Astronomy Notes

Part 9: The Sun

Dr. John P. Pratt
Updated 7 Sep 2004

General Characteristics

• It is 100 times the diameter of the earth; hence about 1,000,000 times the volume
• It is about 3/4 hydrogen and 1/4 helium, and 1-2% other elements
• Being gaseous, it rotates on its axis faster at its equator (27 days) that at its poles (36 days).
• It has sunspots, which are cooler, stormy areas.
• It also has huge solar flares erupting from these sunspots.
• There is a solar wind of particles which are blown out into space, affecting our radio and TV transmission.

The Sun's structure

• There is believed to be an inner core about 15,000,000 K.
• The outer 25% is thought to be a convective zone, with bubbles like boiling water
• The sharp edge of the sun is called photosphere, which is only about 5,700 K.
• Above that is the chromosphere, a hotter layer which is red because of H-alpha emission.
• The very thin extended atmosphere is called the corona, which is about 2,000,000 K.
• Both chromosphere and corona are visible during total eclipses.

The Sun's Energy

• It cannot be heated by chemical burning because it gives off way too much heat for that.
• The energy can be explained by gravitational contraction, but it would only last for a few million years.
• Because geologists believed the earth is billions of years old, astronomers had to find another energy source.
• It is now believed that nuclear reactions produce the sun's energy, which has stopped the sun from contracting.
• The principal reaction in the sun's core is thought to be the proton-proton reaction, turning hydrogen to helium.
• The sun shines because it is hot; if you heat anything up to 5,700 K, it will look like the sun. The nuclear reactions only replenish the heat.
• The one observational proof that there really are nuclear reactions is that neutrinos should be detected. Those which are observed do not match the theory, so there are problems to be solved.

Kirchhoff's Laws of Radiation

1. A solid, liquid, or high pressure gas emits a continuous spectruml.
2. A hot, low-pressure gas has an emission spectrum, meaning it has a few bright lines.
3. Light passing throught a cool, low-pressure gas has an absorbtion spectrum, meaning there are dark lines missing from the spectrum, where light has been absorbed.

The Solar Spectrum

• The solar spectrum has hundreds of absorption lines.
• They are mostly from iron.
• Some are from helium, which was discovered first on the sun.

Sun Spots

• Sunspots look darker only because they are slightly cooler (4,000 K) and hence less bright.
• They have intense magnetic fields.
• Sunspots often occur in pairs, with one being a magnetic north pole and the other a south pole.
• Huge solar flares erupt from sunspots.
• There is a 22-year sunspot cycle: 11 years with a peak in the number of spots, then 11 with opposite magnetic polarity. The magnetic field of the entire sun reverses every 22 years.
• Solar flares can send particles to earth causing the aurora (northern lights) to be visible in the United States in years near the sunspot maxima (like 1999-2001).