Story of the star that got away, escaping death by black hole
By Amanda Jermyn



We tend to think of black holes as having such powerful gravity that they suck in all light and objects, such as stars, that come near enough to them. But here’s the tale of a star that got away.

It all began with a project called the All-Sky Automated Survey for Supernovae (ASAS-SN, pronounced “assassin”), conducted by a team of astronomers from Ohio State University. In January of this year, while trolling the sky for supernovae, the violent explosions of stars that have reached the ends of their lives, the team detected a signal coming from a galaxy about 650 million light years away, in the direction of the constellation Ursa Major. While this was initially thought to emanate from a supernova, on closer inspection, it turned out to be a flare of light emitted by a star as it passed by a supermassive black hole, close enough that some of its mass fell into the black hole but far enough away that the star managed to get away with little damage. This is known as a tidal disruption event.

The astronomers couldn’t see the star itself but observed the flare of light as the black hole swallowed the material it captured, an amount of gas about the size of Jupiter. Though Jupiter may seem pretty big, it is still about a thousand times smaller than the sun. So the star in question got away relatively unscathed. But what caused the flare of light? The gas from the passing star does not immediately vanish into the black hole. It first orbits around the black hole in an accretion disk, moving faster and faster as it swirls toward the center. As the rapid movement heats the gas, it shines brighter, producing the flare of light observed by the astronomers.

According to Christopher Kochanek, co-author of the paper on the star that got away, a black hole consumes a whole star only once every 10,000 to 100,000 years. Given that some supermassive black holes are billions of times more massive than our own star, the sun, how could they grow so large on such a meager diet? The answer to this is still unknown, yet it is possible that black holes may be feeding on parts of stars, like the one that got away, more often than we think, adding to their mass on a regular basis. As very few instances of these tidal disruption events have been observed, it’s hard to know how often they occur. However, given that the group from Ohio State University came upon this event so early in their search for supernovae, co-author Krzysztof Stanek believes this suggests that it may be more common than previously thought for passing stars to lose some of their gas but still escape the clutches of a black hole. Stay tuned next month for yet another tale of a strange star that got away…

While there is no public Stars Club meeting in December, on Friday, January 2nd at 7:30pm, the Stars Club and Springfield Science Museum will host “Stars over Springfield,” an astronomy adventure for the whole family. Jack Megas, an astronomy educator at the Science Museum’s Seymour Planetarium, will give a talk about winter stars entitled “Winter Wonderland.” A fee of $3 for adults and $2 for children under 18 will be charged. For more information about the Stars Club visit the Stars Club – Massachusetts page on Facebook at https://www.facebook.com/groups/929125583765113/ or the Stars website www.reflector.org.