With black holes, astronomers learn to go with the flow

Column: They gain new insights on black holes and distant nebulas by tracking the path of stars and interstellar gases.

S. Gillessen et al./ESO
The central parts of our Galaxy, the Milky Way, as observed in the near-infrared with the NACO instrument on ESO's Very Large Telescope. By following the motions of the most central stars over more than 16 years, astronomers were able to determine the mass of the supermassive black hole that lurks there.

When astronomers can’t 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’s 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’s 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’s 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 “seeing a one euro coin [about the size of a US quarter] at a distance of roughly 10,000 km,” according to the ESO’s announcement.

Dr. Gillessen’s 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 “must be a black hole, beyond reasonable doubt.” He explains that this means “the 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.”

Matt Povich at the University of Wisconsin in Madison and colleagues have been using the Spitzer Space Telescope’s 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’s 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’s infrared photos. Povich likens these bow shocks to “weather vanes, indicating the direction of the stellar winds in the nebula.”

He adds that “these bow shocks serve as a reminder that stars aren’t born in quiet nurseries but in violent regions buffeted by winds more powerful than anything we see on Earth.”

NASA and the ESO have made photos of these effects available on their websites: here and here.

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