Mapping the Milky Way
IF THE MILKY WAY were just an average galaxy, mapping its structure would be easy. The average galaxy is a dim, ghostly dwarf roughly a thousand light-years across, so astronomers could easily chart it by measuring the parallaxes of its stars to determine their distances.
But the Milky Way is a galactic colossus, one of the great Goliaths of the cosmos. From end to end the spiral arms inhabit a disk spanning some 120,000 light-years. So huge is the galaxy that nearly every star the naked eye can see belongs to the local spiral arm in which the Sun and Earth dwell. As a result, many astronomers have long despaired of ever knowing how our galaxy would appear from afar.
In recent years, however, visible, infrared and radio observations have delivered the crispest views yet of the Milky Way’s structure. Surprises abound. At least one spiral arm seems to make a complete turn around the galaxy. And precise parallaxes of far-off stellar nurseries have upgraded the status of the spiral feature that hosts the Sun.
Best of all, the Milky Way may be much more majestic than many of its spiral peers. “We potentially live in quite a lovely barred spiral,” says Thomas Dame (Center for Astrophysics, Harvard & Smithsonian), “and I wouldn’t have thought that 15 years ago.”
Open arms
The Milky Way has four main spiral arms, winding out from its central regions and whipped up by its rapid rotation. If you were above the galactic plane, you’d see the Milky Way’s disk spinning clockwise — the direction planets orbit the Sun. Because of this clockwise rotation, the arms wend their way counterclockwise from the inner galaxy to the outer. The same thing happens when you stir
