Nebula


What is a nebula? How are nebulae formed? What are the types of nebulae, and how do they differ from each other? What is the shape of nebulae, and how do they look? Read on to know more!

WHAT IS A NEBULA?
A nebula is an interstellar cloud of hydrogen, helium, and other ionized gases(gases that carry a positive or negative charge). A nebula can also consist of plasma( state of matter wherein an atom gets so much heated that it's orbiting electrons are thrown out of their shells. They are super-ionized). All this makes nebula quite bright. Below is a picture showing a nebula.

Helix nebula

I know it looks like some weird eye-ball from horror movies, but that's how 'Helix Nebula' ( Planetary nebula) looks like (I have talked more about types of nebulae later in this article).

Nebulae have really low density. For example, think about this: If the earth magically transforms into a nebula of the same size that of earth, it would weigh just a few kilograms. 

FORMATION OF NEBULA

The formation of nebula can be roughly divided into two ways.
Some of them are formed from the matter that is randomly present in the interstellar medium(just a fancy name for empty space). So these ionized gases somehow manage to come together, forming a nebula. These nebulae are really wide-spread; their diameter being hundreds of light-years.

The other type of formation of nebula involves the death of stars. The stars 'die' when they undergo an explosion called a supernova. So, after the explosion is over, a lot of gases and energy is released, along with a neutron star or black hole as the case may be. These gases are ionized, and some of them are so ionized and filled with energy, that they form plasma. These gases and plasma together make up a nebula. Nebulae can also be formed when the stars shed materials and gases during the later stages of the stellar evolution.
Some stars slowly lose their outer layer by pulsotronic emissions. These form the 'planetary nebula'. 

HOW NEBULAE FORM STARS?

Some nebulae even form new stars. Yes, they give birth to stars. Some gases come close together to form a clump, and this happens due to gravity. This process further increases the gravitational force, and more matter comes and joins it. Gradually, this develops into a lot of matter, and nuclear processes such as nuclear fusion begin to take place inside this huge clump due to high temperature and pressure. And then, a star is born. Due to this, nebulae are rightly called 'star nurseries'.

TYPES OF NEBULAE

Nebulae can be classified into 4 types:
  1. Diffuse nebulae (emission nebulae and reflection nebulae)
  2. Planetary nebulae
  3. Supernova remnant
  4. Dark nebulae

DIFFUSE NEBULAE

Most of the nebulae are diffuse nebulae. They have no fixed or particular boundaries. Diffuse nebulae are a source of infrared radiations and have different visibility at optical wavelengths. They basically consist of emission nebulae and reflection nebulae.
       EMISSION NEBULAE
The electrons in the gases constituting this kind of nebula jump into a higher energy level(or shell), due to the energetic ultra-violet rays. These electrons then lose energy and return back to their original shells. The energy that is lost, is in the form of light energy. This is also the basic principle of the photoelectric effect, the theory for which Einstein became famous. These emissions give this type of nebulae their name. Examples for emission nebulae: Rosette nebula, Eagle nebula, Lagoon nebula.
       REFLECTION NEBULAE
These types of nebulae have just the normal matter, nothing special. They work somewhat like our moon. They reflect the light from the nearby star(s). That's it! These types of nebulae are mostly blue because the scattering of light is more for the blue color. 
The Emission nebula and Reflection nebula are often found together, or nearby. Examples for reflection nebulae: Pleiades nebula
Rosette nebula
Pleiades nebula






PLANETARY NEBULAE

A planetary nebula, as discussed before, is formed by the outer layers of the old stars. There are a lot of strong stellar winds, and when they hit an old star, the star expels or removes its outer layer, and what's left behind is a white dwarf, which, once upon a time, was the star's core. A nebula is then formed. The white dwarf then makes the nebula to emit the light, the exact same way as the emission nebula I have discussed previously. But, unlike the diffused nebulae, these are comparatively a little denser. The scientists have predicted that our sun, according to its size, will form a planetary nebula after around 12 billion years after it's formation. The "planetary" nebula has nothing to do with planets. It was named so because earlier, when the astronomers looked at them, they thought of them as planets, due to their(nebula's) small size, and the astronomers' telescopes were not as advanced as we have today. Example: Ring nebula
Ring Nebula

SUPERNOVA REMNANTS

For this too, I have already mentioned its formation before. It is formed from the gases that are emitted from a supernova, which happens after a star dies. Examples are the crab nebula and the veil nebula.

Crab nebula


DARK NEBULA

Now these fall under the dense category of nebulae. Contrary to the general form of a nebula, they are not bright at all. This is what gives them their name. They are dense enough to block the light behind them from passing through them. The coldest and densest part of the clouds are responsible for this blockage of light. These areas and atoms just absorb the light energy. The interesting part about them is that they are covered by frozen carbon monoxide, as well as nitrogen. These two are actually responsible for all that blockage of light. Other than these two, a dark nebula is also made up of ammonia, carbon monosulphide, formaldehyde, an isotope of oxygen, etc. Example: Horsehead nebula.
Horsehead nebula

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