A group of stars, gas, and dust held together by gravity is called a nebula.

Here's a breakdown:

* Stars: The bright, luminous objects that emit light and heat due to nuclear fusion.

* Gas: Mostly hydrogen and helium, the most abundant elements in the universe.

* Dust: Tiny solid particles, including grains of rock, ice, and organic molecules.

* Gravity: The force that attracts all matter towards each other.

Nebulae are the birthplaces of stars, and they come in various shapes, sizes, and compositions. Some famous examples include the Orion Nebula, the Eagle Nebula, and the Crab Nebula.