You may well have heard someone say that "we are all made of stardust" - but did you know this is actually true? Many of the elements our bodies contain can only be produced in a supernova, which is an enormous explosion some stars will produce when they die. These elements are scattered across the universe by these explosions, and some eventually end up being a part of you. Other stars may not die in a supernova but might instead turn into dwarf stars. This article explains the various life cycles a star could have, and what determines how a star will behave.
What is a star?
Stars are large celestial bodies that mainly consist of hydrogen and helium, the two lightest elements. They can have different sizes and temperatures and produce energy through continuous nuclear fusion reactions occurring in their core. We benefit from the energy released by our local star, the sun, as it heats and illuminates the earth. Stars are formed in a nebula and go through different stages in their life cycle depending on their mass. These stages will be explained in more detail below.
Facts about the life cycle of a star
The life cycle of a star is the sequence of events that takes place in the life of a star from its formation to its end. The life cycle of stars depends on their mass. All stars, regardless of their mass, are formed and behave similarly until they reach their main sequence stage. The initial three stages that occur for a star to enter its main sequence are described below.
The step-by-step life cycle of a star
We will now describe the stages of a star's formation in detail.
Stage 1: Formation of a star
A star is formed from a nebula, which is a huge cloud of interstellar dust and a mixture of gases, mostly comprising hydrogen (the most abundant element in the universe). The nebula is so vast that the weight of the dust and gases start to cause the nebula to contract under its own gravity.
Fig. 1: The Carina nebula is visible in a remote location in the southern sky near Indonesia. It is approximately 8,500 light-years from earth.
Stage 2: Protostar
Gravity pulls the dust and gas particles together to form clusters in the nebula, which results in particles gaining kinetic energy and colliding with each other. This process is known as accretion. The kinetic energy of the gas and dust particles increases the temperature of matter in the nebula clusters to millions of degrees Celsius. This forms a protostar, an infant star.
Fig. 2: This image shows a protostar forming, located in the southern Chamaleon constellation.
Stage 3: Main sequence of a star
Once a protostar has reached a high enough temperature through accretion, nuclear fusion of hydrogen to helium begins in its core. This main sequence begins once the temperature of the protostar core reaches around 15 million degrees Celsius. The nuclear fusion reactions release energy, which produces heat and light, maintaining the core temperature so the fusion reaction is self-sustaining.
The nuclear fusion reaction in a star's core fuses two hydrogen isotopes to form helium and large amounts of energy in the form of neutrino radiation.
\[^2_1H+^3_1H=^4_2He+^1_0n\]
Experimental nuclear fusion reactors are being developed by scientists to try to replicate this process on earth as a source of clean energy!
During the main sequence stage, an equilibrium is achieved in the star. The outward force created from the expanding pressure due to nuclear reactions is balanced with the inward gravitational force trying to collapse the star under its own mass. This is the most stable stage in a star's life cycle, as the star reaches a constant size where the outward pressure balances the gravitational contraction.
If the protostar mass is not large enough, it never gets hot enough for nuclear fusion to occur - therefore the star does not emit light or heat and forms what we call a brown dwarf, which is a substellar object.
A substellar object is an astronomical object that is not large enough to sustain the nuclear fusion of hydrogen.
A star spends the majority of its lifespan in the main sequence, ranging from millions to billions of years depending on the mass of the star.
Summary of the life cycle of a massive star
All stars follow a similar initial lifecycle, however, a star's behaviour following the main sequence is highly dependent on its mass. At GCSE level, we consider two general mass categories of stars; sun-like stars and massive stars. To categorise the masses of stars they are often measured in terms of the mass of our Sun.
If the mass of a star is at least 8 to 10 times the mass of the Sun, the star is considered to be a massive star.
If the mass of a star is more similar to the size of the Sun, the star is considered to be a sun-like star.
Stars with larger masses are much hotter, appearing brighter in the sky - however, they also burn through their hydrogen fuel much faster, meaning their lifespans are much shorter than average stars. Because of this, large hot stars are also the rarest.
The colour of a star is determined by its temperature. High-temperature stars will appear blue, and low-temperature stars will appear redder. The Sun has a surface temperature of 5,500 degrees Celsius, hence it appears yellow.
The life cycle of a low-mass star
After several billion years of main sequence behaviour, low-mass, sun-like stars use up the majority of the hydrogen supply in their cores and the nuclear fusion to helium stops. However, the star still contains lots of hydrogen in its outer layers, and fusion begins to occur here instead - heating up the star and expanding it significantly. As the star expands it forms a red giant. At this point, other nuclear fusion reactions begin to occur in the core which fuses the helium into heavier elements such as carbon and oxygen - however, these reactions produce less energy and the star begins to cool.
As the rate of fusion reaction eventually slows to a stop and the temperature decreases, gravity once again becomes the dominant force and the red giant may collapse in on itself to form a white dwarf. The temperature of a white dwarf is significantly lower, in the region of hundreds of thousands of degrees. At this point, the star's life is over and the white dwarf continues to cool down until eventually it no longer emits heat or light and is known as a black dwarf. The flow diagram shown below illustrates the life cycle of a sun-like star on the left side.
The time required for a white dwarf to cool enough to become a black dwarf is estimated to be longer than the current calculated age of the universe. Therefore, scientists predict black dwarfs cannot exist in the universe yet.
Massive stars
Large stars also expand when the hydrogen supply in their core runs out and fusion reactions occur in the outer layers of the star. The heaviest element that can be produced in the main sequence stage of a star is iron, as fusion reactions combining energy heavier than iron no longer release energy. A massive star will expand into a red supergiant, which is the largest type of star we know of. As massive stars burn their hydrogen fuel much more quickly, the red supergiant will collapse rapidly when it eventually runs out of fuel.
The extreme temperatures and pressures created by the rapid collapse cause a massive explosion of the outer layers of the star. This explosion has the conditions for fusion reactions to produce elements even heavier elements than iron, such as gold. This cosmic explosion is known as a supernova.
Planet earth (and your body!) contain elements that are heavier than Iron. This indicates that Earth was formed from the elements created during the supernova of another star.
The supernova ejects its outer layers, scattering the elements produced into space and forming a new cloud of gases which will eventually collapse and form new stars and planets. The dense core of the star remains and can form different objects depending on its mass. If the surviving core of the star is around 3 solar masses, it will contract due to gravity and form an incredibly dense core comprised of neutrons known as a Neutron star.
Fig. 3: Artistic illustration of a Neutron star.
If the surviving core is greater than three solar masses, it will also collapse due to gravity into a very small point of infinite density forming a black hole. The gravitational pull of a black hole is so powerful that not even light can escape its pull.
Fig. 4: Predicted apparence of black hole with toroidal ring of ionised matter.
The life cycle of stars diagram
Fig. 5: Flow diagram showing the life cycle of stars. [Left] Sun-stars sequence. [Right] Massive stars sequence.
The Life Cycle of a Star - Key takeaways
- Stars have different sizes, which determine how their life cycle progresses.
- Stars are born in a nebula and die when they run out of fuel to supply nuclear reactions in the core strong enough to balance their own gravity.
- Low mass stars evolve into red giants and high mass stars evolve into red supergiant.
- Red giants eventually cool to become black dwarfs over incredibly long amounts of time.
- Red super giants eventually explode in a supernova and become either neutron stars or black holes.
- Elements from helium to iron are produced by the fusion reactions that occur in stars.
- Elements heavier than iron are only produced in supernovas.
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