Hello @peter! I'm glad you're curious about the marvels of the cosmos, such as nebulae. Let me elucidate this astronomical phenomenon for you.

A nebula is an enormous cloud of dust and gas suspended in interstellar space, and it is one of the most visually captivating structures in the universe. The term 'nebula' is Latin for 'cloud', and that's what nebulae were thought to be before the development of modern telescopes. Now we know them to be vast and intricate, and often the birthplaces of stars and planetary systems.

There are several types of nebulae, classified mainly based on how they are formed and the nature of their illumination. Emission nebulae, for example, are vibrant and colorful because they emit light from ionized gases. When high-energy ultraviolet light from nearby stars excites the atoms within the gas cloud, these atoms emit light of various colors depending on the gas; hydrogen gives off red light, whereas oxygen emits a bluish hue. The Orion Nebula is a famous emission nebula and a stellar nursery where new stars are being born.

In contrast, reflection nebulae do not emit light on their own but reflect the light of nearby stars, hence their name. They often appear blue because blue light is scattered more efficiently than red by the dust within the nebula. Reflection nebulae are usually found in close proximity to bright stars, like in the Pleiades cluster.

Dark nebulae are dense regions of gas and dust that appear as silhouetted blobs against the brighter background of emission or reflection nebulae. These nebulae can block out the light from whatever lies behind them, making them visible as dark patches in the sky. The Horsehead Nebula is an iconic example, known for its distinctive shape that resembles a horse's head when viewed from Earth.

Planetary nebulae are somewhat misleadingly named; they have nothing to do with planets. They form when medium-sized stars, like our Sun, near the end of their life cycles. The stars expel their outer layers, which drift away and glow brightly from the ultraviolet radiation emitted by the remaining hot core. The Ring Nebula, with its beautiful circular shape, is an example of this type.

Lastly, supernova remnants are the aftermath of a supernova explosion, where a massive star has ended its life in a cataclysmic blast, and its outer layers are thrown into space. These remnants spread heavy elements throughout the cosmos, seeding future generations of stars and planets. The Crab Nebula, for instance, is a supernova remnant that is expanding and illuminating the darkness of space with its complex structure.

In sum, Peter, the variety of nebulae in the universe is as breathtaking as it's fundamental to our understanding of cosmic evolution. From the birth of stars to the final whisper of a dying one, nebulae are the celestial canvases where the artistry of the universe is on grand display, continually reminding us of the vast and wonderful cosmos in which we find ourselves.