Wow, what a treasure trove of southern delights!! Where do I start talking about them? Perhaps with the Jewel Box cluster
(picture found here
Naah. The Jewel Box
is, in comparison with all the other goodies here, a fairly run-of-the-mill circa 15 million-year-old young cluster, nothing special as such clusters go.
Evolutionary path of a solar-mass star.
But let's start with Gamma Crucis, Gacrux! Gacrux is the red star at the top of the constellation Crux, here seen
in a picture where it looks perhaps brighter in relation to the other stars of Crux than it really is. The fascinating thing about Gacrux is that it is an M-class giant, not one of the dime-a-dozen K-class giants like Pollux or Dubhe, and not one of the brilliant M-class supergiants, like Betelgeuse or Antares.
Look at the illustration at left of the evolutionary path of a solar-mass star. After our Sun has exhausted its core hydrogen, it is will leave the main sequence and expand prodigiously as its center contracts and heats up, while its outer atmosphere balloons enormously. At this stage, the Sun will ascend the red giant branch all the way to the top of it. I believe that this stage is relatively
quick and quite dramatic.
But when the Sun has reached the top of the red giant branch, its core has grown hot enough that it can start fusing its core helium into carbon and oxygen. When this happens, the Sun's outer layers shrink, and the Sun will descend from its beastly red glory into the red clump stage of its evolutionary path. The red clump is where you will find all these boring K-type giants. Look at this picture of the Beehive Cluster
, M44, by Bob Franke. See those weakly colored pale, pale orange stars? They are the modest red clump K-type core helium burning stars. Yawn.
When the Sun has reached its red clump stage and started fusing helium to oxygen and carbon in its core, it is going to stay in the red clump region for a relatively long time. The fact that the helium fusion can be produced by modest stars like the Sun, and that it can keep going for a relatively long time, is the reason why we see so many K-class pale orange giants everywhere in the sky. Because the red clump stars are K-class stars, while (perhaps all) the red giant branch stars and all the AGB stars are M-class ones.
Right. But when the Sun has exhausted its core helium, it will begin its second ascent, on the Asymptotic Giant Branch, which will take it to even brighter and redder and more monstrously swollen heights. And after reaching the top of the AGB branch, the Sun will get the Mira hiccups
. And then, planetary nebula and white dwarf death is near
for the Sun.
wrote about Gacrux:
Gacrux's evolutionary -- ageing -- status is uncertain. With a mass perhaps three times solar or less, it may well have given up not just hydrogen fusion in its core (which it must do to become a giant in the first place), but may also have gone through its core helium-fusion stage. If that is the case, Gacrux is in the process of becoming a "second-ascent" giant, one brightening into the ethereal realm of the giant stars for the second time, a speculation reinforced by its variability. Perhaps one day it will appear to us as a full-blown pulsating variable like Mira in Cetus.
My point is that present-day Gacrux could be what the Sun will be in the future, when it becomes as red and bright and large as it will ever be.