Nebulae And Their Types

Nature never fails to astonish us with its beauty and one such beautiful creation of nature are the Nebulae ! So, in today's article of The Basics Astrophysics series, we will delve into the beauty and types of these magnificent objects.

Nebula is basically an interstellar cloud made up of 90% hydrogen, 10% helium and trace amounts of some heavier elements. These are the spooky gas clouds which can both be vibrantly illuminated and sometimes, completely dark too. They play a key role in evolution of our universe as they are the building blocks containing elements from which stars and planets are born. They are visible in night sky either as a distinct patch or as a bright silhouette against other luminous matter. 

Observational History

Observations of nebulae date back to thousands of years. In second century CE, Ptolemy noted five nebulous stars, 800 years later, Abn al-Rahman Suri mentioned a little cloud at a place where we now locate Andromeda Galaxy. The first telescopic observation of nebula was made by Nicolas-Claude Fabri de Peiresc in 1610 when he identified the Orion nebula. Later, there was an avalanche in nebulae sightings in the 18th century by one astronomer after another. Earlier all the cloud like structures were considered to be a nebula. The true nature of nebulae was known mainly through the work of Edwin P. Hubble in 1920.

Classification of Nebulae:

Nebulae are basically classified into two categories, i.e. Galactic Nebulae and Extragalactic Nebulae. Nothing specific is known about the extragalactic nebulae, however galactic Nebulae are further categorized as Diffused and Regular Nebulae.

Diffuse Nebulae

As the name says, these are the dust clouds which do not have any regular shape. Diffuse nebulae are further of two types, luminous and dark nebulae. Luminous nebulae are bright, shiny and easily visible. They are diffuse and irregular and are distributed among spiral arms of our galaxy.

Classification of Nebulae
Classification of Galactic Nebulae

Reflection Nebulae

These are those interstellar clouds which can reflect the light from the nearby stars or from the nearby emission nebula. They are illuminated by B type stars that are very luminous but their temperature is less than 25000K , not sufficient enough to ionize the gas but sufficient to start scattering to make dust particles visible.
Examples include : Witch Head Nebula

The Witch Head Nebula (Image: NASA)

Emission Nebulae

If there is a star earlier than B1 near the galactic plane, then a diffuse emission nebula is associated with it. The strong ultraviolet radiations of these hot stars are energetic enough to ionise the gas , which leads to fluorescence effect , eventually emitting visible light.
There are two types of emission nebulae as follow

H II Region:

An H II region is a large low density cloud of partially ionized gas in which star formation has recently taken place. The short lived blue stars emit enough amount of UV radiations that ionise the surrounding gas . They are named so for the large amount of ionized hydrogen they contain . H II regions may give birth to thousands of stars over a period of thousand million years. In the end, supernova explosions and strong stellar winds from the most massive star in the resulting star cluster may disperse the gases of H II region , leaving behind a cluster of birthed stars.
Example includes : Orion and Eagle Nebula.

The Eagle Nebula (Image: NASA)

Supernova Remnants

Massive stars end their life as a supernova, blasting its outer layers back to interstellar space. Supernova remnants are the nebulae formed by gaseous debris ejected at the time of supernova explosion. The gaseous nebulae consist of the blown off superficial layers of the exploding star as well as the ISM(Interstellar Medium) swept by the passage of shockwave from the exploded star. Although not necessarily visible at the optical wavelengths, supernova remnants tend to be powerful X- Ray and radio emitters due to interaction with surrounding ISM. The thrown off debris sweep away the surroundings ISM suffering a deceleration and mingling its rich abundance of heavy nuclei with interacting medium. The enriched mixture is subsequently used as a material for the next generation of stars.
Example includes: Crab nebula in Taurus

Crab Nebula (Image: NASA)

Dark Nebulae

Dark nebulae refers to an interstellar dust cloud which is dense enough to obscure the visible light from background. The clouds appear to be dark due to the micro dust particles covered with carbon dioxide and frozen nitrogen which blocks the background light in visible wavelength. They possess an average density of 100-300 molecules per cubic centimeter and an internal temperature of 7-15K. The cloud cores are completely undetectable except for the microwave radiations from their constituent molecules. Examples include: Horsehead nebula

Nebulae
The majestic Horsehead Nebula (Image: NASA)

Planetary Nebulae

Unlike the emission nebulae with complete irregular structures, Planetary nebulae are emission nebulae of fairly regular shapes and each having an extremely hot blue star at the centre called the nucleus of the star. It is basically a ring shaped nebula formed by an expanding shell of gas around an ageing star . The term planetary was derived from the planet like round shapes of these nebulae and have nothing to do with planets. Once most of giant’s atmosphere is dissipated , UV radiations from the hot luminous core ionise the ejected material .Absorbed UV radiation then energizes the shell of nebulous gas around the central star, causing it to appear as a brightly coloured nebula.
Examples include: The Eskimo Nebula

The Eskimo nebula (Image: NASA)

Author's Message

Nebulae are of crucial importance to study the evolution of any galaxy. They are not just the starting points of stellar evolution , but can also be the end points. I hope this article gave you a fundamental overview of different types of nebulae existing in our Universe.

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