Surface of the Sun
Written by Fraser Cain
The visible surface of the Sun is called the photosphere. This is the region of the Sun where energy produced in its core can finally escape the star, and fly off into space. It reaches a temperature of almost 6,000 degrees Kelvin, and gives the Sun its white/yellow appearance.
Perhaps the most familiar feature on the surface of the Sun are sunspots. These are relatively cooler regions on the surface of the Sun, where its magnetic field lines pierce the surface of the Sun. Sunspots can be the source of solar flares and coronal mass ejections.
When we look at the Sun, we notice that the center of the Sun looks much brighter than the edges of the Sun. This is called "limb darkening", and it happens because we're seeing light that has passed through the surface of the Sun at an angle, and has been blocked more – and so it's darker.
With a good telescope (and an even better solar filter), it's possible to see that the photosphere isn't smooth. Instead, it's covered in convection cells called granules. These are caused by convection currents of plasma within the Sun's convective zone. Hot plasma rises in columns through this the convective region of the Sun, release some of their energy and then cool down and sink back down. Imagine bubbles rising to the surface in boiling water. These granules can be 1,000 km across and last just 8-20 minutes before dissipating.
Huge coronal mass ejections can also be seen blasting out of the surface of the Sun. These are created when the coiled up magnetic field of the Sun snaps and reconnects. This reconnection releases a tremendous amount of energy, and throws charged plasma into space. When this plasma reaches the Earth, it creates the beautiful auroras best seen near the Earth's poles.
We have written many articles about the Sun on Universe Today. For example, here's an article about amazing images captured by the Hinode spacecraft, and another about magnetic fountains on the surface of the Sun.
Here's an article from NASA about the photosphere, and here's more information from an astronomy 162 lecture.
We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.
Filed under: Astronomy
