The Sun

The Sun’s Size and Composition The Sun is roughly 100 times larger than Earth in diameter, and 300,000 times larger in mass. It is a gaseous sphere composed of 75% hydrogen, 24% helium, and 1% other elements.

The Sun’s Temperature and Energy Output The surface of the Sun has a temperature of roughly 6000 K while the core of the Sun is 15 million K. The total light output of the Sun is 3.8 x Watts. The energy produced in 1 second would be enough to power human civilization for 1 million years!

The Sun’s Differential Rotation Since the Sun is a gaseous body rather than solid, different latitudes can rotate at different speeds. Its equator rotates once every 25 days, while regions near the poles rotate every 30 days.

The Sun’s Magnetic Field Imagine the Sun as a bar magnet, with magnetic field lines cutting through it. Because the equator is rotating faster than the poles, the magnetic field lines near the equator are pulled around the Sun faster, resulting in loops and kinks in the magnetic field.

Creation of Sunspots The magnetic field loops and kinks poke through the Sun’s surface. They absorb energy from areas on the surface that they pass through, making those spots cooler, and therefore darker.

Prominences and Flares Eventually, just like a rubber band, the loops and kinks in the magnetic field will break and release their energy in a huge explosion. Solar Prominence Solar Flare

Effects on Earth When the Sun has a lot of sunspots, solar flares, and prominences  The earth is warmed by all the additional energy  The earth is bombarded with cosmic rays (i.e., high energy hydrogen and helium nuclei that are ejected from the Sun. In other words, a stronger solar wind.) The earth’s magnetic field and atmosphere protects us from these particles; those that get through are funneled into the atmosphere at the poles.

Aurorae When the solar wind hits the earth’s atmosphere, the particles excite electrons bound to atoms of oxygen and nitrogen. When the electrons fall back down, they produce emission lines.

Slides beyond this point contain extra material that you might find interesting but is not covered on homeworks and exams

The Sun in X-rays Because the Sun’s surface temperature is about 6000°, it emits mostly at visible wavelengths. However, solar flares are extremely hot, so they produce lots of X-rays.

The Solar Cycle After the magnetic field lines break, they straighten out and the cycle begins again. One cycle is completed every 11 years, which is seen in the varying number of sunspots over time.

The Solar Cycle Solar Maximum March 29, 2001 Solar Minimum Oct. 11, 2004 After the magnetic field lines break, they straighten out and the cycle begins again. One cycle is completed every 11 years, which is seen in the varying number of sunspots over time.

The Maunder Minimum Sunspots are easy to see –you don’t need a telescope (just project the sun through a pinhole). So good data on the Sun exists all the way to the time of Galileo. In the 1600’s, the earth went through a mini- ice age: Europe and Asia were abnormally cold. This coincided with an unusually low number of sunspots.

Aurorae on other planets

Sun-grazing Comets