Alien stars pass close to home

Stars from an alien galaxy are raining down on our own Milky Way and passing just a few hundred light-years from Earth. That’s the conclusion of astronomers who have mapped the extent of the Sagittarius dwarf galaxy, one of two dwarf galaxies that the Milky Way’s gravity is ripping apart (SN: 11/15/03, p. 307: Available to subscribers at Chow Down! Milky Way gobbles its closest known neighbor).

SHREDDED GALAXY. Map of stars in the Sagittarius dwarf galaxy (red streams) orbiting the Milky Way (blue pinwheel). Yellow dot marks the sun. S. Majewski, et al.

When a dwarf galaxy passes close to the Milky Way, its leading edge gets pulled more strongly by our galaxy’s gravity than its trailing edge does. The unequal tugs stretch the dwarf, pulling stars out in spaghetti-like streams.

A veil of dust in the Milky Way blocks visible light emanating from these streams, but infrared light punches through. Steven R. Majewski of the University of Virginia in Charlottesville and his colleagues mapped Sagittarius by selecting a group of infrared-bright stars. These M stars are rare in the outskirts of our galaxy but plentiful in the dwarf.

The new map reveals that thousands of stars from Sagittarius are now passing through the region of the Milky Way in which the sun resides, the team reports in the Dec. 20 Astrophysical Journal. That’s something of a cosmic coincidence because the sun and its environs take 240 million years to orbit the center of the Milky Way and intersect the Sagittarius stream for only a small fraction of that time.


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