5. What Makes the Sun Shine?

    • The heat and power of the Sun cannot be generated by the Sunís gravitational contraction for more than about 100 million years, but the Earth has received the Sunís light and heat for almost 4.6 billion years.

    • The temperature at the center of the Sun is 15.6 million K. It is measured by determining the kinetic energy of moving protons required to support the entire mass of the Sun.

    • The Sun shines by nuclear fusion reactions in its central core.

    • The mass lost during nuclear fusion provides the energy that heats the center of the Sun, sustains ongoing nuclear reactions, and produces the radiation of the Sun.

    • The nuclear reactions in the center of the Sun, known as hydrogen burning or the proton-proton chain, fuse four protons to make one helium nucleus while also releasing gamma rays and neutrinos.

    • The neutrinos released during hydrogen burning in the Sun were eventually detected, but in fewer amounts than predicted. This solar neutrino problem was resolved when it was found that there are three kinds of neutrinos and only one kind, the electron neutrino, is produced at the center of the Sun.

    • The gamma rays produced in the core of the Sun work their way out through the radiative zone, gradually losing photon energy and increasing in wavelength.

    • The convective zone caps the radiative zone. Energy is transported by convective gas motion in the convective zone.

    • The solar granulation marks the top of the convective zone.

    • The photosphere, or the visible disk of the Sun, lies just above the top of the convective zone.

    • Sound waves produced in the Sunís convective zone travel through the Sun and cause the photosphere to oscillate, or move in and out, with periods of about 5 minutes.

    • Observations of the Sunís five-minute oscillations have determined how the Sun rotates inside. Differential rotation, in which the equator rotates faster than the poles, persists down to the bottom of the convective zone; below that there is uniform rotation at all latitudes.

    • The Sun is gradually getting brighter as time goes on and it therefore had to be dimmer a long time ago; but the Earthís fossil records indicate that the Earth has always had a warm climate. This faint-young-Sun paradox can be resolved if the Earth had a strong greenhouse effect in its youth or if the Sun was more active in its youth.

    • The Sun will run out of hydrogen fuel in about 7 billion years. Its core will then collapse and heat up while the Sunís outer atmosphere expands into a giant star that will engulf Mercury. The luminous giant Sun will then boil the Earthís oceans away.

Copyright 2010, Professor Kenneth R. Lang, Tufts University