Life Cycle of a Star
Stars are not all the same small dots we see in the night sky, they are as "diverse and complex as anything else in our universe"(Cosmonova, n.d.). They go through a natural life cycle like every living being; they are born, go through an expansion during their lifespan, then eventually die. A big difference is that a star's life can take millions to billions of years to complete. However, we do not know what happens to a star after the current known last stage, which can be as a white dwarf, neutron star, or blackhole.
Main Stages in a Star's Life Cycle
There are a total of 7 main stages in the life cycle of a star. The last stages of a star can be different from other stars depending on the size and mass it develops to during it's time as a main sequence star. If the star has a greater mass than our Sun, its last stage will be different from that of a star that has a smaller or same mass as our Sun.
All stars are born the same way, they are formed from a nebula. A nebula is a large cloud of gas and dust that is mostly made of hydrogen. The temperature of a nebula is low enough to allow molecules, like hydrogen, to form. Some of these molecules, like hydrogen, light up and allow us to see the nebula in space. The rest of the star's life cycle is determined by star's mass, which comes from the amount of matter from its nebula. The Orion Cloud Complex in the Orion system is an example of a stellar nebula.
Next the hydrogen from the nebula pulls together by gravity, starts spinning increasingly faster, and heats up to form the huge molecular cloud or warm clump of molecules called a protostar (Nasa, n.d). A protostar can be seen with infrared vision because they are warmer than other materials in the nebula. Depending on the size of the nebula several protostars can form. It should be noted that a protostar is not a star, it is the mass before a star, and can last for 100, 000 years before the next stage. The increase in pressure and gravity causes the protostar to collapse on itself, like the gas did previously to form a protostar, to leave a pre-main-sequence star.
These stars are named after a young star in the Taurus star-forming region. A star in the t-tauri phase is also known as a pre-main-sequence star. The star releases the huge amount of energy they get from the gravitational pressure that holds them together. Though, this pressure is not enough and the star doesn't have a high enough temperature for nuclear fusion to occur at the core. A star in this stage almost has the same temperature as a main sequence star, except their larger size allows them to shine brighter. The star produces strong winds that push away the surrounding gas, allowing it to be visible to scientists for the first time. The t-tauri stage lasts about 100 million years.
Main Sequence Stars
This stage is the longest as a star will "spend 90% of its life in this stage"(Roberts, 2019). Our Sun and most of the stars in the night sky are main sequence stars. The core temperature reaches a point where nuclear fusion can occur to convert hydrogen into helium, but this reaction causes the star to release lots of energy. At this stage the star's gravitational pull balances out the outward push with high pressure, coming from the high temperatures. The star shines brightly, contracts a little, and is stable in this stage.
After all the hydrogen is used up, the nuclear fusion stops and there is no longer the outward pressure to balance the inward pull from gravity. So the star contracts inward, but gravity causes the outer shell that is mostly made of hydrogen to expand. At this stage the helium that was produced will start fusing at the core. A red giant can be 100 times larger than it was in as a main-sequence star, but will only last a few hundred million years before the star uses up all the helium layers. A red giant appears more red because it's surface is cooler than that of a main-sequence star.
Last Stage of a Star Smaller or the Same Size as Our Sun
For a smaller star that is the same size or smaller than our Sun, a white dwarf forms after the red giant phase. Once all the helium is used up the core shrinks and starts fusing carbon. When there are no more elements to fuse and iron starts appearing in the core, the core will collapse then contract into a white dwarf. The star will start cooling down, but still shine brightly from all its heat. It will take hundreds of billions years for a white dwarf to completely cool down so we do not know what happens next in this type of star's cycle.
Last Stage of a Star Bigger Than Our Sun
After its red giant phase, a larger star will continue the nuclear fusions at its core, like a small star, but the implosion in the core will cause an an explosion called a supernova. Star material is flung out into space leaving the core behind.
Neutron Star or Blackhole
Next a neutron star or blackhole forms at the last stage, depending on the star's size. For a star 1.35 - 2.1 times the size of our sun, the remaining core will become a neutron star. A neutron star is completely made of neutrons because the powerful gravitational pull of the star combines the electrons and protons together to form neutrons. For a star larger than 2.1 times the size of our sun, the supernova will turn into a blackhole.
- BBC. (n.d.). The formation and life cycle of stars - The life cycle of a star - AQA - GCSE Physics (Single Science) Revision - AQA. BBC Bitesize. Retrieved February 2, 2021, from https://www.bbc.co.uk/bitesize/guides/zpxv97h/revision/1
- ByJu’s. (n.d.). GDPR. Retrieved February 2, 2021, from https://byjus.com/physics/life-cycle-of-stars/
- CosmoNova. (n.d.). The Different Types of Stars. Retrieved February 2, 2021, from https://cosmonova.org/different-types-stars-stellar-evolution/
- NASA. (n.d.). Background: Life Cycles of Stars. Imagine The Universe. Retrieved February 2, 2021, from https://imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/background-lifecycles.html
- Roberts, B. (2019, March 2). 7 Main Stages of a Star. Sciencing. https://sciencing.com/7-main-stages-star-8157330.html