The Evolution of our Sun

By: Grace Lam and Rina Sor

The center of our universe that is known as the Sun, did not appear all of a sudden one day. It became what it is today through a series of stages. The stages of evolution can be followed in the Hertzsprung-Russell (H-R) diagram. First is the pre-solar evolution stage and then is it the post solar evolution stage.

Pre-Solar Evolution Stage 

The pre-solar evolution consists of the Cloud, Globule and Protostar.

The Cloud

About 4.5 million years ago, there was a gas cloud with a diameter of 480 trillion kilometers (that is about 50 light years). It wasn't very dense. It could have either dissipated further into outer space or contracted into a denser configuration. It begun to condense and contract under the force of gravity then it became disturbed by either an outer impact or by a random condensation of a large number of cloud particles.

Gas Cloud from here.

The Globule 

Dark patches formed inside the cloud that contained nebulae and molecular clouds. This causeed the temperature of the cloud to raise slowly, but it was still not enough to radiate light. One of the globules eventually became the sun and it continued to condense, which then caused the temperature to increase.

Globule image from here.

Protostar

Within 400,000 years, the globule condensed to about one-millionth its original size. At the center of the globule, a core developed what had the ability to radiate energy into the less dense outer region of what used to be the globule.As the core radiates energy, it causes the condensation of the matter to slow down.The matter becomes opaque and the free fall is stopped; the core has now developed into a stable protostar.After about a few thousand years the protostar collapsed and the temperature reached to about 150,000 degrees (56,000K); this led to the ionization of atoms. At this point, the sun appeared very red. The size of the protostar after the collapse about the size of the orbit of Mars.

Image from here.

The Sun Forms

The protostar contracted enough for its temperature to be high enough to burn deuterium to form helium-3. While burning the deuterium, the contracting process slowed down. As the protostar continued to contract, the central temperature began to increase and the radiation temperature began to decrease. The process of convection stops and the radiative core grew outward. With the ignition of hydrogen, the protostar became a star, which is characterized with the gravitational stabilized fusion reactor located at its center. Nuclear reactions released vast amounts of subatomic energy, causing the sun to vary in luminosity and surface activity. After about 30 million years its structure stabilized into the structure of one solar mass, and it had enough fuel in the form of hydrogen to maintain shining for a period of time.

The Sun (currently)

The sun is currently is in the middle of its lifecycle (main sequence star).

This is the longest stage of the sun’s life and it will spend about ninety percent of its lifetime as a main-sequence star. The sun is no longer contracting because the intense heat in its core produces an outward pressure that balances the inward force of gravity.

Post-solar Evolution Stage

This stage consists of the Red Giant, White Dwarf, and Black Dwarf.

Red Giant

While sun is aging, helium is collected in the center. In 9 billion years, about 10% of the hydrogen will have converted to helium and nuclear fusion reactions will stop making energy. Under its gravitational attraction, the Sun slowly collaspses and the gravitational energy will convert into heat. That heat will burn the shell, swelling the Sun's outer layers. By now, the sun appears red and is now a red giant. In several million years to follow, the Sun will have swallowed Mercury, Venus and Earth.

White Dwarf

The solar core shrinks and has radiated the energy from when it was collaspsing.

Black Dwarf

When a white dwarf, it emits yellow light and red light. This light draws out thermal energy. The nuclei packs very tightly until its collaspsing is impossible. It will be come dark and cold then float through space until it will possibly find another giant gas cloud.  

Sources:

http://www.columbia.edu/~ah297/unesa/sun/sun-chapter5.html

http://astrosun2.astro.cornell.edu/academics/courses//astro201/evol_sun.htm

http://www.telescope.org/btl/lc1.html 

http://uk.encarta.msn.com/media_461518106_761562112_-1_1/evolution_of_the_sun.html

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