A billion years ago the center of the Milky Way experienced an extraordinary burst of star formation, which led to supernovae fireworks, with more than 200,000 explosions in an astronomically-speaking short amount of time.
Astronomers have witnessed thousands of supernova explosions over the last century, but all have been in galaxies too distant for the ideal view. Since the invention of the telescope, our own galaxy has been quiet. Those frustrated by the situation can only dream of how Sagittarius appeared a billion years ago.
The Milky Way was born 13.5 billion years ago, which led to a period of rapid star formation. We know subsequent encounters with other galaxies caused surges in star formation. Nevertheless, astronomers had thought the overall trend had been fairly steady for a long time.
Dr Francisco Nogueras-Lara of the Max Planck Institute for Astronomy led a team that studied the galactic center in unprecedented detail using an infrared camera on the European Space Agency's Very Large Telescope and found something different. "Contrary to what had been accepted up to now, we found that the formation of stars has not been continuous,” he said in a statement.
Approximately 80 percent of the stars in the galactic center were formed at least 8 billion years ago, Nogueras-Lara and co-authors report in twin papers in Nature Astronomy and Astronomy & Astrophysics. After that came a drought lasting 6 billion years. Then things restarted for reasons we cannot yet explain. For 100 million years this small region spawned almost as many stars as the entire galaxy does today. Five percent of the center's current star mass appeared during this short (astronomically-speaking) period, equalling tens of millions of times the Sun's mass.
Many of the stars born in this era are still with us today for Nogueras-Lara to find. However, the larger a star is, the faster it burns its fuel and the shorter its life. Any stars with masses eight times that of the Sun or more born during this starburst period have passed through their entire lives, ending them in supernova explosions. Nogueras-Lara found more than 100,000 stars lived and died this way.
A galaxy the size of the Milky Way usually averages a supernova a century, so 200,000 in 100 million years is actually not that fast, but this is what was going on just in the galactic center, an area with much less than 1 percent of the galactic volume. The rest of the galaxy would have continued its development, supernovae included, at the usual rate.
The Nature Astronomy paper's authors interpret their findings as indicating the Milky Way's central supermassive black hole had acquired most of its current mass by 8 billion years ago. They also think our galaxy's bar only appeared quite recently, as an after-effect of the starburst.