One half of the Milky Way is essentially the mirror image of the
other, and a whole structure has an uncommon and perfect
symmetry, CFA astronomers say. Thomas Dame and Patrick Thaddeus
of the Harvard-Smithsonian Center for Astrophysics in Cambridge,
Mass. discovered a vast structure at the Milky Way's outskirts,
which is most likely the outer extension of the Scutum Centaurus
arm from the inner galaxy. It appears that Scutum-Centaurus
wraps all the way around the Milky Way, making it a symmetric
counterpart to the galaxy's other major star-forming arm,
Perseus. The newfound structure lies about 49,000 light-years
from the galaxy's center, and one of the arm's many large
molecular clouds contains an amount of molecular hydrogen
equivalent to that of 50,000 suns! The two arms of the Milky Way
extend from opposite ends of the galaxy's central, bar-shaped
cluster of stars, each winding around the galaxy. Thomas Dame
found the new structure while reviewing galactic data on atomic
hydrogen gas, which radiates at a