Stellar Painting Petals Around Supermassive Black Holes

Figure | The general running track of "rose knot" (Source: ESO / L. Calçada)

On April 16, foreign media revealed that astronomers used the Very Large Telescope (VLT) of the European Southern Observatory (ESO) to observe a star orbiting the supermassive black hole in the center of the Milky Way. This finding validates the results predicted by Einstein's general theory of relativity. This long-awaited result was finally achieved by astronomers following nearly 30 years of tracking observations and relying on scientific observation methods that have developed more and more accurately in recent years. Newton predicted in his gravitational theory that the orbit of a star should be similar to an ellipse, but this time the observed star orbiting a black hole looks like a rosette. This supports the prediction in Einstein's general theory of relativity: the binding orbit of one celestial body around another is not closed, unlike Newton's law of gravitation, but moving forward on the plane of motion.

Previously, the shape of Mercury's orbit around the Sun has been the first evidence supporting general relativity. As early as 1859, the French astronomer Le Ville discovered that the precession value of Mercury's perihelion calculated according to Newton's law of gravitation is different from the observed result. He found that the actual observation value was 38 seconds faster than the theoretical value calculated according to Newton's law, which caused widespread concern by astronomers at that time. They speculate whether this difference will be caused by gravitational interference caused by a planet closer to the sun than Mercury? But after years of hard work searching, no one found this "mysterious planet". Later, Einstein published the famous general theory of relativity in 1915, successfully explaining this problem. Today, more than a hundred years later, astronomers discovered a star (S2) around Sagittarius A* (Sagittarius A*, a very bright and dense radio source) that surrounds the center of the Milky Way. This observation provides correct evidence for the new Einstein's general theory of relativity, and further verifies that Sagittarius A* should be a supermassive black hole 4 million times larger than the mass of the sun.

Figure | The image of the center of the galaxy contains a black hole called SgrA* in the middle, marked with an orange cross, and S2 is the star surrounding Sgr A* observed this time (Source: ESO / MPE / S. Gillessen et al.)

Sagittarius A* is 26,000 light-years away from the sun. It is likely to be the closest supermassive black hole to the earth, so it is also considered to be the best target for studying the physics of black holes. Sagittarius A* is surrounded by this denser group of stars, which provides physics with a unique laboratory for testing in extreme gravity environments that have never been explored. This discovery by an astronomers-the star known as S2 is also located in this high-density group of stars, it orbits Sagittarius A* this "supermassive black hole" at a distance not exceeding 20 billion kilometers One of the closest stars to A* Sagittarius discovered so far. Of course, perhaps the degree of "intimacy" is also limited, the distance between the two is about 120 times the distance from the sun to the earth.

In 2018, through the observation of ESO's super-large telescope, astronomers discovered that when S2 is closest to Sagittarius A*, its moving speed will exceed 25 million kilometers per hour, almost 3% of the speed of light, and complete an orbit every 16 years. "After tracking this star in its orbit for more than 25 years, our precision measurement technology finally found strong evidence in its orbit around Sagittarius A* and successfully detected the Schwarzschild precession effect in S2." Astrophysicist Stefan Gillessen of the Max Planck Institute in Germany said. He led the measurement and analysis of S2 and published it in the journal Astronomy & Astrophysics.

Figure | The trajectory of S2 when it is very close to the "supermassive black hole": when it is close to the black hole, the strong gravitational field will cause the star's color to shift slightly to red, which is the role of Einstein's general theory of relativity; in this figure , For clarity, the color effect and size of the object are exaggerated (Source: ESO / M. Kornmesser)

Most stars and planets have a non-circular orbit, so they will get closer or farther away from the celestial body around which they rotate. The stellar S2's orbital precession means that whenever it reaches the point closest to the "supermassive black hole", its position will change with each rotation . In this way, the next orbit will rotate relative to the previous one, thus forming a rose shape. Einstein's general theory of relativity provides an accurate prediction of its orbital changes, and the latest measurement results of this study are in complete agreement with theoretical calculations. This effect is called "Schwarzschild precession", and people have never measured the stars around massive black holes before.

This study of the European Southern Observatory's super-large telescope also helps scientists learn more about the situation near the "supermassive black hole" in the center of the Milky Way, and further helps people demystify the black hole. "Because the measurement of the stellar S2 follows the general theory of relativity well, we can strictly limit how much invisible matter exists around Sagittarius A*, such as distributed dark matter or possibly smaller black holes. The formation and evolution of mass black holes are of great significance, "said Guy Perrin and Karine Perraut, the French chief scientists involved in the project.

The results of the research disclosed today are the result of scientists' observation of the star S2 for 27 years. Most of the study was conducted at the Southern European Observatory in the Atacama Desert in Chile. The research team conducted more than 330 mid-level measurements through the Very Large Telescope (VLT) and GRAVITY, SINFONI, and NACO instruments. The number of data points marking the position and velocity of the star proves the comprehensiveness and accuracy of the study.