Nebula: Definition, Formation and 4 Important Types

NEBULA : Definition, Formation and Types

nebula, nebulæ or nebulas is an enormous collection cloud of dust and gas suspended in space. Originally, Nebula was the term used to describe every interstellar/astronomical entity. The nebulae have enormous size and range over hundreds of light-years. Even though they occupy large spaces, the density of the nebulae is relatively low. This is because the clouds of dust and gas are scattered and spread out. 

The Orion Nebula is the brightest nebulæ that are visible to the naked eye. 

Formation of Nebula

Most nebulæ form as a result of the explosion of gas and dust produced during the death of star {supernova explosion}. A supernova explosion releases dust and gases that get ionized by the energy of its core. The crab Nebula is an example of such formation.  

Planetary nebulae are formed when a low-mass star, at its last stages evolves into a red-Giant and lose their outer layers. When a sufficient amount of matter is released from the star, the UV radiations ionize the surrounding it. Our Sun would form such a planetary nebulæ when it dies. 

Another way of nebula formation takes place during the birth of new stars from dust. When a massive molecular cloud collapses to form a star, the process emits UV radiations that can ionize the surrounding gases and forming a nebulæ.

This is the reason why some nebulae are called “star nurseries.”

Formation of stars in a Nebula

Nebulae consists of clouds of dust and gases—typically hydrogen and helium. Under the influence of a strong gravitational field, the scattered dust and gases in a nebulæ slowly start to come together by forming clumps. As the size of the clump increases, their gravity becomes stronger. Ultimately, these dust and gas clumps become compact as its density increases and collapses from its gravity. As a result of the collapse, the gaseous core starts to heat, thereby leading to a star’s birth.

Where are nebulae found?

Nebulae are found between the stars in the space i.e., the interstellar space. The Helix nebulæ is the nearest known Nebula to the Earth. It is the relic of a dying star that is comparable to the Sun. The Helix Nebula is around 700 light-years away from Earth. 

How do we know about the appearance of a nebula?

Astronomers utilize the help of very powerful telescopes and satellites to capture pictures of the faraway nebulae. Powerful space telescopes such as Hubble Space Telescope and NASA’s Spitzer Space Telescope have taken many images of faraway nebulae that assisted the astronomers in their research. The information collected through these pictures is then further developed by using computer simulators. 

Types of Nebulae

The entities that can be termed as nebulæ belongs to 4 major groups. 

  • · H II regions, [these are the large diffuse nebulæ containing ionized hydrogen]
  • · Supernova remnant (e.g., Crab nebulæ)
  • · Planetary nebulae
  • · Dark Nebula

Earlier, when there was little knowledge about nebulæ, Galaxies and star clusters were also termed as nebulæ.   

  1. HII Regions

The H II region is basically a cloud of interstellar atomic hydrogen (present in its ionized form.). Such nebulæ are formed when a new star is born from partially ionized gases. These nebulæ are irregular in terms of shape and color due to the uneven distribution of gases.

Over a period of millions of years, H II regions may be responsible for giving birth to stars. These regions play a vital role in studying the chemical composition of galaxies. 30 Doradus and NGC 604 are examples of such nebulæ.

H II region
H II Region
NASA, Hui Yang University of Illinois ODNursery of New Stars, Nursery of New Stars – GPN-2000-000972, marked as public domain, more details on Wikimedia Commons

2. Planetary Nebula

Planetary nebulae are basically the remains of the lower-density stars after their last formation stage. Like the H II regions, these are also composed of ionized hydrogen. However, planetary nebulae are dense and compact.

Planetary nebulae are called so because initially, astronomers were unable to separate them from planets. They would often confuse distant, dim planets as nebula. Our sun is most likely to form a planetary nebulæ after 12 billion years since its birth.

Planetary nebula
Planetary Nebula
ESA/Hubble, The Oyster NebulaCC BY 4.0

3. Supernova Remnants

When the nuclear fission at the core of a very massive star stops, it collapses. The heated gas from the core explodes as it expands outwards. The formation of an ionized h=gaseous shell due to expansion results in a supernova remnant.

A supernova remnant is basically formed by ionized gases, non-thermal radio emissions, and high-velocity oscillating electrons. The Crab nebula is an example of a supernova remnant.

4. Dark Nebula

Dark nebulae are a form of diffused nebulae i.e., nebulae that lack a distinct boundary. Dark ones are called so because they do not emit a significant amount of light. These either lack visible radiation or are enveloped by opaque clouds block light.

Diffuse Nebula
Diffuse Nebula
ESO, Carina Nebula by ESOCC BY 4.0

Andromeda Nebula [now, galaxy]

The Andromeda Nebula is one of the closest neighbor of the Milky Way. The Andromeda galaxy is the brightest galaxy that can be seen by the naked eye. The dimensions of the nebulae extend that of ours but are seen to depict similar stellar composition and shape. The Andromeda one is surrounded by four small elliptical galaxies that display a movement similar to that of a satellite around the Nebula.

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About Sanchari Chakraborty

I am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers.

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