What’s the exact shape of stellar halo in the Milky Way galaxy? 

Just like all the other galaxies in the universe, the one to which we belong, called the Milky Way, also has a wide array of stars. But so far, the exact shape of these stars has largely remained unknown, until recently.  

While the inner shape of the stars is still a mystery, the outer shape of their halo (cloud-like diffusion surrounding them) has finally been discovered. Unlike previously thought, it isn’t spherical but oblong and titled (better known as triaxial ellipsoid), much like a football, or a rugby ball or even a zeppelin. A brand-new finding has been published in The Astronomical Journal by astrophysicists based at Harvard University and Smithsonian Institution, USA. and throws light on the evolution and history of Milky Way galaxy apart from valuable insights on the mysterious dark matter.  

For years, scientists had presumed that the stellar halo was both spherical and isotropic (same in all directions), something that now needs to be corrected. What’s interesting however is that, this stellar halo is likely full of invisible dark matter, whose presence is only known thanks to the measurable gravity it exerts on the Milky Way galaxy. While the dark matter behaves like a stellar covering and remains out of sight, the stars turn visible and take proper shape. And now that the shape is finally traced, it will help researchers better analyse dark matter.  

But why did the shape remain a mystery so far? Because the Earth is embedded with it. On top of that, the stars are many hundred thousand light years away and in between lies our solar system and other celestial objects. To put matters into perspective, for other galaxies, it’s easy to just look at them and measure their halos. But the same is rather difficult for our own Milky Way galaxy as we lack similar aerial point of view. The stellar halo of our Milky Way is too diffused and comprises only 1 per cent of the mass of all the galactic stars. So, the only way to deduce the shape of the stars is to analyse them up close and study their chemical setup, alongside measuring their distances and motions across the sky.  

This major astronomical breakthrough guided by cutting-edge technology also helps understand two major datasets gathered over the recent times surrounding the stellar halo. The first set came from Gaia (a revolutionary spacecraft launched by European Space Agency in 2013) that has compiled precise measurements of the positions, motions and distances of millions of stars in the Milky Way galaxy. The next is from H3 (Hectochelle in the Halo at High Resolution) that was a ground-based survey conducted by University of Arizona and had analysed in details tens of thousands of stellar halos that were farther from Gaia’s reach. And now, compiling all the data from three different sources, astronomers have confirmed that the shape of the stellar halo is anything but spherical. Not only that, the stellar halo is not even uniformly distributed as previously thought.  

Interestingly, the shape of the stellar halo may have been influenced by a lone dwarf galaxy that had collided with our Milky Way 7 to 10 billion years ago. Guess what the name of the dwarf galaxy was? Gaia-Sausage-Enceladus or GSE; where “Gaia” refers to the aforementioned spacecraft, “Sausage” refers to the pattern appearing when analysing the Gaia data and “Enceladus” refers to the Greek mythological giant who was buried under a mountain — much like how GSE was embedded in the Milky Way. When this collision occurred, the dwarf galaxy was torn apart and its constituent stars turned into a dispersed stellar halo for our Milky Way. Not only that, the tilt of the stellar halo signifies that GSE came face to face with the Milky Way at a bent angle and not straight. This suggests that the underlying dark matter is also essentially tilted, thus retaining the “football shape” of the stellar halo as well.