Stars crossed for meeting in far-away galaxy
By Amanda Jermyn

Last month I wrote about two massive black holes orbiting each other in a binary system and heading towards a cataclysmic collision. This month, we’ll look at two massive stars orbiting each other very closely in a binary system, with an as yet unknown outcome.

This binary star system, known as VFTS 352, is about 160,000 light-years away in the Tarantula Nebula, in one of the most active star-forming regions in our local group of galaxies. The two young stars are massive, hot and bright, and orbit each other in just over a day. With their centers only about seven and a half million miles apart, the stars are so close that their surfaces overlap, forming a bridge between them. Star systems orbiting so closely are known as contact binaries, and VFTS 352 is the hottest and most massive such system ever discovered. The stars’ combined mass is about 57 times that of our sun, and they both have similar surface temperatures of over 70,000 degrees Fahrenheit. By comparison, our sun’s photosphere, the layer we see as sunlight, is a mere 10,000 degrees Fahrenheit.

When stars of unequal mass orbit each other closely, the gravity of the one with greater mass may suck in matter from the smaller one. However, in the case of VFTS 352, where both stars have about the same mass, matter is shared. In fact it is estimated that the two stars are sharing about 30% of their material. Because the stars are so close together it seems likely that strong tidal forces facilitate the enhanced mixing of the material in the stars’ interiors.

This star system was recently discovered by an international team of astronomers using the European Southern Observatory’s Very Large Telescope located in Chile. Catching such binary stars in the act of sharing material this way is pretty rare because this phase in the system’s evolution is so brief. So this is a significant discovery. According to the lead author of the study, Leonardo Almeida of the University of Sao Paulo, Brazil, “The VFTS 352 is the best case yet found for a hot and massive double star that may show this kind of internal mixing.”

So what comes next in the evolution of this star system? Two possible outcomes have been predicted. In one scenario, the two stars merge, most likely producing a rapidly rotating, possibly magnetic, massive single star. According to Hugues Sana, lead scientist in the study, “If it keeps spinning rapidly it might end its life in one of the most energetic explosions in the Universe, known as a long-duration gamma-ray burst.”

In the second scenario, according to Selma de Mink, lead theoretical astrophysicist in the study, “If the stars are mixed well enough, they both remain compact and the VFTS 352 system may avoid merging. This would lead the objects down a new evolutionary path that is completely different from classic stellar evolution predictions. In the case of VFTS 352, the components would likely end their lives in supernova explosions, forming a close binary system of black holes. Such a remarkable object would be an intense source of gravitational waves.”

Either way, the outcome should be dazzling. Meanwhile, observing the process is providing astronomers with intriguing new insights into the evolution of these exotic star systems.

Join the Springfield Stars Club on Tuesday, November 24th at 7:30pm at the Springfield Science Museum for a talk by Dr. David Wexler on Space Weather. Dr. Wexler will discuss the transition from Earth to Space, and how the near-Earth space environment is strongly influenced by the Sun. A Stars Club board member, Dr. Wexler studied neuroscience and engineering at Case Western Reserve University, and completed medical studies at Stanford University. He works as an ear, nose and throat specialist at Baystate Wing Hospital in Palmer. Dr. Wexler has a Master’s degree in astronomy and is currently participating in solar research at MIT’s Haystack Observatory. 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 Thursday, December 17th at 12.15pm, Art a la Carte at the Springfield Museums presents a talk on The Star of Bethlehem. Richard Sanderson, Curator of Physical Science at the Springfield Science Museum, and Science Museum planetarium educator Jack Megas investigate the history of the “Christmas Star” and evaluate some possible scientific explanations. The talk will take place in the Science Museum’s Seymour Planetarium. The fee is $2 for members and $4 for non-members of the Springfield Museums.