Life and Death of the

The Sun, like all other stars, was born from a cloud of interstellar dust and gas that began to condense under gravity. As the pressure increased at the center of the cloud, temperatures rose as a result of a release of gravitational energy and when sufficient mass, temperature and pressure had been built up, nuclear fusion began to take place at the core of the newborn Sun. From the time the first contraction of the interstellar cloud takes place to the first fusion takes approximately 30 million years for a star the mass of the Sun. Material not pulled into the core of the newborn star may make smaller stars, brown dwarfs or planets. In the simplest and most efficient form of fusion, two hydrogen atoms are fused together to make one helium atom. The energy released as a result of this reaction causes the star to emit radiation across the entire electromagnetic spectrum, including the visible range. Depending upon how much hydrogen is available to a given star, it will burn somewhere along the main sequence. Ironically, the larger the star, the hotter it will burn and the exponentially faster it will run through it fuel. The Sun is estimated to have approximately a ten billion year supply of hydrogen to fuse during its main-sequence lifetime. Massive, hot blue stars like Rigel will only live for a few hundred million years before exhausting their fuel, while tiny, cool red stars like Proxima Centauri can burn for many times longer than the Sun can ever hope to live without ever changing.

The Sun has been converting hydrogen into helium for some five billion years and will continue to do so for about five billion more. In the meantime the Sun is converting billions of tons of hydrogen each second into helium and that helium is accumulating in the core of the Sun. At present the pressure in the Sun is insufficient to fuse helium into heavier elements so it sits in the Sun's core accumulating. The energy radiating outward from the core precisely balances against the mass of the Sun trying to collapse inward under gravity, maintaining the Sun in a steady state. When the Sun's supply of hydrogen begins to run out, the energy output will fall off and the core of the Sun will collapse under gravity. As the core collapses, gravitational energy will cause the core to again begin fusing hydrogen, but only in a narrow ring around the outer edge of the still collapsing core. The renewed fusion will then begin to push the outer layers of the Sun away into space, enormously increasing its surface area while temperatures at the surface cool dramatically. The combination of increased surface area will be roughly offset by cooling temperature causing the Sun's total luminosity to remain constant.

Meanwhile at the center of the Sun, inside the shell of fusing hydrogen, the helium rich core is still collapsing and still increasing in temperature. When core temperatures reach about 100 million degrees Celsius, the helium begins to fuse into beryllium, then carbon and oxygen. This reaction is not nearly as efficient as was the hydrogen fusion so as helium becomes the Sun's dominant source of output; the Sun will cool and become red in color. The outer layers of the Sun will escape into space forming a shell that reflects light from the dying core called a "planetary nebula." Eventually when most of the helium has been fused into heavier elements, fusion ceases and all that is left is the solid collapsed core. The Sun lacks sufficient mass to fuse carbon and oxygen into heavier elements. More massive stars may have multiple shells surrounding their cores performing fusion of hydrogen, helium, beryllium, carbon, oxygen, silicon, sulfur and finally a core of iron. What will happen then, we'll discuss in a later chapter. For the Sun, its continually collapsing core will create pressures so extreme that even atoms cannot stand up to it. Electrons will be crushed into the nuclei of their atoms creating heavy neutrons (called "degenerate matter") and emitting white light from the Earth-sized remnant. The degenerate core, consisting entirely of neutrons will become so dense that a teaspoon of it will weigh a ton! The dead Sun will continue to glow in this way for many billions of years before extinguishing into a dark heap of degenerate matter.

While the Sun will end its life in an astronomical whimper, for today it is the great source of power and life in the inner solar system and the greatest natural nuclear physics laboratory available to us. Let's take a closer look at our amazing Sun and unlock some of its secrets.

Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment