With black holes, astronomers learn to go with the flow
When astronomers can鈥檛 observe a phenomenon directly, they sometimes try to go with the flow. Like the flow of interstellar gases that help point the way cosmic winds are blowing. Or there鈥檚 the flow of orbiting stars that give insight into the unseen black hole at the center of our galaxy. New research on both flows shows how this indirect approach pays off.
It has taken 16 years of persistent study to get a more accurate fix on our Milky Way galaxy鈥檚 mysterious central black hole known to astronomers as Sagittarius A. A black hole is so dense not even light can escape from within it. But by following the motion of 28 stars orbiting it, Stefan Gillessen at the Max Planck Institute for Extraterrestrial Physics in Germany and an international research team have pinned down the mass and distance of Sagittarius A with unprecedented accuracy. The team determined it鈥檚 27,000 light years away and weighs in at 4 million times the mass of our sun.
The details of how the team worked this out are explained in the Astrophysical Journal and in an announcement this week from the European Southern Observatory (ESO) headquarters in Garching, Germany. Observations made over 16 years with various instruments, including infrared light to penetrate obscuring dust clouds, have gone into the present calculations. As the precision of the measurements has increased, so, too, has confidence in the accuracy of the calculations. That precision now is equivalent to 鈥渟eeing a one euro coin [about the size of a US quarter] at a distance of roughly 10,000 km,鈥� according to the ESO鈥檚 announcement.
Dr. Gillessen鈥檚 colleague, Reinhard Genzel, says that the details of how those 28 stars move shows that the mass in the center of our galaxy that controls their motion 鈥渕ust be a black hole, beyond reasonable doubt.鈥� He explains that this means 鈥渢he most spectacular aspect of our long-term study is that it has delivered what is now considered to be the best empirical evidence that super-massive black holes really do exist.鈥�
Matt Povich at the University of Wisconsin in Madison and colleagues have been using the Spitzer Space Telescope鈥檚 infrared vision to trace gas flows in the Swan nebula 6,000 light years away in the constellation Sagittarius. Their work, reported this week in the astronomical Journal and in an announcement from the National Aeronautics and Space Administration, traces the roiling rush of winds that flow from massive stars at the nebula鈥檚 center at speeds of 4.5 million miles an hour.
Dr. Povich explains that the gas flows around stars like a river rushing past rocks. It interacts with other gas piling up shock waves of gas and dust. These show up clearly in Spitzer鈥檚 infrared photos. Povich likens these bow shocks to 鈥渨eather vanes, indicating the direction of the stellar winds in the nebula.鈥�
He adds that 鈥渢hese bow shocks serve as a reminder that stars aren鈥檛 born in quiet nurseries but in violent regions buffeted by winds more powerful than anything we see on Earth.鈥�
NASA and the ESO have made photos of these effects available on their websites: and .
