Open star clusters are groups of stars in loosely-bound gravitational associations. The stars are further apart than the stars in their cousins, the globular clusters. The weak gravity from the loose clusters means open clusters take on irregular shapes. They usually contain only a few thousand stars.
The Hubble Space Telescope captured images of two clusters in the Large Magellanic Cloud.
There are about 700 open clusters in the Large Magellanic Cloud (LMC,) one of the Milky Way’s satellite galaxies. NGC 1858 is a large and particularly bright open cluster that’s only about 10 million years old. You can see it with a backyard telescope or binoculars, but not from the northern hemisphere.
This Hubble image captures some of NGC 1858’s interesting traits. Though astronomers think that the stars in open clusters form from the same giant molecular cloud and have roughly the same age, NGC contains stars in different stages of stellar evolution.
The variety of different-aged stars makes NGC 1858 a complex collection. There’s a very young, emerging protostar, which means that star formation in the open cluster might be active or has only stopped very recently.
But NGC 1858 is also an emission nebula, as evidenced by the blue glow. That means that a particular type of star has recently formed here. Only hot stars that live a short time can create an emission nebula. They produce the powerful UV photons needed to ionize the gas between the stars. Once ionized, the gas releases its own photons at optical wavelengths. Helium is typically responsible for the blue in nebulae, though dust can play a role.
The open cluster contains a handful of massive stars, which stand out in the center of the images. But the cluster is also situated in a tightly-packed region. All of the stars around the nebula make it difficult to study.
Hubble also spotted the open cluster NGC 2002. It’s also in the LMC, about 160,000 light-years from Earth.
NGC 2002 is also pretty young, only about 18 million years old. It’s a little unusual because its shape is more spherical than other open clusters.
Open clusters are important observational targets for studying clusters in general. That’s because astronomers can resolve individual stars more easily than in globular clusters. While globular clusters can contain millions of stars, with many of them tightly packed into the dense center, open clusters are more loosely associated with each other.
NGC 2002 contains about 1100 stars, and as the cluster has evolved, the more massive stars have gravitated toward the center. The less massive ones congregate away from the center. The red stars in the center are five red supergiants, stars that have left the main sequence and are in the advanced stages of stellar evolution. Their presence helps date the cluster’s age.
Unlike NGC 1858, NGC 2002 is not an emission nebula. It doesn’t have the right types of stars that give off the powerful UV radiation needed to ionize the gas between the stars.
Both of these open clusters are in the Dorado constellation. They were both discovered by Scottish astronomer James Dunlop in 1826 at an observatory in Australia.