By definition, the term "character" does not apply to non-fiction works. Most dictionaries define "character" (in the sense of literature) as a human in a novel, play, or movie. Animals can also be characters, as in George Orwell's classic novel Animal Farm and the film Babe. Talented artists can even turn inanimate objects into characters (e.g., The House of Seven Gables, by Nathaniel Hawthorne).

So what should we call the light, black holes, and gamma rays featured in your book? They are nouns, and in a sense, they are subjects, or topics, of the book, especially if your book devotes a separate chapter to each. In the natural world, they are called elements, or maybe even phenomena (observed facts that are mysterious or remarkable).

Character in literature is defined as persons, animal, things, or natural forces presented as persons in short stories, novel, plays, or narrative poems.  Since character is thus limited to fiction in this definition, it may not be a good idea to use this word in your book report.  Perhaps you could use something like contributing element or meaningful/explanatory element.  Other words such as concept/idea may also serve your purposes. In such as book as that which you mention are there also not theories? At any rate, it does seem advisable to avoid the use of the word character which finds itself applicable only to literature.