NEBULA

NEBULAE

     If we translate the Latin word ‘nebula’, it would simply mean ‘cloud’. The nebula that we are about to talk about, however, is so much more than just a cloud. A nebula is an interstellar cloud in outer space that is made up of dust, hydrogen and helium gas, and plasma. It is formed when portions of the interstellar medium collapse and clump together due to the gravitational attraction of the particles that comprise them.

Many nebulae or stars 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 ionizes the surrounding gas, making it visible at optical wavelengths. Examples of these types of nebulae are the Rosette Nebula and the Pelican Nebula.

1. Classical types

Objects named nebulae belong to four major groups. Before their nature was understood, galaxies ("spiral nebulae") and star clusters too distant to be resolved as stars were also classified as nebulae, but no longer are.

H II regions, large diffuse nebulae containing ionized hydrogen

Planetary nebulae

Supernova remnant (e.g., Crab Nebula)

Dark nebula

2. Diffuse nebulae

Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries.In visible light these nebulae may be divided into emission and reflection nebulae. Emission nebulae emit spectral line radiation from ionized gas (mostly ionized hydrogen);[16] they are often called HII regions (the term "HII" is used in professional astronomy to refer to ionized hydrogen).

Reflection nebulae themselves do not emit significant amounts of visible light, but are near stars and reflect light from them.[16] Similar nebulae not illuminated by stars do not exhibit visible radiation, but may be detected as opaque clouds blocking light from luminous objects behind them; they are called "dark nebulae".[16]

Although these nebulae have different visibility at optical wavelengths, they are all bright sources of infrared emission, chiefly from dust within the nebulae.[16]

3. Planetary nebulae

Planetary nebulae form from the gaseous shells that are ejected from low-mass asymptotic giant branch stars when they transform into white dwarfs.[16] They are emission nebulae with spectra similar to those of emission nebulae found in star formation regions.[16] 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.[16] 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.[17]

4. Protoplanetary nebula

A protoplanetary nebula (PPN) is an astronomical object which is at the short-lived episode during a star's rapid stellar evolution between the late asymptotic giant branch (LAGB) phase and the following planetary nebula (PN) phase.[18] During the AGB phase, the star undergoes mass loss, emitting a circumstellar shell of hydrogen gas. When this phase comes to an end, the star enters the PPN phase.

The PPN is energized by the central star, causing it to emit strong infrared radiation and become a reflection nebula. Collaminated stellar winds from the central star shape and shock the shell into an axially symmetric form, while producing a fast moving molecular wind.[19] The exact point when a PPN becomes a planetary nebula (PN) is defined by the temperature of the central star. The PPN phase continues until the central star reaches a temperature of 30,000 K, after which is it hot enough to ionize the surrounding gas.[20]

 5. supernove nebulae

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.[16] The expanding shell of gas forms a supernova remnant, a special diffuse nebula.[16] 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.[16] This emission originates from high-velocity electrons oscillating within magnetic fields.