Stars are the powerhouses of the Universe. It may be hard to believe but stars are the source of life on earth (almost all the life. Life also thrives in the deep thermal vents in the ocean). And where there is time, there is evolution. Stars, albeit not living like us, also go through phases of evolution. This is the life cycle of a massive star. How the star is formed, it’s a life of stability, the inevitable death, and what happens after it.
Below mentioned are the seven stages of star formation and death, the complete evolution of stars, and all about red and blue giants. We have also added the fate of our star, the sun. What would happen to the sun after death? This is the story of the stellar life cycle.
What are the stars?
Before we learn about the life cycle of a massive star, first we have to know what a star is. Stars are heavenly bodies that have a core that can fuse hydrogen and produce heat and light. They work because the laws of the universe allow them to, like gravity and nuclear forces. Stars are formed wherever a sufficient amount of dust is present.
Stars come in different types. First, there are small stars which aren’t too bright or hot. Then there are medium-sized stars, like our sun. And finally, there are huge stars that are very hot and very bright. An example is Betelguese in the constellation of Orion. When it comes to the life span of a star, the bigger the star, the shorter its life span as it uses its fuel quickly.
The life cycle of a massive star
This section will discuss the stages of star evolution for a large star, many times larger than the sun. There is a brief description of each stage with an appropriate illustration. The life cycle of the sun is also explained in the following section.
This is the first stage of the life cycle of a star, both for large and small stars. This is also how our sun was formed. The nebula is a large cloud of dust that contains mostly hydrogen and some scarce amount of other elements. This large cloud comes closer and becomes concentrated due to the gravitational pull.
This is the second stage in this process. Due to gravity, the gases collapse to form a core and a loosely bound structure called a star. The core is pressured and heated due to the condensation of the mass of the star. The temperature starts rising and reaches 15,000,000 degrees Celsius. The temperature is enough to begin the fusion of hydrogen atoms and this is the initiation of the third stage, the main sequence.
The Main Sequence
This phase is the longest in the life cycle of a massive star, often continuing for millions to billions of years. At this stage, the inward gravitational pull of the mass of the star is balanced by the outward push of the fusion core. The balance between the inward and outward force keeps the star stable and shining for eons.
The star now is called the Main Sequence star and it stably gives light and heat. All the hydrogen is fused together and helium is produced. The sun has a main sequence span of nearly 10 billion years! But massive stars have less period, some even a few million years.
Red Giant Phase
The amount of helium inside the core is limited. After the main sequence phase of the star nears the end, the hydrogen starts depleting. This slows down the fusion process and the core begins to collapse due to the pressure. The outer layer of the star is mostly hydrogen and it starts to expand. Expansion results in cooling and hence, it stars glowing red (due to the decrease in temperature). The name red giant is because of its appearance. It is red due to cooling and giant due to swelling. This is the phase where stars of different masses go in different ways. For the life cycle of a massive star, it goes supernova.
Stars that are ten times more massive than the sun undergo supernova, a gigantic stellar blast that is sometimes brighter than an entire galaxy! Small stars do not go supernova, their life cycle is described later in the article.
As the core runs out of hydrogen to fuse, it stars fusing helium to form carbon atoms. Again, as the pressure increases, it fuses carbon atoms to form other elements. This happens until the iron is formed. After the elements are fused and iron is formed, no more fusion can occur. But the gravity gets more powerful, collapsing the star further on itself.
Finally, when the core cannot take anymore, it gives in and it blasts. This is called a supernova and it expels more energy in mere seconds than the sun would in its entire lifetime. The supernova can be seen fro thousands of light-years and any planet nearby will be destroyed.
Black Hole or Neutron Star
There is again a sub-division for different massive stars. For large stars, after the supernova explosion, the core remains while the outer shell is expelled in the explosion. This core is a dense collection of tightly packed neutrons. It is said that a pin-head sized portion of the neutron star could weigh as much as ten elephants.
The most massive stars are so massive that the core becomes so dense, it turns into a black hole. Blackhole is a super dense stellar object with such a powerful gravitational pull that it can stop light from escaping from it. This isn’t the final stage of a massive star per se, but in essence, this is where the star that existed for eons disappears.
The outer shell that was expelled in the supernova scatters all around the dead star. Some of it goes even further. This is called the pre-planetary dust. The mixture of iron, oxygen, carbon and other elements condense and form planets. Earth was formed in a similar way and since we came from the earth, humans are literary stardust.
The life cycle of the sun
Our sun, like every other star in the universe, has a life cycle. It was born, it is in the main sequence phase and slowly but surely, it is marching towards its inevitable death. So how will our star die? First, let’s see how it was born.
The sun is a third-generation star. This means that it was born after two previous stars completed their life cycle. The sun is a medium-sized star, not too massive, not too small. Some stars are almost 450 times smaller than the sun. There are some hyper-giants like UY Scuti whose radius is around 1,700 times larger than the sun.
Since the sun is average-sized, it has a longer life span. The sun is currently 5 billion years old and it still has 5 billion years left. But what happens after 5 billion years? Will it go supernova? Or will the sun turn into a white dwarf?
The Death of the sun
The sun will turn into a red giant after the core hydrogen starts depleting. It will burn as a red giant for around a billion years until the helium in its core starts fusing into carbon and oxygen. Slowly, the outer shell will expand and shed off, turning the sun into a small and warm white dwarf.
The elements from its shell will turn into a planetary nebula. This might, after some million years, turn into planets. A new solar system will be formed, with new planets. But sadly, we won’t be here to see it, neither will be the earth or the moon (add link).
The life cycle of the star in stages
First, the star is born from a nebula.
Then forms the Protostar, precursor to the star
Main sequence star, or the normal star stage.
Supernova/Blackhole/Neutron star/White dwarf.
Remnant or pre-planetary nebula.
The life span of a star
The word life span is different from the life cycle. The life cycle means the different phases of a star. Life span means how long the star lives, from the initiation of fusion to supernova. The life span depends on two main factors and they are interdependent.
The size of the star and the rate of fusion. If the size of the star is massive or it has a high mass, the star will have a large amount of hydrogen. But this also means more pressure. So the rate of fusion is also very high in high mass stars. The life span of large stars is much less than small and medium mass stars. Here are some examples;
Eta Carinae, which is one of the largest stars in the observable universe is estimated to have a lifespan of only about 3 million years. The start could go supernova anytime. UY Scuti is also one of the largest stars and it has a lifespan of a few million years.
Listening to millions of years as “short life” is bizarre. But it won’t sound that bizarre when you see the life span of small stars. The sun, which is a medium-sized star, has a life span of 10 billion years, a thousand times more than the large stars. But some of the smallest stars, like red dwarfs, have a life span of more than 10 trillion years. Suddenly millions of years are not looking that long isn’t it?
Red giant stars examples
The universe is filled with stars. Some stars are easily understood but some are mysterious, like Tabby’s star. There are plenty of Red giant stars too. And we have listed a few of them. Here are some examples;
- X Cygni
The sun will also turn into a red giant in 5 billion years. But we won’t be here to see it. Maybe we won’t be anywhere, or maybe we’ll stream the destruction of the sun online from some other planet. No one knows.