Milky Way Arm Containing Our Solar System May Be Bigger Than We Thought

The spiral structure of the Milky Way might look like NGC 1232, pictured here. European Southern Observatory

Studying the Milky Way as a whole has always been difficult as we literally can’t see the forest for the trees. Now, new observations of its spiral structure (known as "arms") have provided a clearer picture of what our galaxy really looks like.

An international team of astronomers, led by Ye Xu from the Chinese Academy of Science, has discovered that the galaxy’s Local Arm, where the Solar System is located, is much longer than previously thought, stretching for about 20,000 light-years.

For a long time, the Local Arm was considered a minor spiral feature in the Milky Way, a spur coming out of the Sagittarius Arm. This new study, published in Science Advances, collected new distance measurements for eight regions undergoing huge episodes of star formations.

They have found that it is the same size as the Milky Way's major arms, such as Sagittarius and Perseus, which originate from the galaxy's core. The measurements were performed using radio waves, and they allowed the team to better estimate the local spiral structure.

Artist's impression of the previous view of the Milky Way. The Local Arm, also called the Orion Spur, appears to be a small feature. The latest research suggests that its size is similar to the Perseus and Sagittarius arms. NASA/JPL-Caltech/ESO/R. Hurt

We see the galaxy as a flat band across the sky, so to truly understand its shape we need to carefully estimate the three-dimensional position of stars that we can see. This is not a simple task; there are stars up to 60,000 light-years from us, and our galaxy is full of gas and dust that makes the measurements difficult.

The idea of the Milky Way being a spiral galaxy was first proposed in 1852 by American astronomer Stephen Alexander, but it took another hundred years for scientists to begin finding out the details of this spiral structure.

Since then several different models have been proposed, but there’s no general agreement on the actual number and properties of the galaxy’s spiral arms. Only in the last 10 years have astronomers begun measuring distances with enough precision to finally produce a more accurate picture of the Milky Way.

These new measurements were collected by the Very Long Baseline Array, a system of 10 radio telescopes spread across the continental US all the way to Hawaii, that operates as a single instrument with a baseline of 8,611 kilometers (5,351 miles). This set up utilizes the separation between the different radio dishes to extract more information than would be possible with a single one, yielding the most accurate measurements to date.

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