Read more: “Mysteries of the Milky Way“
Step out into a clear night, far from the city lights, and you’ll see a creamy streak of stars splashed across the sky: the “Milky Way” that has come to stand for our island universe. We see it as we stare through the flat, star-dense disc of our galaxy where we are also quartered. But what does our home look like from outside?
The short answer is we’re not sure. Our telescopes unveil other galaxies in majestic detail, but introspection is much trickier. We think we live in a spiral galaxy of the sort we see scattered throughout the cosmos, but our lowly viewpoint in the galactic disc means we struggle to trace how its arms are furled, or even count how many there are.
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Interstellar dust doesn’t help: it blocks our view over distances of more than a few thousand light years, so we cannot map out distant spiral arms by their stars. We instead trace clouds of hydrogen atoms, which emit radio waves with a characteristic wavelength of about 21 centimetres. This long-wave radiation penetrates the dust, and by measuring the change in its wavelength – its Doppler shift – we can work out a cloud’s speed towards or away from us. Comparing that with the ways in which different parts of the galaxy should rotate allows us to pinpoint a cloud’s distance.
The resulting tentative maps suggest that the galaxy is a complicated, messy, many-armed spiral (see diagram). But even that sketch is arguable. For a start, the galaxy’s rotation is not precisely known, and individual clouds need not follow the average motion; different models produce different maps. And when we look towards or away from the galactic centre, where the clouds are moving almost sideways relative to us, the Doppler method is no help in determining their distance. “Arms can only be identified in segments,” says galaxy mapper of the University of Wisconsin-Whitewater. “The task of piecing them together is left to the discretion of the astronomer.”
A parallel mapping effort, which suffers from similar limitations, uses radio emission from carbon monoxide gas that hangs around parts of the galaxy. Since 2008, this method has revealed more details of the galaxy’s structure, including what seem to be new arm segments. Better landmarks may be interstellar clouds where molecules of water or methanol act as lasers, amplifying a narrow line of microwave emission to produce bright pinpoints. These “masers” are so well localised that we can see how their position shifts as Earth orbits the sun, and thus triangulate their distance from us precisely. There are too few of them to map out the galaxy on their own, but they can be used to test the results of other methods. Maser range-finding could finally reveal the true face of the Milky Way.