Astronomy 100 -- The Nearest Star: The Sun

THE SUN -- Our Closest Star

  • Diameter is about 100 times that of Earth's.
    So 1,000,000 Earths would fit inside!

  • Mass = 300,000 times the Earth's mass.

  • Distance from the Earth is:
    150,000,000 km or
    8.3 light-minutes or
    1 Astronomical Unit (AU)

  • Surface Temperature = 6100 K

  • Age of the Sun = 4.6 Billion Years!

  • Composition: 98% Hydrogen and Helium.


        Element    Symbol    Percentage
       Hydrogen        H        78
       Helium        He        20
       Oxygen        O          0.8
       Carbon        C          0.3
       Nitrogen        N          0.2
       Neon        Ne          0.2
       Nickel        Ni          0.2
       Silicon        Si          0.04
       Sulfur        S          0.04
       Iron        Fe          0.04

    Mass of the Sun = 2 x 1030 kg!!!
    (Read as two times ten to the thirty kilograms)

    2,000,000,000,000,000,000,000,000,000,000 kilograms!

    Average Density of the Sun = 1.4 grams/cc
    (just a little higher than water)


    The Source of the Sun's Energy

    What are the possibilities?

    Nuclear Fission and Fusion

    Iron is the most stable element in Nature. All atoms want to become part of an iron atom. This can happen by two processes.

    The Proton-Proton (P-P) Cycle

    The PROTON-PROTON CYCLE, or P-P Cycle, is a nuclear fusion reaction that combines 4 hydrogen nuclei (which are protons) into a single helium nucleus. These are the reactions that take place:

    1H + 1H ==> 2H + e+ + neutrino
    2H + 1H ==> 3He + photon
    3He + 3He ==> 4He + 1H +1H + photon

    Here is a diagram.

    For each 3He made, 3 1H are used. So when the 2 3He combine (a total of 6 1H), 2 1H come back out when the 4He is made.

    This energy comes from the fact that 1 helium nucleus has less mass than the 4 hydrogen nuclei that made it (by about 0.7%). That missing mass is converted to energy (those emitted photons) via Einstein's wonderful equation:

    E = mc2
    Since c2 is the speed of light squared, a little mass can make a lot of energy. To produce the energy emitted by the Sun, the Sun has to convert 4 million tons of mass to energy each second!! The Sun has enough mass to produce this amount of energy for 9 to 10 billion years!

    How do we know the Sun uses the P-P cycle?

    The only direct method to test this is to look at neutrinos.


  • What are Neutrinos or "ghost particles"?
  • How many neutrinos reach the Earth?

  • Neutrino Detectors:

    Light and Radiation.

    Basic Properties of Stars.

    Back to The Front Page.