Introduction
For millenniums, stars have fascinated the human race. In medieval times, these heavenly bodies were thought to possess mystical powers and some civilizations even worshiped them. This supernatural view was caused by the lack of information on the true nature of stars. Modern science has enabled man to study stars and come up with scientific explanations of what they are and why they shine. Astronomers in the 20th century have been able to come up with a credible model of the entire life cycle of stars.
Green and Burnell (2004) state that the life cycle of a star takes place over a timescale that appears infinitely long to human beings. Astronomers are therefore unable to study the complete life cycle of stars since the changes occur at a very slow rate to be observed. The evolutionary pattern of stars is therefore deduced by observing their wide range at different stages of their existence. This paper will set out to provide a detailed description of the life-cycle of a star.
Birth of a Star
Stars are born from vast clouds of hydrogen gas and interstellar dust. This gas and dust clouds floating around in space are referred to as a nebula (NASA2010). Nebulas exist in different forms with some glowing brightly due to energizing of the gas by previously formed stars while others are dark due to the high density of hydrogen in the gas cloud.
A star is formed when the gas and dust making up the nebula start to contract due to their own gravitational pull. As this matter condenses due to gravitational pull, the gas and dust begin to spin. This spinning motion causes the matter to generate heat and it forms a dull red protostar (Krumenaker, 2005).
When the protostar is formed, the remaining matter of the star is still spread over a significant amount of space. The protostar keeps heating up due to the gravitational pressure until the temperature is high enough to initiate the nuclear fusion process (NASA, 2010). The minimum temperature required is about 15 million degrees Kelvin and it is achieved in the core of the protostar. The nuclear fusion process uses hydrogen as fuel to sustain the reaction and helium gas is formed from the fusion of the hydrogen nuclei.
At this stage, the inward pull of gravity in the star is balanced by the outward pressure created by the heat of the nuclear fusion reaction taking place in the core of the star (Lang, 2013). Due to this balance, the star is stable and because of the nuclear fusion, considerable heat and a yellow light is emitted from the star, which is capable of shining for millions or even billions of years depending on its size.
Mature and Ageing Stars
The newly formed star is able to produce energy through nuclear fusion of hydrogen into helium for millions to billions of years. During the nuclear fusion process, the heavier helium gas sinks into the core of the star. More heat is generated from this action and eventually, the hydrogen gas at the outer shell also begins to fuse (Krumenaker, 2005).
This fusing causes the star to swell and its brightness increases significantly. The closest star to the Earth is the Sun and scientists predict that it is at this stage of its life cycle. The brightness of a star is directly related to its mass since the greater the mass, the greater the amount of hydrogen available for use in the process of nuclear fusion.
Death of a Star
A star dies when its fuel (hydrogen) is used up and the nuclear fusion process can no longer occur. Without the nuclear reaction, the star lacks the outward force necessary to prevent the mass of the gas and dust from crashing down upon it and consequently, it starts to collapse upon itself (Lang, 2013). As the star ages, it continues to expand and the hydrogen gas available for fuel is used up.
The star collapses under its own weight and all the matter in the core is compressed causing it to be being heated up again. At this stage, the hydrogen in the core of the star is used up and the star burns up more complex elements including carbon, nitrogen, and oxygen as fuels. The surface therefore cools down and a red giant star, which is 100 times larger than the original yellow star, is formed. From this stage, the path followed in the cycle is determined by the individual mass of a star.
Path for Low Mass Stars
For low mass stars, which are about the same size as the Sun, a helium fusion process begins where the helium making up the core of the star fuses into carbon. At this stage, a different heating process from the original hydrogen nuclear fusion process occurs. Al-Khalili (2012) explains that due to the compression heat, the helium atoms are forced together to make heavier elements.
When this occurs, the star begins to shrink and during this process, materials are ejected to form a bright planetary nebula that drifts away. The remaining core turns into a small white dwarf star, which has an extremely high temperature. The white dwarf is capable of burning for a few billion years but eventually it cools. When this happens, a black crystalline object referred to as a black dwarf is formed.
Path for High Mass Stars
For high-mass stars which are significantly bigger than the Sun, the carbon produced from helium fission fuses with oxygen. More complex reactions occur and eventually an iron core is formed at the center of the star. Since this iron does not fuel the nuclear fission process, the outward pressure provided by the previous nuclear process does not occur and the star collapses.
The collapse leads to a supernova explosion. Green and Burnell (2004) describe a Supernova as the “explosive death of a star” (p.164). During this explosion, the star produces an extreme amount of energy, some of which is carried away by a rapidly expanding shell of gas. The exploding star attains a brightness of 100 million suns although this amount of energy release can only last for a short duration of time.
For stars that are about five to ten times heavier than the sun, the supernova is followed by a collapse of the remaining core to form a neutron star or pulsar.
As the name suggests, neutron stars are made up of neutrons produced from the action of the supernova on the protons and electrons previously available in the star (Krumenaker, 2005). These stars have a very high density and a small surface area since their diameter stretches for only 20km (Al-Khalili, 2012). If the neutron star exhibits rapid spinning motion, it is referred to as a pulsar.
For stars that are 30 to 50 times heavier than the Sun, the explosion and supernova formation lead to the formation of a black hole. In this case, the core of the star has a very high gravitational pull that prevents protons and neutrons from combining.
Due to their immense gravitational pull, black holes swallow up objects surrounding them including stars and they lead to a distortion of the space. Parker (2009) observes that the gravity of the black hole is so strong that even light is unable to escape from this pull. The only substance thing that black holes emit is radiation mostly in the form of X-rays.
Conclusion
This paper set out to provide an informative description of the life cycle of a star. It started with nothing but modern astronomy has made it possible for mankind to come up with a convincing sequence for the life cycle of a star. The paper has noted that all stars are formed from a nebula cloud.
It has revealed that the life expectancy of stars can vary from a million to many billions of years depending on their mass. A star begins to die when it runs out of hydrogen and the fusion reaction can no longer occur. The paper has also demonstrated that the death of a star is dependent on its mass. If a star is the size of the Sun, it will die off as a white dwarf while if it is significantly bigger, it will have an explosive death as a supernova.
References
Al-Khalili, J. (2012). Black Holes, Wormholes, and Time Machines. Boston: CRC Press.
Green, S.F., & Burnell, J. (2004). An Introduction to the Sun and Stars. Cambridge: Cambridge University Press.
Krumenaker, L. (2005). The Characteristics and the Life Cycle of Stars: An Anthology of Current Thought. NY: The Rosen Publishing Group.
Lang, R.K. (2013). The Life and Death of Stars. Cambridge: Cambridge University Press.
NASA. (2010). The Life Cycles of Stars: How Supernovae Are Formed. Web.
Parker, K. (2009). Black Holes. London: Marshall Cavendish.