Nah, seriously, they really are cool. Some of them are cooler than Venus, and they don’t fuse... not even deuterium. In fact, some red dwarves don’t come anywhere near the temperatures required for fusion (roughly 108°K standalone; with the help of a few other factors, maybe 3,000,000°K), which raises an obvious question.
Why do they glow?
“Gravitational collapse” can only get one a part of the way towards an explanation. In fact, Chandra detected a brown dwarf throwing a big Xray flare in 1999, which presents an even more stark dilemma. What’s a star (LP 944-20, ≅60 Jupiter masses) that can’t even fuse, that’s gravitationally collapsing (adiabatic compression) doing throwing flares?
Even more MiB: why haven’t we been told that our mainstream astrophysics has many such issues and is well past due for a revolution?