The Sun as Our Star Last class we talked about how the Sun compares to other stars in the sky Today's lecture will concentrate on the different layers of the Sun's interior and its atmosphere We will also talk about the behavior of the Sun over time and some of the cycles it goes through Properties of the Sun Mass Radius Average Density Surface Gravity Surface Temperature Luminosity 1.99 x 10 30 kg 696,000 km 1.41 g/cm 3 274 m/s 2 5780 K 3.85 x 10 26 W 332,000 x Earth 109 x Earth 0.255 x Earth 28 x Earth ~ 300 K for Earth - Solar Composition Element Hydrogen Helium Oxygen Carbon Nitrogen Silicon Magnesium Neon Iron Abundance (by mass) 71.0 % 27.1% 0.97 % 0.40 % 0.096 % 0.099 % 0.076 % 0.058 % 0.14 % 1
The Solar Interior We already know that the core of the Sun is where fusion takes place There are two other main levels in the solar interior, the radiation zone and the convection zone The Solar Core The central core of the Sun is about 200,000 km in size Core All fusion of hydrogen into helium takes place inside the core The next layer out from the core is the radiation zone (about 300,000 km thick) The Radiation Zone Radiation Zone In this layer, the energy produced in the core is transported by radiation (light) Gamma rays scatter repeatedly off atoms in this layer, losing energy and making their way to the surface 2
The Convection Zone The convection layer is the final interior layer and is about 200,000 km thick Here, the energy of the Sun is transported by the swirling motion of the heated gas Hot cells of gas rise to the surface, release their energy, cool, and drop down toward the interior Convection Zone The Solar Atmosphere The outer layers of the Sun are comprised of the material we can actually see The 'surface' of the Sun is the point at which light is free to escape, without bumping into more and more atoms The Photosphere The photosphere is the first layer in the solar atmosphere and is usually thought of as the 'surface of the Sun' (only 500 km thick) The photosphere can be viewed using a filter which blocks out the majority of the light from the Sun Images of the photosphere will show sunspots on the surface of the Sun 3
Features of the Photosphere Sunspots are features in the photosphere created by interactions with the Sun's magnetic field Just like a huge magnet, some bodies in the Solar System (like the Sun, Earth) will have a magnetic field Sunspots usually come in pairs, each member of the pair can be thought of a north and south component of a magnetic field line Umbra Penumbra Features of the Photosphere Close up pictures of the Sun's photosphere reveal granulation - the Sun's surface appear to be broken into small cells Each cell represents a convective 'bubble' that has risen to the surface The bright interior is where hot material is coming to the surface, the darker exterior is where cooler material is sinking below Solar Granulation The Chromosphere The next layer of the solar atmosphere is the chromosphere, about 1,500 km thick This region is not easily visible since the light of the photosphere is so much greater The Sun using an H-alpha filter 4
Features of the Chromosphere Close up images of the chromosphere reveal small jets of gas shooting up from the surface, called spicules Spicules On much larger scales, long stringy filaments of gas can also be seen in the chromosphere Filaments The outermost layer of the Sun's atmosphere is called the corona, and extends 100,000's of km away from the Sun's surface The Corona Surprisingly, the corona is much hotter than the other layers of the atmosphere, in the millions of Kelvin The corona become visible during a total solar eclipse The Solar Wind The solar wind is most dense near the Sun, and dissipates as you move further and further away The solar wind is easily detectable at the Earth, and is still strong at Jupiter and beyond 5
Solar Activity Very frequently, large amounts of gas will erupt from the surface of the Sun Solar flare A stream of material coming off the Sun is usually called a flare, while a loop of material that falls back to the Sun is called a prominence Solar Prominence Coronal Mass Ejection Occasionally (a couple times a week), very large eruptions will explode off the Sun's surface into space These coronal mass ejections (CME's) travel very fast outward through the solar system A large CME directed toward the Earth can affect satellites, the space station, and electrical grids on the Earth's surface SOHO 6
Solar Sunspot Cycle During the 1600's, there was a lack of activity now known as the Maunder Minimum After that, it became quite apparent that sunspot activity follows an 11-year period What Causes the Sunspot Cycle? Observations of the Sun's rotation have revealed that it does not rotate at the same speed in all places At its equator, the Sun rotates significantly faster than at the poles The magnetic field of the Sun is tied to the rotating gas Twisted Magnetic Fields Differential rotation causes the magnetic field lines of the Sun to become twisted The more twisted they become, the higher the amount of activity on the Sun's surface 7
The Sunspot Cycle Eventually, the magnetic field lines become so disrupted that the field disappears and then reforms Each time the magnetic field is reformed, the field lines are straight and solar activity decreases Also, the polarity of the Sun's poles change (the north magnetic pole becomes the south and vice versa) 8