The brightest stars of globular clusters are normally orange giants. The HR-diagram of an old cluster shows that all the massive stars have moved away from the main sequence to the giant stage (Many good examples are shown in Burnham's celestial handbook).
A few years ago I realised that I could see the orange colour of the brightest stars of some globular clusters. It started with the relatively coarse (and relatively close to the solar system) cluster, M4. I observed this object for the first time from La Palma, and immediately noticed an orange double standing out from the cluster population (I doubt that this is a binary star. Probably just two bright stars at nearly the same line of sight). Later, when observing M22, I was struck by some 20 or 30 bright stars of the cluster, all orange. Somehow these stars appeared to float in front of the cluster. This is an illusion, of course, but very beautiful, since the "background" star cloud appeared whitish. Last weekend I observed the same in M13, but not in M15 and M92.
Have anybody else noticed this? I use 12.5 inches of aperture, but I guess that a keen-sighted observer can do with less. And with big scopes I would expect colour to be even more easy to detect.
Inge S
A few years ago I realised that I could see the orange colour of the brightest stars of some globular clusters. It started with the relatively coarse (and relatively close to the solar system) cluster, M4. I observed this object for the first time from La Palma, and immediately noticed an orange double standing out from the cluster population (I doubt that this is a binary star. Probably just two bright stars at nearly the same line of sight). Later, when observing M22, I was struck by some 20 or 30 bright stars of the cluster, all orange. Somehow these stars appeared to float in front of the cluster. This is an illusion, of course, but very beautiful, since the "background" star cloud appeared whitish. Last weekend I observed the same in M13, but not in M15 and M92.
Have anybody else noticed this? I use 12.5 inches of aperture, but I guess that a keen-sighted observer can do with less. And with big scopes I would expect colour to be even more easy to detect.
Inge S
