WE CALL it home, but in some ways we know less about the Milky Way than about neighbouring galaxies. Trying to determine what it looks like has been like trying to identify the structure of a forest while sitting in it. A map of our galaxy鈥檚 spiral arms should redress the balance.
Evan Levine and his colleagues at the University of California, Berkeley, created the map of the outer part of the galaxy鈥檚 spiral arms using radiotelescope measurements of the amount of atomic hydrogen in the galaxy from the joint Leiden/Argentine/Bonn survey.
The team confirmed that the galaxy has a non-symmetric spiral arm structure like those seen in nautilus shells and cyclones. 鈥淚t鈥檚 cool that nature repeats these patterns,鈥� Levine says.
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The arms are made up of dense regions of hydrogen gas. Unlike some other galaxies, which have perfectly defined arms 鈥渁s though they have been drawn by hand鈥�, says Levine, the Milky Way鈥檚 arms are ragged and patchy along their length.
Moreover, the arms stretch out to distances 80,000 light years from the galactic centre, well beyond the stars (Science, DOI: 10.1126/science.1128455). This is surprising because the spirals are thought to be caused by gravitational effects, but these would be quite weak in such far-out regions, says co-author Leo Blitz.
