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The Sun’s Surface is highly mottled, or granulated, with bright and dark gas. The bright areas are called granules . They are 1000 km across (the size of a continent on Earth). They only last 5 to 10 minutes before dissipating.
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The Sun’s Surface is highly mottled, or granulated, with bright and dark gas.The bright areas are called granules. They are 1000 km across (the size of a continent on Earth). They only last 5 to 10 minutes before dissipating.
A granule is the topmost part of a convection cell, they are bright and upward moving. The darker edges are moving downward. The granule wells up in the center and sinks at the edges. The brightness differences are due to differences in temperature (500K temperature difference).
Supergranulation - similar to granules, but these cells are 30,000 km across. Supergranules may represent a deeper, larger tier of convection cells.
The edges of the Sun appear darker than the rest of the Sun. These edges are called the limb of the Sun. Limb darkening is caused by a difference in the temperature which causes the light. The limb area is not as deep; therefore it is cooler.
The chromosphere emits little light because of its low density. It is not normally visible (the photosphere dominates), but is seen during a solar eclipse. During an eclipse the chromosphere’s reddish hue can be seen.
This reddish color is due to the Ha emission line of hydrogen. Small solar storms erupt in the chromosphere every few minutes. Hot jets of material being ejected are called spicules.
Spicules only cover about 1% of the total area of the Sun. They seem to accumulate around the edges of supergranules. The magnetic field of the Sun is stronger than average in these areas.
If the Moon blocks the photosphere and the chromosphere during an eclipse, the corona is visible.
The temperature in the corona decreases to 4500K at 500 km above the photosphere, then rises steadily.
About 1500 km above the photosphere, the temperature rises rapidly reaching 1,000,000 K at 10,000 km. It then remains constant.
One of the biggest questions in astronomy was: What is the heat source for the upper corona?
Scientists today believe that sound produced on the surface of the sun combines with the energy of the magnetic field to produce the energy needed to heat the corona.
Electromagnetic radiation and particles (protons and electrons) are continually emitted from the Sun. Light is emitted at C (8 min to Earth), particles at 500 km/s (a few days to Earth).
This stream of light and particles is the solar wind. It is caused by the high temperature of the corona which heats gases until they are hot enough to escape the Sun’s gravity.
The solar atmosphere is constantly replenished, otherwise the corona would be gone in one day.The Sun is “evaporating”. One million tons of solar matter is lost each second.
Even at this rate of depletion, the Sun has lost less than 0.1% of its mass since its formation.
The light emitted by the hot coronal gas is primarily in the form of X-rays.
X-ray images of the Sun show that most of the solar wind escapes through solar “windows” in the corona called “coronal holes”. These are areas deficient in matter in the Sun’s atmosphere.
