
Many nebulae form from the gravitational collapse of gas in the
interstellar medium or ISM. As the material collapses
under its own weight, massive stars may form in the center, and
their ultraviolet radiation ionises the surrounding gas, which
creates plasma, making it visible at optical wavelengths. An
example of this type of nebula is the Rosette Nebula or the
Pelican Nebula. The size of these nebulae, known as HII regions,
varies depending on the size of the original cloud of gas, and
the number of stars formed can vary too. As the sites of star
formation, the formed stars are sometimes known as a young,
loose cluster. Some nebulae are formed as the result of
supernova explosions, the death throes of massive, short-lived
stars. The material thrown off from the supernova explosion is
ionized by the supernova remnant. One of the best examples of
this is the Crab Nebula, in Taurus. It is the result of a
recorded supernova, SN 1054, in the year 1054 and at the centre
of the nebula is a neutron star, created during the explosion.
Other nebulae may form as planetary nebulae. This is the final
stage of a low-mass star's life, like Earth's Sun. Stars with a
mass up to 8-10 solar masses evolve into red giants and slowly
lose their outer layers during pulsations in their atmospheres.
When a star has lost a sufficient amount of material, its
temperature increases and the ultraviolet radiation it emits is
capable of ionizing the surrounding nebula that it has thrown
off. It is 97% Hydrogen and 3% Helium. The main goal in this
stage is to achieve equilibrium