Stars merge for mystery in the Milky Way
By Amanda Jermyn



Last month I wrote about a star in a galaxy 650 million light years away that came close to a black hole but managed to escape the clutches of its powerful gravity relatively unscathed. Here’s the tale of another star that got away, but one that happened much closer to home.

Our story begins with a mystery object, known as G2, passing close to the supermassive black hole at the center of our very own galaxy, the Milky Way. When G2 was discovered in 2011 there was speculation that it was a huge cloud of hydrogen gas or a star surrounded by gas, but it turned out to be both, and more. Astronomers now believe it to be a pair of binary stars orbiting each other that merged into a single massive star surrounded by gas and dust. This merged star orbits the black hole.

Observing the mystery object’s encounter with the black hole helped establish what it was. According to Andrea Ghez of UCLA who led the observations at the Keck Observatory in Hawaii, “G2 survived and continued happily on its orbit; a simple gas cloud would not have done that.” With a gas cloud, there would have been far more “spaghettification” or stretching due to the black hole’s gravity than was observed. This is because a gas cloud doesn’t have much gravity relative to its size, so it has a hard time holding itself together. However, a pair of stars that has merged constitutes a much denser and better-held together object. The nature of the mystery object’s orbit around the black hole also made it look like a star, but there had to be a layer of gas surrounding it because of the tidal stretching that was observed.

According to Ghez, the object appears to be a recently recognized type of star found near a black hole that is created because the black hole’s powerful gravity causes two stars in a binary relationship to merge. In the process of merging, the star expands for over a million years before settling down. G2 was observed to be in this inflated stage, and to be elongated from the black hole’s gravity, though it suffered only minor damage to its outer layer, and was not sucked into the black hole.

While most astronomical events occur over very long time periods, G2’s encounter with a black hole could be observed over just a few months, generating great interest recently among astronomers. It should be pointed out, however, that the close encounter actually occurred about 25,000 years ago, but we are only able to observe it now because that’s how long it has taken for the light from the event to travel the vast distance involved to reach us. Observing such encounters helps provide new insights into the physics of black holes and the stars that surround them.

Join the Springfield Stars Club on Tuesday, January 27th at the Springfield Science Museum for a talk by amateur astronomer Alan Rifkin on his Observatory of the Future. Rifkin is president of the Springfield Stars Club and owner of FAR Laboratories, currently the only telescope dealership in Massachusetts. In addition, Naomi Volain will display and present NASA moon rock samples and meteorites. Volain is a member of the NASA Network of Educator Astronaut Teachers and a science teacher at Springfield Central High School. For more information please visit the Stars Club – Massachusetts page on Facebook at https://www.facebook.com/groups/929125583765113/ or the Stars website www.reflector.org. Refreshments will be served and the public is welcome. The meeting is free of charge for members, with a suggested donation of $2 per non-member.

Also, on Friday, February 6th at 7:30pm, join the Stars Club and Springfield Science Museum for “Stars over Springfield,” with guest speaker Paul Cardone. A fee of $3 for adults and $2 for children under 18 will be charged.