Here's why:

* Irregular masses: Nebulae are vast clouds of gas and dust, often with no defined shape.

* Particles, gases, and plasmas: Nebulae are composed of hydrogen, helium, and trace amounts of heavier elements, often in ionized (plasma) states.

* Reflect light from nearby stars: Some nebulae, called reflection nebulae, are illuminated by nearby stars. The starlight scatters off the dust particles in the nebula, making it appear as a glowing cloud.

There are different types of nebulae, including:

* Emission nebulae: These glow because their gases are heated by nearby stars, causing them to emit light.

* Dark nebulae: These are dense clouds of dust that block light from behind them, appearing as dark patches in the sky.

* Planetary nebulae: These are formed when a star like our Sun reaches the end of its life and expels its outer layers into space.

Let me know if you'd like to know more about a specific type of nebula!