Nebula
Many nebulae are visible due to fluorescence caused by embedded hot stars, while others are so diffused that they can be detected only with long exposures and special filters.
Originally, the term "nebula" was used to describe any diffused astronomical object, including galaxies beyond the Milky Way.
However, the first detailed study of the Orion Nebula was not performed until 1659 by Christiaan Huygens, who also believed he was the first person to discover this nebulosity.
[14] This number steadily increased during the century, with Jean-Philippe de Cheseaux compiling a list of 20 (including eight not previously known) in 1746.
From 1751 to 1753, Nicolas-Louis de Lacaille cataloged 42 nebulae from the Cape of Good Hope, most of which were previously unknown.
[17][18] A third category was added in 1912 when Vesto Slipher showed that the spectrum of the nebula that surrounded the star Merope matched the spectra of the Pleiades open cluster.
[20] Both Hubble and Henry Norris Russell concluded that the nebulae surrounding the hotter stars are transformed in some manner.
Examples of the latter case are planetary nebulae formed from material shed by a star in late stages of its stellar evolution.
Massive stars may form in the center, and their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths.
Other nebulae form as the result of supernova explosions; the death throes of massive, short-lived stars.
The materials thrown off from the supernova explosion are then ionized by the energy and the compact object that its core produces.
The compact object that was created after the explosion lies in the center of the Crab Nebula and its core is now a neutron star.
When a star has lost enough material, its temperature increases and the ultraviolet radiation it emits can ionize the surrounding nebula that it has thrown off.
[25] Planetary nebulae are the remnants of the final stages of stellar evolution for mid-mass stars (varying in size between 0.5-~8 solar masses).
Evolved asymptotic giant branch stars expel their outer layers outwards due to strong stellar winds, thus forming gaseous shells while leaving behind the star's core in the form of a white dwarf.
[25] Planetary nebulae were given their name by the first astronomical observers who were initially unable to distinguish them from planets, which were of more interest to them.
The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode.
