Free Astronomy Magazine November-December 2015

SPACE CHRONICLES galaxies. The lanes are the shredded remnants of a canni- balized galaxy. Other observa- tions provided circumstantial evidence for such encounters, showing that the cores of many of these galaxies had active supermassive black holes fueled by the collision. Many of the galaxies also re- side in dense environments at the heart of galaxy clusters, a prime area for mergers. The telltale clue was strong dust lanes piercing through the centers of several of them. The location of the superno- vae in relation to ancient gal- axies indicates that the orig- inal stars must have been old, too, Foley reasoned. And if the stars were old, then they must have had companions with them that provided enough material to trigger a supernova blast. How does a double-star sys- tem escape the boundaries of a galaxy? Foley hypothesizes that a pair of supermassive black holes in the merging galaxies can pro- vide the gravitational sling- shot to rocket the binary stars into intergalactic space. Hub- ble observations reveal that nearly every galaxy has a mas- sive black hole at its center. Accord- ing to Foley's scenario, after two galaxies merge, their black holes migrate to the center of the new galaxy, each with a trailing a cluster of stars. As the black holes dance around each other, slowly getting closer, one of the binary stars in the black holes' entourage may wander too close to the other black hole. Many of these stars will be flung far away, and those ejected stars in sur- viving binary systems will orbit even closer after the encounter, which speeds up the merger. "With a sin- gle black hole, occasionally a star will wander too close to it and have an extreme interaction," Foley said. "With two black holes, there are two reservoirs of stars being drag- ged close to another black hole. This dramatically increases the like- lihood that a star is ejected." While the black hole at the center of the Milky Way may eject about one star a century, a binary super- massive black hole may kick out 100 stars a year. After getting booted out of the gal- axy, the binary stars move closer to- gether as their orbits con- tinue to accelerate, which speeds up the binary stars' aging process. The binary stars are likely both white dwarfs, which are the burned out relics of stars. Eventually, the white dwarfs get close enough that one is ripped apart by tidal forces. As mate- rial from the dead star is quic- kly dumped onto the surviv- ing star, an explosion occurs, causing the supernova. The time it takes for one of these ejected stars to explode is relatively short, about 50 million years. Normally, these kinds of binary stars take a long time to merge, probably much longer than the age of the universe, which is more than 13 billion years. "The interaction with the black holes shortens that fuse," Foley explained. While scientists think they have found what causes these outcast supernovae, some mysteries remain unsolved, such as why they are un- usually weak. These superno- vae produced more than five times as much calcium as other stellar explosions. Normally, supernova explo- sions have enough energy to create much heavier elements, such as iron and nickel, at the expense of producing the lighter calcium. However, for these atypical explo- sions, the fusion chain stops mid- way, leaving lots of calcium and very little iron. "Everything points to a weak explo- sion," said Foley. "We know that these blasts have lower kinetic en- ergy and less luminosity than typical supernovae. They also appear to have less ejected mass, whereas a more energetic explosion should completely unbind the star." T his illustration offers a plausible scenario for how vagabond stars exploded as supernovae outside the cozy confines of galaxies. 1) A pair of black holes comes together during a galaxy merger, dragging with them up to a million stars each. 2) A double-star system wanders too close to the two black holes. 3) The black holes then gravitationally catapult the stars out of the gal- axy. At the same time, the stars are brought clos- er together. 4) After getting booted out of the galaxy, the binary stars move even closer to- gether as orbital energy is carried away from the duo in the form of gravitational waves. 5) Even- tually, the stars get close enough that one of them is ripped apart by tidal forces. 6) As mate- rial from the dead star is quickly dumped on- to the surviving star, a supernova occurs. [NASA, ESA, and P. Jeffries and A. Feild (STScI)] n