jueves, 31 de mayo de 2007

Streams of Stars Reveal Cannibal Nature of Milky Way

Three new streams of stars were discovered ringing the Milky Way. The two closest streams are thought to be star clusters, while the huge arcing stream is thought to be a dwarf galaxy. Credit: Caltech

Newly discovered stellar streams that arc around our galaxy might be the remnants of cannibalized star clusters and galaxies, scientists announced today.

The stellar streams findings, described by Caltech's Carl Grillmair here at a meeting of the American Astronomical Society, reveal our galaxy can be a dangerous place for passersby. .

Stellar streams are thought to form over billions of years as our galaxy's gravity slowly tears apart globular clusters and even dwarf galaxies. The stars, which were once packed tightly together, are now separated by light-years, trailing one another as they jet at high speeds through the galactic halo.

Grillmair and his colleagues analyzed data from the Sloan Digital Sky Survey (SDSS), comparing the colors and luminosities of stars and grouping similar stars together.

Two of the discovered streams are about 13,000 light-years from Earth and are likely the remains of ancient globular clusters, spherical collections of hundreds of thousands of old stars. Astronomers have identified only about 150 globular clusters orbiting the Milky Way, though they think thousands may have existed in the past.

The third stream is about 130,000 light-years from Earth and could be the closest dwarf galaxy to our Milky Way ever discovered. To date about 20 dwarf galaxies have been identified in the Milky Way and astronomers have wondered why they haven't found more.

"It might tell us why we don't see them, because they all get ripped up like this," Grillmair told SPACE.com. He added that perhaps there is a safe distance from the Milky Way, and any closer dwarf galaxies would be "toast."

Containing up to 100 million stars, dwarf galaxies are also thought to be chock-full of dark matter, the glue thought to hold our galaxy together.

Grillmair suspects stellar streams will garner loads of scientific attention for years to come, as they are windows into our galaxy's past, present and future and likely hold evidence of dark matter.

One theory for how our galaxy formed says that lots and lots of dwarf galaxies merged, and are continuing to merge, and ultimately gave rise to the Milky Way. The dwarf-galaxy stream could be one such merger that's slowly succumbing to the Milky Way's gravitational lure.

"This is a very exciting time for galactic archaeology, and finding more of these ancient streams will really help us to piece together the structure of our galaxy and how it evolved over time," Grillmair said.