Life Cycle of an Average Star

The life cycle of an average star, or any star for that matter, is such an exciting and mysterious thing – it can’t help but invoke curiosity and wonder.

For an average human, life begins, takes place and ends over the course of 80 to 100 years, if you’re lucky. For our brains, that’s completely comprehensible, but as you think about objects with longer life spans, it can become difficult to put into perspective.

For an average star, and when we refer to an average star we’re talking about a star that’s up to 1.5 times the size of our Solar System’s star, the Sun. Nonetheless, for an average star around the size of the Sun in our Solar System, the expected life span is around 10 billion years. Can you believe that?



“Sun white” by Geoff Elston – Society for Popular Astronomy, Solar section,


Generally, a star of our Sun’s size will fuse hydrogen for around 8 to 10 billion years, putting out light and heat. An average star’s surface temperature can range between 4,760 (2,900 Kelvin) and 12,140 (7,000 Kelvin) degrees Fahrenheit, hot!

The size of a star significantly impacts its expected life span and can greatly increase or decrease by millions or even billions of years. Average sized stars, equal to or less than 1.5 times the size of our Sun can live between; 2,000,000,000,000 and 2,000,000,000 years. Stars several times larger than our Sun can live for only 10 million years… Can you image if our Sun only lived for 10 million years? Goodbye humans!

The Life Cycle of an Average Star – Birth:

The life cycle of an average star begins like all stars, with their birth. All stars have the same mommies and daddies; giant molecular clouds and they are amazing.



Photo by, Judy Schmidt


Giant Molecular Clouds, or Nebulae, are enormous. They can span 100 to even 600 light years in diameter! That’s the equivalent of 587,862,537,318,360 to 3,527,175,223,910,164 miles, in diameter. Your brain can’t even comprehend the true scale of how large that is.



Photo by, NASA’s Marshall Space Flight Center


Giant Molecular Clouds are big and cold collections of gas and dust. They get so big that they eventually begin to collapse on themselves. When they do, magic happens; stars begin the birthing process. They start out as little baby protostars and turn into main sequence stars.



Photo by Hubble Heritage,


The Life Cycle of an Average Star – The Middle Years:

Depending on the size of the star, the period of which thermal energy is created through the process of nuclear fusion can vary. However, for the life cycle of an average star, this can be roughly a few billion to 10 or even 15 billion years.

The way a star generates heat and light is pretty simple, through the process of nuclear fusion.



“Sun920607” by NASA – Licensed under Public Domain via Wikimedia Commons –


You are probably very familiar with nuclear fusion and may or may not even know it. Nuclear fusion is mostly simply described in one of the most famous equations of all time; E=mc2, by Albert Einstein.

Nuclear fusion is the process in which hydrogen atoms turn into helium. This occurs through a sequence called the proton-proton chain. Nuclear fusion relies on carbon, nitrogen and oxygen atoms to convert hydrogen into helium atoms.

This process creates the output of Energy (E) through the conversion of one element to another, in this case, hydrogen to helium. The amount of Energy (E) is proportionate to the Mass (M) of the object multiplied by the variable of the Speed of Light, squared (C2). The mass energy equivalence.



Photo by DonkeyHotey,


Because of this, you can see the light illuminated from a star. If you’re close enough to the star, you can also feel the heat that this event creates. This process happens over and over and over and is basically all a star does in its middle years. Stars are basically professional makers of energy through the process of nuclear fusion and they’re awesome at their job.

Over the course of time, the proportion of hydrogen being converted into helium begins to shift. The hydrogen converting into helium makes the core of the star more and more dense, which ages it and increases its size. This begins the twilight years of a star’s life cycle.

The Life Cycle of an Average Star – The Golden Years:

As the life cycle of an average star begins to age and get old, it begins a beautiful process of death. Now, that may not sound very nice or exciting, but as you’ll see, it very much so is.

The latter years of an average star goes from converting hydrogen into helium through nuclear fusion, creating heat and light and in some cases, life. After the hydrogen runs out, its core is full of helium and begins to expand and cool off.



“Solar internal structure” by I, Sakurambo. Licensed under CC BY-SA 3.0 via Wikimedia Commons –


The Red Giant Phase:

As the star’s core contracts and its outer layers expand, its heat and light output decreases and it becomes dimmer and cooler – this starts the Red Giant phase of a star’s life cycle.



Photo by NASA’s Marshall Space Flight Center,


The helium atoms in the star’s core begin to fuse together, forming carbon atoms and releasing energy again! The stage of a Red Giant is massively huge in its size and mass. The outer layers expand and expand and expand. When our Sun begins this process of its life cycle, say goodbye to planet Earth, we’ll be toasted!

Planetary Nebula Phase:

After the expansion of the star in the red giant phase, its outer layers keep expanding. Eventually the outer layers drift on out to space and are no longer part of the inner core of the star, once held together by the nuclear fusion process.

Keep in mind, even through planetary is in the name of the part of the life cycle of an average star, it has nothing to do with a planet.



Photo by NASA’s Marshall Space Flight Center,


As the star loses its outer layers, the star continues to cool off and decrease in size. What was once a massively large object, eventually shrinks down to just thousands of miles in diameter. This is the end of the planetary nebula phase.

White Dwarf Phase:

Once the star converts all of its hydrogen into helium and the core of the star has fused to carbon atoms, the star begins the white dwarf phase of the life cycle of an average star’s life. This is close to the end and not a very exciting time of its existence.



“Artist’s impression of debris around a white dwarf star” by ESA/Hubble. Licensed under CC BY 3.0 via Wikimedia Commons –


The star no longer is converting atoms, no longer converting elements and creating light or heat and is basically in the state of radiating its remaining heat for up to billions of additional years.

Black Dwarf Phase:

The black dwarf phase of a star’s life is the end. It’s the equivalent of an old person in the hospital bed hooked up to tubes, plugs, devices and everything else keep its pulse pulsing.

At this stage in the life cycle of an average star, it’s basically dead. There is no fusion taking place, it’s lost all of its heat to the cold temperatures of the voids of space and is dark and basically invisible to the human eye.

A star will remain in the Black Dwarf state until forever, basically.



Featured image by Zach Dischner,

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