He was immersed in his work all day.

He was working all day.

An open cluster is a cluster of stars that consists of dozens to thousands of stars.

Astronomers have discovered a new globular cluster.

It is important to understand the importance of health.

I often go birdwatching in the park on weekends.

He committed another crime while on probation.

He has a deep interest in astronomy.

He has astronomical knowledge.

Please use the emergency stairs in case of a fire.