47 Tucanae is a very interesting globular cluster, and I'm going to need many pictures to show it to you!
Let's start by comparing 47 Tuc, which is the Milky Way's second largest globular, with Omega Centauri, which is the largest globular of our galaxy:
Can you spot a difference? You can, can't you? 47 Tuc has an incredibly compact and concentrated center. Omega Centauri doesn't. It shows no central concentration at all.
Why the difference? Personally I don't find 47 Tuc's central concentration so surprising. It is a known fact that the most massive stars tend to sink to the center of a globular cluster, whereas the most lightweight stars run a not negligible risk of being ejected altogether. The most massive stars of 47 Tuc have all died, but their ghostly and still moderately massive remnants still inhabit the center of this magnificent globular in the form of pulsars and even a black hole.
It is actually stranger that Omega Centauri is so "fluffy" and "unconcentrated". But Omega Centauri is believed to be the center of a dwarf galaxy
that once collided with the Milky Way, leaving the center of the galaxy intact but shredding its outer regions. Note that small galaxies often have fluffy centers. Even the third largest galaxy in the Local Group, M33, may have such a loose center that it even lacks a central supermassive black hole
. The "naked" center of the dwarf galaxy that once collided with the Milky Way is now in orbit around around our galaxy as globular cluster Omega Centauri.
But 47 Tuc is strange in other ways. Let's compare it with the great northern globular cluster, M13.
Can you tell the difference between the colors of these two clusters? You can, can't you?
There is a large population of blue stars in M13. In 47 Tuc, such stars are completely absent.
The two images are not directly comparable, because different filters were used for them. For the 47 Tuc image, NASA used an ultraviolet filter but not a blue one, while for M13 NASA used a blue filter but not an ultraviolet one. This difference enhances the visible concentration of "moderately blue" stars (say, A- to F-type stars) in M13 and suppresses it in 47 Tuc. But really, there is very little to suppress in 47 Tuc, because this globular completely lacks the class of blue stars that is so eminently visible in M13: the blue horizontal branch stars.
Compare the color-magnitude diagram of 47 Tuc at left with the illustration of the evolution of a one solar mass star at right. First you must identify the main sequence, which is the diagonal track running from lower right to upper left. In the illustration at right, this track is extremely short. In the color-magnitude diagram at left, it is moderately long. Stars that are on the main sequence fuse hydrogen to helium in their cores.
Then the star reaches the turnoff point, when it has (or is about to) exhaust the hydrogen in its core. The turnoff point is located right across from the letter "V" in the color-magnitude diagram at left. In the illustration at right, there are only the words "Subgiant branch".
After the star has completely exhausted the hydrogen in its core, the star's core shrinks, which in itself releases a lot of energy. In response, the stars grows very much bigger and redder. It is now on the red giant branch.
When the core of the star has grown sufficiently hot, it initiates helium fusion. Helium is fused to carbon and oxygen. As this happens, the star shrinks considerably and its outer layers also become somewhat hotter. This is the red clump stage. You can find it in the illustration at right. This is a relatively stable evolutionary stage, and many (if not most) K-type stars in the sky belong to the red clump.
After the star has exhausted the helium in its core, its core shrinks again, the star rises again on the so called asymtotic giant branch, becomes larger and redder again, until it starts pulsating, sheds its outer layers and becomes a white dwarf.
Anyway. Here is my point. Can you see the red clump in the color-magnitude diagram of 47 Tuc? It is right across the number 14 on the Y-axis.
Now let's take a look at the color-magnitude diagram of a more "normal" globular cluster:
The main difference between the color magnitude diagrams of 47 Tuc and most other globulars is that most other globulars have a long, mostly blue horizontal branch reaching to the left, whereas 47 Tuc only has a small red clump of stars.
And that is exactly why we could see no blue stars in 47 Tuc.
The reason why 47 Tuc lacks a long blue horizontal branch is that this globular is too metal-rich. That is, its stars were made from a nebula that contained a too high concentration of elements heavier than hydrogen and helium. Most other globulars do contain blue horizontal stars, because their stars were made from more metal-poor gas.
The fact that 47 Tuc lacks a long blue horizontal branch also means that it lacks RR Lyrae
stars. These pulsating stars can be found in the middle of the horizontal branch, on the so called instability strip. The pulsations of RR Lyrae stars reflect the absolute magnitude of them, in a manner reminiscent of Cepheid stars, and they can be used to infer the distance to the globulars that contain RR Lyrae stars. But 47 Tuc hasn't got them, so its distance can't be constrained that way.