Main Sequence Rules: Definition & Interpretation

[Saturn_V]

When viewing an HR diagram, the main sequence curve is apparent, and the general shape of it is obvious. However, in this truncated version, it's unclear to me exactly which stars should be considered main sequence. I've added a shaded grey area as what I think I should count as main sequence, but there's a bit of room for interpretation. Are there hard-and-fast rules, or is it a matter of opinion? What do you think of this shaded area?

 

[Ken G]

There are two possible definitions of the main sequence, one is empirical and is the luminosity class V, but more commonly the definition is physical rather than empirical-- it is stars that are fusing hydrogen in their core. So if we take the latter meaning, the "subgiants" that are between the main sequence and the red giant branch have stopped core fusion and are instead fusing in small shells around an inert core, but the core is not yet tiny and degenerate. As the core shrinks further and goes degenerate, the shell gets really tiny and even higher in temperature, and the envelope will puff out and the star will move up and to the right into the giant branch. So I think you'd have to use interior models to decide where the main sequence ends and the subgiants begin, but your shaded area is probably the right ballpark.

Another key point to bear in mind is that many stars are actually double or triple stars, and this normally cannot be determined with certainty when the stars are plotted on the H-R diagram, unless you can separate the spectra based on their Doppler shifts for "spectroscopic" binaries (close binaries seen not from above the orbits). So you can get points that are actually a kind of average of different stellar types, creating confusion as to exactly what type of star you are looking at.

 

[Nik_2213]

Slightly OT: IIRC, a bunch of 'weird' Hipparchos distances have been attributed to orbital motion of unresolved binaries skewing the measured parallax. Some 'oops' seem due to mega-flares offsetting the nominal positions...

As mentioned above, spectroscopic binaries aligned such their orbital Doppler shifts are masked by surface activity are complex targets. Worse, contamination by mass-transfer or merger may skew determinations...

Must be said, watching such riddles and apparent paradoxes being patiently un-picked is excellent sport !