey starts to spiral away slowly. The white dwarf is also expected to become a planet about the size of Jupiter, as the mass goes on decreasing at a slow pace. But it takes more than a trillion years making the process as slow as possible.

Several tidal disruption events in which a black hole encounter completely tear out a star have been observed in many years. This raised the probability that the fate of a star near a black hole is to die in it. However very few cases where the stars are thought to survive opens a door to something beyond scientific explanations. But the chances are only a few mainly due to the reasons that a supermassive black hole more massive than the one in GSN 069 may engulf the star before it takes orbit to move around or the star takes a long time to complete its orbit, making the observation of repeated bursts unseen and undetected. These two reasons leave the astronomers in great despair.

From the studies, King was also able to predict several features on the blackhole-white dwarf pair including their orbit, mass and composition. The white dwarf possesses a mass of about two-tenth of the solar mass and is enriched in He, as it is the core leftover. King expresses his remarkable concern on the detection of this tiny little one far away from us. The predictions on the “precess” that the white dwarf might show during its orbital motion is also appreciable. If so, the wobble os anticipated to repeat every two days of motion.

All these breakthrough discoveries appeared in the March 2020 issue of the Royal Astronomical Society, which is available online for those interested.