Telescope Reviews: Why aren't globulars red?

OK, look at it this way:
Your eye has low sensitivity to red frequencies, and high sensitivity to blue-green frequencies.
Though a reddish star emits most of its energy in the orange-red-infrared part of the spectrum, it still emits light in the green-yellow end of the spectrum.
When you look at such a star through a telescope, that star will appear yellowish because that's where your eye is most sensitive.
Add to that the fact that our night vision shifts the greatest sensitivity of the eye toward the blue (from 550nm to about 500nm), so we lose even more red sensitivity at night (which is why we can use red LED flashlights without significantly damaging our night vision).
Then, put together a large number of stars, each of which has a fair amount of output in the green-yellow, and, at best, we will see such a group of stars that is somewhat yellowish (as in pictures of globular clusters or the stars in the cores of galaxies).
But, the eye will combine all the light frequencies into a perception of white light, though missing some of its blue output.
Even through a telescope, Antares appears redder than most stars.
But, in the scheme of stellar evolution, a star spends relatively little time in its red giant phase. It spends correspondingly more time as a main sequence star or as a white dwarf after the red giant phase is over. So, the odds are that in any large group of stars of nearly the same age, you have a low likelihood of catching a significant number of them in the red giant phase. That corner of the HR diagram actually has more stars in it, in a typical globular cluster, than in general in space, yet it is still not an appreciable portion of the stars in the cluster at any one given time.

Then, add to all that the idea that globular clusters formed from gas clouds that were essentially primordial, so that the stars formed entirely of hydrogen (75%) and helium (25%) and no heavier elements. Such stars would have cores that would take longer to arrive at the heavier elements that lead to the burning of heavier elements (such as the carbon cycle in the core of our sun), so would have extended lifetimes over equivalently massive stars that formed in a second generation gas cloud containing a lot of heavier elements.

And, billions of years down the road, the most massive stars would all be gone, leaving the stars of lesser mass to shine on.

So when we look at a globular cluster, we see mostly the less massive stars that have not yet entered the red giant phase, a smattering of stars in the red giant phase, and a small amount of light from the dwarf stars we know are there, but which are also very blue. The massive blue stars, or blue stragglers as they are called, likely formed from collisions and so number only a small percentage of all stars seen.

I hope that all that I've said helps explain why we see a globular cluster as white, with perhaps a little yellowish bias.

Imaging, as you can see from pictures, catches a lot more of the red giants, but even in the images they are not a large percentage of the stars seen. Nor will they ever be.

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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member