The nature of white dwarfs is covered in some detail in order to answer the title question.
This month we also have a blog post related to the topic: Can merging white dwarfs form variable stars?
The monthly focus tag is an opportunity for us to learn more about a topic and improve the site by taking a look at a particular area. Please take this opportunity to look at the questions and answers in white-dwarf. If you see anything that could be improved, take the opportunity to edit. If you have follow-up questions, go ahead and ask them (with the appropriate tag, of course). If you happen to know the answer to an unanswered question, we'd appreciate your input!
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Feel free to use the answer space below to make community wiki lists of posts in need of editing, or to brainstorm ideas of what to add to the tag excerpt and wiki. Not sure about a particular white-dwarf question you have? Go ahead and share your concerns below, so we can help guide you to framing the question on the site.