This add-on adds Hoag's Object, an unusual ring galaxy in Serpens Caput. It is a ring galaxy, and from Earth it appears nearly face-on. It is very symmetrical for a ring galaxy, unlike other galaxies like Arp 147.
The add-on is made of two parts. There is a ring component of Hoag's Object, which is defined using a CustomTemplate. The core component is defined using a special CMOD model, which is essentially just a bunch of nearly transparent "shells" nested inside each other.
Requires Mobile Celestia 1.5.23 or later for the addon to work "as is". Both galaxies and nebulae should be enabled.
How to install add-ons? Find out here.