Some additional notes on your stellar comments, Aramis:
Several class M9 and L0-2 dwarfs are thought to be large enough to sustain, but not enough to initiate, fusion.
EG: Gliese 569 - 116 Jupiter masses - but no evidence of fusion.
Metallicity (the ratio of hydrogen and helium to everything else in a star) is the main factor in determining that. At Solar metallicities, the cutoff point for proton-proton fusion is about 80 Jupiter Masses, or anything dimmer than an M6.5V red dwarf. Stars with much higher metallicities will ignite at lower masses (perhaps as low as 68-70MJup), while low metallicity brown dwarfs will stay dormant at even up to 92MJup (as is the case with brown dwarf SDSS J010448.46+153501.8).
As metallicity generally tracks with age (the lower the older), overly-large brown dwarfs are almost certainly always cold, quiet, weird old objects, and very hard to spot if you don't know where to look. Probably a good place to stick a Zhodani forward base, 'lost' penal colony, dark money research project, or just plain old Ravenous Horror Rising from the Depths of Time Itself. I've been toying with the idea of putting one somewhere in the Lanth Rift.
For a while, the size VI Subdwarf was for over-hot under-mass main sequence stars.
There are two main categories of subdwarf, the overwhelming majority of which are the low-metallicity 'cool' subdwarfs. Lower metallicity stars burn at hotter temperatures for their mass (and smaller radius too, making them less luminous overall). The highest metallicity any cool subdwarf star will be is about 1/100th that of Sol, with some 'ultracool' subdwarfs observed to be as low as 1/100,000.
Cool subdwarfs are nearly always extremely ancient stars. In the Traveller sense, this would mean that any naturally-occurring worlds with oxygen atmospheres (Atm 2-9) orbiting a subdwarf would almost certainly feature some of the oldest biospheres in Charted Space, not a few of which would have been around since well before the Solar System was even an accretion disk. Bring sunscreen along if you choose to visit, though; subdwarfs overproduce in the ultraviolet range, so their worlds will be colder but extra cancerous, by human standards.
There are also things called blue subdwarfs, stars who operate at type O or B temperatures despite having masses about half that of Sol. They are believed to be the result of what happens if a red giant is stripped of its outer shell right before it begins fusing helium, leaving the blue core exposed. I imagine Imperial scientists would find any such star near Charted Space pretty interesting, if only for the fact that they give us a window on to what red dwarf stars are supposed to evolve into eventually, given a few hundred billion more years. They are probably also the only blue stars with any chance of hosting terrestrial planets.
Size IV are larger stars - technically not main sequence - but aside from being larger, having shorter lives, being brighter, and ending in a bang... are very similar to Size V. They're expected to end in degenerate cores - neutron stars - rather than white dwarves.
Not quite. That's
kind of right, in that a few high mass subgiants are still defined that way out of habit, but the vast majority of subgiants are what is known as evolutionary subgiants, a stage between the main sequence (V) and red giant (III) phases. The Sun will become a subgiant in about 4.5-5 billion years, and will stay there for about a billion or two years before evolving into a red giant, and finally a white dwarf.
The nearest evolutionary subgiant to Terra is Procyon, I think. I count five in the Spinward Marches (Lanth, Tizon, Narsil, Gungnir and Jinx) according to the Wiki, four of which feature more-or-less breathable atmospheres. I would guess the middle three (all but Lanth and Jinx) are all fairly ancient ecosystems too, unless they were terraformed.
Sizes O, I, II, III (and subtypes thereof) are bigger. They are expected to end in Neutron Stars and/or black holes.
Type III red giant stars are not necessarily bigger than Sol, at least by mass. Most of them currently are, because very few stars of Solar mass are old enough to have reached post-Main Sequence yet (and virtually
no stars smaller than Sol have); but there should be a few Type III's out there who evolved from early-universe G or F-type stars, and they should be at or significantly below Solar mass.
All type III stars will degenerate into basic white dwarfs; they simply do not have the gravitational oomph to go full-on black hole or neutron star. And for that we should all be very, very grateful.
