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In space, a nebula is a massive cloud of dust and gas. The word was originally applied to any diffused astronomical object, such as galaxies beyond the Milky Way. Some nebulae (multiple nebulae) are formed by the gas and dust ejected by a dead star's explosion, such as a supernova. Other nebulae are places in which new stars are forming.
In space, a nebula is a massive cloud of dust and gas. Some nebulae (multiple nebulae) are formed by the gas and dust ejected by a dead star's explosion, such as a supernova. Other nebulae are places in which new stars are forming. Some nebulae are dubbed "star nurseries" because of this. Dust and gases, chiefly hydrogen and helium, make up nebulae.
The dust and gases in a nebula are very dispersed, yet gravity can gradually bring clumps of dust and gas together. The gravitational pull of these lumps grows greater as they become larger. The cluster of dust and gas eventually becomes so large that it collapses under its own weight. The material at the cloud's core heats up as it collapses, and this hot core is the start of a star.
The area between the stars, commonly known as interstellar space, is home to nebulae. The Helix Nebula is the nearest known nebula to Earth. It's the shattered remains of a dying star, possibly the Sun. It's about 700 light-years away from the Earth. That means that even if you could travel faster than light, it would take 700 years to get there!
Photographs of faraway nebulae are taken with extremely powerful telescopes. Many photos of distant nebulae have been acquired by space telescopes such as NASA's Spitzer Space Telescope and Hubble Space Telescope. The various forms of nebulae have a range of formation mechanisms. Some nebulae are created by stars, while others are formed by gas already present in the interstellar medium.
Giant molecular clouds, the coldest and densest phase of interstellar gas, can arise as a result of the cooling and condensation of more diffuse gas. Planetary nebulae, for example, are produced from material shed by a star in the last phases of its stellar evolution. Emission nebulae linked with massive molecular clouds are known as star-forming areas.
These are formed when a molecular cloud collapses under its own weight, resulting in the formation of stars. Massive stars may emerge in the core, ionising the surrounding gas and making it visible at optical wavelengths. An H II region is an area of ionised hydrogen that surrounds massive stars, while photodissociation regions are the shells of neutral hydrogen that surround the H II region.
The Orion Nebula, the Rosette Nebula, and the Omega Nebula are examples of star-forming areas. Star formation feedback in the form of enormous star supernova explosions, stellar winds or ultraviolet radiation from large stars, or outflows from low-mass stars may destabilise the cloud, eventually destroying the nebula after several million years.
