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