Please press “1” to test your transmitter. The Sun Our Star Please press “1” to test your transmitter.

General Properties Average star Average star Only appears so bright because it is so close. 109 times Earth’s diameter 333,000 times Earth’s mass Consists entirely of gas (av. density = 1.4 g/cm3) Central temperature = 15 million 0K Surface temperature = 5800 0K

Which parts of the sun can only be seen during a total solar eclipse? Prominences The solar corona Sun spots 1 and 2 All of the above.

Structure of the Sun Only visible during solar eclipses Only visible during solar eclipses Apparent surface of the sun Heat Flow Temp. incr. inward Solar interior

The Sun’s Interior Structure Photosphere Energy transport via convection (explained soon) Flow of energy Energy transport via radiation Energy generation via nuclear fusion Temp, density and pressure decr. outward

Do we have a direct view of the sun’s energy source? :09 Yes, because the sun is just a transparent gas ball. Yes, because most of the energy is produced very close to the surface. Yes, because the sun’s center is so bright that the light is shining through any material. No, because the sun has a non-transparent solid surface. No, because the radiation produced in the center is scattered around many times on its way towards the surface. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 0 of 30

How is energy produced in an H bomb? :09 (Chemical) Burning of hydrogen. Nuclear fusion of hydrogen into heavier elements. Nuclear fission of hydrogen. Nuclear fission of heavier elements into hydrogen. Nuclear fission of heavier elements into elements heavier than hydrogen. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 0 of 30

Energy generation in the Sun: Fusion of Hydrogen into Helium Need large proton speed ( high temperature) to overcome Coulomb barrier (electromagnetic repulsion between protons). Basic reaction: 4 1H → 4He + energy 4 protons have 0.048*10-27 kg (= 0.7 %) more mass than 4He. T ≥ 107 K = 10 million K Energy gain = Dm*c2 = 0.43*10-11 J per reaction. Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second, to resist its own gravity.

How do we know that the sun is made mostly of Hydrogen? :09 Space probes have taken samples of solar material and analyzed it. The sun’s spectrum shows strong emission lines from Hydrogen. The sun’s spectrum shows strong absorption lines from Hydrogen. Hydrogen is very easily flammable; this explains the sun’s brightness. Nonsense! The sun is actually made mostly of Nitrogen and Oxygen. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 0 of 30

Absorption Lines

Analyzing absorption spectra Each element produces a specific set of absorption (and emission) lines. Comparing the relative strengths of these sets of lines, we can study the composition of gases. By far the most abundant elements in the Universe

Which Hydrogen Lines appear in visible light? :09 The Balmer Lines (from/to the first excited state) The Balmer Lines (from/to the ground state) The Lyman Lines (from/to the first excited state) The Lyman Lines (from/to the ground state) The Einstein Lines (from/to the second excited state) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 0 of 30

The Balmer Lines Ha Hb Hg … Transitions from 2nd to higher levels of hydrogen n = 1 n = 4 n = 5 n = 3 n = 2 Ha Hb Hg The only hydrogen lines in the visible wavelength range. 2nd to 3rd level = Ha (Balmer alpha line) 2nd to 4th level = Hb (Balmer beta line) …

The Cocoon Nebula (Ha emission)

Energy Transport Radiative energy transport Convection g-rays Energy generated in the sun’s center must be transported to the surface. Inner layers: Radiative energy transport Outer layers (including photosphere): Convection Cool gas sinking down Bubbles of hot gas rising up Gas particles of solar interior g-rays

Granulation … is the visible consequence of convection

Which every-day phenomenon is another example of convective energy transport? :08 Gas bubbles rising up in a soda drink. Gas bubbles rising up in boiling water. Giant waves moving onto the sea shore. Earthquakes. All of the above. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 0 of 30

Which every-day phenomenon is another example of radiative energy transport? :09 The heat of a bonfire warming you when you’re sitting close to it. Heating food in the microwave oven. The air around a light bulb heating up when the light is on. All of the above. None of the above. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 0 of 30

The Sun’s Interior Structure Photosphere Energy transport via convection (explained soon) Flow of energy Energy transport via radiation Energy generation via nuclear fusion Animation Temp, density and pressure decr. outward

Very Important Warning: Never look directly at the sun through a telescope or binoculars!!! This can cause permanent eye damage – even blindness. Use a projection technique or a special sun viewing filter.

Cooler regions of the photosphere (T ≈ 4240 K). Sun Spots Active Regions Visible Ultraviolet Cooler regions of the photosphere (T ≈ 4240 K).

Considering that sunspots are cooler regions on the photosphere with a temperature of ~ 4240 K, how would you think a sunspot would appear if you could put it on the night sky without the sun surrounding it? It would be invisible. It would glow very faintly, similar to the faint red glow of the eclipsed moon. It would appear moderately bright, comparable to the brightest stars. It would appear very bright – even brighter than the full moon. It would be almost as bright as the sun itself.

Solar Activity, seen in soft X-rays

What can we infer from the fact that we see the gas above active regions (sun spots) mostly in ultraviolet light and X-rays? The gas must be very dense. The gas must be very dilute. The gas must be very hot. The gas must be very cold. The gas must consist mostly of Helium.

Magnetic field in sun spots is about 1000 times stronger than average. Sun Spots (III) Magnetic North Poles Magnetic South Poles Related to magnetic activity. Magnetic field in sun spots is about 1000 times stronger than average. In sun spots, magnetic field lines emerge out of the photosphere.

Magnetic Field Lines Magnetic North Pole Magnetic South Pole Magnetic North Pole Magnetic South Pole Magnetic Field Lines Mass ejections from the sun often follow magnetic field loops.

After 11 years, the direction of magnetic fields is reversed The Solar Cycle Solar Maxima 11-year cycle Full 22-year cycle Reversal of magnetic polarity (during solar minima) After 11 years, the direction of magnetic fields is reversed => Total solar cycle = 22 years

Are we currently near a solar maximum or a solar minimum? 0 of 5

The Solar Corona Very hot (T ≥ 1 million 0K), low-density gas

Prominences Looped Prominences: gas ejected from the sun’s photosphere, flowing along magnetic loops

Eruptive Prominences Extreme events, called coronal mass ejections (CMEs) and solar flares, can significantly influence Earth’s magnetic field structure and cause northern lights (aurora borealis). (Ultraviolet images)

(Ultraviolet images) Eruptive Prominences