The Sun – A Typical Star

The Sun – A Typical Star

In Astro 3, we’ll do a very brief chapter on the Sun. More detail… take Astro 4.

Here’s the Story we’ll Unfold for you…• The layers of the sun; core, radiative zone,

convective zone, photosphere, chromosphere, and corona

• Sunspots and magnetic fields, the sunspot cycle

• Solar activity and how it influences the Earth

Gravity vs. pressure

A Star: A Balance between Gravity and

Pressure• Self-regulating…

• Higher fusion rate would expand star, lowering core’s self-gravity and thence density, pressure, temperature and thus lowering fusion rate. And vice versa

Sun layers

Layers of the sun

• Core = where temperature exceeds fusion point (10 million Kelvin)

• Radiative Zone = nothing much goes on here. It just acts as an obstacle course for the photons created in the core and random-walking their way upward

• Convection Zone = temperature gradient is so steep that photon diffusion can’t carry the heat outward fast enough. The rising temperature expands the gas, lowering density and causing it to rise (helium-balloon-like) to the surface, where it cools, gets denser, and falls back down to get reheated and start all over again. Think – soup cooking on a stove.

• Photosphere = visible surface. This is where the mean free path now gets so long the material is transparent above here.

Sunspots

• Places where the sun’s magnetic field is concentrated and inhibits the normal convective flow of hot material from below. So the material sits on the surface and cools off as it radiates to the sky.

• Charged particles in a magnetic field feel a force sideways to their motion, binding the gas to the field.

• Sunspots are like “magnetic scabs” of gas unable to be recirculated to lower, hotter levels. They are bound to the magnetic fields in the photosphere, cooling as they radiate to the cold universe, and hence cool and darken.

• Vertical structure of a sunspot

Sunspots optical

Sunspot group optical

Marching sunspots

Sun pic at max,min

Sunspot cycle 1760-1965

Sunspot cycle #vs time

How Does The Solar Cycle Affect Earth?

• Two important ways…

• 1. The solar wind creates aurorae (more later)

• 2. Climate. We have seen that lower solar activity goes with lower average temperatures on Earth

How Does Solar Activity Change Earth Temperatures?

• Not well understood – clearly a very complex mechanism is involved. However,

• Lower solar activity -> Lower solar luminosity, so that’s one way. But probably inadequate to fully explain historical climate record, since the effect is far less than 1%

• Could solar activity explain current climate change? No. In the last 10 years, climate change has become so dramatic and so accelerated that it far exceeds any historical correlation between sunspot numbers and average temperatures.

chromosphere

Chromosphere all around

Diamond ring

Total eclipse corona

Eclipse composite

Total eclipse corona

Powerful solar flare

cme

solarwind

Sun Halpha + big prominence

Sun halpha + big prom; later

EarthOnionMagField

Earth magnetosphere

SOHO wide angle

Aurorae – GiNormous Flourescent Lights!

• Caused when high speed solar wind particles impact the Earth’s atmosphere

• Collisionally excites the nitrogen and oxygen atoms

• These atoms then de-excite (electrons fall back down through the energy levels) giving off photons

• Exactly the same as how flourescent lights work!

greenpurpleAurora

Aurora westlake

Aurora lindersen

Aurora hoffman

Aurora c/IkeyaZhang

Aurora ewoldt

Aurora tricolor

Aurora from space iss

Comet NEAT kinky tail

Long Term Change…

• As the sun ages, its core collapses as hydrogen converts to helium, and this increases the gravity and pressure and fusion rate in the core

• So, the sun is getting brighter long term

• During the life of the solar system, the sun has increased in luminosity by 25%.