subgiant branch
When a star has just used up the hydrogen in its core and begins burning hydrogen in a shell instead, it does not leap straight to becoming a huge red giant. It goes through an in-between stage, like a person stepping from one room to the next through a doorway. That doorway is the subgiant branch: the relatively brief phase when the star is growing larger and a touch redder, but has not yet swollen into a full giant. The name says it plainly — a subgiant is on its way to being a giant, but not there yet.
On the Hertzsprung-Russell diagram, the subgiant branch is the bridge that runs from the main-sequence turnoff rightward to the base of the red-giant branch. During this phase the star's overall brightness stays roughly steady while its surface cools and its radius slowly grows — physically, the helium core is contracting and heating while the hydrogen shell around it does the burning. For a star like the Sun this crossing takes a few hundred million years, short compared with its 10-billion-year main-sequence life, which is why relatively few stars are caught in the act.
Because the subgiant phase is brief, finding subgiants in a star cluster helps trace exactly how stars transition off the main sequence, testing our models of stellar interiors. Subgiants are also prized for measuring ages: their brightness and temperature change quickly and predictably, so a single subgiant's properties can pin down its age more sharply than a main-sequence star can. A subtle point: 'subgiant' describes a stage of life, not a size class fixed forever; the very same star will later be a red giant, and the term simply marks where it is on its journey.
The star Procyon, the eighth-brightest in the night sky, is a subgiant just slightly more massive than the Sun. It has finished core hydrogen burning and is beginning the slow swell across the subgiant branch — a preview of the Sun's own future a few billion years from now.
A subgiant is a star caught mid-transition, between the main sequence and the red giants.
The subgiant phase is short, so subgiants are rare in any snapshot of the sky — not because few stars pass through it, but because each star spends so little time there.