Astronomers Discover Class of Strange Objects Near Our Galaxy’s Black Hole

Artist’s impression of G objects, with the reddish centers, orbiting the supermassive black hole at the center of our galaxy. The black hole is represented as a dark sphere inside a white ring (above the middle of the rendering).
(Image: Jack Ciurlo)
Artist’s impression of G objects, with the reddish centers, orbiting the supermassive black hole at the center of our galaxy. The black hole is represented as a dark sphere inside a white ring (above the middle of the rendering). (Image: Jack Ciurlo)

Astronomers from UCLA’s Galactic Center Orbits Initiative have discovered a new class of bizarre objects at the center of our galaxy, not far from the supermassive black hole called Sagittarius A*. They published their research in the journal Nature. “These objects look like gas and behave like stars,” said co-author Andrea Ghez, UCLA’s Lauren B. Leichtman and Arthur E. Levine Professor of Astrophysics and director of the UCLA Galactic Center Group.

The new objects look compact most of the time and stretch out when their orbits bring them closest to the black hole. Their orbits range from about 100 to 1,000 years, said lead author Anna Ciurlo, a UCLA postdoctoral researcher. Ghez’s research group identified an unusual object at the center of our galaxy in 2005, which was later named G1. In 2012, astronomers in Germany made a puzzling discovery of a bizarre object named G2 in the center of the Milky Way that made a close approach to the supermassive black hole in 2014.

Ghez and her research team believe that G2 is most likely two stars that had been orbiting the black hole in tandem and merged into an extremely large star, cloaked in unusually thick gas and dust. Ghez said:

Co-author Mark Morris, UCLA professor of physics and astronomy, added:

But are G2 and G1 outliers, or are they part of a larger class of objects? In answer to that question, Ghez’s research group reports the existence of four more objects they are calling G3, G4, G5, and G6. The researchers have determined each of their orbits. While G1 and G2 have similar orbits, the four new objects have very different orbits.

Orbits of the G objects at the center of our galaxy, with the supermassive black hole indicated with a white cross. Stars, gas and dust are in the background. (Image: Anna Ciurlo, Tuan Do/UCLA Galactic Center Group)

Orbits of the G objects at the center of our galaxy, with the supermassive black hole indicated with a white cross. Stars, gas, and dust are in the background. (Image: Anna Ciurlo, Tuan Do/UCLA Galactic Center Group)

Ghez believes all six objects were binary stars — a system of two stars orbiting each other — that merged because of the strong gravitational force of the supermassive black hole. The merging of two stars takes more than 1 million years to complete. Ghez added:

Ciurlo noted that while the gas from G2’s outer shell got stretched dramatically, its dust inside the gas did not get stretched much, adding:

The researchers made observations from the W.M. Keck Observatory in Hawaii and used a powerful technology that Ghez helped pioneer, called adaptive optics, which corrects the distorting effects of the Earth’s atmosphere in real time. They conducted a new analysis of 13 years of their UCLA Galactic Center Orbits Initiative data. In September 2019, Ghez’s team reported that the black hole is getting hungrier and it is unclear why.

View an animation below of the orbits of the G objects, together with the orbits of stars near the supermassive black hole. (Image: Advanced Visualization Lab, National Center for Supercomputing Applications, University of Illinois)

The stretching of G2 in 2014 appeared to pull off gas that may recently have been swallowed by the black hole, said co-author Tuan Do, a UCLA research scientist and deputy director of the Galactic Center Group. The mergers of stars could feed the black hole. The team has already identified a few other candidates that may be part of this new class of objects, and are continuing to analyze them. Ghez noted the center of the Milky Way galaxy is an extreme environment, unlike our less hectic corner of the universe, saying:

Ghez said this research will help to teach us what is happening in the majority of galaxies.

Provided by: Stuart Wolpert, University of California, Los Angeles [Note: Materials may be edited for content and length.]

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