What does the Milky Way look like? Are there two spiral arms like our sister spiral galaxy Andromeda, or four? We cannot exactly get a “bird’s eye” view as the Galaxy is so vast that if we were to send a satellite out a useful vantage point it would take millions of years.
Even if we could start such a venture and build a satellite with parts and batteries good to last millions of years then what are the chances that our future descendants would remember to check back to look at the pictures?
It seems instead that we must make do with mapping out our home galaxy from Earth. On top of the poor vantage point, mapping out the Milky Way has proven difficult in part because Earth is a moving platform situated amongst the stars and clouds which are themselves also in motion.
This introduces some ambiguities in our distance measurements. For example, when we look straight through the center of the Galaxy with the intent to study stars on the other side, we are unable to sort out which stars are on the near side and which are on the far side.
A breakthrough in measuring an unambiguous distance came when Alberta Sanna and colleagues recently used a natural long wavelength laser beam in space called a maser (instead of a laser) as a tool to measure the distance to the star forming region (October 13th issue of Science).
They took images of the position of the maser relative to the positions of stars superimposed in the background. They then repeated the measurement when the Earth was on the opposite side of its orbit six months later.
They found a difference between these two images, or parallax, which they related to the maser's exact distance from us. In sum, it turns out that this maser, and thus also the star forming region in which it sits, are on the other side of the Galaxy. It looks like the Milky Way has newly discovered arm!