A team of Chinese Academy of Sciences space researchers from both the Purple Mountain Observatory and the National Astronomical Observatory found that the traditional view that the Milky Way galaxy has four arms is incorrect.
In the paper published in The Astrophysical Journal, the group describes the analysis they did of several data sources in an attempt to get a true shape of the Milky Way.
For many years, space scientists imagined the Milky Way galaxy as a spiral, with a central bulge and four main spiral arms—with several other smaller arms branching off.
In recent years, telescope technology has improved, and with it, the scientific community has realized that the vast majority of galaxies only conform to one of three main shapes: spiral, irregular, and elliptical. Also, most have two main arms, with the spirals dividing into smaller arms.
The Milky Way would have only two main arms
These splits are thought to have occurred as a result of 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 so, there would have to be some unique attributes that would have led to such a unique form.
The researchers behind this new effort suggest that it’s more likely they got the shape wrong to begin with. They believe that, like most other galaxies, the Milky Way has only two main arms.
The team of researchers reached this conclusion by analyzing data from a new generation of space instruments, all of which have technology that allows measuring the distance at which individual stars are from us, writes Phys.org.
One of these instruments, the team notes, has long-baseline interferometry, which can measure distances to microwave-emitting stars with high precision.
The most likely shape of the Milky Way
They used this instrument to measure 200 such stars, which they used to begin building a map of the Milky Way. The team also used data from the Gaia space observatory, which is used to map the location of stars relative to Earth.
They focused their attention on OB stars, which proved to be useful because they don’t move much after formation. They collected data on 24,000 of them and added them to the map they were building. They also added data for nearly a thousand stars in open clusters, also courtesy of Gaia.
The team then adjusted the arrangement of the stars to a spiral, and in doing so discovered that the most likely shape of the Milky Way is a spiral with two main arms stretching outward. Other shorter arms are more distant and irregular and not connected to the main structure.
The study was published in The Astrophysical Journal.
- New measurements suggest rethinking the shape of the Milky Way galaxy: https://phys.org/news/2023-05-rethinking-milky-galaxy.html
- What Does the Milky Way Look Like?: https://iopscience.iop.org/article/10.3847/1538-4357/acc45c
