Just an ordinary galaxy

The Sun is embedded in the Milky Way, about 9 kiloparsecs, or roughly 26,000 light-years, from the galactic center. “That’s a really bad perspective for trying to see what the Milky Way looks like,” Levine said. So astronomers peer edge-on into the galactic disk, looking for areas dense with hydrogen.

Levine did not do the looking himself; he and his colleagues analysed data from a state-of-the-art survey of the galaxy by radio astronomers. By analysing the shift in spectral signals through the Doppler effect—the phenomenon that makes a car horn’s pitch appear to change as the car speeds up or slows down relative to the listener—the researchers were able to determine how far away the hydrogen clouds were.

The result, published online this month by the journal Science, is the most detailed map yet of the structure of the galaxy. Like other galaxies, the Milky Way has arms that closely approximate logarithmic spirals, the same kind of curvature seen in nautilus shells. The researchers found the outer arms were not as tightly wound as those in the inner part, Levine said.

The Milky Way’s irregular structure may be due to the gravitational pull of other galaxies or collisions with smaller ones. Astronomers have seen those effects with other imperfect spiral galaxies.

“We actually live in a very typical place in the universe,” Levine said.

Henry Fountain