One day, when I was a little boy in the kitchen, I knew that Alpha Centauri A was a yellow dwarf. Then I speculated that Beta Centauri must be a blue dwarf (as far as I remember).
Turns out, I was correct, kinda...
Beta Centauri is a blue main-sequence star.
But it turns out, the word "blue dwarf" is not used for small blue main-sequence stars (which often are hard to distinguish from their giant and hypergiant counterparts, like the O-type star Alpha Camelopardalis, which, by the way, is the furthest star from Earth with a Bayer designation.) "Blue dwarfs" is used for a tentative proposed evolved stage of a red dwarf star.
Because red dwarf stars are fully convective, millions of tons of hydrogen fuel is delivered to their cores, meaning these stay on the main-sequence for trillions of years - far longer than the universe has been existing (13.8 billion years). This means literally none of them have left the main-sequence, so we can only speculate what happens if they do.
Because of their small mass, red dwarf stars may not undergo helium fusion and expand into giants, but rather, it is believed their temperatures may increase so they become "blue dwarfs".
However, stars today that do become red giants shrink into white dwarfs once fusion stops. In the process, they become Subdwarf B and O stars. Well-known examples include Kepler-70 (which may or may not have planets very close to it that are subsequently hotter than the Sun) and V391 Pegasi.
Now, I have a proposal to make: what if we call these Subdwarf B and O stars blue dwarfs instead? I know the terms "dwarf" and "main-sequence" are synonymous, but there is no point to reserve a term that is more likely to resonate with the public ("blue dwarf") for a purely hypothetical stage that we have to wait trillions of years just to observe. Indeed, the word "dwarf" implies a low luminosity and size - exactly what Subdwarf B and O stars are, especially relative to their main-sequence counterparts.
I have seen professional resources that dispute the proposed "blue dwarf" stage for red dwarfs, instead claiming that they will just collapse straight into white dwarfs. Furthermore, the term "blue dwarf" for the hypothesized evolved stage is a misnomer - these stars would mostly only reach temperatures rivalling that of the Sun, making them more like "yellow subdwarfs".
Can we please use the more easy-to-say name "blue dwarf" with a fitting class of star that exists today rather than for a tentative evolved stage that may or may not exist in the future?
Only time will tell if red dwarfs will "bluen up".
All Glory to God!
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