1. Unit 1 Physics Detailed Study 3.2
Chapter 11: Astrophysics

2. Section 11.2 Our favourite star
Our best-known star
It wasn’t until after Galileo and Newton that stars were Sun-like objects, a long way away.
A good way to learn more about distant stars, was to learn more about the star closest to us, the sun.
Galileo observed sunspots on the surface of the sun, this lead to the discovery that the sun rotates on an axis, and is not solid, but in fact gaseous.

3. Section 11.2 Our favourite star
Our best-known star
Tycho Brahe and Johannes Kepler were able to find distances in the solar system relative to the sun, and other planets, but these distances weren’t the ‘actual’ distances.
To do this we need a scale to work with.
This was achieved by using the known diameter of the earth.
Struve showed that Vega showed a parallax of 1/8 of an arc second ( AU from Earth)

4. Section 11.2 Our favourite star
Our best-known star
The distance to the sun was determined to be 1AU (astronomical unit), this gives the solar system scale.
Using this information we can determine other properties of the sun such as density.

5. Section 11.2 Our favourite star
Where does the energy come from?
The origin of the suns energy was a mystery for some time. We know it has been radiating energy for a really long time, but even the most efficient chemical reaction would have only lasted years.
A simple calculation based on the mass and energy output of the sun showed that the energy being produced by the sun was huge compared to a chemical reaction.
Something very different is going on in the sun.

6. Section 11.2 Our favourite star
Where does the energy come from?
Einstein's famous equation, E=mc2, tells us that there is a huge amount of energy locked up in the mass of the sun.
But how is the mass turned into energy?
This is done at the center, or core, of the sun, and is achieved through the process known as nuclear fusion.
A nuclear fusion reaction occurs when two nuclei (hydrogen nuclei in the case of our sun) fuse together creating a new nucleus (helium), this releases a huge amount of energy in the process.

7. Section 11.2 Our favourite star
Modelling the sun
Astrophysicists use computers to model the conditions within the sun. In doing so, they have been able come up with ‘facts’ about our sun. (These facts are all theoretical as it is impossible to test them)
These facts include:
Fusion occurs within 0.25R where temperatures reach above 10 million degrees.
Radiative diffusion is the main mechanism for energy transfer to about 0.7R.
At the surface, the temperature is ~5800 K.
The light produced in fusion, takes year to reach the surface of the sun, but only 8 minutes to reach earth from the surface.

8. Section 11.2 Our favourite star
The Sun’s atmosphere
Even though the sun is gaseous and there is no real surface, we still say it has an atmosphere.
The photosphere is a hot (5800 K) thin layer from which the sun’s visible light is emitted from.
Above that there are two more layers, though they are not as readily visible, mostly in eclipses.
The Chromosphere, which is coloured and extremely thin.
And the Corona, which has a white glow and is so thin would be considered a high vacuum if produced in a Lab.

9. Section 11.2 Our favourite star
… and its magnetic field
The sun has a very dynamic magnetic field compared to the earths magnetic field.
The suns magnetic field reverses every 11 years, which gives rise to phenomena such as sunspots, solar flares and coronal mass ejections.
These ‘magnetic storms’ result in solar winds, which if they reach Earth can be hazardous to astronauts or even disrupt communications and electronics.