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The Sun – A Typical Star. 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.
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The Sun – A Typical Star • 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
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
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 off and darken. • Vertical structure of a sunspot
Solar Physics has advanced to the point of being able to make rough predictions of solar activity
How Does The Solar Cycle Affect Earth? • Two important ways… • 1. The solar wind creates aurorae (more later) • 2. Solar luminosity changes during the cycle. We have seen that lower solar activity goes with lower average temperatures on Earth
How Does Solar Activity Change Solar Luminosity? • Higher solar activity produces higher solar luminosity. • Mechanism – more surface magnetic field energy, which thermalizes (i.e. becomes random kinetic energy, as the 2nd Law of Thermodynamics requires) and produces more net solar radiation. • Some of the magnetic field energy directly impacts Earth by high speed solar wind particles hitting Earth. • However, the luminosity changes are tiny; less than 0.1%, as measured by satellites above the atmosphere.
Maunder Minimum – The “Little Ice Age”; Connected? • The Maunder Minimum in sunspot numbers corresponds to a cool period in climate on Earth. • The cooler climate is very likely caused in part by the lower solar activity, but only in part, as the direct solar luminosity changes are too small. • The Little Ice Age also corresponds to a period of unusually intense volcanic activity, and also to the time when human population dropped by ~20% due to the Plague and mass loss of life in the New World due to transported disease – leading to reforestation which lowered CO2 levels. • However, the exact apportionment to these differing causes is not well constrained by the data • Regardless, solar activity changes cannot be responsible for the climate change of the past 60 years, for multiple reasons we’ll discuss later
The most intensely spotted star yet discovered – and discovered right here at Cabrillo Observatory!
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!
Long Term Solar Evolution… • 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 more luminous on the long term • During the 4.56 billion year life of the solar system, the sun has increased in luminosity by about 25%. • This will continue, and gradually accelerate…
