G2, a mysterious object near the supermassive blackhole at the center of the galaxy, hosts what was once a pair of stars that have been merged by the enormous gravitational influence of their near neighbor.
Strange things happen at the center of galaxies. Black holes millions of times the mass of the sun produce enormous gravitational forces and emit x-rays. Stars crowd so close together, they are in danger of bumping into each other. Ionized streams of gas light up like auroras.
Yet even in such an intense environment, G2 is something special. It has puzzled astronomers, who initially suspected it of being a gas cloud approaching Sgr A*, the black hole at the galaxy's center. Debate raged as to how much of it would survive the encounter.
However, UCLA's Professor Andrea Ghez challenged the assumption on which these ideas are based. G2, she argues in The Astrophysical Journal Letters, is not a gas cloud of roughly three Earth masses, as others have speculated, but instead contains a central star, one she suspects of a very interesting history.
Ghez's conclusions are based on observations of G2 as it reached its closest approach (periapse) to Sgr A* in the course of its orbit. This happened earlier this year when G2 reached a distance of 3000 times the event horizon from Sgr A*. "It was one of the most watched events in astronomy in my career," says Ghez.
A gas cloud lacking a central object, “should be tidally disrupted during periapse passage,” Ghez and her co-author's suggest. However, using the 10m Keck telescopes and laser guide star adaptive optics to adjust for the effects of the Earth's atmosphere, the authors found G2's brightness and size remained the same and its orbital characteristics were consistent with a condensed object.
They propose the center of G2 is a star. “This star has a luminosity of ~30L☉ and is surrounded by a large (~2.6 AU) optically thick dust shell,” they argue.
Stars 30 times the brightness of the sun are not unusual; Vega and Sirius are a little above and below that respectively. However, Ghez says although we cannot see it directly, the team believe G2's star is distinctive for its expansive size, the product of two stars merging together. This prompts an expansion that lasts around a million years before returning to a more normal diameter.
"This may be happening more than we thought. The stars at the center of the galaxy are massive and mostly binaries,” syas Ghez. “It's possible that many of the stars we've been watching and not understanding may be the end product of mergers that are calm now."
The combination of the dense packing of stars at the galactic center, and the influence of Sgr A*'s gravity could cause stars to collide and join on a regular basis in a way that almost never happens in the outer reaches of the galaxy. G2's collision occurred recently enough for us to see the process in action.