A nebula is a cloud of gas and dust in space. Some nebulae (more than one nebula) are regions where new stars are being formed, while others are the remains of dead or dying stars. Nebulae come in many different shapes and sizes. There are four main types of nebulae: planetary nebulae, reflection nebulae, emission nebulae, and absorption nebulae. The word nebula comes from the Latin word for cloud.
Some nebulae are formed as the result of supernova explosions, the death throes of massive, short-lived stars. The materials thrown off from the supernova explosion are ionized by the energy and the compact object that it can produce. One of the best examples of this is the Crab Nebula, in Taurus. The supernova event was recorded in the year 1054 and is labelled SN 1054. The compact object that was created after the explosion lies in the center of the Crab Nebula and is a neutron star.
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 enough material, its temperature increases and the ultraviolet radiation it emits can ionize the surrounding nebula that it has thrown off. The nebula is almost 97% hydrogen and 3% helium, plus trace amounts of other elements.
The more masses clump together, the greater their gravitational attraction will be to other bodies and particles in their vicinity. As the particles clump further to form larger and more massive structures, they attract more dust and gas. The pressure inside then gets so high that nuclear fusion ensues. This results in the emission of high-energy electromagnetic radiation, which in turn ionizes the outer layers of gas. Ionized gas is plasma, and so plasma and electromagnetic radiation are now added to the mix. This now becomes the earliest stages of star formation, and is what some scientists are most interested about. Actually, nebulae are not just the starting points of stellar evolution. Ironically, they can also be the end points. Stars that evolve into red giants can lose their outer layers during pulsations in their outer layers, known as their atmospheres.
The first true nebula, as distinct from a star cluster, was mentioned by the Persian/Muslim astronomer, Abd al-Rahman al-Sufi, in his Book of Fixed Stars (964). He noted “a little cloud” where the Andromeda Galaxy is located. He also cataloged the Omicron Velorum star cluster as a “nebulous star” and other nebulous objects, such as Brocchi’s Cluster. The supernova that created the Crab Nebula, the SN 1054, was observed by Arabic and Chinese astronomers in 1054.
Planetary nebulae form from the gaseous shells that are ejected from low-mass asymptotic giant branch stars when they transform into white dwarfs.They are emission nebulae with spectra similar to those of emission nebulae found in star formation regions. Technically they are HII regions, because most hydrogen will be ionized, but they are denser and more compact than the nebulae in star formation regions. Planetary nebulae were given their name by the first astronomical observers who became able to distinguish them from planets, who tended to confuse them with planets, of more interest to them. Our Sun is expected to spawn a planetary nebula about 12 billion years after its formation.
A supernova occurs when a high-mass star reaches the end of its life. When nuclear fusion in the core of the star stops, the star collapses. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode. The expanding shell of gas forms a supernova remnant, a special diffuse nebula. Although much of the optical and X-ray emission from supernova remnants originates from ionized gas, a great amount of the radio emission is a form of non-thermal emission called synchrotron emission. This emission originates from high-velocity electrons oscillating within magnetic fields.
Reference: Calthech University Astronomy Department; NASA; OpenUniversity source.