Observing dark nebulae is difficult.

This problem is very difficult.

Observing a nebula is a wonderful experience that makes you feel the mystery of the universe.

He was observing the behavior of birds.

He was observing the behavior of the birds.

His heart was full of darkness.

He was able to find the way in the darkness.

The Horsehead Nebula is one of the celestial bodies that symbolize the beauty of the universe.

I often go birdwatching in the park on weekends.

He committed another crime while on probation.