What Is A Planetary Nebula?

Imagine that you’re on a camping trip. Away from all the pollution and hustle-bustle of the city, somewhere on the hills. You’ve been hiking all day and now it’s getting dark. So, you decide to set up your camp and lay down in the open. Soon, the sunsets and the night sky reveal themselves. Much clearer than it would have been in the cities. You stare in awe at the dark sky embellished by twinkling stars and beautiful hues. But there is a melancholic twist to this beautiful sight. It takes years and ages for the light from these stars to reach us, so many of the stars that we see today would have already vanished and many new stars would’ve been born.

Nebulae and Life of Stars

Quite like us, Stars have a defined life cycle. They are born, they grow and then they age and die. And for many stars, the start of their lives and the end of it happen amidst Nebulae.

Nebulae are massive clouds of dust and gases (mostly hydrogen). They are enormous- some of interstellar and some of the intergalactic proportions. These Nebulae are integral to the life cycle of stars. The birth of many stars takes place in these glowing clouds. This starts when gravity pulls together the gases and creates a ball of gas and dust. This ball now possesses its own gravity which continues to pull in more gas and dust. The ball grows in size and the process continues for a long period of time. And soon, the gravity, density, and heat reach the point which ignites the fusion reaction within this ball, turning it into a burning, glowing ball of gas- a Star.

We can imagine it like a snowball down a mountain. That is how when we roll a snowball down a mountain it keeps on rolling and gaining mass until it becomes massive enough to cause destruction. Even though it’s quite inaccurate and the snowball would turn into a wheel before falling apart, that’s how the idea goes.

This type of Nebulae is known as Stellar Nebulae or Star-forming Nebulae. But there are three more types of Nebulae and one of them is Planetary Nebulae.

Death Average or Small Star

Before we move on to Planetary Nebulae, we need to take get a quick understanding of the life cycle of average and small-sized stars. The life of these stars is similar to other stars up until their youth and middle life of the stars. But next the path that these stars follow is different from the other stars. We know that the stars constantly burn Hydrogen in the fusion reaction which makes Helium. This generates energy that pushes outwards. The gravity from the mass of the star pushes inwards. The interaction of these two forces creates a balance that is important for the survival of the star. The beginning of the death of a star is marked by an imbalance between these two forces. From this point on the average and small-sized stars follow the path that leads to them becoming White Dwarf Stars.

When these stars run out of fuel for the fusion reaction, that is Hydrogen, they start the fusion of helium into heavier elements like Carbon, Oxygen, and Neon. This creates an imbalance. The Stars start swelling up and turn into Red Giants. They start shedding their outer layers as the forces of gravity become incapable of holding the star together. These layers that are expelled form some of the most beautiful objects in the Universe- Planetary Nebulae.

Planetary Nebulae

Planetary Nebulae have little to nothing to do with planets but they were named so because when early astronomers like William Herschel discovered them they assumed them to be planets.

Planetary Nebulae, like other Nebulae, are massive glowing clouds of dust and gases. As discussed earlier, these clouds are formed from the shedding away of the mass of a dying star. In the center of the Planetary Nebulae what remains is a remnant of the star- a White Dwarf Star.

While most other Nebulae are made up of Hydrogen and Helium, these Planetary Nebulae contain in them heavier elements like Oxygen and Carbon made from the fusion reaction in the dying star. As the White Dwarf Star in the center energizes the different elements of the nebulae, they give off glows of different colors. Hence why the Planetary Nebulae are vibrantly colorful.

These heavier elements made in the dying star and beautifully displayed by the Planetary Nebulae, make up only 2% of the total matter in the visible universe. And are responsible for the existence of all life as we know it. So, if it weren’t for them then maybe life in the Universe would have been made of Hydrogen and Helium rather than Carbon and Oxygen. What would life be like in that world?