Explain why nuclear fusion takes place only in the centers of stars rather than on their.....?

Question

Explain why nuclear fusion takes place only in the centers of stars rather than on their surface as well. What is the major fusion process that takes place in the Sun?

Answer 1

A star is born when atoms of light elements are squeezed under enough pressure for their nuclei to undergo fusion. All stars are the result of a balance of forces: the force of gravity compresses atoms in interstellar gas until the fusion reactions begin. And once the fusion reactions begin, they exert an outward pressure. As long as the inward force of gravity and the outward force generated by the fusion reactions are equal, the star remains stable.

Clouds of gas are common in our galaxy and in other galaxies like ours. These clouds are called nebulae. A typical nebula is many light-years across and contains enough mass to make several thousand stars the size of our sun. The majority of the gas in nebulae consists of molecules of hydrogen and helium--but most nebulae also contain atoms of other elements, as well as some surprisingly complex organic molecules. These heavier atoms are remnants of older stars, which have exploded in an event we call a supernova. The source of the organic molecules is still a mystery.

STAR BIRTHS are started when the interstellar matter in gas clouds, such as the Eagle Nebula shown here, compresses and fuses

Irregularities in the density of the gas causes a net gravitational force that pulls the gas molecules closer together. Some astronomers think that a gravitational or magnetic disturbance causes the nebula to collapse. As the gases collect, they lose potential energy, which results in an increase in temperature.

As the collapse continues, the temperature increases. The collapsing cloud separates into many smaller clouds, each of which may eventually become a star. The core of the cloud collapses faster than the outer parts, and the cloud begins to rotate faster and faster to conserve angular momentum. When the core reaches a temperature of about 2,000 degrees Kelvin, the molecules of hydrogen gas break apart into hydrogen atoms. Eventually the core reaches a temperature of 10,000 degrees Kelvin, and it begins to look like a star when fusion reactions begin. When it has collapsed to about 30 times the size of our sun, it becomes a protostar.

When the pressure and temperature in the core become great enough to sustain nuclear fusion, the outward pressure acts against the gravitational force. At this stage the core is about the size of our sun. The remaining dust envelope surrounding the star heats up and glows brightly in the infrared part of the spectrum. At this point the visible light from the new star cannot penetrate the envelope. Eventually, radiation pressure from the star blows away the envelope and the new star begins its evolution. The properties and lifetime of the new star depend on the amount of gas that remains trapped. A star like our sun has a lifetime of about 10 billion years and is just middle-aged, with another five billion years or so left.

Answer 2

The fusion process is brought about by huge volumes of gas pressing down onto the center of the ball. All of that pressure causes heat. The immense pressure and intense heat reaches some specific point and when that point is exceeded, fusion begins. The fusion that is most common is of Hydrogen gas into Helium gas. That nuclear fusion reaction produces more heat and a broad spectrum of light radiation which ranges from the visable to the ultra and infra red rays and includes X Rays and Gamma Rays.

To give you some idea of the pressures involved...The Sun has a diameter of 864,900 Miles. Compare that with the measley diameter of the Earth which is 7,926 Miles. So, on the Sun, you are dealing with (if you just look at a single slice of the star for purposes of discussion) a stack of hydrogen and helium gas 432,450 miles high and that big of a stack of gas weighs lots and lots of tons.

Answer 3

Brad was present at school that day I see....lol
Simply put. Gravity. The inside of stars are under pressures unimaginable on earth.
Everything a star burns, makes something else. A star like our sun is burning mostly hydrogen, that makes helium.
While a star acquires a helium core, as a result of core hydrogen burning, this core contracts and becomes hotter. Eventually, helium is ignited and burns. The product of helium burning is carbon and oxygen.
Astronomy was my favorite class and i did well at it. I should have pursued it, but everyone said a business degree was what I needed.

Answer 4

I know the others above me have gone into much greater detail, but I don't think they summarized their points very clearly.

Fusion occurs in the core specifically because this is the only place where the pressure and gravitational forces are strong enough for the atoms to break-apart into plasma (the 4th state of matter, arguably), and then start under-going fusion.

Mind you though, the others deserve the best answer for this for sure - they've gone into far more depth.