Why is there a hole in the heart of this nebula?

[ahlam1399]

For years, researchers have wondered why there is a hole in the heart of the Rosette Nebula, 5,000 light-years from Earth in the Monoceros (unicorn) constellation of our Milky Way galaxy.

أكثر... (https://www.cnn.com/2018/02/14/world/rosette-nebula-heart/index.html)