Updated Spiral  Arms Of the Milky Way Determining the structure of the Milky Way is a  difficult task for astronomers because we are inside it. To resolve this  problem, astronomers turned to the National Science Foundation’s Very  Long Baseline Array (VLBA) and its ability to make the most accurate  measurements of positions in the sky. Thelma’s capabilities allowed us  to use a technique that yields accurate distance measurements  unambiguously through simple trigonometry. By observing objects when  Earth is on opposite sides of its orbit around the Sun, astronomers can  measure the subtle shift in the object’s apparent position in the sky,  compared to the background of more-distant objects. This effect is  called parallax, and can be to directly determine distances much farther  from Earth. Recently, Xu et al.(2013, ApJ, 769, 15) using VLBA finds  that the Local Arm, previously thought to be only a small spur between  two large branches known as the Sagittarius and Perseus Arms  (Fig.1),instead should appear as a prominent feature of the Milky Way.  In fact, the Local Arm looks as 15000 light-years in length and 3000  light-years in width. Based on both the distances and the space motions  measured, it is a major structure, maybe a branch of the Perseus Arm, or  possibly an independent arm segment. This striking result was an  upgrade to the status of the Local Arm within which our Solar System  resides. It was presented at the 222nd meeting of the American  Astronomical Society in Indianapolis on June 3, 2013, and also was  reported by U.S．News and World’s Report, SPACE, Universe Today, and  NRAO. This work is a part of the BeSSeL Survey (Bar and Spiral Structure  Legacy Survey). The BeSSeL Survey is a VLBA Key Science project and the  goal is to study the spatial structure and kinematics of our own  Galaxy. It plans to measure accurate distances and three-dimensional  motions of up to 400 high-mass star forming regions in the Milky Way  between2010 and 2015 where water and methanol molecules are boosting  radio waves. This will result in a catalogue of accurate distances to  most Galactic high-mass star forming regions visible from the northern  hemisphere and very accurate measurements of fundamental parameters such  as the distance to the Galactic center, and the rotation curve of the  Milky Way.

By with  XU Ye

(Left) Old picture: Local Arm a small "spur" of Milky Way. (Right) New picture: Local Arm　　probable major branch of Perseus Arm.