When the star S4716 reaches its shortest distance of just 100 astronomical units (AU) from the four-million-solar-mass black hole, it is traveling at speeds of up to 8,000 km/s, or nearly 2.7 percent of the speed of light.
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| One AU is roughly 150 million kilometers, i.e. the distance between the Earth and the Sun. |
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All this was revealed by a study that was recently published by a professional journal Astrophysical Journal. Experts from Brno’s MUNI, the German Max Planck Institute for Radio Astronomy and the University of Cologne took part in it.
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“Like a tall person in a movie theater who gets in the way”
The black hole at the center of the Milky Way galaxy – i.e. the object named Sagittarius A* (Sgr A*) – is not isolated, it is surrounded by one of the densest star clusters, the so-called nuclear star cluster. Its central part consists of fast-moving stars, the so-called S stars. These stars have different masses and luminosities.
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“One of them – the S2 star – behaves like a big person sitting in front of you in the cinema, blocking your view of what’s important,” noted Florian Peißker, lead author of the new study.
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“Thus, the view into the very center of our Galaxy is often blocked by the very bright star S2. However, there are brief moments when we can observe the close vicinity of the central black hole,” he added.
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The newly identified star S4716 (circled in green) and its compact orbit with the shortest period of 4 years around the supermassive black hole at the center of the Galaxy. The bright star S2 (in the black circle) and its 16-year orbit are also marked.
Photo: MUNI Faculty of Science
Advances in analysis methods and long observation times over the past 20 years have made it possible to detect the star S4716 and confirm its four-year period.
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During its orbit, the star always stays close to the central black hole Sgr A* at a distance comparable to the dimensions of the Solar System, with the shortest distance reaching – as already mentioned – only about 100 AU. This is really little by astronomical standards, scientists point out.
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The fact that a star in a stable orbit is so close and orbiting so fast near a supermassive black hole is “completely unexpected” and represents the limit of what can be reached with traditional telescopes.
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A mysterious track
In addition, the discovery sheds new light on the origin and orbital evolution of fast-moving stars at the heart of the Milky Way.
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“The short-period, compact orbit of the star S4716 is quite mysterious,” said astrophysicist Michal Zajaček from the MUNI Faculty of Science, who participated in the study.
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“Stars cannot easily form so close to a black hole – S4716 had to migrate inward, for example by undergoing a series of close approaches with other stars and objects in the S cluster, which caused its orbit to shrink significantly,” explained Zajaček.
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