Is the sun the Lake of Fire?
The Lake of Fire cannot be the sun, because the Lake of Fire will exist after the sun and the rest of the Universe has been destroyed:
Notice the heavens and earth (meaning all the universe) are gone in v.11, yet the Lake of Fire still exists following their departure (in vs.14-15). For more information on the Lake of Fire, please listen to our Revelation Bible study online. You may also find the following article helpful: Where is the physical location of the Lake of Fire?