New measurements suggest rethinking the shape of the Milky Way

A team of space scientists from the Chinese Academy of Sciences at both the Purple Mountain Observatory and the National Astronomical Observatories have determined that the traditional four-armed view of the Milky Way is incorrect. In her article published in The Astrophysical Journalthe group describes their analysis of multiple data sources in an attempt to extract a true form of the Milky Way.

For many years, space scientists have imagined the Milky Way as a spiral shape with a central bulge and four main spiraling arms — with several other smaller branching arms. Telescope technology has improved in recent years, and with it has come the realization that the vast majority of galaxies conform to just one of three main shapes: spiral, irregular, and elliptical. Also, most have two main arms with spirals that split into smaller arms.

Such a split is believed to have occurred due to collisions with other galaxies or clusters. Such observations suggest that if the Milky Way is a four-armed spiral galaxy, it would be extraordinarily rare. And if that were the case, there would have to be some unique attributes that would have resulted in such a unique shape. The researchers with this new effort suspect it’s more likely we misunderstood the shape from the start. They believe that the Milky Way, like most other galaxies, has only two main arms.

The research team came to this conclusion by analyzing data from a new generation of space instruments, all of which have technology that can measure how far away individual stars are from us. One such instrument, the team says, has long-baseline interferometry that can measure very accurately the distances to microwave-emitting stars. They used it to measure 200 such stars, which they used to begin creating a map of the Milky Way.

The team also used data from the Gaia space observatory, which is used to map the position of stars in relation to Earth and each other. They focused their attention on OB stars, which have proven useful because they don’t move much after formation. They collected data on 24,000 of them and added it to the map they created. They also added data for nearly a thousand open star clusters, also courtesy of Gaia.

The team then adjusted the arrangement of the stars into a spiral, finding that the Milky Way’s most likely shape is a barred spiral with two main arms extending outward from the pole. Other shorter arms are more distant and irregular and not connected to the main structure.

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