There isn't a perfect one-word antonym for "nebula." However, depending on the context, you could use:

* Star: If you're referring to the nebula's role as a birthplace for stars.

* Planet: If you're contrasting the diffuse nature of a nebula with the solidity of a planet.

* Void: If you're emphasizing the emptiness of space compared to the cloudiness of a nebula.

* Clear sky: If you're discussing the appearance of a nebula against a backdrop.

Ultimately, the best antonym depends on the specific meaning you want to convey.