To provide more information or to identify a word, a noun or pronoun is placed with another noun or pronoun, referred to as an appositive phrase.
An appositive phrase may be nonessential or essential.
In essential appositive, the words are not set off with commas.
Whereas, nonessential appositive refers to the addition of meaning in a sentence which is already clear and is set off with commas.
Hence, in the above sentence, the nonessential appositive phrase is 'a dense dead star'. Herein, 'dense dead' are modifiers which modify 'star' and provides detail about it.