The Life Cycle of Stars
A star can be described as a luminous globe of gas that emits its own light and heat via nuclear fusion. Stars are formed from nebulae, and are largely made up of helium and hydrogen gas (Williams 2015). A star's life cycle is identified by its own mass; if it is larger, its life cycle will be shorter. To begin with, the nebula is the place where stars are produced. Nebulas are either due to stars either going supernova or collapsing, or the remainder of the big bang (Jessa 2016). As stars near their end of life, they become unstable they swell and explode, leaving behind a neutron star or black hole. The life cycle of stars has two paths: the medium stars and the massive stars. In each of these paths, the life of a star begins in a nebula when the star loses its nuclear fuel, becomes weak, and ends up as a white dwarf or neutron star (NASA 2015).
A nebula is an enormous cloud of dust and gas (mainly hydrogen) in space. We have various forms of the nebula. For instance, an Emission Nebula usually glows brightly as stars responsible for its formation energise the gas in the nebula. Orion nebula is a good example of an Emission Nebula. In a Refection Nebula, another type of nebula, starlight illuminates and is reflected on a nebula owing to the presence of grains of dust on it. A perfect example of a Reflection Nebula is the nebula that surrounds the Pleiades Cluster (Williams 2015). The third type of nebula is known as Dark Nebula. This refers to dense clouds consisting of molecular hydrogen. These either completely or partially adsorb the light emitted by stars behind them. A good example of a Dark Nebula is the Horse head Nebula (Williams 2015).
Star formation occurs inside molecular clouds, which consist of a comparatively dense concentration of dust and interstellar gas (Schneider & Arny 2014). Star formation following the collapse of the dense sections of the cloud core, triggered by its own gravity/weight. Since the outer cloud is less dense than the core, cores are usually the first to collapse. Following their collapse, cores fragment into clumps which then aggregate to form protostars in a process approximated to take some 10 million years (Schneider & Arny 2014).
Over time, the hydrogen gas gets pulled by the gravity in the nebula and begin to spin. When the gas spins faster, its heat rises up to form the early stage in the evolution of the protostar. Thereafter, the star becomes stable and contracts a bit. This is the main sequence star, and it will stay in this form for billions of years. Thereafter, nuclear fusion occurs, in which hydrogen is converted into helium (Cain 2015). When the hydrogen begins to run out of the star, it becomes unstable. The outer layers of the star which are mostly hydrogen start to grow. As it extends, it cools down and becomes a red giant star. At this stage, the hydrogen gas temperature in the outer layers keeps on rising, as does the temperature in the core of the star. The hydrogen gas in the outer layers is then blown away, thereby forming a ring called a planetary nebula. Thereafter, the medium size star begins to die. Gravity causes the last of the star’s matter to collapse, and this constitutes the last stage which is the white dwarf star.
The white dwarf star glows with white light. As the years pass, the white dwarf cools down and begins to dim. However, it is important to note that it will take billions of years for the white dwarf to stop glowing (NASA 2015). Once the white dwarf star runs out of energy, its light will stop glowing and it will die, turning into a black dwarf. It then remains in this state forever.
Massive stars are able to fuse with heavier elements inside their core. The star gets bigger until it fuses with elements that include iron. Once an iron core has been formed, there is no more energy left to facilitate fusion. Consequently, the core collapses. Thereafter, the supernova explosion happens, making the outer layer of the star blasted into space for the supergiant star (NASA 2015). All that is now left is the core which will turn into a neutron star. Furthermore, when the core collapses, the gravity is so strong that it becomes a black hole
Stars have two paths: the medium star path, and the massive star. In each path, the life star starts in the nebula where the protostar forms and grows to become a red giant in a medium star. The outer layers of the star separate, with the outer layers forming a planetary nebula. The core then turns to a white dwarf which eventually dies and becomes the dark dwarf in the massive star. The star grows to become a red supergiant. As the red supergiant grows, it becomes unstable. Thereafter, the supernova explosion happens, and all that remains now is the black hole or neutron star, depending on the star mass and the explosion.
References
Cain, F (2015). Nuclear fusion in stars. [Online].
Jessa, T (2016). Where are stars born? [Online].
NASA (2015). The Life Cycles of Stars. [Online]. NASA (2015). The Life Cycles of Stars: How Supernovae Are Formed. [Online].
Schneider, S & Arny, T (2014). Pathways to Astronomy, 4th Edition. New York: McGraw-Hill.
Williams, M (2015). Nebulae: What Are They Where Do They Come From? [Online].
