"White dwarf" is short for "white-dwarf star", so that "dwarf" is really thought of in its adjective form — it is a "substantive adjective". Substantive adjectives are pluralized as though they were nouns. Note that "Dwarves" is the plural form for characters from fantasy books, like Tolkien's The Hobbit. "Dwarfs" is the plural for real people affected by dwarfism. Perhaps the use of "white dwarfs" suggests their existence in the real world, rather than fantasy.