Transcript File - YEAR 11 EBSS PHYSICS DETAILED STUDIES

Unit 1 Physics
Detailed Study 3.2
Chapter 11: Astrophysics
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.
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.
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.
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 10 000 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.
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.
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 170 000 year to reach the surface
of the sun, but only 8 minutes to reach earth from the surface.
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.
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.