If stargazing is your favorite pastime, you may look up and wonder how exactly a star forms and how old they are. You might be surprised to hear that, unlike other celestial bodies in our universe, stars do not last forever. However, they do live a long and full life and are ever-changing, moving through various forms. The life cycle of a star is a fascinating process like no other. Every star goes through the same process, although the ending of the process and the time that the life cycle takes varies based on the star’s mass.
Curious to find out more about stellar evolution? Keep reading to learn about the life cycle of a star, from star formation to its final state, to fully understand what makes these celestial bodies so special. Plus, you’ll be able to recognize the current stage of the life cycle of our sun.
What Is a Star?
Let’s start with the basics: What is a star?
Put simply, a star is a bright ball of gas made up of mostly helium and hydrogen atoms. This ball of gas produces heat and light from the churning nuclear forges in its core. Astronomers estimate 300 billion stars in our Milky Way galaxy alone.
An interesting fact that many don’t know is that the dots we see in the sky are thousands of light-years, the distance that light travels in a year, away from Earth. That means that, for the most part, the stars we see with the naked eye are what they looked like light years ago. It’s possible that some of the stars we see don’t exist anymore.
Now that you know what exactly a star is, let’s take a closer look at the full life cycle of a star.
What Is the Life Cycle of a Star?
A star goes through several different stages before it ends up in its final form, which differs depending on the size of the star.
Without further ado, let’s walk through the life cycle of a star and determine which stage of the cycle our Sun is currently in.
A star starts as a nebula, a giant molecular cloud of gas and dust from which it is born. Gravity pulls together the hydrogen gas, causing it to spin. This causes it to heat up and transition into the next part of the cycle, a protostar.
Once the temperature of the protostar gets hot enough, around 15,000,00 degrees, nuclear fusion begins to develop in the molecular cloud’s core. From there, the nuclear cloud stabilizes and becomes stable. This is the stage of a star we are most familiar with – a main sequence star.
Main Sequence Star
The star will remain in its main sequence state for millions to billions of years, which is the stage that the sun is in at this very moment, as well as most stars in the galaxy. It’s the fact that the sun is in a state which has provided the necessities for life to evolve on Earth. What sustains this star during the main sequence star era is that its hydrogen converts into helium by nuclear fusion.
Red Giant Phase
Once the hydrogen supply at its core begins to run out, the core becomes unstable and begins to contract while the star’s outer shell expands. As the star expands, it begins to cool down and glow red. This is called the red giant phase, which is named after the physical state of the star. It is giant because it is expanded and red because it has cooled down from the main sequence star phase. This is the last phase that every star has in common.
Let’s take a look at how the death of a star occurs.
The Death of a Star
All stars eventually run out of hydrogen fuel and die. How a star dies depends on its mass. As mentioned, the hydrogen runs out, and a star with a similar mass to our sun will expand and become a red giant.
The life cycle is determined by its mass – the larger the mass, the shorter its life cycle. The mass is initially determined by the amount of matter available in its nebula.
In low-mass or smaller stars, the core experiences gravitational collapse after a fusion reaction where helium fuses into carbon. As the core collapses, the star’s outer layers are expelled, and the outer layers form a planetary nebula. From here, the core becomes a white dwarf, eventually cooling to become a black dwarf. Here’s a fun fact: Despite its large size in our solar system, the sun is actually a low-mass star.
For high-mass stars, which are typically ten times the size of our sun, their life cycle ends with a supernova explosion. Depending on the size of the remnants of the blast, it will either become a black hole or a neutron star.
A black hole occurs when, after the explosion, the star’s core is about three times the mass of our sun as the force of gravity will push the boundaries of the nuclear forces, which keep neutrons and protons from combining. This results in the core being swallowed by its own gravity, and thus a black hole is born. As you may know, black holes attract all energy and matter that come near them.
Conclusion: A Star Is Born
To wrap up, star formation is a unique process that, depending on the size of the star, can take billions of years to complete. The stars populating our sky are main sequence stars, with at least millions of years left in their life cycle.
Smaller stars become white dwarfs and then black dwarfs and massive stars become black holes, which contain a significant amount of mass that pulls so much that even light can’t get out. However, before stars reach this final stage, they will have lived exceptionally long lives and greatly brightened our universe, making a permanent impact.