One of the Milky Way鈥檚 star-studded spiral arms lies twice as close to Earth as some previous estimates suggested. New research has produced the most accurate distance measurement ever made of the arm, which could help astronomers understand how our galaxy鈥檚 spiral structure formed.
The Milky Way appears to be made up of four main arms that curve around its centre like a pinwheel. 鈥淗owever, our view from the interior makes it difficult to determine its spiral structure,鈥� writes a team led by Ye Xu of the Shanghai Astronomical Observatory in China, in Science.
Measuring the distance to the spiral arms can be particularly tricky. This is because astronomers can only measure the speed of an astronomical object in terms of how fast it is moving towards or away from the Earth. Comparing this speed to theoretical models, which assume the objects travel on circular paths around the centre of the galaxy, allows astronomers to deduce the object鈥檚 distance from Earth.
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Astronomers using this technique had previously estimated the distance to Perseus, the arm immediately beyond the Sun, at more than 13,000 light years. But other researchers arrived at half that distance using a method that compares the apparent brightness of massive, young stars with estimates of their intrinsic brightness.
Now Xu鈥檚 team has used a third technique 鈥� 100 times more accurate than the other two 鈥� to conclude the Perseus arm is indeed relatively close, at just 6400 light years from Earth.
Hawaii to the Caribbean
They used a system of 10 radio dishes that boasts the sharpest vision of any telescope in existence. Called the Very Long Baseline Array (VLBA), the dishes 鈥� each spanning 25 metres 鈥� are scattered from Hawaii to the Caribbean Sea.
They focused on a star-forming region called W3OH inside the Perseus arm. Bright, young stars in the region heat methanol vapour in gas clouds around them, which in turn emits radio waves in what are called 鈥渕asers鈥�.
The team tracked the masers at five intervals over the course of a year, determining their distance by 鈥渢riangulating鈥� their observed positions from different points along Earth鈥檚 orbit.
鈥淲e used our changing vantage point to form one leg of a triangle,鈥� says team member Mark Reid, an astronomer at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, US. 鈥淭hen, measuring the change in angle of the source as the Earth orbits the Sun, we can calculate the source鈥檚 distance by simple trigonometry.鈥�
They found that W3OH is not moving in a perfectly circular orbit but instead follows an elliptical path, as if drawn along the Perseus spiral arm. 鈥淚t seems to be indicating that the spiral arms may have a higher density than previously guessed,鈥� Reid told New 杏吧原创.
The team will now use the VLBA to measure the distances to a dozen star-forming regions spread across several of the Milky Way鈥檚 spiral arms. 鈥淲e hope to use such data to better understand how spiral arms form,鈥� says Reid.
Journal reference: Science (DOI: 10.1126/science.1120914)