Paul Schlyter wrote:

If "star" is defined as a "natural body with thermonuclear reactions
as its energy source", then a white dwarf isn't a star, because
thermonuclear reactions have ceased in a white dwarf. They are
former stars...... now, does this make white dwarfs "planets"? g

Hello, and thank you for raising a possibly jovial question which reminds us
that the IAU has yet to address the more general question of defining a
"planet" in terms that can apply to objects outside our own solar system,
to which the scope of the current definition is restricted.

I understand that a simple answer to your question is that a "star" -- or
possibly a bit more broadly a "fusor" (including brown dwarfs) -- is any
natural body which at any point in its life history has sufficient mass to
initiate nuclear fusion. Obviously a white dwarf has met this test. A planet
could be defined as, among other things, a "non-fusor" at all points in its
life history. That would set an upper limit of about 13 Jupiter masses --
although 75 or so are required to produce a main sequence star. The
intermediate range is the realm of fascinating objects called "brown dwarfs"
-- fusors under this definition, and thus not planets.

This raises the taxonomic issue of whether a brown dwarf is a special kind of
star ("star = fusor") or a distinct category from either "star" or "planet."

Getting back to the main white dwarf question, we know that a white dwarf has
had a very different history than a planet which in a standard scenario grows
by accretion from planetesimals and at no point attains enough mass to become
a fusor. Thus at least to a certain degree, cosmogony and history do play a
role in these definitions.

Anyway, thank you for a question which gives me a chance to see how well I
understand some of these concepts.

Most respectfully,

Margo Schulter