According to the Big Bang theory, after the initial "explosion" the universe began to cool due its rapid expansion. As temperatures decreased, the simplest possible atomic structure formed: a single proton which makes up the hydrogen nucleus. When the universe's temperature dropped below about 3 billion Kelvin several minutes after the Big Bang, an isotope of hydrogen known as deuterium (one proton and one neutron) could form and remain stable. Soon the deuterium collected another neutron and became tritium, yet another isotope of hydrogen. As the energy continued to dissipate, tritium molecules collected a proton to become a helium nucleus.
This process, known as cosmic nucleosynthesis, continued to convert elemental hydrogen into helium. After about 15 minutes, the temperature of the universe had decreased such that there was not sufficient energy to maintain the nuclear reactions that fuse helium into heavier elements. Soon, not even hydrogen reactions could be sustained. At this point, the universe was approximately 90% hydrogen, 10% helium, with traces of of the next two elements, lithium and beryllium. That leaves 88 naturally occuring elements unaccounted for and uncreated.
As the universe aged, stars formed and began the creation of elements anew. In fact, the nuclear reactions (and sometimes violent explosions) that occur within stars create all of the naturally occuring elements except hydrogen. Not only do stars give birth to the heavier elements, but they have a seeding mechanism to spread the elements throughout the galaxies. In turn, this dispersed matter forms other stars, their companion planets, and us. I find it quite interesting that the process is not really that different from the processes that perpetuate and spread life on our planet: birth, growth, maturity, procreation, death.
Stars are initially composed primarily of hydrogen. As a star forms, the gravity compresses the matter in the star until sufficient pressure and temperature is reached to initiate nuclear fusion. The process converts hydrogen to helium, releasing energy. The energy that reaches Earth from our sun is caused by the fusion of hydrogen atoms into helium atoms. The gas pressure caused by high temperatures within a star counteracts the gravity pulling the star's matter into its center. For most stars, this balance between collapse and explosion is maintained for billions of years.
Stars evolve differently depending on their mass. A star like Earth's sun has modest mass and will burn for about 10 billion years. Stars that contain about 10 times as much mass as our sun, however, burn through their fuel very quickly when compared to our sun. These stars last only about 35 million years before turning into what is known as Type II Supernova.
As the massive star's hydrogen decreases, the star contracts, resulting in higher temperatures. This, in turn, fuses helium atoms into carbon and oxygen atoms. As the helium is exhausted, the star continues to contract and the carbon and oxygen atoms are fused into neon and magnesium. Eventually, the neon and magnesium are fused into silicon and sulfur. The silicon and sulfur then fuse into iron.
During this process, the heavier elements displace lighter elements at the center of the star and the star continues to contract. If a cross-section of the star could be examined, it would look much like an onion with the heavier elements at the core of the star and the lighter elements composing layers further and further from the center.
At the point of iron-creating fusion, however, the situation changes. The nuclear properties of iron are different than the lighter elements from which it came. Although iron can fuse with other elements, the reaction does not generate enough energy to sustain itself, so nuclear fusion stops. Now the delicate dance between gravity and pressure is disrupted. With no expanding gas to counter the effect of gravity, the iron core collapses within a fraction of a second. It collapses to its maximum density and then rebounds, accompanied by titanic shock waves.
As the shock waves move through the layers of the star, they cause a huge explosion to occur. The explosion completely blasts the outer layers away at about 10,000 miles per second. The tremendous heat caused by this explosion allows iron to fuse into even heavier elements. Indeed, a Type II Supernova is the only way that elements heavier than iron are created and seeded throughout the universe. All the gold, lead, silver, and uranium on earth was, at one time, iron atoms seething in a distant star.
The story of a Type II supernova does not stop with the explosion however. A huge amount of matter is left behind after the explosion. This completely collapses onto itself until all atomic structure is broken and the electrons and protons are combined into neutrons, resulting in a rapidly spinning neutron star.
A Type 1a supernova, on the other hand, begins as a star about the same size as our sun (but no more than about five times its mass) orbiting a second, similar star. This is known as a binary star system.
When a binary star about the size of our sun begins to run out of hydrogen, the balance between gravity and the expanding gas within the star shifts. As a result, the star begins to expand, eventually turning into what is called a red giant. The core of the red giant continues to contract however, fusing helium into carbon and then into other elements, pushing the outer layers of the star away. Eventually only the dense, luminous core remains as a white dwarf.
Eventually, its partner star in the binary system also turns into a red giant. As the second star expands, however, the gravity of the white dwarf draws the outer layers of the red giant towards itself, much like water circling a drain. Over time this process continues until the white dwarf has a mass about 40% more than our sun. At this point, a runaway nuclear reaction causes the white dwarf to explode.
Type 1a supernova are very important to astronomers because they always explode at 1.4 times the mass of the sun and all have the same characteristics, including how bright they shine. This allows astronomers to measure the distance to a Type 1a supernova by simply measuring its brightness.
The explosion of a Type 1a supernova is about 15 billion times brighter than our sun and completely destroys the white dwarf. The core of the red giant is catapulted out of orbit and its outer layers are blown into space where they will, over time, collect and create yet another star system.
Recently (December 2009), evidence has surfaced of another type of supernova known as a pair-instability supernova. Observation and analysis of a 2007 supernova (SN2007bi) indicate that a star containing about 200 times the mass of the sun exploded with a brightness 50 to 100 times that normally seen in a supernova.
This event seems to validate that super-massive stars can form and that pair-instability supernovii are possible. In such a supernova, the super massive star has exhausted its supply of hydrogen and helium, leaving a core of mostly oxygen. In smaller stars, this process continues until the core is iron and it explodes in a Type II supernova. In this case, however, while the core is still oxygen, super energetic photons are released that create electrons and positrons (electron anti-matter opposite particle). When the matter and anti-matter meet, they completely annihilate each other, reducing the star's pressure and forcing a collapse and burning up the oxygen so completely (in a run away nuclear explosion) that nothing of the star remains. The gas and material generated by the explosion spreads throughout its immediate stellar neighborhood.
God has created a mechanism by which the stars regenerate themselves through a process remarkably similar to the organic processes we see on earth. Everything we touch or see around us, including the elements that make up our own physical bodies, was transformed in the nuclear crucible of a distant star into something new and then blasted into space in an ongoing process of solar and planetary genesis.
The creation story in Abraham 4 (http://scriptures.lds.org/en/abr/4) is quite interesting in its wording. In verses 10, 12, and 18, the creation seems to be defined more as a process than an event. The Gods started a process and watched as they were "obeyed". I believe these eternal processes are still in operation as they continue to obey God's initial commands. What a blessing it is to live in a time when we're understanding more and more of God's majestic creations.