A nebula (Latin for "cloud" or "fog" is an interstellar cloud of dust, hydrogen, helium and other ionized gases,Originally, nebula was a name for any diffuse astronomical object, including galaxies beyond the milky way.
Most nebulae are of vast size, some are hundreds of light years in diameter[not in citation given] A nebula that is barely visible to the human eye from Earth would appear larger, but no brighter, from close by. The Orion Nebulae, the brightest nebulae in the sky that occupies a region twice the diameter of the full Moon, can be viewed with the naked eye but was missed by early astronomers. Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created on Earth – a nebular cloud the size of the Earth would have a total mass of only a few kilograms. Many nebula are visible due to their fluorescence caused by the embedded hot stars, while others are so diffuse they can only be detected with long exposures and special filters.
Nebulae are often star-forming regions, such as in the "Pillars of Creation" in the Eagle Nebula. In these regions the formations of gas, dust, and other materials "clump" together to form denser regions, which attract further matter, and eventually will become dense enough to form stars.
There are a variety of formation mechanisms for the different types of nebulae. Some nebulae form from gas that is already in the interstellar medium while others are produced by stars. Examples of the former case are giant molecular clouds, the coldest, densest phase of interstellar gas, which can form by the cooling and condensation of more diffuse gas. Examples of the latter case are planetary nebulae formed from material shed by a star in late stages of its stellar evolution.
TYPES OF NEBULAE
1.Diffuse nebulae
Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries. Diffuse nebulae can be divided into emission nebula, reflection nebulae and "dark nebulae." Visible light nebulae may be divided into emission nebulae that emit spectral line radiation from excited or ionized gas (mostly ionized hydrogen)they are often called HII regions (the term "HII" refers to ionized hydrogen.
2.Planetary nebulae
The Oyster Nebula is a planetary nebula located in the constellation of Camelopardalis
Planetary nebulae form when low-mass asymptotic giant branch stars nova. A star that novas pushes the outer layers of the star's mass outward forming gaseous shells, while leaving behind the star's core in the form of a white dwarf.The hot white dwarf illuminates the expelled gases producing 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 found in star formation regions.
3.Protoplanetary nebula
The Red Rectangle Nebula is an example of a protoplanetary nebula located in the constellation of Monoceros
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.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.
- Supernova remnants
The Crab Nebula, an example of a supernova remnant
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.
Facts About Nebulae
*Most nebulae contain the “stuff of stars and planets”, including gases, dust, and complex molecules.
*As stars die and lose their materials to space, their gases and dust mix with clouds of gas, creating the complex nebulae we see.
*Nebulae are always in motion, even though they look quiescent in images. The clouds mix and churn, which creates magnetic fields.
*There are several types of molecular clouds: dark globules, emission nebulae, and reflection nebulae. Emission nebulae glow as their gases are heated. Reflection nebulae are mostly dust which reflects the light from nearby stars.
*Our Sun and planets formed in a nebula some 4.5 billion years ago.
*Nebulae exist in other galaxies. Astronomers have observed them in all spirals as well as the nearby Magellanic Clouds.
Source; https://en.wikipedia.org/wiki/Nebula
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