A new study has revealed the true shape of the diffuse cloud of stars surrounding the disk of our galaxy.

Jiwon “Jesse” Han, a PhD student at the Center for Astrophysics | Harvard & Smithsonian explained:

“The shape of the stellar halo is a very fundamental parameter that we’ve just measured to greater accuracy than was possible before. There are a lot of important implications of the stellar halo not being spherical but instead shaped like a football, rugby ball, or zeppelin — take your pick!” 

For decades, astronomers have thought that this cloud of stars — called the stellar halo — was largely spherical, like a beach ball.

The Milky Way’s stellar halo is the visible portion of what is more broadly called the galactic halo. This galactic halo is dominated by invisible dark matter, whose presence is only measurable through the gravity that it exerts. Every galaxy has its own halo of dark matter. These halos serve as a sort of scaffold upon which ordinary, visible matter hangs. In turn, that visible matter forms stars and other observable galactic structure. To better understand how galaxies form and interact, as well as the underlying nature of dark matter, stellar haloes are accordingly valuable astrophysical targets.

 Charlie Conroy, professor of astronomy at Harvard University and the Center for Astrophysics said:

“For decades, the general assumption has been that the stellar halo is more or less spherical and isotropic, or the same in every direction. We now know that the textbook picture of our galaxy embedded within a spherical volume of stars has to be thrown out.”

The new study by CfA researchers and colleagues uses two major datasets gathered in recent years that have plumbed the stellar halo as never before. 

“The tilt and distribution of stars in the stellar halo provide dramatic confirmation that our galaxy collided with another smaller galaxy 7-10 billion years ago,” says Conroy. 

“The tilted stellar halo strongly suggests that the underlying dark matter halo is also tilted. A tilt in the dark matter halo could have significant ramifications for our ability to detect dark matter particles in laboratories on Earth.”

Click on this link to access The Stellar Halo of the Galaxy is Tilted and Doubly Broken published in The Astronomical Journal