The Sun l l l The Sun is

The Sun l l l The Sun is a star. It is 4, 500 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

What are sun spots? l l l Temporary “blemishes” on the photosphere Caused by intense magnetic activity which reduces convection, forming areas of reduced temperature (3000– 4500 K). Sunspots expand contract as they move round the Sun and can be as large as 80000 km in diameter making them clearly visible from Earth.

The Sun As A Star The Sun from the dwarf-planet Sedna

What powers a star? • A star is ‘powered’ by nuclear fusion reactions in its core. • Nuclear fusion involves light atomic nuclei fusing together to form heavier ones. • This process releases huge amounts of energy - each second, the Sun produces 4 x 1026 joules of energy! • It would take 2000 million nuclear power plants a whole year to produce the same amount of energy on Earth.

What’s inside an atom? Inside an atom, there are three smaller things: l Protons - positively charged l Neutrons - neutral (no charge) l Electrons - negatively charged l The protons and neutrons form the nucleus in the center of the atom and this is surrounded by electrons.

Nuclear Fusion l l l We can add neutrons to a nucleus without changing the way an element reacts. These different variations are called isotopes. Deuterium and tritium are two- and three- times as heavy as normal hydrogen.

Nuclear Fusion l l Inside stars, a proton (hydrogen) fuses to a neutron to make deuterium. This then fuses with another proton (hydrogen) to make helium -3.

Nuclear Fusion l l Once we have two helium-3 nuclei, we combine them to make helium-4 (the most common isotope of helium). Lots of energy is released at every stage of this process, in the form of light, making the Sun shine.

How are elements made? • Nuclear fusion in stars produces new atoms. • In the early stages of a star’s life, light elements such as helium are mainly formed. • When all the hydrogen has been used, other elements are fused together to make the heavier elements of the Periodic Table. However, not all elements are made in the early stages of a star’s life. Some of the heavier elements are only made when a star explodes at the end of its life.