A star that circles the supermassive black hole at the heart of our galaxy every four years has been found by astronomers. That is the shortest orbit around a supermassive black hole that has ever been seen.

The recently found star, known as S4716, is almost four times as big and twice as hot as our Sun. It endures extreme circumstances while orbiting this black hole at a distance of as little as 100 astronomical units, or less than three times Pluto’s distance from the Sun.

The finding should aid astrophysicists in better understanding the environment around the Milky Way’s gravitational giant and in determining its mass and radius.

The idea that a supermassive black hole may be present in the Milky Way’s core, its powerful gravitational field heating gas and dust and mutilating nearby stars, was first put out in 1971 by British astronomers Donald Lyndon-Bell and Martin Rees. Three years later, the strange object was first observed.

Black hole discovery

Our Milky Way’s supermassive black hole, known as Sagittarius-A* (pronounced Sagittarius-A-star), is around 26,000 light years away from Earth and is about 4 million times more massive than the Sun. The Event Horizon Telescope team published the first photograph of Sagittarius A* in May 2022, providing unequivocal proof of the object’s existence.

The attraction of adjacent stars to such a large object is not surprising. In fact, astronomers have long noted stars in orbits with periods ranging from 10 to 166 years. The orbits of these objects, generally referred to as “S stars,” provide information about Sagittarius Asize *’s and mass. As an illustration, S2, which has an orbital period of 16 years, moves at a maximum speed of 17 million mph (28 million km/h), or about 3% of the speed of light. The supermassive black, according to its motion, cannot be bigger than Uranus’ orbit in our solar system.

The fact that the circling stars are all concentrated in a compact area of the sky and that the black hole also draws in dust and gas, which shines brilliantly as a result of frictional heating caused by the strong gravitational forces, makes observations challenging. Though none orbit as quickly as S4716, astronomers have managed to dissect the motion of a number of nearby stars.
The discovery of S4716 was initially hinted at by Florian Peiker and colleagues at the Institute of Physics at the University of Cologne in Germany using infrared photos from the Keck Observatory in Hawaii.

They re-examined photos of the same location collected by multiple telescopes as far back as 2003 to corroborate their concerns and search for potential missed evidence of S4716.

Algorithmic solution

Each image is a snapshot rather than a video displaying the motion of the stars, which makes the work difficult. To determine the stars’ orbits and their speed between photos, the scientists thus employed a variety of techniques. With the stars traveling at substantial fractions of the speed of light, that becomes considerably more challenging.

However, the scientists discovered a wealth of evidence supporting S4716, including several instances when it had been misidentified for other stars. According to Peiker and colleagues, “we detected this new source, which we name S4716, in 16 epochs between 2003 and 2020 by analyzing published and archival data.

That research is fascinating because it sheds light on the extraordinarily complex environment that surrounds the supermassive black hole that sits at the center of our galaxy, the remarkable stars that manage to survive in it, and the potent methods that astronomers have developed to dissect the phenomenon at play.

The task is still far from finished. According to Peiker and colleagues, the orbit of S4716 indicates that Sagittarius A* is surrounded by gas, dust, rocks, and other debris with a total mass that is several orders of magnitude higher than the Sun. In fact, Sagittarius A* abruptly brightened 75 times more than usual in 2018, very definitely as a result of swallowing an asteroid-sized object.
Clearly, there are yet more mysteries to unearth in this remote region of the Milky Way galaxy.

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