Astronomy Ch.14 Our Star (The Sun) PDF

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photosphere chromosphere corona convection (zone) radiation (zone) sunspot sunspot group prominence filament spicule(s) granules granulation proton-proton chain models of the Sun sunspot cycles 11-year cycle 22-year cycle Maunder minimum differential rotation of the Sun sudden ionospheric disturbance aurora geomagnetic field solar wind solar wind and coronal holes

14.1 A Closer Look At the Sun Why was the Sun’s energy source a major mystery?

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Late 19th century astronomers suggested that the Sun generates energy by slowly contracting in size a process called gravitational contraction.

Why does the Sun shine? 

The Sun converts mass into energy through the process of nuclear fission.

The Stable Sun     

For the Sun to shine steadily, it must have a way of keeping the core hot and dense. Maintains these internal conditions through a natural balance between two competing forces: gravity pulling inward and pressure pushing outward. This balance is called gravitational equilibrium (or hydrostatic equilibrium). The Sun’s internal pressure precisely balances gravity at every point within it, thereby keeping the Sun stable in size. Pressure must increase with depth. The energy released by fusion, in turn, heats the gas and maintains the pressure that keeps the Sun in balance against the inward pull of gravity.

How Fusion Started 

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When the central temperature and density finally grew high enough to sustain nuclear fusion, energy generation in the Sun’s interior came into balance with the energy lost from the surface in the form of radiation. The Sun is therefore only about halfway through its 10-billion-year lifetime. Gravitational contraction made the Sun hot enough to sustain nuclear fusion in its core. Energy liberated by fusion has maintained the Sun’s gravitational equilibrium and kept the Sun shining steadily, supplying the light and heat that sustain life on Earth.

What is the Sun’s structure? 

Sun is a ball of plasma – a gas in which many of the atoms are ionized because of the high temperature.

Basic Properties of the Sun 

Spectroscopy tells you that the Sun is made almost entirely of hydrogen and helium.

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From the Sun’s angular and distance, you can determine that its radius is just under 700,000 kilometers, or more than 100 times the radius of Earth. Sunspots, which appear as dark splotches on the Sun’s surface, can be larger in diameter than the Earth. Measure Sun’s mass using Newton’s version of Kepler’s third law. It is about 2 X 10^30 kilograms, which is about 300,000 times the mass of Earth and nearly 1000 times the mass of all the planets in our solar system put together. You can observe the Sun’s rotation rate by tracking the motion of sunspots or by measuring Doppler shifts on opposite sides of the Sun. Solar equator completes one rotation in about 25 days, and the rotation period increases with latitude to about 30 days near the solar poles. The Sun’s total power output, or luminosity, is an incredible 3.8 X 10^26 watts.

Basic Properties of the Sun Radius – 696,000km (about 109 times the radius of Earth) Mass – 2X10^30kg (about 300,000 times the mass of Earth) Luminosity – 3.8X10^26 watts Composition (by percentage of mass) – 70% hydrogen, 28% helium, 2% heavier elements Rotation rate – 25 days (equator) to 30 days (poles) Surface temperature – 5800K (average); 4000K (sunspots) Core temperature – 15 million K The Sun is Not on Fire  

The Sun’s surface shines because it is hot enough to emit thermal radiation that includes visible light. Sun keeps shining because it surface is kept hot by the energy rising from its core.

The Sun’s Atmosphere  

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Solar wind – the stream of charged particles continually blown outward in all directions from the Sun. Corona, the outermost layer of this atmosphere, extends several million kilometers above the visible surface of the Sun. Temperature of corona is about 1 million K, explaining why this region emits most of the Sun’s X rays. Corona’s density is very low. Chromosphere – middle layer of the solar atmosphere and the region that radiates most of the Sun’s ultraviolet light. 10,000K. Photosphere – lowest layer of the atmosphere, the visible surface of the Sun. Temperature is under 6000K and its surface seethes and churns like a pot of boiling water. Where you’ll find sunspots, regions of intense magnetic fields that would cause your compass needle to swing about wildly.

The Sun’s Interior  

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Convection zone – where energy generated in the solar core travels upward, transported by the rising of hot gas and falling of cool gas called convection. Convection is the cause of the Sun’s seething, churning appearance. Radiation zone – about a third way down to the center, where energy moves outward primarily in the form of photons of light. 10 million K. X rays trillions of times more intense than visible light. Solar core – find the source of Sun’s energy (nuclear fusion transforming hydrogen into helium). At the Sun’s center, the temperature is about 15 million K, the density is more than 100 times that of water, and the pressure is 200 billion times that on the surface of Earth.

14.2 The Cosmic Circle  

Nuclear fission – the process of splitting a nucleus into two smaller nuclei. Nuclear fusion – the process of combining nuclei to make a nucleus with a greater number of protons or neutrons.

How does nuclear fusion occur in the Sun?  

Strong force – which binds protons and neutrons together in atomic nuclei, is the only force in nature that can overcome the electromagnetic repulsion between two positively charged nuclei. The key to nuclear fusion, therefore, is to push the positively charged nuclei close enough together for the strong force to outmuscle electromagnetic repulsion.

The Proton-Proton Chain 

Proton-proton chain – the sequence of steps that occurs in the Sun, begins with collisions between individual protons (hydrogen nuclei) - Step 1 – Twos protons fuse to form a nucleus consisting of one proton and one neutron, which is the isotope of hydrogen known as deuterium. Converts proton to neutron, reducing the total nuclear charge from +2 for the two fusing protons to +1 for the resulting deuterium nucleus. The lost positive charge is carried off by a positron (antielectron). A neutrino – a subatomic particle with a very tiny mass is also produced in this step. This step must occur twice in overall reaction since it requires a total of four protons. - Step 2 – deuterium nuclei collides and fuses with a proton. The result is a nucleus of helium-3, a rare form of helium with two protons and one neutron, along with the production of a gamma-ray photon. This step also occurs twice in the overall reaction. - Step 3 – addition of another neutron to the helium-3, thereby making normal helium-4. Collision of two helium-3 nuclei. The final result is a normal helium-4 nucleus and two protons.

The Solar Thermostat  

Nuclear fusion is the source of all energy the Sun release into space. The Solar thermostat – Gravitational equilibrium regulates the Sun’s core temperature. Everything is in balance if the amount of energy leaving the core equals the amount of energy produced by fusion. A rise in core temperature triggers a chain of events that causes the core to expand, lowering its temperature to its original value. A decrease in core temperature triggers the opposite chain of events, also restoring the original core temperature.

The Gradually Brightening Sun 

Each fusion reaction converts four hydrogen nuclei into one helium nucleus. The total number of independent particles in the solar core therefore gradually decreases with time.

How does the energy from fusion get out of the Sun?  

The amount of nuclear energy generated in the core equals the amount of energy radiated from the surface as sunlight. Each time a proton and electron collide, the photon gets deflected into a new and random direction. The photon therefore bounces around the dense interior in a haphazard way outward from the Sun’s center. Radiative Diffusion - this slow outward migration of photons. To diffuse means to spread out and radiative refers to the photons of light or radiation.

Mass-Energy Conversion in Hydrogen Fusion    

Single proton has a mass of 1.6726X10^-27 kg, so 4 protons have 4 times as much mass, or a total of 6.690X10^-27kg; a single helium-4 nucleus has slightly lower mass of 6.643X10^-27kg When four protons fuse to make one helium-4 nucleus, the amount of mass that disappears and becomes energy is therefore 6.690X10^-27kg – 6.643X10^-27kg = 0.047X10^-27kg. Sun’s luminosity is 3.8X10^26 watts, which means it produces 3.8X10^26 joules of energy each second which can replace E in E=mc^2 (c= 3X10^8 m/s) Number of fusion reactions (per second) = total mass lost through fusion (per second)/ mass lost in each fusion reaction

Pressure in the Sun: The Ideal Gas Law 

Pressure also depends on how many particles are colliding with each unit area of that surface each second, which means that pressure depends on the number density of gas particles.

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P=nkT P represents gas pressure, n represents the number density of particles (in particle per cubic centimeter), T represents gas temperature (on the Kelvin scale), and k = 1.38X10^-23 joule/K which is known as Boltzmann’s constant.

How do we know what is happening inside the Sun? Mathematical Models 

Mathematical models that use the laws of physics to predict internal conditions.

Solar Vibrations 

Can learn about inside of Sun by observing vibrations of the Sun’s surface that are somewhat similar to the vibrations that earthquakes cause on Earth.

Solar Neutrinos   

Observe subatomic particles made by fusion reactions in the core. Neutrinos produced in the Sun’s core pass outward through the solar interior almost as though it were empty space. Neutrinos come in three distinct types, called electron neutrinos, muon neutrinos, and tau neutrinos.

14.3 The Sun-Earth Connection 

Because sunspots and other features of the Sun’s surface change with time, they constitute what we call solar weather, or solar activity.

Sunspots and Magnetic Fields 

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Magnetic fields are invisible, but we can represent them by drawing magnetic field lines. These lines represent the directions in which compass needles would point if we placed them within the magnetic field. The lines are closer together where the field is stronger and farther apart where the field is weaker. Charged particles, such as ions and electrons in the solar plasma, cannot easily move perpendicular to the field lines but instead follow spiraling paths among them. Gas in the Sun’s chromosphere and corona becomes trapped in loops, making solar prominences.

Solar Storms 

Solar flares – sends bursts of X rays and fast-moving charged particles shooting into space.

Heating of the Chromosphere and Corona  

Some regions of the corona, called coronal holes, barely show up in X-ray images. These particles streaming outward from the corona make up the solar wind, which blows through the solar system at an average speed of about 500 kilometers per second and has important effects on planetary surfaces, atmospheres, and magnetospheres.

How does solar activity affect humans?  

These particles travel outward from the Sun in huge bubbles that we call coronal mass ejections. Once a coronal mass ejection reaches Earth, it can create a geomagnetic storm.

How does solar activity vary with time? The Sunspot Cycle    

Sunspot cycle – a cycle in which the average number of sunspots on the Sun gradually rises and falls Solar maximum – when sunspots are numerous Solar minimum – few if any sunspots at a time Average of 11 years between maximums and the Sun’s complete magnetic cycle averages 22 years.

Longer-Term Changes in the Sunspot Cycle  Maunder minimum – period where astronomers observed virtually no sunspots between the years 1645-1715. Putting Chapter 14 into Context  The Sun shines with energy generated by fusion of hydrogen into helium in the Sun’s core....