In the last lecture we reviewed the evidence that the Universe began some 10-20 billion years ago in a massive explosion known as the Big Bang. Now we turn to the time since.
At the end of the first three minutes, particles such as protons, neutrons and electrons were stable and began to bang into one another. Protons are positively charged, massive particles that together with neutrons (which have no electrical charge) make up the nucleus of atoms. Electrons are very light, negatively charged particles that scoot around in orbits around the nucleus.
Elements are defined by the number of protons that exist in the nucleus of an atom of each element. A lonely proton is the nucleus of the element hydrogen (H), oxygen (O) has eight protons and iron (Fe) has 26.
Also, each element comes in one or more flavors, depending on the number of neutrons mingling with the protons in the nucleus of the atom (the number of neutrons is always close to the number of protons, but does vary a little). These flavors are called "isotopes" of the given element. For example, carbon always has six protons, but can have six, seven or eight neutrons. The "atomic weight" of a particular atom is the sum of the weights of the protons and the neutrons in the nucleus of that atom, the isotope of carbon which has 6 protons and 8 neutrons in the nucleus is called "carbon-14". This is the radioactive isotope of carbon which is used to date archeological findings among other things (e.g. the shroud of Turin).
The important isotopes for the early universe are three isotopes of hydrogen -- normal hydrogen, with one proton and no neutrons, "deuterium", which has one of proton and one neutron, and "tritium", which has one proton and two neutrons -- and the isotope of helium which has two protons and two neutrons and is called "helium-4".
Electrical charges of the same sign repel one another. Protons are positively charged and therefore tend to fly apart, unless they get very very close to one another. If this happens, another force takes over and they stick to one another. The trick is that they have to collide with a tremendous speed to overcome the electrical force trying to keep them apart. This only happens at very high temperatures (60 million degrees or so). After the first three minutes after the Big Bang, things were cool enough for protons, neutrons and the like to exist but still hot enough for them to bang together and stick. However, with continued expansion, the universe cooled down and only about 10% of the protons were able to stick together to form helium (and a very tiny bit of other heavier elements). By weight, the universe was originally about 24% He and 76% H (with that tiny bit of somewhat heavier elements).
The universe didn't expand uniformly in all directions, but had patches with more and less material. A picture of microwave radiation taken by the COBE telescope reveals the texture of the early universe. These clumps collapsed eventually to form galaxies as seen in the deep field view from the Hubble Space Telescope.
Hydrogen and helium are nice, but how do you live on it? Somewhere, somehow the elements like oxygen, iron, magnesium, calcium, aluminum, etc. that we know and love had to be made. The hydrogen/helium soup that was made in the big bang collapsed to make stars. As the stars contract, gravitational energy is transformed into heat and once again temperatures were reached sufficient to "burn" hydrogen to make helium.
Once the atomic furnace ignites, the making of helium actually releases a tremendous amount of energy. This is because of the famous Einstein equation E=mc2. What does this equation mean? That when matter is converted to energy it does so proportional to the speed of light squared (a very big number). The mass of four hydrogen atoms (protons) is 6.696x10-24 g. The mass of one helium atom is 6.648x10-24 g. The incredibly tiny mass difference is converted to a large amount of energy. Burning one gram of hydrogen gives 250 trillion calories -- the same as eating about 5 billion Big Macs!
Our sun is mostly burning hydrogen, and the temperature at the center of the sun is about 60 million degrees. It gets harder and harder to burn elements heavier than hydrogen and helium (a star has to be about 3000 million degrees in order to burn silicon!), and eventually, it gets too hard. Energy is only released in making elements up to iron (26 protons). Heavier elements actually require energy to be put in, instead of releasing energy. Elements heavier than iron are only made in the violent death of the biggest stars - the supernova explosion (well, also in nuclear bombs, too). So the elements we are made of were actually made during the life and death of stars -- we really ARE star dust!
About 4.6 billion years ago, after perhaps a dozen generations of larger stars, a big gas cloud ("nebula") began to form. This was the beginning of our solar system. The nebula, mostly made of hydrogen and helium, but with some other stuff too, began to collapse into a disk. As the gas cloud cooled, minerals began to form and coalesce into dust particles. Slowly but surely, a big central clump of dust and gas collected, and when it got big enough and hot enough, it turned on -- the Sun was born.
When the sun ignited, it blasted off most of the lighter stuff from the inner solar system, leaving the heavy stuff like silicon, iron, and the like. The dust in the inner solar system began to stick together, or accrete, to make larger and larger bodies (planetesimals). The first few 100 million years was a raging bumper car festival of planetesimals and larger bodies, until by about 3.8 billion years ago, only the biggest bodies remained. While we are still bombarded by meteorites, it is now a pretty rare event that a large one hits us. The last really big one was a 10 km-wide body that hit the Earth some 65 million years ago, wiping out the last of the dinosaurs.
Perhaps during intial accretion, perhaps shortly thereafter, the Earth became a layered body, with much of the iron and nickel dropping down into the center (called the "core"), leaving the lighter silicon and oxygen (among other elements) in a "mantle" around the outside of this metallic core. The lightest stuff floated (and is still floating) to the top to form the crust on which we live.
The moon was probably created by one of the early impacts. Scientists now believe that the Earth was hit by a Mars sized body, which vaporized on impact. The core of the body joined our own core and the rocky outer mantle coalesced to form the moon.
Birth of the Moon
This picture is from a computer simulation of the origin of the Moon by the glancing impact of a Mars-sized body with the Earth at the end of the accretionary phase of the planets. Both bodies have already differentiated into mantle (metallic iron) and core (silicates). Following the collision, the mantle of the impactor is tossed into orbit and coalesces to form the Moon. The core of the impactor is swallowed up by Earth. (see Taylor, S.R., The origin of the Moon, American Scientist, 75, 469-477, 1987 for more).