Chapter 5 part 2 — Stellar Lifecycle

Published in

Project Bluestar

7 min readJun 13, 2021

Introduction

Despite stars being non-living things, they too have a lifecycle just like us humans and pretty much every other organism on the planet. However their lifecycle is a bit funky.

There are different forks in their lifecycle and a star doesn’t progress to the next stage based on age like how we do. The entire lifecycle a star goes through is based on their mass.

Stars have a lifecycle much like animals. They get born, they grow, they go through a definite internal development, and finally they die, to give back the material of which they are made so that new stars may live.
- Hans Bethe

The above image is a simple chart that shows the life cycles of all types of stars. I’ll keep referring to that as it’s very handy.

THE NEBULA

That’s right! Nebulas, stars, black holes and supernovae are all connected, God this is a better crossover than Infinity War.

We start off with the very first stage of the lifecycle. The Nebula. Think of this as a newborn baby.

Nebulas are clouds of gas and dust, they’re also extremely pretty to look at; sometimes you can barely spot them if the sky is extremely clear with no clouds.

The gas present in a Nebula is primarily Hydrogen. As time goes by, Gravity starts pulling on the various stellar “stuff” that make up the Nebula and clump them together. As they start clumping, they start generating temperatures of up to millions of degrees. We then reach the second stage of the lifecycle which isn’t present in the chart.

PROTOSTARS

A protostar isn’t technically a star as of yet. We start getting to the “starry” bit when temperatures inside the core is hot enough to fuse Hydrogen into Helium.

As time goes by, the protostar gets bigger due to collisions of gas particles and dust, as they get bigger the pull of Gravity on the protostar gets stronger. This sort of squeezes the protostar which makes it more dense. As you might guess, this raises the temperature of the star and the process repeats till temperatures become hot enough to fuse Hydrogen into Helium.

MAIN SEQUENCE

We now reach the first fork. The protostar can either become an average star or a massive star based on it’s mass. An average star is a star with a mass between 0.8 to 8 times the mass of the Sun.

A massive star is a star with a mass of more than 8 times the mass of the Sun. The mass of the star depends on how much gas, dust and other particles there were in the nebula. The more particles there were, the greater the mass.

Regardless of mass, the same thing happens in the core of every Main Sequence star. Hydrogen is constantly fused into Helium. This fusion process releases some energy which is what powers up solar panels and forms wind.

An interesting effect known as the “Mass defect” takes place. Basically, if you find the mass of Hydrogen before the fusion process and find the mass of the Helium after, the Helium will actually be lighter. The mass by which Helium is lighter is the m in E = mc².

Like I mentioned, the fusion process releases some energy. This energy balances the force of Gravity. Essentially the two quantities cancel each other out and the star is said to be in Hydrostatic Equilibrium.

This is the longest stage in the lifecycle as stars can remain in this stage for millions of years. Our Sun will last 10 billion years in this stage.

The lower the mass of the star, the longer it’s lifecycle is. We’ll get to why larger mass stars have shorter lifecycles in a moment.

Stars eventually start running out of Hydrogen. As you can imagine, this means that the energy released by fusion cannot offset the force of Gravity, this causes the star to collapse.

As the star collapses, it becomes denser which increases it’s temperature. It becomes so hot that the Helium in the core can now fuse into heavier elements which causes it to expand. We now reach the next stage.

RED (SUPER)GIANT

The name of the next stage depends on the star. If we were looking at an average star, it would become a Red Giant. A massive star on the other hand would become a red supergiant.

Red supergiants are so large that they can have radii up to 1500 times that of the Sun. However Red giants are massive as well. When the Sun becomes a Red giant, it will completely engulf Mercury, Venus and Earth, effectively killing any organism on Earth if there happens to be one. Don’t worry, this will only happen in about a billion years so you’ll be long dead by then.

Eventually Red giants run out of Helium to fuse which causes it to expel a layer which becomes a planetary nebula. The nebula at the start of the lifecycle is known as the stellar nebula.

The remaining star starts to cool down and we reach the next stage of the lifecycle for average stars.

White Dwarfs

White dwarf are extremely dense, they have a mass close to 1 solar mass but they are only as large enough as the Earth. No fusion takes place in White Dwarfs, they are essentially dead. As time goes by White Dwarfs cool and eventually become Black Dwarfs.

Black dwarfs are yet to be discovered, the reason for this is the time that a White dwarf takes to cool to a Black dwarf is greater than the age of the universe.

We have reached the end of the lifecycle for average stars. What about the massive stars?

SUPERNOVAE

A red supergiant eventually starts fusing heavier elements. One of these elements happen to be Iron. Iron is known as the Star killer because once a star starts fusing Iron, it has dug its grave. The reason Iron is so deadly for stars is because fusing Iron releases extremely high amounts of energy.

This energy is greater than the force of Gravity which causes the star to expand. The star then explodes, which is known as a supernova.

When Betelgeuse, a star about 700 ly from Earth, explodes, it’ll be so bright that we’ll be able to see the colours in the sky for weeks on end.

The supernova ejects the entrails of the star, such as iron, gas and other elements. We reach our final fork in the lifecycle.

Neutron Stars

If the star was considered to be an “average massive” star. The remnants would form a dense core resulting in a Neutron star.

Neutron Stars are said to be so dense that even a teaspoon of it would have a larger mass than that of the Himalayan Mountains.

Black Holes

If the star that exploded was an extremely massive massive star. The remnants would collapse in on itself and form a Black Hole.

The Black Hole at the centre of the M87 galaxy

Black Holes have such a great gravitational pull that they manage to absorb light in its surroundings. Every galaxy has a supermassive black hole in its centre.

Conclusion

That’s it for today’s post. This is one of my favourite topics and I hope you enjoyed reading this. I didn’t delve too much into some parts of the lifecycle like nebulas, black holes and neutron stars because I plan to make separate posts for each of them. Thanks for reading!

Images can be found at:

Chapter 5 part 2 — Stellar Lifecycle

Written by Madhav Menon