Most Nebulae are very large and can be hundreds of light years long. Though they seem so, they are not very dense. A cloud the size of Earth would only weigh a few kilograms. There are four types of nebulae; planetary, reflection, absorption, and emission. Reflection nebulae are nebulae that reflect nearby stars, while emission nebulae emit their own light in wavelengths. Absorption nebulae are nebulae so dense that they obscure light from objects behind it such as stars and other nebulae. 

The Helix Nebula is the first nebula discovered that contains knots. Cometary knots, or globules, are a feature of evolution found in planetary nebulas, and are strands of molecular hydrogen. Not much about their origin is known, but they are about the size of our solar system, and the Helix Nebula contains about 40,000 of them. When observed with binoculars, the nebula is seen as a hazy, greenish cloud. The ring shape can only be seen with larger telescopes, and the radial streaks can only be seen using large ground-based telescopes.