Space ‘rain’ and black holes solve galactic star-making mystery

File photo - an expanding shell of debris called SNR 0519-69.0 is left behind after a massive star exploded in the Large Magellanic Cloud, a satellite galaxy to the Milky Way.

File photo - an expanding shell of debris called SNR 0519-69.0 is left behind after a massive star exploded in the Large Magellanic Cloud, a satellite galaxy to the Milky Way.  (REUTERS/NASA/CXC/SAO/Handout via Reuters)

For decades, the reason why some galaxies produce more stars than others has remained a mystery. The Milky Way, for instance, produces one new star a year while the Baby Boom Galaxy produces 4,000 per year, or one star every 2.2 hours. On the opposite end of the spectrum, some galaxies are barren, producing little to no stars anymore. However, a team of astronomers at Michigan State University now believes they may have finally solved the riddle. The answer: galactic “rain” in the form of cool gas clouds which mist the cosmos, and their interaction with black holes. “It's like we all of a sudden got really good at predicting the ‘weather’ in galaxies,” Team Leader and MSU Professor Mark Voit told

It’s no mystery that cool gas clouds help galaxies form stars, but it was unknown why the gas in less “fertile” galaxies remained un-cooled.  “Astronomers have known for decades that galaxy clusters are filled with hot gas that looks as if it should be cooling off and forming lots of stars,” Voit said. “However, searches for these newly-forming stars would only turn up one to ten percent of the expected number, meaning that something is preventing most of the hot gas from cooling.”

Using NASA’s Chandra X-ray Observatory, Voit and his team of space weathermen set about solving the mystery, carefully studying X-rays of over 200 galaxy clusters, which are structures consisting of hundreds to thousands of galaxies bound together by gravity. The team focused on galaxies at the center of these clusters, which, according to Voit, are the largest in the universe. In the middle of these galaxies, the team focused on a prime suspect: massive black holes.

“We and our colleagues have suspected for years that massive black holes in the centers of these giant galaxies are somehow responsible for preventing the cooling,” Voit said. “When matter falls into a black hole, it can release lots of energy on the way in.  The puzzle was to figure out how the black hole knows just how much energy to release to stop the cooling. What we are finding is that strong black-hole energy outbursts are triggered when cold clouds start to ‘rain’ down into a galaxy out of the hot gas surrounding the galaxy.  If there is no such rain, then the black hole is relatively dormant.  However, as the hot gas starts to cool down, it develops a ‘mist’ of cool clouds.”

Some of those clouds form stars while others start falling toward the central black hole, according to Voit.  “When those clouds fall into the black hole, the resulting energy release reheats the hot gas and limits the amount of ‘rain.’  Therefore, the amount of star formation is limited, and the galaxy is continually suspended in a misty state, unable to produce a true downpour of cold, star-forming clouds,” he said. In other words, when the gas clouds fall into a black hole, it cranks up the galaxy’s “heater” by producing jets. These heat-blasting jets prevent the galaxy from cooling, thus preventing star formation. “One of the tricky things to figure out was how these galaxies limit the amount of new star formation without completely shutting it off,” Voit added. “The key was to recognize that the black-hole heat source does not turn on until some cold clouds have already formed, meaning that a modest amount of star formation is inevitable.  However, the galaxy can never make a whole lot of cold clouds without some of them falling into the black hole and triggering the ‘heater’.” He added that most of the giant galaxies the team studied are barren, having no star formation at all. “In those galaxies, the hot gas is not able to cool fast enough for ‘rain’ to begin.”

So when, if ever, will the Milky Way shut down its star-making factory, which-at one star per year- seems already close to going out of business? According to Voit, not for a very, very long time. “The Milky Way has enough gas surrounding it to form stars for ten to thirty billion more years, if star formation continues to proceed at the current rate. It's quite likely, though, that its star-formation rate will tail off more gradually, lasting up to 100 billion years at an ever-decreasing rate.”