Thanks for the explanation.

Love the website and Astronomy Cast.

**Brent A. Archinal and Steven W. Hynes, 2003. Star Clusters. Willmann-Bell, Inc., Richmond [ADS: 2003stcl.book.....A]

Class G stars are probably the best known, if only for the reason that our Sun is of this class. Most notable are the H and K lines of Ca II, which are most prominent at G2. They have even weaker hydrogen lines than F, but along with the ionized metals, they have neutral metals. There is a prominent spike in the G band of CH molecules. G is host to the "Yellow Evolutionary Void". Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the G classification as this is an extremely unstable place for a supergiant to be. G stars represent about 1 in 13 of the main sequence stars in the solar neighborhood.

"We have checked the reality of the yellow evolutionary void (which is an area in the Hertzsprung-Russell diagram where atmospheres of blueward evolving super- and hypergiants are moderately unstable), by comparing one star inside the void: HD 33579 (= R76), and two at the low-temperature border of it: HR 8752 (= HD 217476, V509 Cas) and IRC+10420. We found that the first star has a large mass and a fairly stable behaviour over time. These aspects suggest, together with abundance determinations by others, that it is a fairly young, still redward-evolving supergiant. For such a star the void is not forbidden. The two other stars, HR 8752 resp. IRC+10420, have low masses which places them in the post-red blueward loop. They show indications of the expected bouncing effect for blueward returning red supergiants: when approaching the void they eject mass, resulting in a sudden reduction of T_eff and a decrease of the atmospheric acceleration to g_eff =~ zero. Thereafter T_eff and g_eff increase again. For HR 8752 two such recent `bounces' have been identified. The photometric variations of HR 8752 and of HD 33579 are due to high-l gravity-wave pulsations. Based on observations at the La Palma Observatory and the ESO Observatory in Chili."

**Nieuwenhuijzen, H.; de Jager, C.

To make a long story short, a G-type giant is an oddity that's slipped through the stellar evolutionary cracks – well worth observing M25 just for that reason!

"contains two G-type giants"

I thought type G stars (and all other main-sequence stars) were dwarfs. What's a G-type giant?