In another example of how difficult it is to see the forest for the trees, or in this case, what our galaxy looks like from the inside, astronomers have just discovered one of the largest pieces of the galaxy never before observed. The new “arm” is a huge arc of hydrogen gas a few thousand light years thick, extending 77,000 light-years in the outer reaches of the galaxy. That’s big enough to subtend 90 degrees of the night sky – if you could see it with the naked eye! The new map of the galaxy shows this as a chunk of gas that may be connected by thinner material to one of the four known spiral arms of the Milky Way.
The observing team was led by Naomi McClure-Griffiths of the Australia National Telescope Facility. They had been mapping the distribution of hydrogen gas in the galaxy, and found an unexpected ridge of bright emission in the radio map. Apparently there is so much new astronomical data out there that there could well be other major components of the Milky Way already in the data, just waiting to be analyzed – this according to Robert Benjamin, an astronomer at the University of Wisconsin (as reported by the New Scientist). The discovery of the new arm will be published in Astrophysical Journal Letters.