THE SUN

• We now turn to the Sun, the object which dominates in the Solar System. The Sun is the nearest example of a star, so it provides a good introduction to our next topic, stars.

• The Sun contains most of the mass in the Solar System. This is critical, because only objects which are sufficiently massive get hot enough in their centers that nuclear reactions occur there. The Sun stands apart from the planets because it is the only object in the Solar System in which nuclear reactions are occurring.

• What is a nuclear reaction?
  • Under extreme conditions, it is possible for several atoms of one type of element to merge together to form a different element, or for a single atom to be broken up into several individual atoms of other elements. These are known as nuclear reactions, and the two types are known as fusion and fission reactions.
  • Nuclear reactions can release large amounts of energy because for some reactions, the total mass of the product atoms have slightly less mass than the total mass of the atoms that went into the reaction. During the nuclear reaction, this extra mass is converted into energy, where the amount converted is given by:
    E = mc²
    where c is the speed of light. Since the speed of light is a very large number, even the conversion of a small amount of mass results in a large amount of energy.
    • Generally, putting together two small atoms to make a bigger atom releases energy for atoms up to the mass of an iron atom, but putting together two bigger atoms actually requires energy. The fusion of smaller atoms is what produces energy in stars.
    • Conversely, splitting apart a big atom to make two smaller atoms releases energy for atoms more massive than iron, but requires energy for splitting smaller atoms. The fission of large atoms is what produces energy in nuclear power plants.
    • Lower mass atoms release energy by fusion reactions, higher mass atoms release energy by fission reactions. Iron falls in between the two groups; no nuclear reaction involving iron releases energy.

  • However, it is very hard to get nuclear reactions to occur. To understand why, we need to know how atoms are held together.
    • Inside an atom, protons and electrons have a property called electric charge. Electric charge can come in two different signs, positive and negative. Protons have positive charge, and electrons have negative charge. There is a fundamental force in nature which acts as an attractive force between objects with opposite charge and an repulsive force between objects with the same charge. In an atom, the electrons are held to the nucleus by this electromagnetic force.

    • Inside the nucleus, all of the protons have positive charge. They manage to stick together despite the repulsive electromagnetic force. This is because there is another fundamental force called the strong force which can hold protons and neutrons together. The force is stronger than the repulsive electromagnetic force between the protons, but only if two protons are brought very close together.

    • The only way to get protons in different atoms close enough to each other that the strong force can overcome the electromagnetic force and allow the two nucleii to stick together is if the two atoms collide with each other at a high speed. Since the speed of atoms is related to their temperature, this means that nuclear reactions can only occur in objects which are very hot.

• In the Sun, the nuclear reaction which occurs is called the proton-proton cycle, in which four Hydrogen atoms are combined in a series of reactions to form one Helium atom; this chain of reactions also produces energy (in the form of gamma rays) and some other particles called neutrinos. These reactions only occurs in the central regions of the Sun where it is hottest. Energy is generated in this central region and keeps it hot.

• The energy that is generated in the central regions of the Sun gradually works its way out through the Sun. In the inner parts of the Sun, energy gets out by radiation, while in the outer regions it gets out by convection. Eventually, it makes it to the surface of the Sun, and finally is radiated out into space. The energy which we receive from the Sun was originally generated by nuclear reactions in the core of the Sun.
  • Convection is responsible for some of the observed features on the Sun, such as granulation (granulation movie)
  • Magnetic fields are also important for some of the observed features of the Sun, such as sunspots. The Sun has a 22 year long magnetic cycle, which affects the number and location of sunspots and also the amount of solar activity, which can affect things, e.g., radio communications, on Earth.

• The basic structure of the Sun can be characterized by several layers: core, radiative zone, convective zone, photosphere, chromosphere, and corona, going from inside to out. Somewhat peculiarly, the very outer layers of the Sun, the chromosphere and the corona, are hotter than the photosphere. They are probably heated by a process related to the magnetic field on the Sun.

• It is only because the inner parts of the Sun are hotter that the Sun doesn't collapse under its own gravity. The atoms in the central regions move faster than those in outer regions and consequently they push outwards with more force, holding the Sun up. The force which they exert is described by the pressure; the internal pressure is higher than the external pressure, so the Sun is held up against gravitational collapse.

• How do we know that this is what is going on in the Sun?
  • We know that the Solar System is at least several billion years old. Nuclear reactions are the only energy source we know of that can sustain the energy production of the Sun for such a long time.

  • We can probe the internal structure of the Sun by studying solar oscillations (movie). It turns out that the Sun is vibrating continually, in some respects similar to the vibrations which occur in the Earth because of earthquakes. In the same way we used earthquakes to probe the internal structure of the Earth, we can use these solar oscillations to probe the internal structure of the Sun. We find that the observed structure closely matches that of our model which has nuclear reaction in the core of the Sun.

  • Nuclear reactions produce, in addition to energy, a special kind of particle called a neutrino. Although difficult, it is possible to detect neutrinos coming from the nuclear reactions which occur in the core of the Sun, and this strongly supports the idea of energy generation from nuclear reactions. In detail, the observed number of neutrinos is slightly different from the prediction of our model; for some time, people wondered whether this indicated that we had a few details in the solar model wrong, but it has recently been discovered that actually the model is very good, but that there were some properties of neutrinos that we hadn't understood correctly!

• What does our model predict for the evolution of the Sun?
  • We expect that the Sun will eventually change, because at some point, all of the hydrogen in the core of Sun will get converted into helium, so there won't be any left for nuclear reactions. The hydrogen in the very center runs out first, leaving a helium core with a shell of nuclear reactions around it. During this stage in the Sun's evolution, the pressure forces change, and the outer regions of the Sun will expand to be very large.

  • When the hydrogen is depleted, there will be nothing keeping the center of the Sun hotter than the outside, so there will be no pressure which balances gravity. Consequently, the central regions of the Sun will begin to contract.

  • As the core of the Sun contracts, the central regions will heat up. Eventually it will get hot enough for another nuclear reaction to start. This reaction changes three Helium atoms into one Carbon atom. This nuclear reaction keeps it hotter in the center, so the Sun stabilizes for a while.

  • Eventually, the helium in the core of the Sun is all converted into carbon. Again, the core of the Sun will contract, because nothing balances the inward gravitational force.

  • As the core of the Sun gets denser and denser, the core material eventually changes state into a kind of matter called degenerate matter. Degenerate matter has the property that it provides outward pressure even when energy is not being generated. This electron pressure will stop the collapse of our Sun. When this sets in, the core of the the Sun will be essentially pure carbon. The outer regions of the Sun will most likely be blown off of the Sun during the later stages of its evolution and return to interstellar space. We observe this for other stars that have reached the end of their life ( Planetary nebulae gallery)

  • Consequently, at the end of its lifetime, our model predicts that the Sun will be a dense, hot, carbon core which will gradually cool off. We actually observe many stars that have the properties we expect the Sun will eventually have: these are called white dwarfs.