Globular clusters are roughly spherical collections of extremely old stars, and around 150 of them are scattered around our galaxy. Hubble is one of the best telescopes for studying these, as its extremely high resolution lets astronomers see individual stars, even in the crowded core. The clusters all look very similar, and in Hubble’s images it can be quite hard to tell them apart – and they all look much like NGC 411, pictured here.
And yet appearances can be deceptive: NGC 411 is in fact not a globular cluster, and its stars are not old. It isn’t even in the Milky Way.
NGC 411 is classified as an open cluster. Less tightly bound than a globular cluster, the stars in open clusters tend to drift apart over time as they age, whereas globulars have survived for well over 10 billion years of galactic history. NGC 411 is a relative youngster — not much more than a tenth of this age. Far from being a relic of the early years of the Universe, the stars in NGC 411 are in fact a fraction of the age of the Sun.
The stars in NGC 411 are all roughly the same age, having formed in one go from one cloud of gas. But they are not all the same size. Hubble’s image shows a wide range of colours and brightnesses in the cluster’s stars. These tell astronomers many facts about the stars, including their mass, temperature and evolutionary phase. Blue stars, for instance, have higher surface temperatures than red ones.
The image is a composite produced from ultraviolet, visible and infrared observations made by Hubble’s Wide Field Camera 3. This filter set lets the telescope “see” colours slightly further beyond red and the violet ends of the spectrum.
Interesting. I don't have my software here as I am writing this, but my software isn't that good for checking up clusters anyway. I note that an unusual set of filters were used to produce this image: 336 nm (ultraviolet), mapped as blue, 475 nm (blue), mapped as green, and the old favorite 814 nm (infrared), mapped as red. It is quite unusual for HEIC to put so much effort into detecting blue and ultraviolet objects.
Anyway, it is clear from the visual appearance of the cluster that NGC 411 could not possibly be a globular cluster, at least not a typical very metal-poor globular. Such a globular cluster would have a rich population of blue horizontal branch stars, which are fainter than the red giants but clearly brighter than the main sequence stars. Here, there are a few bright blue stars that almost certainly don't belong to the cluster. There are, in fact, a few semi-bright blue stars too, but they are scattered far from the most star-rich part of the cluster in a most un-globular-cluster-like way. In the center there is a large population of faint mildly bluish main sequence stars and quite a lot of giant stars, which are all more or less orange in color.
Here is a picture of The Small Magellanic Cloud, courtesy of Stephane Guisard. Two foreground globulars can be seen. The large bright one is 47 Tuc. The smaller one is NGC 362.
NGC 411 is located in the part of the Small Magellanic Cloud which is seen between globular cluster NGC 362 and the line of pink nebulae "straight above" the globular.
