The Formation of the Galaxies:

After the Big Bang, The brand-new universe immediately began to expand---and expand, and expand...Althought the early universe was unimaginably hot, it cooled as it swelled. A few second after the explosion, the temperature of the universe was reduced to a few billion degrees, allowing for the formation of tiny subatomic particles---protons, neutrons, electrons, nuetrinos, and phototons. After about 100 seconds, the temperature was about one billion degrees, and protons and neutrons came togethr to make the nuclei of the lightest chemical elements, hydrogen and helium. Within munitess the temperature of the universe was only 100 million degrees, too low for more nuclei to form. Hydrogen and helium are still the most abundant elements in the universe.

Although the universe continued to expand, gravity brought some of the matter together in giant swirling clouds of gas--clouds that would eventually become galaxies. Even now, astronomers are able to see the birth and evolution of galaxies, since looking farther into space is the same as looking back in time. Light waves must travel through space across billions of light years before they reach earth. When the astronomer sees (throug a telescope) as galaxy at the edge of the universe, billions of Light years away, she is actually seeing what went on billions of years ago; since it has taken billions of years for the light waves from the distant galaxy to travel to the Earth. All galaxies, including our Milky Way, formed a few million years after the universe began.

Matter was not evenly distributed within the clouds of turbulent gas and dust that comprised the early galaxies. At some point, however, a small regioj with a higher density of material than that of surrounding regions (and therefore, a stronger gravitational pull) began to attract more and more material into itself. As matter comes together, particles impact and release energy; ultimately the center of this cloud becames a dense, hot prostar. Once the prostar became hot enough, it began to fuse hydrogen into helium, becoming a full-fledged star. Once the outward pressure exerted by the cloud of burning gases reached the point at which it balances the inward pull of gravity, the universe saw the birth of its first stable star.

Our sun's life thus far has been pretty typical for a star of its size. It turns out that it's probably not uncommon for stars to have planets in orbit around them. How did the planets of our solar system come about?

About five billion years ago, as shock wasve from a nearby supernova explosion broke a cloud of interstellar gas and dust into smaller clouds. One small cloud---our solar nubula--contained all the chemical elements (and even some organic molecules) necessary to form our planet. Astronomers have concluded that at least one supernova exploded in our region of space some time before the formation of our solar system because our Earth contains gold (AU), lead (Pb), uranium (u) and other heavy elements know to form only when particles come together, they release gravitational potential energy; when they stop, they release kinetic energy. Therefore, the nebular cloud grew hotter as it collapsed. Particles combined, increasing their gravitational pull and drawin in other particles to join them. The collapsing cloud also began to spin due to angular momentum caused the rest of the material to spread outward from the center in a great flat disk.

The center of the cloud was massive. As more and more particles were drawn into the cloud, more adn more energy was continuously released. Eventually the cloud became quite hot and became a protostar. After about 50 million years the protosun became so hot it ignited, triggering a chain of conversions of hydrogen to helium. Our star was born. Gas pressure built up in the burning star, ending the process of gravitational collapse and creating the Sun as a stable star.

At the same time that the Sun was forming, material was coming together to form what would become the planets. As the nebula cooled, matter condensed; first the metals, then the rock, and later the ices. The heavier materials---metals and rocks were pulled in closer to the center, while the ligher materials-gases and ices (called volatiles)--were left in the outer reaches. Volatiles near the Sun immediately vaporized. the materials swirling around the Sun collided with each other until dust particles became pebbles, pebbles became rocks, and rocks became boulders. The larger an object got, the greater was its gravitational pull on the other object; and the more material the object could attract to itself, the larger it grew. Boulders eventually became plantertesimals, asteriod-like bodies whose collisions would ultimately result in the planets we see orbiting our Sun today.

The Sun and planets formed simultaneously about 4.6 billion years ago. Accretion of each of the planets and a little shorter for the smaller inner planets. After formation, planey orbits were still full of debris, which for the first few hundred million years bombarded the planets. Most of the solar system's bodies carry the scars of these frequent impacts.

Earth was also created from asteroids and comets, dust and plantesimals. The numerous impact, gravitational potential energy and radioactivity involved in this aggregation of matter created an intense heat that cause the new planet ot melt. Volatiles vaporized and were lost, while the rocks and metals formed a dense molten soup. Gravity pulled the heaviest molecules--primarily iron and nickel metals into the center. Meanwhile, the lighter stony liquids rose to form an early crust, and the heavier stony materials became the mantle. The formation of core, mantle, and crust--called differentiation-- was the basic process underlying the formation of each of the other planets and satellites in the solar system.

NEXT LESSON: NEBULAR HYPOTHESIS OF STAR FORMATION.




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