Hubble's Star Clusters
Billions of trillions of stars illuminate the galaxies of our universe. Each brilliant ball of hydrogen and helium is born within a cloud of gas and dust called a nebula. Deep within these clouds, knots can form, pulling in gas and dust until they become massive enough to collapse under their own gravitational attraction.
Open Clusters
Open clusters contain between a few dozen and a few thousand stars, all formed from the same initial cloud of gas and dust. The density of stars is low enough in these clusters that individual stars are visible with a telescope, or sometimes the unaided eye, giving them an “open” appearance. Most open clusters reside in the arms of spiral galaxies, and their stars are usually relatively young. Their shape is more irregular than spherical, with large amounts of gas between the stars. Over time, as these clusters rotate around a galaxy, gravitational disruptions from passing cosmic objects can cause the stars to disperse. The Milky Way is home to more than a thousand of these clusters, and even our Sun may have formed in an open cluster.
Globular Clusters
Globular clusters are much larger and denser than open clusters, containing several thousand to millions of stars all formed from a shared nebula. Unlike open clusters, the density of stars at their centers is so high that individual stars are hard to discern, even with powerful telescopes. Globular clusters lie on the dusty outskirts of galaxies and their stars are older than those in open clusters. In fact, globular clusters contain some of the oldest known stars in a galaxy. Because old stars tend to have a reddish glow, globular clusters generally appear redder than open clusters.
The large number of stars in a relatively small area causes the shape of a globular cluster to appear spherical, as stars’ intense gravitational attraction pulls them together. These gravitational ties grant globular clusters more stability than open clusters, helping them keep their structure instead of breaking up over time. The Milky Way alone has over 150 globular clusters, and our nearest neighboring galaxy Andromeda has over 400.
Embedded Clusters
Embedded clusters are a precursor to open and globular clusters. As the youngest type of star cluster, they contain newly born and forming stars surrounded by cosmic gas and dust. As with open and globular clusters, all of the stars formed from the same initial nebula. Embedded clusters are likely the basic unit of star formation since a significant fraction of all stars form within them. Once star formation ends, embedded clusters resemble open clusters, but are often disrupted by passing objects due to their weaker gravitational bonds. The embedded phase typically lasts between 2-7 million years. Since embedded clusters are heavily obscured by dust, they are rarely observed in visible wavelengths of light. However, Hubble’s infrared instruments can detect the longer wavelengths of infrared light that aren’t as easily scattered by clouds of gas and dust. Hubble’s unique capabilities are essential for learning more about these young clusters.
