Brian Clark Howard at National Geographic writes that “Our Part of the Milky Way is Four Times Bigger Than We Thought.”
Published in the journal Science Advances this week, a new study reported that our surrounding area of stars, gas, and dust—called the Local Arm, Orion Spur, or Orion–Cygnus Arm—is actually about 20,000 light-years long.
The Local Arm is still considerably shorter than the Milky Way’s four main arms, which wind around the galaxy’s center for roughly 80,000 light-years. The total diameter of the galaxy is often reported as about 100,000 light-years, although Reid says more recent evidence suggests it is probably more like 70,000 light-years, at least to the edge of where star density starts to really drop off.
The abstract from the paper in “Science Advances,” “The local spiral structure of the Milky Way” by Ye Xu, et al.
The nature of the spiral structure of the Milky Way has long been debated. Only in the last decade have astronomers been able to accurately measure distances to a substantial number of high-mass star-forming regions, the classic tracers of spiral structure in galaxies. We report distance measurements at radio wavelengths using the Very Long Baseline Array for eight regions of massive star formation near the Local spiral arm of the Milky Way. Combined with previous measurements, these observations reveal that the Local Arm is larger than previously thought, and both its pitch angle and star formation rate are comparable to those of the Galaxy’s major spiral arms, such as Sagittarius and Perseus. Toward the constellation Cygnus, sources in the Local Arm extend for a great distance along our line of sight and roughly along the solar orbit. Because of this orientation, these sources cluster both on the sky and in velocity to form the complex and long enigmatic Cygnus X region. We also identify a spur that branches between the Local and Sagittarius spiral arms.