Mapping from within
With their gently unfurling arms and ongoing star formation, spiral galaxies are some of the most beautiful star collections in the universe. But it is far easier to calculate the characteristics of distant galaxies than it is to understand the features of our own Milky Way.
"Determining the structure of the Milky Way has been a long-standing problem for astronomers because we are inside of it," Xu said. "While astronomers agree that our galaxy has a spiral structure, there are disagreements on how many arms it has and on their specific location."
Mark Reid, a researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts who was not involved in the study, compares the Milky Way to a dinner plate with an interesting design on its face. While the pattern is easy to spot from above, it can be difficult to interpret when the plate is edge-on.
"All of the structures are projected on top of each other, and without accurate distances to these structures, it is impossible to infer the design," Reid told Space.com by email.
To measure how far parts of the arm sit from the sun, scientists search for telltale signals in star-forming regions. As gas enters galactic arms, gravitational forces squeeze the gas to produce newborn stars. In other galaxies, blobs of bluish light that are produced by the birth of stars trace out spiral arms.
In the Milky Way, star-forming regions are more challenging to map. As part of the new research, the scientists identified bright spots of radio emission known as masers, whose shift in light researchers can measure to identify their movement and distance from Earth. Masers can be made up of clouds of gas that contain trace amounts of molecules such as water and methyl alcohol.
Reid compared the microwave emissions produced by masers to the spots of red light streaming from a hand-held laser.
"All they need is a source of energy — analogous to the battery in a laser pointer — and long path-lengths to amplify the emission," Reid said. "In star-forming regions, the more massive and very young stars provide the energy."
Using the National Radio Astronomy Observatory's Very Long Baseline Array (VLBA), a suite of 10 telescopes operating in Socorro, New Mexico, the scientists identified and measured eight new masers in the Orion Arm, setting its new length at about 25,000 light-years long. (A light-year is the distance light travels in a year.) Although measurements of the arm vary, Xu’s team set the distance as being just over 16,000 light-years in 2013.
"This characterization of the Local Arm will change the image of the Milky Way," Xu said.
The new research, which was published in the journal Science Advances in September, reveals the Milky Way as more complex than scientists have previously estimated. The galaxy is typically classified as a grand-design spiral, which Reid said is often very symmetrical, often boasting only two arms.
"The Milky Way, while probably a 'pretty galaxy,' has significant irregularities," Reid said. "Based on our observations, it is clear that there are four major spiral arms and some non-symmetric structures like the Local Arm."
Further studies are needed to determine how irregular the Milky Way might be: "Without a complete map of the Milky Way, however, it is not clear how symmetric the four arms are," Reid said.
Instruments like the VLBA, located in the northern hemisphere, are limited in their ability to study the Milky Way. According to Reid, they can only map a bit more than half of the galaxy.
"We need more observations, particularly from the Southern Hemisphere, so that we can map the entire Milky Way," Reid said.
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