Life Cycle of Stars
Stars are just as mortal as us, they also born and they die too, after weaving a long span of electromagnetic spectrum with waves like radio waves, microwaves, or ultraviolet waves, besides emitting light. The humans witness this fascinating life of stars from a safe distance and even enjoy a frugal part of it by watching VIBGYOR here and there!
Hues as Clues
Each group of waves has its own wavelength, and thus different amount of energy, though all of them travel at the speed of light. It is this uniqueness of the each group of waves provide the clue regarding the present state of a star, from which the astronomers work out its past and future alongside its possible impact on the universe.
A Comic Breather
While astronomy remains a serious and ever challenging subject, the scientists don’t miss the scope to entertain them by churning amusing titles for serious jobs. As for example, if someone hears an astronomer saying, “Oh be a fine girl! Kiss me!”, one should not be perturbed with any misconception about the nature of that astronomer. Because, “Oh be a fine girl, kiss me” is just a mnemonic created by the letters denoting the classification of the stars (O, B, A, F, G, K, M). These seven letters distinguish the stars into seven major categories by evaluating their spectra (ordered array of the waves) and temperature.
However, the Hertzsprung -Russell (H-R) Diagram, essentially a graph, divides the stars into three very different types after considering the color of a star (spectral type or its surface temperature) against its luminosity (its natural brightness, or absolute magnitude). These three groups are known as ‘Main Sequence Stars’, ‘Supergiants’ and ‘White Dwarf’ (Neutron stars, or Black Holes).
Scientists estimate that the Universe was born 15000 million years ago, by virtue of a colossal explosion, which has been named as ‘Big Bang’. It was that explosion which had created energy, space, time and matter. It was very shortly after the explosion, atomic particles got together to form the gases like helium and hydrogen, which then went on to create the galaxies, stars and the Universe over millions of years and still they are at it. Thus the process that was started with expansion and change is still goes on.
The astronomers have estimated the estimated timeline of the events till now and it goes like below:
1000 million years after the Big Bang : The matters started coming together.
3000 million years after the above: Galaxies began to form, the quasars came into being a little later, which are the forerunner of galaxies, evolves further through another 5000 million years, before our galaxy, the Milky way formed into a shape of disc! Our galaxy is a member of a community of 100,000 million galaxies.
Thus the celestial hierarchy looks like below:
Big Bang to Nebula;
Nebula to Protostar;
Protostar to Tauri type star
Further consolidation of T.Tauri star leads to the Main Sequence star, which after the increase in its luminosity turns as Red Giant star.
Red Giant star turns Cepheid star, which gradually turns White/Black Dwarf, before becoming a Black Hole.
This estimation through Big Bang theory is still accepted in the astronomy world. However, it is still difficult to ascertain the exact nature of the events or to specifically deny any missing event that might have taken place somewhere in the middle.
Giant Nuclear Reactors
A tremendous atomic collision takes place in the center of the stars, which rip apart the atoms and alter their structure, in the process of which, they release an enormous amount of energy. This makes them hot and bright. Nuclear fusion at their core generates the power in them, mostly by converting hydrogen into helium. In spite of this constant process, stars have a steady period during their life span, when they transform hydrogen into helium. When they run out of hydrogen, they enter their last phase of life. Then the formed helium in them transforms into larger elements like carbon, oxygen or neon.
After the formation of the nebula, known as the ‘cosmic placenta’, stars are born in groups within a galaxy, which is then called ‘stellar nursery’. Most of them break up, while the rest are kept together by gravity. The rest of a star’s life depends on how big it is, but in opposite way – the bigger it is in size, the quicker it spends its stock of hydrogen fuel and thus dies earlier after a stormy life. Some even explode due to its massive size. However, most of the stars, like our own Sun, manage to maintain a stable life when they shine steadily.
The luminosity of a star helps to estimate the amount of energy it radiates. It varies with its surface temperature and with its radius, where the stars with higher surface temperature, like the ‘Blue Giants’, generates higher luminosity than the ‘Red Giants’, which are relatively colder. The observed brightness of a star is dependent on the factors like emission, intensity and distance.
Long life span
Most stars spend their life by fusing hydrogen into helium, so is the case of our sun, which has been doing it for some five billion years, and is expected to continue doing it for another five billion or so years. This hydrogen burning starts from the very center of the star, and moves its way out, leaving a core of helium behind.
The Last Days
With age, the stars expand. Their cores gradually run out of hydrogen and then helium, expanding the core contacts and the outer layers, which become cool and loose its brightness. This is the last stage of the stars, before meeting the death according to their size:
Sun-like Stars (under 1.5 times the mass of Sun) –> Red Giant –> Planetary Nebula –>White Dwarf –> Black Dwarf ;
Huge Stars (between 1.5 to 3 times the mass of Sun) –> Red SuperGiant –> Supernova –> Neutron Star;
Giant Stars (over 3 times the mass of Sun) –> Red SuperGiant –> Supernova –> Black Hole.
The life cycle of stars resembles everyday recycling process in our lives: certain components (gases) are gathered and then, with the help of a catalyst (gravitational force), those components become bound into cohesion to give birth to a new material (star), all to worn out, before regrouping again under the same influences.
Stars (2003). 3 Nov, 2007. http://curious.astro.cornell.edu/stars.php
Space (1993). Universe (pp. 273-284). London, Great Britain: Dorling Kindersley.