A galaxy and its halo

This simulated image shows a galaxy and its enormous stellar halo. Surrounding the galaxy's central bright disc (blue and yellow) lies a huge spherical region filled lightly scattered with stars, depicted black on a white background.  

Importantly, the structure of a galaxy’s halo can reveal a lot about how the galaxy formed. For example, stellar streams such as the arc visible above the central disc in this image are the remnants of smaller galaxies or star clusters that were torn apart while merging with the host galaxy. Other small satellite galaxies appear as dark spots below and to the left of the central galaxy. 

ESA's Arrakihs mission plans to study the stellar haloes of at least 80 nearby galaxies with a similar mass to the Milky Way galaxy. This is a large enough number to get statistics on how a ‘typical’ galaxy forms, allowing us to understand how unique our home galaxy is. 

[Image description: In the centre of this image lies a spiral galaxy, glowing in blue and yellow. It is surrounded by a roughly round, hazy black region that stands out against the white background. This region is darker closer to the central galaxy, as well as along a plume pointing up and to the right. There are some concentrated dark spots scattered around the image, most notably below and to the left of the centre.]