Small red giants

[Orange Sheets]

Found couple, both having solar radius around 0,45. Relatively small to have an "outer athmosphere [that] is inflated and tenuous".

Spoiler

Shouldn't these bad boys be a tad bit bigger?

 

[Silent_Star]

A whole lot bigger, but it's the same with the mass of white dwarfs, neutron stars and black holes.

 

[Ziljan]

Well it could be a star with a tiny mass of say 0.2 solar masses, and maybe it just recently ran out of hydrogen in its core and is now just beginning the process of expanding. It would have to be very old star since smaller stars burn hydrogen much more slowly, and at nearly 10 billion years, it kind of is a Methuselah of first gen stars, but it takes about 400 billion years to run out of hydrogen in its core.

So yeah, the size of this tiny old red "giant" is plausible. But it might take several hundred billion years to actually start turning into a red giant.

 

[Orange Sheets]

Well it could be a star with a tiny mass of say 0.2 solar masses, and maybe it just recently ran out of hydrogen in its core and is now just beginning the process of expanding. It would have to be very old star since smaller stars burn hydrogen much more slowly, and at nearly 10 billion years, it kind of is a Methuselah of first gen stars, but it takes about 400 billion years to run out of hydrogen in its core.

So yeah, the size of this tiny old red "giant" is plausible. But it might take several hundred billion years to actually start turning into a red giant.

That's the explanation that I gave myself when saw these stars These kinds of borderlines/in process objects/anomalies make the galaxy so much more living. "What's the story of this star? Is it expanding, or is it cooling already?" I find myself visiting wikipedia and astronomy sites quite regularly when exploring and encountering "new" data.