On Black Dwarfs

You know, this is probably one of maybe 5 posts having anything to do with astronomy, and yet astronomy is listed prominently in my tagline. So consider this a concession to keeping astronomy in my tagline. Oh, and it is something I find pretty interesting, too.

I really enjoy working with teachers -- I teach a workshop for science teachers every summer, and they have been (with a few individual exceptions) a great group to work with. They are very engaged in the course, they ask great questions, share interesting insight from different points of view, and in general are just a fun crowd.

Last year, though, I think I blew the mind of one of the teachers -- one incident in particular really caused her distress. It happened during a lesson on stellar evolution. We teach that Sun-like stars become red giants, then expel planetary nebulae, then become white dwarfs.

I grew up reading kids' astronomy books, though, and I remember vividly seeing illustrations of the lifecycle of a star, and it always ended with the white dwarf becoming a "black dwarf". The idea was that a white dwarf that has cooled to the point where it is no longer visible becomes too dark to see, hence "black dwarf".

Now that I've learned a bit of stellar evolution and a bit of observational astronomy, I taught the teachers two things -- (1) in the lifetime of the universe, no white dwarf has cooled beyond say 4000 K or so, and (2) the dividing line between white dwarf and black dwarf has to be some arbitrarily assigned value for "no longer visible". To me, this means that there is no star currently in the universe that you would call a "black dwarf".

When I taught this last year, the 5th grade teacher was aghast, because she has her students act out a stellar evolution play in her class, and the kid that gets the black dwarf part gets to do a melodramatic death scene, which is the highlight of the whole play. She was *very* disappointed, even after I told her that all she needed to do was decide that in her classroom the dividing line between white dwarf and black dwarf was at a relatively high luminosity. Since the line is arbitrary, who says it can't be arbitrarily high?

I taught this again this year, and because of my experience last year, I asked the teachers -- how common is it in middle school textbooks for the book to give the endpoint of stellar evolution as the black dwarf stage? They almost universally agreed that their books used this terminology, so they taught it that way. I pointed out that I *never* teach this in Penn State courses, and I guessed that almost no one else did, either. We took an informal survey of the ~5 other faculty that they saw during the workshop, and none of them teach black dwarfs, either.

In the end, I think that it really doesn't matter, but it does seem interesting that the concept of black dwarfs is so widespread at the K-12 level and so uncommon at the university level and in the refereed literature (my unscientific study shows that if you search for the exact phrase "black dwarf" in the abstracts of refereed articles in ADS, you get a few articles from the 80s that seem to use the term the way K-12 books due in the '00s).

Maybe we should start working on the K-12 textbook publishers so that in the '20s, they'll be up to date.


Blogger Pat D. said...

Hmm...very interesting. I *think* I have seen mention of black dwarfs in some introductory astronomy texts, and I have an ever-so-brief statement on them in my notes. I think even one of the lecture tutorials I use has a reference to a black dwarf as the end of the life cycle of a star.
But yes, I certainly mention that no such thing exist yet. Odd that people get frustrated over things that don't exist... Wait a minute...
Did the teacher say anything about how melodramatic a black dwarf death spiral should be? Like...William Shatner melodramatic?

Dr. D

6:47 PM  

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