A nebula isn't a stage in the life cycle of a star, but rather a cloud of gas and dust that can become a star.

Here's how it works:

* Nebulae are the birthplaces of stars. They are vast, swirling clouds of mostly hydrogen and helium, with traces of other elements.

* Gravity pulls the gas and dust together. This clumping creates denser areas within the nebula.

* As the material clumps, it heats up. This eventually triggers nuclear fusion, which is the process that powers stars.

* A star is born! The star then continues to evolve through various stages.

So, to answer your question directly, a nebula is not a stage in a star's life cycle, but the precursor to it.