Nature never fails to astonish us with its beauty and one such beautiful creation of nature is a nebula. So, in today’s article of the Basics Astrophysics series, we will delve into the beauty and types of these magnificent objects called nebulae.
A nebula is basically an interstellar cloud made up of about 90% hydrogen, 10% helium, and trace amounts of some heavier elements. These are the spooky gas clouds that can both be vibrantly illuminated and sometimes, completely dark too. They play a key role in the 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.
Observations of these magnificent structures date back to thousands of years. In the second century CE, Ptolemy noted five nebulous stars. About 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 a nebula was made by Nicolas-Claude Fabri de Peiresc in 1610 when he identified the Orion nebula. Later, there was an avalanche in their sightings in the 18th century by one astronomer after another. Earlier most people identified all the cloud-like structures as nebulae. However, in 1920 Edwin P. Hubble discovered their true nature.
Classification of Nebulae:
Nebulae are basically classified into two categories, i.e. Galactic Nebulae and Extragalactic Nebulae. Nothing specific is known about the latter, however, galactic Nebulae are further categorized as Diffused and Regular Nebulae.
Classification of Galactic Nebulae:
As the name says, these are the dust clouds that do not have any regular shape. These are further of two types, luminous and dark ones. Luminous ones are bright, shiny, and easily visible. They are diffuse and irregular and we can find them among spiral arms of our galaxy.
These are those interstellar clouds that can reflect the light from the nearby stars or from a nearby emission nebula. They are illuminated by B type stars that are very luminous but their temperature is less than 25,000 K, not sufficient enough to ionize the gas but sufficient to start scattering to make dust particles visible.
Example: Witch Head Nebula
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 ionize the gas, which leads to the 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 ionize the surrounding gas. Their name is related to a 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 the H II region, leaving behind a cluster of birthed stars.
The example includes Heart and Eagle Nebula.
As we read in the article about the formation of white dwarfs, 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 the 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 the interacting medium. Then the next generation of stars uses the mixture as a material.
The example includes the Crab Nebula in Taurus
Dark nebula refers to an interstellar dust cloud that is dense enough to obscure the visible light from the 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 the 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 the Horsehead nebula.
Unlike the emission nebulae with complete irregular structures, Planetary nebulae are emissions of fairly regular shapes, each having an extremely hot blue star at the center called the nucleus of the star. They are basically a ring-shaped nebula formed by an expanding shell of gas around an aging star.
The term planetary comes from the round shapes of those nebulae, similar to planets, but they have nothing to do with them. Once most of the giant’s atmosphere is dissipated, UV radiations from the hot luminous core ionize the ejected material. Absorbed UV radiation then energizes the shell of nebulous gas around the central star, causing it to appear as a brightly colored nebula.
Example: The Eskimo Nebula
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 these fascinating objects existing in our Universe.