Lesson 1 The Sun Objectives l To know

Lesson 1 – The Sun Objectives l To know the size of, and distance to, the Sun. l To know the temperature and describe the nature of the Sun’s visible surface. l To appreciate the dangers of observing the Sun and explain how the Sun can be observed safely.

The Sun l l l The Sun is a star. It is 4, 600 million years old It takes 8 minutes for its light to reach the Earth. 98. 6% mass of the Solar System Consists of Hydrogen (74%) and Helium (24%) • Radius: 695, 000 km • Mass: 333, 000 Earths! • Surface Temperature: 5, 500 °C • Core Temperature: 15, 500, 000 °C

Star Size Comparison

Internal Structure of the Sun Radiative Zone photons travel out through it Convective Zone temperature falls with increasing distance towards the surface Core Hottest part where hydrogen fusion occurs Photosphere granular appearance. EM radiation emitted.

Photosphere l The Sun’s surface - the layer of the Sun where most of the visible light comes from. l The energy released in the Sun's core takes over 100000 years to reach the photosphere.

• A magnified portion of the solar surface. • Energy from below the surface is transported by convection and resulting in granulation. • The lighter areas reveal where gases are rising from below, while the darker areas show where cooler gases are sinking back down.

Chromosphere l l The 2 nd of the three main layers in the Sun's atmosphere ~2, 000 km deep - just above the photosphere Normally invisible - can only be seen during a total eclipse with special equipment (the photosphere is too bright). The temperature varies from ~6000 K to +35000 K

It is not yet fully understood what causes the chromosphere to increase in temperature as you move away from the Sun

Corona l l The plasma "atmosphere" of the Sun Extends millions of kilometres into space Most easily seen during a total solar eclipse Much hotter (nearly 200 times) than the surface of the Sun: the photosphere's average temperature is 5800 K compared to the corona's 1 -3 million K.

The solar wind is a stream of protons and electrons which flow outwards from the coronal holes. The particles get accelerated up to 200 - 400 km/s. The solar wind particles flow through-out the solar system beyond Pluto

• Prominences (aka coronal loops) are large, bright loop -shaped features on the photosphere that extend out into the corona • A prominence takes ~1 day to form and may persist for several months. • A typical prominence extends over many 1000 s of kms; the largest recorded was over 800000 kms long

• Coronal loops are found in the lower corona resulting from the Sun’s magnetic field. • Their frequency is linked with the solar cycle. • They are often found with sunspots at their footpoints.

l A solar flare is a sudden brightening observed over the Sun's surface • Involves a large energy release of ~160 billion megatons of TNT • The frequency of solar flares varies from several per day when the Sun is "active" to less than one per week when the Sun is "quiet“.

• Filaments are prominences viewed from above instead of side on. • As they are cooler than the photosphere, they appear darker