Over 100 trigonometric parallaxes and proper motions for masers associated with young , high-mass stars have been measured with the Bar and Spiral Structure Legacy Survey , a Very Long Basline Array key science project , the European VLBI Network , and the Japanese VERA project .
These measurements provide strong evidence for the existence of spiral arms in the Milky Way , accurately locating many arm segments and yielding spiral pitch angles ranging from about 7 ^ { \circ } to 20 ^ { \circ } .
The widths of spiral arms increase with distance from the Galactic center .
Fitting axially symmetric models of the Milky Way with the 3-dimensional position and velocity information and conservative priors for the solar and average source peculiar motions , we estimate the distance to the Galactic center , { R _ { 0 } } , to be 8.34 \pm 0.16  kpc , a circular rotation speed at the Sun , { \Theta _ { 0 } } , to be 240 \pm 8  km s ^ { -1 } , and a rotation curve that is nearly flat ( i.e. ,   a slope of -0.2 \pm 0.4 km s ^ { -1 }  kpc ^ { -1 } ) between Galactocentric radii of \approx 5 and 16 kpc .
Assuming a “ universal ” spiral galaxy form for the rotation curve , we estimate the thin disk scale length to be 2.44 \pm 0.16 kpc .
With this large data set , the parameters R _ { 0 } Â and \Theta _ { 0 } Â are no longer highly correlated and are relatively insensitive to different forms of the rotation curve .
If one adopts a theoretically motivated prior that high-mass star forming regions are in nearly circular Galactic orbits , we estimate a global solar motion component in the direction of Galactic rotation , { V _ { \odot } } = 14.6 \pm 5.0 km s ^ { -1 } .
While \Theta _ { 0 }  and { V _ { \odot } } are significantly correlated , the sum of these parameters is well constrained , { \Theta _ { 0 } } + { V _ { \odot } } = 255.2 \pm 5.1 km s ^ { -1 } , as is the angular speed of the Sun in its orbit about the Galactic center , ( { \Theta _ { 0 } } + { V _ { \odot } } ) / { R _ { 0 } } = 30.57 \pm 0.43 km s ^ { -1 }  kpc ^ { -1 } .
These parameters improve the accuracy of estimates of the accelerations of the Sun and the Hulse-Taylor binary pulsar in their Galactic orbits , significantly reducing the uncertainty in tests of gravitational radiation predicted by general relativity .