Further Down The Spiral

December 9th, 2005 | researchmaterial

One of the Milky Way’s 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’s spiral structure formed.

The Milky Way appears to be made up of four main arms that curve around its centre like a pinwheel. “However, our view from the interior makes it difficult to determine its spiral structure,” 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’s distance from Earth.

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’s team has used a third technique – 100 times more accurate than the other two – 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 – each spanning 25 metres – 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 “masers”. The team tracked the masers at five intervals over the course of a year…


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