The human life cycle (“cycle” taken literally, e.g. genes passed on to the next generation; or figuratively, e.g. reincarnation) is comparable to the life cycle of a star. Just as the “star” begins as a protostar, then a main sequence star, a red giant or a supergiant, and eventually a white dwarf or a black hole; human life begins in the form of an embryo then a fetus, then through infancy, adulthood, old age and eventually, death.
Life Begins: Protostar = Embryonic and Fetal Development
The life of a star begins in the nebula, where the clumping of interstellar medium (gas and dust) takes place. Eventually, nuclear fusion takes place where hydrogen is fused into helium in the core. For humans, life begins when the union of sperm and egg cells takes place. This union is called fertilization.
|A star is born|
In order to achieve life as a star, the protostar will need to achieve equilibrium, where a balance between gravity pulling atoms toward the center and gas pressure pushing heat away from the center should be maintained. Without this balance, the star ends up as a brown dwarf (never making it to “star status”) or worse, the star eventually dies. For humans, for a normal fetus to take shape, the number of chromosomes from both parents should be 46 (23 from each). Without this balance, abnormalities take place and worse, the fetus eventually dies.
Life Carries On: Main Sequence Star= Infancy through Adulthood
The majority of life of a star is spent being a main sequence star. After thousands of years, some stars eventually achieve nuclear fusion (thus giving them “star status”) and they readily radiate energy into space. For humans, after nine months, some fetuses are successfully born, readily radiating energy into the family and into the human gene pool.
|Enjoying the best days of youth|
The star’s main goal in life is to achieve stability, or equilibrium. In a stable star, the gas pressure pushing out from the center is equal with the gravity pulling atoms inward to the center. Humans, also, also aim to achieve balance in their lives: physically, emotionally, mentally, spiritually and socially. They employ different techniques to achieve stability, which include going in a married life to “balance out” their personal (and financial) lives.
Just as the battle between gravity pulling in and gas pressure pushing out will go on over the entire life span of the star, oxygen being inhaled in and carbon dioxide being exhaled out will also go on over the entire life span of a human being. For humans, the Earth’s gravity against one’s weight can also be considered a “battle.”
Longevity: A Matter of Mass
While larger stars have more fuel, they have to burn it faster in order to maintain equilibrium. Large stars, then, use all of their fuel in a shorter length of time. This means that bigger is not better with respect to how long a star will live. A smaller star has less fuel, but its rate of fusion is not as fast. Therefore, smaller stars live longer than larger stars because their rate of fuel consumption is not as rapid. This could be comparable to humans where being big is not always better. In this day and age, obesity equates with susceptibility to non-infectious diseases and thus shorter lives; while being in a healthy weight equates with a longer and more fulfilling life.
The Senior Life: Red Giant or Supergiant = Old Age
Fusion reactions need a fuel, and there are three main fuels that a star uses for fusion: hydrogen, helium, and carbon. As the star ages, if it still wants to maintain equilibrium between gravity and gas pressure, it needs increased temperatures in the core to re-ignite fusion. The star is forced to burn helium in an effort to maintain stability. This results in a red giant, which is the first step in a star’s old age. Eventually, the core will run out of helium, and in order to maintain equilibrium, the core will contract again to produce a supergiant.
|The struggle to survive at its peak|
It takes a temperature of 10×107 °K to initiate helium burning, whereas it only takes a temperature of 2×107 °K to initiate hydrogen burning. Helium burning can be compared to the metabolism of older people, where they need to exert more effort in burning calories because of their decreased metabolism. Since older people lose muscle in a faster rate than younger people do, they need to exercise more and eat less to help them maintain their weights and strengths.
Goodbye, Universe: White Dwarf or Black Hole = Death
In theory, gravity wins.
With the decrease in fuel for fusion, the temperature decreases and the rate of collapse increases. A white dwarf results when the solar mass turns 0.5 to 3.0; anything larger than this results in a black hole. Just before the star totally collapses, there is a sudden increase in temperature, density, and pressure. This is comparable to the human’s last resort to keeping up with life, when they give their “last breaths” just before succumbing to death.
|Everything, eventually, shall come to an end|
When a star dies, its outer layers are puffed off to produce expanding planetary nebulae. These nebulae, which are the remains of dying stars, serve as the birthplace for future protostars. This is no different from reproductively-fit humans where even in death, they are able to leave something behind to the future generations through the genes they have passed on.
A protostar is similar to human pregnancy – the star is forming, gaining mass, and it’s not truly "born" yet. Once nuclear fusion begins, the star is born. Stars live the majority of their lives in the main sequence phase, and then as they destabilize and burn fuels other than hydrogen, they expand, reduce in core temperature, and eventually die.
It is interesting to think that while we get too preoccupied with our personal struggles, trying to stay sane while balancing all the responsibilities on our shoulders; the universe we live in also has its own struggles while trying to ease out its own differences to make the world a better place for us to live in. We are but young inhabitants enjoying, exploiting, the billions and billions of years of struggle of these cosmic elements.
What a sacrifice to make for a universe that we barely strive to get in touch with.
Campbell, N. 1999. Biology 5th ed. California, USA: Benjamin/Cummings.
Morris, R. 1993. Cosmic Questions. Canada: John Wiley and Sons, Inc.
Seeds, Michael. 2001. Stars and Galaxies 2nd ed. Canada: Thomson Learning.