Runaway stars to fill in the blanks in Milky Way map

GUIDES to the galaxy might call it Zona Galactica Incognita ? the half of our home galaxy we know little about. Indeed, the Milky Way is one of the least charted spiral galaxies in the nearby universe. Now it seems that stars kicked out of their birth clusters can help fill in the void and create the first proper map of the entire galaxy.

Young star clusters and clouds of hydrogen that formed in our galaxy help trace the shapes of the Milky Way's arms, so astronomers are reasonably certain that it has a spiral structure (see right). Observations of stellar motion show that there is a supermassive black hole at its core.

But figuring out how fast the arms rotate or even counting how many there are is tricky, in part because we are embedded in one of its arms and unable to get an outsider's view. In addition, everything behind the galactic centre is shrouded by a dense wall of stars and dust, blanking out a whole area of the Milky Way map.

"It's quite difficult to see the actual structure," says Manuel Silva of the University of Lisbon in Portugal. "I'm a little upset, really, that we don't know our own galaxy that well."

A space telescope called Gaia, scheduled for launch later this year, will map the positions and distances of about one billion stars on our side of the Milky Way, plotting the three-dimensional structure in unprecedented detail. But even Gaia won't be able to pierce the material that blocks our view of the far side.

Instead of trying to look across, Silva and his colleagues suggest looking up, where hundreds of runaway stars fly high above the disc of the galaxy. These stars are born in clusters inside the Milky Way but get ejected during gravitational jostling with other stars. Precise measurements of their velocities, ages and distances would allow astronomers to trace the stellar fugitives back to their homes, even on the far side.

"The idea is that the runaway stars act as signal flares, showing the position of the spiral arm, the same way someone lost in the middle of a dense forest could fire one to the sky to show his or her location to an outsider," says Silva.

His team traced the origins of about 40 runaway stars, observed by the Hipparcos satellite, ranging from roughly 1000 to 100,000 light years above the galactic plane (arxiv.org/abs/1302.0761v1). Although none of these stars came from the far side, the technique seems to work because the results agreed with previous studies that mapped star clusters in the visible section of the galaxy.

"The idea is a new one, and is an interesting one," says Jacques Lepine of the University of S?o Paulo in Brazil, who was not involved in the new study. Comparing Gaia's view of stars with the runaways will be helpful, he adds. "It is good to have different methods, to compare results. If the results are similar, we get more confident."

Jacques Vall?e of the Canadian National Research Council in Victoria, British Columbia, agrees that the proof of concept is impressive. But that doesn't stop him fantasising about easier ways: "Wish I had a friend on a planet around a runaway star in the halo, sending me back a photo."

This article appeared in print under the headline "Runaway stars flesh out Milky Way map"

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