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Ultra-precise radio measurements from the Very Long Baseline Array (VLBA) have given astronomers a detailed look at the motions of the Milky Way’s young stars and reveal that they are orbiting much slower than expected in strangely elliptical paths. "Almost all of our targets seem to have been accelerated opposite to the direction of the galaxy's rotation," says Mark Reid of the Harvard-Smithsonian Centre for Astrophysics. Reid speculates that the accelerating force came from the galaxy's spiral arms, which acts as an astronomical traffic jam. Just as cars stuck behind a slow-moving truck catch up with each other and slow down, gas clouds approaching a spiral arm catch up with the cloud in front, and slow down. As they slam on the celestial brakes they compress, birthing the hot, young stars that Reid studied. The slowing motion also causes the stars’ orbits to wind down into an elliptical orbit, a surprising revelation since all stars were thought to move in circular orbits around the galactic centre, an assumption upon which all previous maps were built. These maps, therefore, contain intrinsic errors and are now out of date.
This map of the near side of the Milky Way is a combination of an artist conception showing multiple arms in the galaxy, and real data points. Our Sun is marked by the red circle and blue circles show the star-forming regions whose distances have been accurately measured in the new mapping project. Image: Bill Saxton (NRAO) and Mark Reid (CfA). In the new study, Reid used the VLBA to measure the apparent shift in position on the sky as the Earth orbits the Sun for masers – very bright radio sources pumped by energy from a young star – in about a dozen star-forming regions. He then applied basic geometry to calculate highly accurate distances to each region. It was also possible to gain precise information on the motion of each maser in the plane of the sky. Combining those data with motions along the line of sight yielded the true three-dimensional motion of each target through space. The new mapping technique, which has given astronomers a first good look at the finer structure and motions of the Milky Way and its constituents, will ultimately help answer basic questions about the anatomy of the Milky Way. "Right now, our map of the Milky Way still has large areas marked 'Here there be dragons,'” says Reid. “Ten years from now, those areas will be filled in.” ESA’s Gaia mission, due for launch at the end of 2011, will map the positions of up to 1 billion stars located as far as 30,000 light-years from Earth, significantly improving the current maps of our Milky Way neighbourhood.
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