He continued this work on open clusters for the next twenty years.

Open clusters generally survive for a few hundred million years.

This open cluster is particularly ancient, estimated to be nearly one billion years old.

Only open clusters are near enough for this technique to be useful.

At the time of the discovery it was the only known planet in an open cluster.

The open cluster M50 is less spectacular, but easy to locate.

This makes it one of the oldest open clusters currently known.

The outskirts of the galaxy have many young open clusters.

It is considered to be one of the more inconspicuous open clusters.

The older open clusters tend to contain more yellow stars.

Because of their high density, close encounters between stars in an open cluster are common.

Under exceptional conditions, open clusters can remain intact for up to 100 million years.

Open clusters are key objects in the study of stellar evolution.

Many open clusters also play host to what have been called "blue stragglers."

It is common for two or more separate open clusters to form out of the same molecular cloud.

Some open clusters contain hot blue stars which seem to be much younger than the rest of the cluster.

The open cluster M26 is in the same field with Delta.

It lies close to the direction of the open cluster Messier 52.

This is one of the brightest of all open clusters.

The closest open clusters can have their distance measured directly by one of two methods.

In this case the corresponding question is: does an infinite open cluster exist?

This suggests that they were born in the same location, and perhaps initially formed an open cluster.

Given sufficient magnification, of course, all open clusters can be resolved into stars.

The smaller, denser open cluster Messier 67 can also be found here.

Open clusters are a crucial step in this sequence.