Chapter 9 The Sun. Nuclear fusion: Combining of light nuclei into heavier ones Example: In the Sun is conversion of H into He

Transcription

1 Our sole source of light and heat in the solar system A common star: a glowing ball of plasma held together by its own gravity and powered by nuclear fusion at its center. Nuclear fusion: Combining of light nuclei into heavier ones Example: In the Sun is conversion of H into He Plasma: Ionized material composed of electrons, protons and ions Chapter 9 The Sun An image of the Sun and the large sunspot taken on October 22, 2014

2 The Stellar balance The outward pressure (from heat caused by nuclear reactions) in the core balances the gravitational pull toward the Sun s center. This balance is called Hydrostatic equilibrium This balance leads to a spherical ball of plasma, called the Sun. What would happen if the nuclear reactions in the core ( burning ) stopped?

3 Main Regions of the Sun Core Radiation Zone Convection Zone Photosphere Chromosphere Transition Zone Corona Solar wind Radius of the Sun = 696,000 km (The thickness of the regions are not to scale)

4 Solar Properties Radius = 696,000 km (100 times Earth s radius) Mass = 2 x kg (300,000 times Earth s mass) Av. Density = 1410 kg/m 3 Rotation Period = 25 days (equator) 36 days (poles) Surface temp = 5780 K The Moon s orbit around the Earth (Radius around 385,000 km) would easily fit within the Sun!

5 Luminosity: Total light energy emitted per second (Power) Luminosity of the Sun = L SUN L SUN ~ 3.96 x W Watt (W) is a unit of power. Power is energy emitted per unit of time. Joule is a unit of energy 1 W = 1 Joule/sec How do we determine the luminosity of the Sun? - First, we measure the amount of energy from the Sun received at the Earth per squared meter per second. This is power in W/m 2 It is called the Solar constant = 1400 W/m 2 d - Second, we multiply this by 4 d 2 (surface of a sphere of radius d), where d is the distance between the Earth and Sun (1 AU, ~ 150 million km).

6 The standard solar model 1 g/cm3 = 1000 kg/m3 The Standard Solar Model The temperature of the core must be least 10 million K in order to be able to convert H into He. The Sun s central core temperature is about 15 million K The temperature of the layer that we see from the Sun (Photosphere) is about 6,000 K

7 Energy Transport within the Sun Extremely hot core, million K. All the matter is completely ionized (plasma) Radiation zone The temperature is so high that no electrons are left on the atoms to be able to capture photons radiation zone is transparent to light. Energy here is transported by radiation Convection zone Temperature falls further away from the core at lower temperatures, more atoms are not completely ionized. The electrons left in the atoms can capture photons The gas becomes opaque to light. Energy is transported here by convection Farther out, the low density in the photosphere makes it transparent to light - radiation takes over again

8 Solar Granulation: Evidence of Convection Solar Granules are the tops of convection cells. Bright regions are where hot material is upwelling (1000 km across). Dark regions are where cooler material is sinking. Material rises/sinks at a rate ~1 km/sec (2200 mph) Detected by Doppler effect.

9 The solar spectrum has thousands of absorption lines (The scale is wavelength in nanometers ) More than 67 different elements are present! Hydrogen is the most abundant element followed by Helium (1 st discovered in the Sun!) The Solar Atmosphere Spectral lines only tell us about the composition of the part of the Sun that forms them. But these elements are also thought to be representative of the entire Sun.

10 The composition of the Sun

11 The chromosphere and the photosphere The chromosphere can only be seen in a total solar eclipse when the size of the disk of the moon is slightly larger than the disk of the Sun so it will block the light from the photosphere The layer of the Sun that we see is the photosphere. The photosphere has higher temperature (5,800 K) and higher density. The chromosphere has lower temperature (4,500 K) and lower density The photosphere forms the continuous spectrum The chromosphere produce the absorption lines. (Remember Kirchhoff s laws)

12 Transition Zone and Corona

13 Transition Zone & Corona The Corona has very low density but high temperature T ~ 10 6 K From the corona we see emission lines from highly ionized elements (Fe +5 Fe +13 ) which indicates that the temperature here is very HOT Why does the Temperature rise further from the hot light source? magnetic activity - spicules and other more energetic phenomena (more about this later )

14 Because the coronal plasma has high temperature (1,000,000 K), it escapes the gravitational attraction of the Sun Solar wind Corona (seen only during total Solar eclipse)

15 Solar Wind

16 Solar Wind The radiation (light or electromagnetic waves) emitted by the Sun travel at the speed of light and take about 8 minutes to reach Earth. The plasma (electrons, protons and ions) ejected from the Sun travel slower, ~500 km/s and take a few days (~ 3 days) to reach the Earth Solar coronal plasma has enough temperature (kinetic energy) to escape the Sun s gravity. This stream of particles ejected from the Sun is called the solar wind Radiation and fast moving particles (electron and protons) continuously leave the Sun. The Sun is evaporating via this wind The Sun loses about 1 million tons of matter each second! However, over the Sun s lifetime, it has lost only ~0.1% of its total mass.

17 Hot coronal plasma (~1,000,000 K) emits mostly in X-rays. Coronal holes are sources of the solar wind (lower density regions) Coronal holes are related to the Sun s magnetic field. Open magnetic field line generate the coronal holes CME: Coronal Mass Ejection Ejection of plasma through the coronal holes

18 An example of a coronal hole showing the magnetic field lines structure Coronal hole

19 An example of a CME The animation was recorded by the SOHO (Solar Heliospheric Observatory) spacecraft

20 Sunspots Granulation around sunspot Umbra: dark center of sunspot Penumbra: grayish area around the umbra

21 Sunspots Size typically about 10,000 km across At any time, the Sun may have hundreds (around solar sunspot maximum) or none (around a solar sunspot minimum) Dark color because they are cooler than photospheric plasma (4,500 K in darkest parts, compared to 5, 800 K in the photosphere.) Each spot can last from a few days to a few weeks or a month Galileo observed these spots and realized the Sun is rotating differentially (faster at the equator, slower at the poles)

22 Rotation of the Sun: An animation

23 Sunspots & Magnetic Fields The magnetic field in a sunspot is 1000 times strongest than the surrounding area Sunspots are almost always in pairs at the same latitude with each member having opposite polarity All sunspots in the same hemisphere have the same magnetic configuration. They have opposite polarity in north and south hemisphere

24 Why the sunspots have lower temperature? The charged particles in the plasma (electrons, protons and ions) from the solar atmosphere interact with the magnetic field and prevent plasma to reach the sunspot zone. A charge particle in a magnetic field will follow helical trajectories. The plasma in and around the sunspot radiates energy and cool off. The temperature of a sunspot is around 4,500 K. The temperature of the photosphere is around 5,800 K Why the sunspots look darker? The ratio of the flux F between the photosphere (Fph) and the sunspot (Fss) can be calculated by the formula (Stefan s Law): Fph/Fss = (Tph/Tss)^4 Fph/Fss = (5800/4500)^4 Fsp/Fss =2.76 The photosphere emit 2.76 times more flux than the sunspots

25 The Sun s differential rotation distorts the magnetic field lines Minimum of sunspot cycle Maximum of sunspot cycle The twisted and tangled field lines occasionally get kinked, causing the field strength to increase A tube of lines bursts through atmosphere creating sunspot pair

26 Sunspot Cycle and Solar Cycle During a solar maximum there is an increase of solar radiation, ejection of solar material, sunspots numbers and flares Solar maximum is reached every ~11 years The sunspot cycle last for about 11 years Solar Cycle is 22 years long. The direction of the magnetic field polarity of the sunspots flips every 11 years (back to original orientation every 22 years)

27 The sunspot number last on average about 11 years but occasionally sunspots may disappear (sunspot number drop to low or zero value) as it happened between 1645 and This is called the Maunder minimum This period of 70 years of minimum sunspot activity coincided with a period of cold temperatures called the Little Ice Age

28 A recent plot of the sunspot numbers including data until 2018 Some sunspot cycle have two maximum.

29 Heating of the Corona Charged particles (mostly protons and electrons) follow helical path and are accelerated along magnetic field lines above sunspots. This type of activity, not light energy, heats the corona.

30 Charged particles follow magnetic fields between sunspots: Solar Prominences Sunspots are cool, but the gas above them is hot!

31 Solar Prominence Typical size is 100,000 km May persist for days or weeks Earth

32 Very large solar prominence (1/2 million km across base, i.e. 39 Earth diameters) taken from Skylab in UV light. When seen against the bright solar surface, prominences appear as dark filaments.

33 Solar Flares Eruptions on the Solar surface resulting from stresses applied to the magnetic field lines, usually near sunspots. Emission of X-rays in a solar flare Flares such as these emit enormous amounts of X-ray and ultraviolet radiation as well as high energy particles both of which have important effects on the Earth. Those high energy particles produce intense auroral emission They also compress the magnetic field of the Earth. That induces a voltage (and current) in power lines. This may activate the power lines protections disconnecting the power from the transmission lines and may create a black out. The high energy particles can damage satellites which can disrupt communications, and TV transmissions Astronauts in interplanetary space are subject to this high energy particles and radiation originated in a solar flare

34 Solar Flares violent magnetic instabilities 5 hours The particles ejected in a flare are so energetic (High speed), the magnetic field cannot keep them trapped close to the Sun they escape Sun s gravity

35 Solar Flare (September 10, 2014) The Sun on Sept. 10, 2014 An animation of the flare X-Ray emission

36 Coronal activity increases with the number of sunspots.

37 What makes the Sun shine? Nuclear fusion: combining light nuclei into heavier ones An example: In the core of the Sun, the conversion of H into He. Four H nuclei are combined to produce one nucleus of He Atomic nuclei are positively charged and repel one another via the electromagnetic force. Merging nuclei (protons in Hydrogen) require high speeds. How it is possible to get high speed protons? NuclearFusion! Nuclear fusion requires temperatures of at least 10 7 K (10 million K) why? Higher temperature faster motion At very close range, a force called strong nuclear force takes over, binding protons and neutrons together (FUSION). Neutrinos are one byproduct. neutron proton

38 More on Nuclear Fusion 4 H Proton Proton He The conversion that take place on the Sun: The proton-proton chain Proton: nucleus of an H atom, positive charge Deuteron: nucleus of a deuterium (one proton, one neutron), an isotope of H Positron: antiparticle, same mass of an electron but has positive charge Neutrino: elementary particle with virtually no mass or charge. It hardly interact with mass Helium-3: Isotope of Helium (Two protons and one neutron) Helium-4: Stable nucleus of helium (Two protons and two neutrons) Gamma rays carry the energy produce by fusion Note: fusion is conversion of a light element into a heavier element. There is another process called nuclear fission in which heavier nucleus split into lighter nuclei releasing energy. This process is used to generate energy and power nuclear reactors

39 The production of energy is an example of the law of conservation of mass and energy But where does the Energy come from!? Mass lost is converted to Energy: E = m c 2 (c = speed of light) The total mass decreases during a fusion reaction. Relativity! c 2 is a very large number! A little mass equals a LOT of energy. Mass of 4 H Atoms = kg Mass of 1 He Atom = kg Difference = kg (% of original mass converted to E) = (0.71%) The Sun has enough mass to fuel its current energy output for another 5 billion years

40 The energy output from the core of the Sun is in the form of gamma rays. These are transformed into visible and IR light by the time they reach the surface (after interactions with particles in the Sun). Visible and IR Neutrinos are almost non-interacting with matter So they stream out freely. Gamma rays Neutrinos provide important tests of nuclear energy generation.

41 Solar Neutrino Problem: Neutrino detectors found only 30-50% of the predicted number that were expected from the Sun! A discrepancy between theory and experiments could mean either Detecting Solar Neutrinos These light detectors measure photons emitted by rare electron-neutrino reactions in the fluid (Fluid is purified water). 1) standard solar model incorrect or 2) standard particle theory incorrect. This discrepancy appears to have been resolved In 2002, Sudbury Neutrino Observatory in Canada showed that neutrinos oscillate into different flavors during their trip to Earth from the Sun. Previous neutrino experiments only detected one type of neutrino. The fluid used by this detector is heavy water. The hydrogen in the water molecule is replaced by deuterium) If all types of neutrinos are accounted for, the total number of neutrinos agrees well with the standard solar model prediction.