Chapter 19.2 The Sun

1. Chapter 19.2 The Sun "The sun, with all those planets revolving around it and dependent on it, can still ripen a bunch of grapes as if it had nothing else in the universe to do." Galileo Galilei

2. The Structure of the Sun • The sun is essentially a giant ball of gas and plasma that gets hotter and denser as you travel from the outer rim to the centre. Temperatures run from a mere 5780K on the outer visible layer (the photosphere) to about 15 MILLION Kelvin in the middle! (0K = - 273°C)

3. The Core • The core of the sun is the the real powerhouse. With temperatures at 15 million K and a density of 160,000 Kg/m3, this is the place where the nuclear fusion that generates massive amounts of energy takes place.

4. The Radiative Zone • Between the core and the convective zone, the radiative zone extends to about 70% of the sun's radius. The energy flowing from the core through the radiative zone, traveling in a very haphazard path, losing energy in the process.

5. The Convection Zone • Is a region about 200,000 km thick where gases circulate in convection currents. Hot gases rise from the interior while cooler gases sink toward the interior.

6. The Photosphere • Sunlight as we know it - the visible white light, is emitted from the photosphere. The photosphere is one of the coolest regions of the Sun (about 6000 K), and it is here that we can see the granulation caused by the bubbling gas in the convection layer and the sunspots caused by strong magnetic fields.

7. The Chromosphere • Is the thin region below the corona. • Only 3,000 km thick. • Too faint to see unless there is a total solar eclipse

8. The Corona • This is the outer layer of the sun and is the whitish halo seen around the disc in a total solar eclipse. This can be seen in the picture of the solar eclipse above. Temperatures range from 2 to 3 million °.

9. Visible Features of the Sun

10. Sunspots • Sunspots are the dark spots you can see when you look at the sun in white light (through an appropriate filter) or when you project an image of the sun on to a screen. These spots are dark because they are cooler than their surroundings The sun generates very strong magnetic fields, and it is a localised concentration of these magnetic fields that causes the cooling that we see as sunspots. Sunspots usually occur in pairs or groups of opposite magnetic polarity that move in unison across the face of the sun as it rotates. They can last anything from a few hours to a few weeks, or even months for the very biggest. Interestingly sunspot activity exhibits an 11 year cycle in terms of the position and number of spots.

11. Flares • Solar flares are huge explosions on the surface of the sun, throwing out massive amount of material as matter is heated to millions of degrees in a few minutes. It can last from minutes to hours.

12. Einstein and the discovery of nuclear fusion • At the beginning of the 20th century Albert Einstein demonstrated that matter and energy are interchangeable. • Matter can be converted into energy according to his famous formula E = mc2 E is energy M is mass C is the speed of light

13. This idea paved the way for an understanding of a very powerful source of energy. Nuclear Fusion

14. Energy production in the Sun • Nuclear fusion is the process by which two or more nuclei with small masses ( such as hydrogen) join together, or fuse, to form a larger, more massive nucleus (such as helium). • During the process, energy is produced-a lot of it.

15. Biology Connection • At the time Darwin introduced his theory of evolution, scientist thought that the sun was a few million years old at most. • Some scientist argued that evolution – which takes place over billions of years – was therefore impossible because the sun could not have been shining that long. • The nuclear fusion that fuels the sun, however, gives it a lifespan of at least 10 billion years.