russ_watters said:Welcome to PF!
Some nebulae are simply too diffuse for the gravitation forces to be high enough to pull them together...and ones that are dense enough eventually collapse into stars.
Nebulae are located in clouds because they are formed from the gas and dust found in these clouds. As these clouds collapse and condense, the gravitational force causes the gas and dust to clump together, forming a nebula.
Nebulae are typically found in molecular clouds, which are large, dense clouds composed of molecular hydrogen and other molecules. These clouds can span hundreds of light years across and can contain enough material to form thousands of stars.
Nebulae get their different colors from the types of gases and elements present within them. For example, a nebula with a lot of hydrogen gas will appear red, while a nebula with a high concentration of oxygen will appear blue.
Some nebulae are visible with the naked eye, depending on their size and brightness. For example, the Orion Nebula is easily visible in the night sky and is a popular target for amateur astronomers.
We are able to detect nebulae through various telescopes and instruments that are designed to capture different wavelengths of light. For example, infrared telescopes can detect the heat emitted by nebulae, allowing us to see them even if they are not visible to the naked eye.