Milky Way Keeps Grip on Neighbor

By Ken Croswell

Published in New Scientist (August 16, 2008, page 8)

Our empire has struck back. Last year, astronomers got a shock when it emerged that our Galaxy's brightest companion, the Large Magellanic Cloud (LMC), appeared to be speeding through space so fast that we wouldn't be able to hold onto it (New Scientist, 13 January 2007, p 13). Now two new measurements suggest the Milky Way will manage to keep its companion after all.

The first shows our Galaxy to be spinning faster than we had thought. A star near the Sun on a circular orbit round the Galactic centre now appears to travel at 251 kilometres per second, compared with a previous figure of 220 km/s. And the centre of the Galaxy now appears to be 27,400 light-years from Earth, slightly farther than the older figure of 26,100 light-years.

Taken together, these findings imply that the Milky Way has about 50 percent more mass than previously estimated, say Genevieve Shattow and Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. This would make its gravity correspondingly stronger. And because astronomers measure the LMC's speed relative to that of the Sun, its estimated speed goes down 10 percent, implying that it orbits the Milky Way every 6 billion years and reaches a maximum distance of 1.1 million light-years. It is now 160,000 light-years from Earth. The astronomers have submitted their work to Monthly Notices of the Royal Astronomical Society.

Carlton Pryor of Rutgers University in New Jersey thinks the new work is probably correct. "When I saw this result, it was a little bit of an 'aha' moment," he says. The Milky Way has torn a 300,000-light-year-long stream of gas out of the LMC. If it were approaching us for only the first time, Pryor says there wouldn't have been enough time for the stream to have grown so large.

Ken Croswell is an astronomer and the author of Magnificent Universe and Ten Worlds.

"Magnificent Universe by Ken Croswell is elegant and eloquent."--Washington Post. See all reviews of Magnificent Universe here.

"On the basis of its striking design and photographs, this handsome, large-format volume is well worthy of praise. And astronomer Croswell's concise yet conversational, information-packed text wins it sky-high accolades in the narrative sphere as well."--Publishers Weekly, starred review. See all reviews of Ten Worlds here.

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