THE CHEMISTRY OF STARS

“Bright star, would I were stedfast as thou art—

         Not in lone splendour hung aloft the night

And watching, with eternal lids apart,

         Like nature’s patient, sleepless Eremite,

The moving waters at their priestlike task

         Of pure ablution round earth’s human shores”.

So wrote the English poet John Keats hinting perhaps that stars could be regarded as symbols of constancy. In a sense such an inference would be justified as stars do seem so unchanging. If you go to sleep tonight, like Rip Van Winkle, and wake up after thousand years you would doubtless find quite a lot of changes on earth. But you would find that the stars in Orion, or in any other constellation, still look virtually the same. A thousand years is but a moment in the life of a star.

Stars are created from whirlpools in the clouds of dust and gas that drift through interstellar space. As the whirling stuff of each protostar gathers material from the surrounding cloud, the temperature in its dense core increases, until one day it is sufficient to break up the nucleus of an atom. This process, known as nuclear fusion, releases tremendous amounts of energy. Light and heat pour out. A star is born. Stars behave in an erratic manner in their youth; they sputter and flash and get themselves sorted out. Soon most stars settle down to a stable career on what Astrophysicists call the “main sequence”. A star is held together by gravity. The energy being radiated from its core prevents it from collapsing.

One fantastic aspect of stars is their spectacular energy output. Even the sun, which is just an ordinary star converts five millions tons of matter into energy every second. The earth intercepts only a tiny fraction of this radiation — about a millionth of one per cent; this alone is sufficient to support all life on this planet, warm us in the morning and give us a nasty burn or even a heat stroke if we stay out all day. But all this energy is perhaps a by-product of what would appear to be the real work of a star, the conversion of simple elements into more complicated ones.

The cloud from which a star is formed is made up mostly of hydrogen. The core of the star breaks down the nuclei of hydrogen atoms and recombines their particles into the heavier nuclei of more complicated elements. The star is thus a machine for turning hydrogen into helium, into carbon, oxygen, iron.

A normal star has its limitations. Its alchemical feats ex­haust themselves at the level of iron. The nuclei of iron atoms are bound together so firmly that not even the heat in the core of a main sequence star can remodel them into something heavier. Since elements heavier than iron exist, where did their atoms come from? The answer is that they were forged in the death throes of stars!

A star’s life-span is determined by its mass. Ordinary stars such as the sun, burn their hydrogen fuel at a relatively frugal rate, and may perk along the main sequence for many millennia; the sun which is a “white dwarf” is  about 4.7 billion years old, and may have another five billions years of stable life left in it.

The brilliant Indian astrophysicist Chandrasekhar worked out the “Limit” which is approximately 1.4 times the mass of the sun below which a “white dwarf” (a star like our sun), will stay a “white dwarf” forever. If however a star exceeds this mass, it is destined to end its life in that most violent of explosions, a supernova, burn more vigorously and squander its fuel much more rapidly. Such stars race through their careers in as little as a few million years. Chandrasekhar was awarded the Nobel Prize for discovering the “Chandrasekar limit”.

Stars die in accordance with their stature. Ordinary stars, no longer able to maintain the balance between gravity and radiation that sustained them on the main sequence, shrug off their outer layers as a shell of gas that expands into surround­ing space and mingles with the interstellar clouds from which succeeding generations of stars and planets are born. Dying stars in our galaxy give out enough material every year to make half-dozen solar systems; Gas shells litter our neighbourhood like skins shed by snakes. They contain billions of tons of iron and lighter elements, concocted within the stars. But where did the elements heavier than iron come from?

Massive stars die violently. When their cores exhaust their fuel and can no longer prop up the walls of the star, the outer envelope collapses. Temperatures at the centre sky­rocket resulting in a supernova (referred to above). Supernovas erupt spectacularly so as to become conversation pieces for observers in galaxies millions of light years away.(In the year 1054 a supernova was first observed by Chinese astronomers, and it remained visible until April 1056. The event was recorded by them; references to it are also found in a Japanese document, of the 13th century).

Upon the forge of their eruptive violence atoms of heavier elements than iron are hammered out. Important elements like Nickel, Copper Molybdenum, Gold and Uranium are made here.

Proxima Centauri, the closest star to our own, is 4.25 light years away (a light year is the distance travelled by light at the speed of 186000 miles per second for a whole year i.e. 5.879 multiplied by 10 raised to the power of twelve!

Therefore the voyager space craft travelling at a speed of 40000 miles per hour would be able to reach Proxima Centauri, in 73,000 years. In biological terms it would mean about 300 generations of humans! This automatically implies that humans can never travel to Proxima Centauri except in Science fiction books and films!

However if someone asked you how you would touch a star, you would probably say that the question should be answered by mystics. But we must realise that Metals were bequeathed us by stars that died as supernovas before the creation of the solar system itself. The skins of those dead stars strewn across the interstellar medium became part of the substance from which out solar system was formed. Each time we use a metal in some form we enjoy the benefits of a stellar inheritance. When we touch metals, we are actually touching what was once part of a star. The medals that Olympic athletes, and Nobel Laureates are given are made of the stuff of stars.

So are the materials used to build telescopes to look at the stars and the metal used to make iPads and computers that type these lines about stars. So you can touch a star by touching almost anything a pen, a typewriter or a watch. In fact the entire cosmos is made of the same materials, and obeys the same physical laws as earth. The more we learn about stars, the more we learn about the origin of the universe, the creation of the galaxies, the solar system, and the appearance of the spark of life, on this tiny planet. The study of stars represents one of the most exciting chapters in the history of Physics. It is a field in which already several Nobel Prizes have been won by some of the greatest brains — Eddington, Hewish, Ryle, and Chandrasekhar, to name just a few.