At the center of the Milky Way lies a supermassive black hole. Its name is Sagittarius A* (pronounced A-star) and weighs over 4 million times the mass of our Sun, compressed in a region just 44 million kilometers (27 million miles) wide. Decades of observations have uncovered several stars that orbit this incredible object. Now, astronomers have found a new one, and it is the one that gets closest to it.
The star is called S62 and has the shortest known stable orbit of any star around Sagittarius A*, which are referred to as the S-cluster. It orbits the supermassive black hole in just 9.9 years, getting as close as 2.8 billion kilometers (1.74 billion miles), a few hundred miles shorter than the distance between Uranus and the Sun. Given the extraordinary gravity of the black hole, at its closest point during its orbit (the periapse), the star is moving at about 10 percent of the speed of light.
"The discovery itself is amazing. The identification of fainter stars in the dense S-cluster is always challenging. Finding this star on such a highly elliptical orbit was not expected. Especially in combination with the short orbital time period of less than 10 years." lead author Dr Florian Peiβker told IFLScience.
There are uncertainties about the detailed properties of the star, but the team estimates it to be roughly 2.2 times the mass of our Sun. Theoretical models show that it is possible that a star like this formed around a supermassive black hole, but the researchers are perplexed by its very elliptical orbit. One possibility is that S62 had a larger companion, and during a very close passage this companion was thrown out of orbit while S62 remained. Another possibility is the star being the product of a collision, but the team is unconvinced.
“While a collision is indeed a very likely fate of S62 in the near future, the object itself is unlikely to be a product of a disruptive collision," the authors wrote in a paper accepted by The Astrophysical Journal and currently available on the online paper repository arXiv. "In that case, the stellar material would have been stretched out over a large section of the orbit very quickly. The event as such would result in an, at least temporary, luminous and extended trail. S62, however, appears to be of similar brightness compared to other S-star cluster members and is very compact.”
If a collision with another star doesn’t seal the fate of this object, another possibility is that over time it will be pulled apart by Sagittarius A* and become a tidal disruption event, a tail of bright plasma swirling around the supermassive black hole. There are other stars around Sagittarius A*, the second closest is S2 and its orbit has been used to test general relativity (GR).
"We will constantly monitor and analyze data from the Galactic center. Since we expect a gravitational periapse shift of around 10 degrees, this star can also be used to test GR." Dr Peiβke added.
The data for this analysis come from two instruments on the European Southern Observatory’s Very Large Telescope. The observations with the NACO instrument cover 2002 to 2018, and the observations with SINFONI cover 2008 to 2012. S62 can be easily tracked in both datasets. The team also independently confirmed the presence of the star using data from the KECK data set.
[H/T: Bad Astronomy]
1