Summary of Our Improbable Universe
Chapter 4: Star Light Star Bright
When we look into the night sky we can see about four thousand stars. For every one that we can see there are a million in the Milky Way Galaxy alone that are too dim to be seen without a telescope. For every star in the Milk Way, there are a thousand galaxies in the universe. Amongst all of these stars, our sun is very typical. It will burn at a fairly steady rate for a total of nine billion years. This kind of star is an ideal source of long term warmth and energy for any ecosystem that is evolving toward intelligence. Long before it grossly over heats, at the end of its life, the ecosystem will have had plenty of time to evolve intelligent creatures that can transport the seeds of that system to a new star. Once life gets starter, it has the potential to go on for as long as the universe exists. Therefore this universe must be absolutely teaming with intelligent life and this life has an essentially unlimited potential for growth. For all of this we can we grateful for the underlying physical structures that enable typical stars, like our sun, to burn smoothly for billions of years.
The long life of stars depends on the brief lives of two different atomic nuclei. One of these nuclei is known as helium 2 (i.e. He2 in the parlance of nuclear physics). It consists of two protons that are stuck to each other. When two protons collide violently in the core of a star, they can stick together for less than a millionth of a trillionth of a second. During this brief instant they are helium 2. Most of the time they fly apart after this brief time. But on rare occasions the Weak Force converts one of the protons into a neutron before they fly apart. Then the neutron remains stuck to the other proton to form the stable nucleus of heavy hydrogen that is called deuterium. When this happens the first step in the chain of events that burns hydrogen into helium has been taken. Stars burn for a long time because the weak force throttles this crucial step.
If helium 2 was a stable nuclei, the weak force would no longer throttle the burning of hydrogen into helium. When two protons collided, they would remain stuck together after emitting an energetic pulse of light. They would stay stuck and one of them would inevitably be converted into a neutron. The critical step to deuterium would occur billions of time more readily than it does. Without the throttling action of the Weak Force stars would burn hydrogen into helium at a phenomenal rate. Their life times would then be measured in thousands of years instead of billions. Life would not have enough time to evolve anything like us. If the strength of the Strong Force between two protons were only bout 1/2% stronger than it is, helium 2 would be stable, and our universe would be devoid of intelligent life. Stars would burn up way too fast. On the other hand, if the Strong Force between two protons were several percent weaker than it is, then helium 2 would be so unstable that long lived stars like our sun would not burn at all.
There is a similar story to tell about the burning of helium 4 (the common kind of helium) into carbon. It occurs through a path way that creates the unstable nucleus beryllium 8. When two helium nuclei collide they form beryllium 8. In a thousandth of a trillionth of a second it usually splits apart again into two helium nuclei. But every now and then, before it decays, a third helium nucleus joins the party to make a carbon nucleus. If the strength of the Strong fForce between two helium nuclei were one percent stronger, then beryllium 8 would be stable and stars could burn helium into carbon at a tremendous rate. Hydrogen burning would become secondary and again stellar life times would be reduced to the thousand year range.
The long term energy source that stars provide is only one way that they aid in the evolution of life. Of equal importance is the supply of the complex elements of life that they cook up. They do this deep in their cores during the sequence of events that leads to the end of their lives. Very large stars end this sequence with a gigantic supernova explosion. This kind of explosion blows these complex elements out into space. Later the dust and gas can again condense into a second generation stars with habitable planets. If the Weak Force between two helium nuclei were several percent weaker, beryllium 8 would be so unstable that helium would not burn into carbon or any other higher element. Life would be deprived of its chemical raw materials. If the weak force were significantly different from what it is, then supernova explosions would not occur and these elements would not get out of the dying stars that make them.
Another way that the universe could be deprived of carbon is for it all to have been burned into oxygen. This would happen if an excited state of carbon had been at a slightly lower energy level or if an excited state of oxygen were at a slightly higher energy. These excited states are like the ringing of a bell. If the pitch was raised slightly on oxygen or lowered slightly on carbon, then all of the carbon that was produced in a star would be rapidly burned into oxygen. This would occur because the rate of carbon production from helium is greatly increased by the existence of such a state at a pitch that is just right. Correspondingly the rate of burning of carbon into oxygen is suppressed by an out of tune state of oxygen. The exact pitch of these two states could be taken as two stepping stones to life but they have been lumped together as one.
The long lives of stars, their production of carbon, the vast distances between stars, and their ability to blow up are all stellar stepping stones to our existence. Though the night sky is beautiful and poetic, the detailed sub-reality is also beautiful. It took a universe with an incredible structure to make it happen the way that it did. It is a universe that not only produces beauty but that also produces witnesses to its beauty that are beautiful as well.