Object: the Milky Way鈥檚 black hole
Food source: big, gassy stars
Our galaxy鈥檚 central black hole is a fussbudget, refusing to eat most of what it pulls to its lips because the food is too hot. Spitting out its meals may not only stunt its growth 鈥� the finicky black hole may also be preventing new stars from being born nearby.
Most large galaxies like the Milky Way are thought to harbour black holes at their cores that have a mass equal to that of millions or billions of suns. Some of these behemoths are enthusiastic eaters, pulling in surrounding gas with abandon. As the gas falls towards the black holes, it heats up, producing that can be seen across the universe.
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The Milky Way鈥檚 relatively dim black hole, known as Sagittarius A* (pronounced 鈥渁-star鈥�), is decidedly not one of these gorgers. A crowded disc of massive stars spins around it, and researchers had previously calculated that these stars should spew out enough gas in stellar winds to provide the black hole with about four Earths鈥� worth of meals over the course of a year. But it does not seem to be swallowing that much material 鈥� if it were, it would shine 100 million times brighter in X-rays.
Scalding soup
Some scientists noted, though, that the brightness estimate assumed the gas coming from the stars was relatively cool, and that it could easily slip down the black hole鈥檚 gullet, says of the University of Massachusetts in Amherst. To find out what鈥檚 really going on, Wang and his colleagues used to measure the temperature and brightness of gas at different distances from the black hole.
They found that the gas gets hotter and less abundant the closer it is to the black hole. The researchers estimate that less than 1 per cent of the surrounding gas ultimately comes near enough to be eaten. Wang thinks the food is just too hot.
Quasars are champion eaters because they slurp up relatively cool gas, below 1 million 掳C. Such gas is dense and flows in an orderly fashion into a quasar鈥檚 maw, like water swirling into a drain. But the gas around Sagittarius A* is much hotter 鈥� collisions between stellar winds in the starry disc heat the gas to 10 million 掳C before it even starts to fall towards the black hole. This hot gas is tenuous and its particles zip around randomly, making it hard to corral.
鈥淚t鈥檚 very hard to get steam into the sink,鈥� says Wang. That means the black hole should not get the blame for apparently turning up its nose at hot gas on its plate. 鈥淭he black hole wants to suck it in, but it cannot,鈥� says Wang. Bizarrely, the tiny fraction of gas our black hole does imbibe may get in because it has transferred some of its jitteriness to gas particles that are thrown outwards, possibly by the black hole鈥檚 own magnetic field lines.
Snack time
Superhot stellar gas ejected from the black hole might be heating up other gas clouds surrounding the galactic centre, which is bad news for star formation, because they form when gas is cool enough to condense into dense bundles. All the extra heat may be stopping star birth. 鈥淭hat will have an effect on the evolution of the galaxy,鈥� says Wang.
Lest anyone worry that Sagittarius A* will starve as it tries, and mostly fails, to sip up the scalding soup of stellar gas, it may soon get a cooler snack as massive as three Earths.
鈥淚n the next few months, a large cloud of gas is on course to collide with the black hole,鈥� says of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, in an article accompanying the new study. Astronomers will be watching intently to see if Sagittarius A* opens wide.
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