16-18th April 2011 The Sun

Comparison of the Sun and Earth

The Sun has a radius of a little under 700 thousand km which is a bit over 100 times Earth's. Its mass is about 2 x 10³⁰, around 300 thousand times heavier than Earth. This means that it is rather less dense than the Earth, with a density which is only a quarter of Earth's.

How old is the Sun?

The Sun was formed about 4.57 billion years ago when a cloud of  hydrogen molecular gas collapsed.

How far away is the Sun from Earth?

The distance from the Sun to the Earth varies slightly, but is around 1 AU (astronomical unit) which is about 150 million km.

What elements are present in the Sun?

The Sun comprises of mostly hydrogen (90.965%) and helium (8.889%) with small amounts of oxygen (774 ppm), carbon (330 ppm), neon (112 ppm), nitrogen (102 ppm), iron (43 ppm), magnesium (35 ppm), silicon (32 ppm) and sulphur (15 ppm).  [NASA: Sun Fact Sheet ] These are all products of stellar nucleosynthesis, which is the process by which hydrogen fuses together to form the other elements.

When the hydrogen runs out, the Sun will explode and ultimately die. There is enough hydrogen for another 5-7 billion years. [Universe Today: When will the Sun die?]

How long does it take sunlight to reach Earth?

From the surface of the sun, the light takes 8 minutes, but that's only the end of the story. What we see as light starts deep within the Sun, in its core.  When fusion occurs, it not only produces the elements mentioned above, but it also produces photons and neutrinos. The neutrinos pass through pretty much anything, so they disperse immediately. The photons, on the other hand, are absorbed by the first few millimetres of plasma. They are then re-emitted, with slightly less energy, in a random direction. This means that they take a random walk in order to reach the surface of the Sun. Old textbooks estimated it to take 17 thousand to 50 million years but using a more accurate mathematical model gives a estimate of 10-170 thousand years. So what we see as sunlight actually took at least 10 thousand years to get here! [NASA: Ancient Sunlight]

What is the structure of the Sun?

The sun consists of various layers at different temperatures.

Layer	Temperature in Kelvin	Location
Core	15 million	From the centre to 20-25% of radius
Radiative zone	5-10 million	From 20-25% up to 70% of radius
Convective zone	2 million	200 000 km to  the surface
Photosphere	6600-4400	The surface (10-400 km thick)
Temperature minimum	4100	About 500 km above the surface
Chromosphere	4300-30000	Up to 10000 km above the surface
Transition region	0.02-1 million	About 200 km thickness above the chromosphere
Corona	0.5-2 or 3 million	From 2500 km and upwards
Heliosphere		Out to past Pluto!

• Core

Fusion burns hydrogen to produce various elements and release neutrinos and photons.

•  Radiative Zone  

This layer is sufficiently dense for heat to travel by thermal radiation. No thermal convection occurs here. Instead, the photons are absorbed and emitted by hydrogen and helium ions as they travel through the layer.

• Convective Zone

Plasma is heated by the radiative layer. This plasma rises, taking heat with it. When it cools, the plasma sinks back down to again be heated.

• Photosphere

This is the visible surface of the Sun. It's a relatively thin layer. We can not see further into the Sun than this level as deeper down is opaque due to a higher density of H^- ions, called hydrogen anions, in lower levels.

The plasma from the convective layer breaks through to the surface here, allowing light to escape

In 1868, Norman Lockyer hypothesised that absorption lines detected in the photosphere were a new element, unknown on Earth at the time. He called this hypothetical element helium after the sun god Helios. It was 25 more years before helium was isolated on Earth.

• Temperature minimum

As the name suggests, the temperature minimum is the coolest part of the Sun.  It is sufficient cool that the spectra of simple molecules such as carbon monoxide and water can be detected.

• Chromosphere

The chromosphere can only be detected using narrow-band filters. During a total eclipse it can be seen as a thin band of colour around the sun.

• Transition region
  

Below this layer, gravity is the dominant force. Helium is totally ionised in the transition region. Ionised helium slows radiative cooling of the plasma. This allows the rapid rise in temperature in this region.

• Corona

The fourth layer of the Sun's atmosphere. It is visible in a total eclipse. It consists of very low density plasma. It is continually expanding into space to form the solar winds.

• Heliosphere

The fifth and final layer of the Sun's atmosphere stretches out to the edges of the Solar System, providing a protective bubble of solar gas which moves with the Sun. This gas is continually emitted from the Sun as solar winds.  It protects the Solar System from ionised galactic gas and from most of the cosmic energetic particles.

What causes weather on the Sun?

Because the Sun is made of gas, it doesn't rotate in the same way as the Earth, a more solid structure.
The inside of the Sun rotates more quickly than the outside. The centre is being twisted much more quickly than the  top and bottom. This twists all the magnetic lines to such an extend that they cause tension and disruptions on the surface of the Sun, which gives rise to the Sun's weather.

Eventually these lines become so tangled that adjacent areas have opposite magnetic fields. This is instable so some areas have to change their field to match their neighbour's. Overall, this causes a reversal of the magnetic poles. This pole reversal occurs about every 11 years.

Summary of Sources

1 .365 Days of Astronomy: 15th April 2011 Our Sun
2. Wikipedia: Sun
3 .NASA: Sun Fact Sheet
4. Wikipaedia: Stellar nucleosynthesis
5. Universe Today: When will the Sun die?
6. NASA: Ancient Sunlight
7. Wikipaedia: Hydrogen anions
8. Universe Today: Sun's Atmosphere
9. Solanki, S.K.; , W. and Ayres, T. (1994). "New Light on the Heart of Darkness of the Solar Chromosphere". Science 263 (5143): 64–66.
10. Space Ref: The Distortion of the Heliosphere: our Interstellar Magnetic Compass