2. Energizing the Sun
- In one second the Sun emits enough energy to supply civilizationís power requirements for a million years.
- The Sun has been keeping the Earth warm enough to sustain life for at least 3.5 billion years.
- Your grandparents had no clue as to why the Sun could shine so brightly for billions of years.
- If the Sun shines, at its present rate, by converting gravitational potential energy into heat, it can last for only 31 million years.
- The Sun is big and massive; it is 109 times the Earthís diameter and 333,000 times its mass.
- The mean mass density of the Sun is just a quarter of the average mass density of the Earth, indicating that the Sun is mainly composed of lighter material.
- The Sun is about 4.6 billion years old, but the expanding Universe is about 14
billion years old.
- The solar material must become hotter, and move faster, at deeper depths in the Sun to support the overlying weight. At the visible solar disk the temperature is 5780 kelvin and at the Sun center it is 15.6 million kelvin.
- The Sun is mainly composed of hydrogen, and each hydrogen atom consists mostly of empty space with a single proton in the nucleus and a single orbiting electron.
- Hydrogen atoms are torn into pieces by collisions inside the Sun, so their nuclear protons and orbiting electrons are free to move about in an ionized gas called plasma.
- Plasma is the fourth state of matter, in addition to solid, liquid and gas. All stars and most of the Universe are plasma.
- The material at greater depths in the Sun is compressed to larger densities, up to 13 times the density of solid lead at the center of the Sun. It is so hot down there that the protons and electrons behave like a gas in spite of the large density.
- The Sunís energy comes from fusing protons together within the hot, dense core of the Sun, thereby converting mass into energy.
- Two protons tunnel through the electrical barrier produced by the repulsion of their like charges. This tunneling is a quantum mechanical thing, made possible because protons do not have exact positions and energies, and the tunneling does not happen very often.
- The sequence of nuclear reactions in the Sun, called the proton-proton chain, converts four hydrogen nuclei, or four protons, into one helium nucleus.
- Hans Bethe was awarded the Nobel Prize in Physics in 1967 for his contributions to the theory of nuclear reactions, especially his discoveries concerning the energy production of stars like the Sun.
- A helium nucleus is just 0.7 percent less massive than the four protons that went into making it, but this mass loss, denoted by ?m, liberates an awesome amount of energy, E, given by Einsteinís celebrated equation E = ? m c2, where c is the velocity of light.
- When two protons fuse together, they produce a positron, the anti-particle of the electron, and an electron neutrino. The anti-matter positrons immediately collide with the material electrons, disappearing in a puff of energetic gamma-ray radiation, but the neutrinos pass effortlessly through both the Sun and the Earth.
- The helium nucleus contains two neutrons and two protons, so two of the four protons that went into making a helium nucleus have to be neutralized.
- Two positrons and two neutrinos are made each time a single helium nucleus is synthesized.
- Only the core of the Sun is hot enough and dense enough to generate energy by sustained nuclear fusion reactions; they cannot occur within other parts of the Sun.
- Energy generated in the core of the Sun slowly works its way out to the visible solar disk by radiation and convection.
- Just outside the core, energy is transported by radiation, taking 170,000 years to travel through the radiative zone and arrive at the bottom of the convective zone.
- As the radiation works its way out, it loses energy and is changed from short wavelength, energetic gamma rays to longer wavelength, less energetic visible sunlight.
- Radiation is blocked by the convective zone, where energy is transported by currents of heated material, taking about 10 days to go from the bottom of the convective zone to its top.
- The convective zone is capped by a thin layer of gas, the photosphere, where radiation is emitted as the sunlight we see.
- High-resolution, white-light images of the photosphere reveal the tops of gases rising out of the Sun by convection. Known as the granulation, they resemble Benard convection formed when a gas or liquid is heated from below, as with a pot of boiling water.
- At any moment, about a million granules can be seen in the white light of the visible solar disk, the photosphere. They mark the tops of gases rising out of the Sun by convection, each granule lasting for about 15 minutes before another one replaces it.
- Velocity images of the photosphere reveal a large convective pattern, named the supergranulation, which moves mainly in the horizontal direction across the visible solar disk. The granules are superposed on this larger cellular supergranulation.
- Our star began it life shining with only 70 percent of its present luminosity, slowing growing in luminous intensity as it aged. Assuming an unchanging terrestrial atmosphere, with the same composition and reflecting properties as today, the decreased solar luminosity would have caused the Earthís global surface temperature to drop below the freezing point of water during its first 2.5 billion years. Yet, there is clear geological evidence that the Earth was never this cold, and must have had a warm climate in its early history. The discrepancy between the Earthís warm climatic record and an initially dimmer Sun has come to be known as the faint-young-Sun paradox.
- The faint-young-Sun paradox can be resolved if there was a stronger atmospheric concentration of greenhouse gases, such as carbon dioxide, methane or ammonia, in the Earthís early history; the greater heating of the enhanced greenhouse effect could have kept the oceans from freezing. Another solution is that the young Sun was a bigger, brighter, hotter and more massive star than it is now, subsequently losing much of that mass in strong solar winds associated with its youth.
- The Sun is getting brighter as time goes on because there is a steady increase in the amount of helium in its core, resulting in hotter temperatures there. Sunlight will be hot enough in three billion years to boil the Earthís oceans away.
- In about seven billion years, the Sun will use up the hydrogen fuel in its core, and it will then balloon into a red-giant star, melting the Earth's surface and turning the frozen satellites of the outer planets into seas of liquid water.
Copyright 2010, Professor Kenneth R. Lang, Tufts University