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I was reading about the coolest stars, and was surprised to find about stars with a surface temperature lower than a candle, like 2MASS 0939-2448 A/B, of about 100 Celsius like CFBDSIR 1458+10B and even below 0 Celsius temperature where the water freezes like WISE J0855-0714. Then I thought, they might be stars where fusion only happens very deep in the core, and then I found out this:

Brown dwarf

In addition, many brown dwarfs undergo no fusion; those at the low end of the mass range (under 13 MJ) are never hot enough to fuse even deuterium, and even those at the high end of the mass range (over 60 MJ) cool quickly enough that after 10 million years they no longer undergo fusion.

Are these brown dwarfs which don't sustain any type of fusion considered stars? If so, what's the reason?

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    $\begingroup$ No, they're massive planets. Although there is some discussion in the scientific community at the moment where to draw the line between star and planet exactly, something that doesn't do fusion is no star for sure. $\endgroup$
    – AtmosphericPrisonEscape
    Feb 9, 2019 at 18:15
  • $\begingroup$ It's a bit of a grey area. There is evidence from young star clusters that the stellar formation mechanism extends down to a few Jupiter masses, but observationally distinguishing such objects from (ejected) planets is likely impossible. At least with the term "brown dwarf" you could interpret this as being short for "brown dwarf star" or not as you preferred, unfortunately this is not possible with "planet". $\endgroup$
    – user24157
    Feb 9, 2019 at 19:23
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    $\begingroup$ The book I am reading treats them as stars and made a distinction to planet based on mechanism and dynamics of formation. So as for the above comments it should be a grey area indeed. I could add a comment as soon I recover the book, I was right at cloud collapsing yesterday. Very roughly: collapse of matter => stars. Debris and dust collection => planets. $\endgroup$
    – Alchimista
    Feb 10, 2019 at 9:23
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    $\begingroup$ You are confused. Just because they have a catalogue number, doesn't make them stars. They are brown dwarfs. $\endgroup$
    – ProfRob
    Feb 10, 2019 at 21:47

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Brown dwarfs are technically stars, but not in your typical sense, as they cannot start or sustain nuclear fusion. Think about it this way: stars are formed in clumps inside giant gas clouds called nebulae, along with planets, but what if one of these clumps is too big to become a planet and too small to become red dwarfs. These are brown dwarfs. Some consider them to be stars, others consider them to be planets, and some neither. So, opinions vary.

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  • $\begingroup$ No opinions, don't vary. Brown dwarfs and stars are mutually exclusive. Whilst there is a grey area between planets and brown dwarfs there is no such grey area between stars and brown dwarfs. $\endgroup$
    – ProfRob
    Jan 13 at 16:11

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