A nebula, which means “cloud” or “fog” in Latin and can refer to ionized, neutral, or molecular hydrogen as well as cosmic dust, is a distinctively luminous component of the interstellar medium. Nebulae are frequently areas where stars are being formed, as seen in the Eagle Nebula’s “Pillars of Creation.” The gas, dust, and other material formations in these areas “clump” together to form denser regions, which draw in more matter and finally reach a density where stars can form. Planets and other planetary system objects are thought to be formed from the leftover material at that point.
Most nebulae are enormous in size; some have diameters of hundreds of light-years. When close by, a nebula that is visible to the human eye from Earth would appear bigger but not any brighter. [7] Although the Orion Nebula, the brightest nebula in the sky, can be seen with the unaided eye and has an extent twice the angular diameter of the full Moon, it was overlooked by early astronomers. [8] Most nebulae are much less dense than any vacuum formed on Earth, despite the fact that they are denser than the space around them; an Earth-sized nebular cloud would have a total mass of only a few kilograms.The densest nebulae can have densities of 10,000 molecules per cubic centimeter, compared to the air humans breathe, which has a density of about 10 to the power 19 molecules per cubic centimeter. While some nebulae are so diffused that they may only be seen with long exposures and specialized filters, others are visible owing to fluorescence created by embedded blazing stars. Some nebulae have fluctuating T Tauri star illumination.
Initially, any diffused astronomical object, including galaxies outside the Milky Way, was referred to as a “nebula.” Before the real nature of galaxies was established in the early 20th century by Vesto Slipher, Edwin Hubble, and others, spiral galaxies in general were referred to as “spiral nebulae” and the Andromeda Galaxy as the Andromeda Nebula. The majority of nebulae are connected to stars and lighted by starlight, as Edwin Hubble discovered. He also assisted in classifying nebulae according to the kind of light spectrum they emitted.
