Researchers used data from NASA's Wide-field Infrared Survey Explorer (WISE) mission to highlight the X-shaped structure in the bulge of the Milky Way. NASA/JPL-Caltech/D.Lang

The core of the Milky Way has got a little bit more interesting. Not only is there a supermassive black hole millions of times the mass of the Sun, but astronomers have now discovered an enormous group of stars shaped like an X.

Melissa Ness from the Max Planck Institute and Dustin Lang from the University of Toronto have collaborated on a study on the mysterious structure, spotted by Ness in a tweet shared by Lang.

Lang shared an interactive map made using publicly available data from the Wide-field Infrared Survey Explorer (WISE), NASA’s infrared space telescope, and although it wasn’t the focus of the tweet or Lang's research, many commented, including Ness, on the Milky Way’s bulge.

"The bulge is a key signature of formation of the Milky Way," said Ness, the study's lead author, in a statement. "If we understand the bulge we will understand the key processes that have formed and shaped our galaxy."

The research, published in the Astronomical Journal, discusses the importance of seeing this feature in the Milky Way. Our galaxy is a barred spiral – the central density of stars (“the bulge”) is not spherical like in many famous galaxies but more rugby ball-shaped.

As for the X shape itself, this is probably formed due to interactions between the core of the Milky Way and the disc, making the stars protrude from the galaxy in this particular shape.

Many astronomers believe the central bar of the galaxy to be unstable, changing size and shape over time – from box-like to peanut-like depending on the gravitational interactions between the stars in the bulge and the spiral disk.

The core of galaxies can also be influenced by major and minor galaxy collisions, although we know from other sources that the Milky Way hasn’t interacted with any large galaxy in the last 9 billion years.

"We see the boxy shape, and the X within it, clearly in the WISE image, which demonstrates that internal formation processes have driven the bulge formation," Ness said. "This also reinforces the idea that our galaxy has led a fairly quiet life, without major merging events since the bulge was formed, as this shape would have been disrupted if we had any major interactions with other galaxies."

The X structure has been reported before, but this study shows the clearest image of it yet. Future observation and dynamical simulation will help us better understand how it formed.

"To me, this study is an example of the interesting, serendipitous science that can come from large data sets that are publicly available," Lang said. "I'm very pleased to see my WISE sky maps being used to answer questions that I didn't even know existed."