Biography of Our Star: The Sun
By Quamrul Haider
The Sun is a 4.55 billion year old dazzling, luminous, spherical ball of hot gas that we see rising every day at the crack of dawn in the eastern sky. Its home address is the spiral arm of the galaxy called Milky Way. There are billions of stars like the Sun in the galaxy, but it is the closest to Earth only eight light-minutes away, or 150 million kilometres. Thus, when we see the Sun in the sky, we see it as it was eight minutes ago.
By galactic standards, the Sun is an ordinary star neither too small, nor too large, having average qualities among the family of all stars seen in the Universe. Although the Sun accounts for 99.9 percent of all the matter in our solar system, it is an entirely different kind of astronomical object from any planet or moon.
To the naked eye, the Sun may seem to shine peacefully, but through a telescope, its surface will appear violently agitated, with rising fountains of incandescent gas and twisted magnetic field lines. Even greater violence wracks its core where the temperature is about 15 million degrees Celsius. There, a nuclear furnace every second burns 600 million tons of hydrogen into helium, producing in one heartbeat the energy of 100 billion nuclear bombs. Yet, its average surface temperature is a cool 6000 degrees Celsius.
The evolutionary changes that gave rise to the Suns birth happened incredibly slowly. According to Nebular Theory, the Sun was conceived when an interstellar cloud of cold, dark, tenuous mass of gas, mostly made up of hydrogen and helium atoms, collapsed under its own weight. The collapse was caused perhaps by collision with a neighbouring cloud or the explosion of a nearby star.
As the cloud shrank under the influence of its increasing weight, it grew smaller and hotter. After a few million years, gravity flattened the cloud into a spinning disk and hydrogen began to fuse into helium. The energy released by the fusion raised the pressure inside and stopped further collapse. By then, enough matter has accumulated at the center of the disk to give birth to the Sun a yellow, low-mass star. The gestation period lasted about 100 million years
After spending a few million years in the stellar nursery, the Sun moved to the main sequence and will remain there until old age, waiting to die. By the way, a main sequence star is not a type of star but a time period in the evolutionary history of a star, typically lasting 90 percent of its lifetime.
The energy source that powers the Sun lies buried at its core, where thermonuclear reaction converts hydrogen into helium. The reaction is accompanied by the release of an enormous amount of energy that keeps the Sun alive and shining. The energy ultimately leaves its body, that lights up our days and sustains life on Earth.
For its survival, the Sun is locked in a constant struggle against the relentless force of gravity. It resists gravitational collapse by the outward pressure generated from the extremely hot gas in its interior.
According to astrophysicists, the Sun will die after 5 billion years. The cause of death will be energy crisis running out of hydrogen. Before death, it will shrink and gradually become hotter. The rising temperature in the core will make the remaining hydrogen burn faster, generating more energy.
Before its death, the Sun will rapidly grow in size and become much more luminous than it is today. It will become a swollen red giant pumping out a thousand times more energy, transforming Earth into a molten ball of rock. However, the red giant Sun may melt the water-rich but now-frozen moons of Jupiter and Saturn. Humanity, if it is still around, might relocate there.
The Sun will shine as a red giant for perhaps another billion years. It will shrink again and move out of the main sequence, become even hotter and turn into a yellow giant. During this stage of its life, the Sun will pulsate as if it is taking slow, deep breaths, but these will be the dying gasps of an old and frail star. It will once again change into a red giant, but a giant even larger and more luminous than before and swallow up everything within its reach, including the Earth. The final red-giant stage can be maintained for approximately 100 million years.
The Sun will no longer be able to support itself because gravitational force exerted by the material in its outer part will force the inner part to condense and heat up. It will become an expanding shell of gas called planetary nebula.
The high temperature in the core of the nebula will be its death warrant. It will drive the outer half of the Sun streaming away through the solar atmosphere into space as radiation and finally strip the Sun to its bare core. Fiercely hot but with no further energy supply, it will eventually cool down to become a white dwarf, little larger than the Earth, but about 200,000 times more massive.
The Suns obituary should read: From an embryonic patch of drifting gas, waiting to collapse to a central core and burst forth with light 9.55 billion years ago, the Sun expanded, swallowed the Earth and ended its luminous career by dwindling into a white dwarf. n
The writer is a Professor of Physics
at Fordham University, New York.
Photos: Google Image
