Speedy star duo surprises astronomers
By Amanda Jermyn

One of the many delights of astronomy is that new discoveries often defy expectations and challenge our preconceived notions of how the universe should work. There’s nothing better than a good mystery, and the universe is full of them. So when I heard about a recently discovered hypervelocity binary star called PB 3877 I was intrigued.

A binary star may seem to be one star because to the unaided eye it appears as a single point of light. However, it is actually a star system consisting of two stars orbiting each other. Larger systems with multiple stars orbiting one another also occur.

This particular binary star system, PB 3877, is about 18,000 light-years away, and consists of a hot compact star, about five times hotter than our Sun, and a cool companion, much cooler than the Sun. It is traveling extremely fast, at nearly the escape velocity of our galaxy, which means it is moving almost fast enough to escape the enormous gravitational attraction of the Milky Way. So far, only about two dozen hypervelocity stars are known to be escaping the galaxy, but they are all single stars, and PB 3877 is the first wide binary star system found to travel at such high speed.

This discovery, by a team of astronomers at the Friedrich Alexander University in Erlangen, Germany, led by Peter Nemeth, is also unusual in that it challenges the commonly held view that hypervelocity stars are accelerated by the supermassive black hole at the center of our galaxy. The team, together with scientists from the California Institute of Technology, showed that the binary system cannot originate from the galactic center, and there is no other known mechanism that is able to accelerate a wide binary system to such a high speed without it breaking apart. They were able to exclude the galactic center as the place of origin because the binary’s trajectory never came close to it. Other ejection mechanisms, such as the collision of stars or a supernova explosion, would have disrupted the binary system.

They suggest that there must be a lot of dark matter to keep the star system bound to our galaxy or that the binary could have come from another galaxy, and may or may not leave the Milky Way again. According to Nemeth, “PB 3877 may be an intruder from another galaxy. In that case, its prolonged gradual acceleration would not harm its integrity. The outskirts of our galaxy contain various stellar streams that are believed to be the remnants of dwarf galaxies that were torn to shreds by the strong tidal force of the Milky Way.”

We don’t currently have enough information about the stellar streams to pin down the binary’s origin, nor can we determine its future. Whether or not it remains bound to the Milky Way depends on the amount of dark matter in the galaxy. However we don’t yet fully understand dark matter and don’t know exactly how much of it our galaxy contains. PB 3877 is currently being studied in more detail, and the search is on for more such hypervelocity binary stars. Hopefully observing more of them will help resolve their origins and futures but for now these mysteries remain.
Join the Springfield Stars Club on Tuesday, November 22nd at 7:00pm at the Springfield Science Museum for a talk by Dr. David Wexler on “An Inside Look at Astronomical Research.” 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 Friday, December 2nd at 7:30pm, the Stars Club and the Springfield Science Museum will host “Stars over Springfield,” an astronomy adventure for the whole family. Astronomy educators Richard Sanderson, Jack Megas and Dave Gallup will present the program “A Dipper full of Stars.” A fee of $3 for adults and $2 for children under 18 will be charged.