Chapter 16 The Sun: a Nuclear Powerhouse

The closest star is the Sun

Solar Prominences (on upper right) from the Astronomy Picture of the Day site link (photo by the SOHO spacecraft)

The Sun is a layered structure

Vital Statistics of the Sun Diameter is about 109 Earth diameters. Mass is about 333,000 times Earth’s mass. Surface temperature is about 5800 K (or 5500oC). Rotation period is about 25 days at the equator and 35 days at the poles (differential rotation). The core rotates once each week (this is new information!). Average density is about 1.4 times that of water. All the Sun’s energy is produced by fusion in a dense core that is at about 15.5 million Kelvin (or Celsius). The energy travels outward through various zones.

FIGURE 10-21 The Solar Model (a) Thermonuclear reactions occur in the Sun’s core, which extends to a distance of 0.25 solar radius from the center. In this model, energy from the core radiates outward to a distance of 0.8 solar radius. Convection is responsible for energy transport in the Sun’s outer layers. (b) The Sun’s internal structure is displayed here with graphs that show how the luminosity, mass, temperature, and density vary with the distance from the Sun’s center. A solar radius (the distance from the Sun’s center to the photosphere) equals 696,000 km. (c) Ten most common elements in the Sun, by the numbers of atoms of each and by the percentage of the Sun’s total mass they each comprise. (a: NASA)

The thermonuclear core produces energy by nuclear reactions which are caused by the high temperature, hence “thermo-nuclear”. (more on this later) Much of the energy comes out of the core in the form of X-rays or gamma rays, which travel without being absorbed through the radiative zone, which is transparent to these X-rays or gamma rays (g rays). The X-rays are then absorbed in the convective zone, and this heats the plasma in that zone, which undergoes convection, a motion which is similar to convection in any hot fluid. The convective zone has very large convection cells, and then above that zone is the photosphere, which has smaller convection cells.

FIGURE 10-21 The Solar Model (a) Thermonuclear reactions occur in the Sun’s core, which extends to a distance of 0.25 solar radius from the center. In this model, energy from the core radiates outward to a distance of 0.8 solar radius. Convection is responsible for energy transport in the Sun’s outer layers. (b) The Sun’s internal structure is displayed here with graphs that show how the luminosity, mass, temperature, and density vary with the distance from the Sun’s center. A solar radius (the distance from the Sun’s center to the photosphere) equals 696,000 km. (c) Ten most common elements in the Sun, by the numbers of atoms of each and by the percentage of the Sun’s total mass they each comprise. (a: NASA)

ionized but the protons do not have enough velocity to hit each other Nuclear fusion reactions occur at high temperature in the core, which causes particles to slam into each other at high speed. Below about 10 million K hydrogen atoms are ionized but the protons do not have enough velocity to hit each other because of electric force. At higher temperatures, the protons have more velocity, so when they hit each other they can fuse together to form a nucleus of deuterium (and a positron and a neutrino).

Nuclear fusion occurs in several reaction stages, all occurring simultaneously in the core of the Sun.

The players in this drama Proton – the nucleus of a hydrogen atom, an elementary particle with a positive electrical charge. (denoted by p or 1H) Neutron – another elementary particle, found in the nucleus of heavier elements. A neutron is electrically neutral (zero charge). Electron – an elementary particle which normally orbits a nucleus, but is moving freely in the high-temperature plasma of the Sun. The electron has a negative electrical charge. (e-) Positron – the antiparticle to the electron, with the same mass but a positive charge. (e+) Deuteron – nucleus of deuterium, consists of a proton and a neutron bound together. (2H or D or d) Helium-3 – has 2 protons and 1 neutron in its nucleus. (3He) Helium-4 – has 2 protons and 2 neutrons in its nucleus. (4He) Neutrino – a very light particle with no electrical charge and the ability to penetrate through ordinary matter easily. (greek nu n)

Nuclear reactions Nuclear reactions are distinct from chemical reactions. The difference is that chemical reactions combine atoms to form molecules, and different molecules can react to form new ones, whereas nuclear reactions involve nuclei, composed of elementary particles, and/or elementary particles themselves. An example is the proton-proton fusion reaction. Fusion means to join together. If two protons smash together at enough speed, they can fuse together and form reaction products. The products are a deuteron, a positron, and a neutrino. All of these move away at high speed, which means there is a form of energy called kinetic energy (energy of motion). In equation form: p + p --> d + e+ + n + energy The next slide show several other reactions.

See details on following slides

Radiant energy from the core travels quickly outward through the radiation zone to heat the convection zone.

Equivalently, the reactants could be four protons and four electrons (i.e., four ionized hydrogen atoms), and the products are one helium nucleus (also called an alpha particle) which has two protons and two neutrons, and two electrons (an ionized helium atom). Also produced is a large amount of kinetic energy (the high speed of the reactants) and energy in the form of gamma rays or X-rays that quickly leave the core. Another product is the neutrino. Large quantities are produced, but they have little effect on the Sun because they pass easily through large layers of ordinary matter without being absorbed. However, the neutrinos need to be studied because they can tell us something about the core of the Sun.

This neutrino detector near Kamioka, Japan, is called the Super-KamiokaNDE. (Kamioka Neutrino Detection Experiment) (Workers are seen inspecting the phototubes, with some of the water drained out of the large tank, which is deep underground.) This detects neutrinos from the core of the Sun.

Cross-section of the Kamioka NDE detector (which is located in a mine) Cross-section of the Kamioka NDE detector (which is located in a mine). The big tank is filled with pure water and the walls have the PMT tubes, to detect light from the arrival of a neutrino

The rare interaction of a neutrino with the water will produce a burst of light which fans out in a cone shape. This light is detected by a device called a photomultiplier tube (PMT) and recorded by computers for later analysis.

A Solar Neutrino Experiment in the Sudbury Neutrino Observatory, in an old nickel mine near Sudbury, Canada. This and other experiments confirm the Solar Model described on the previous slides. FIGURE 10-22 The Solar Neutrino Experiment Located 6800 feet underground in the Creighton nickel mine in Sudbury, Canada, the Sudbury Neutrino Observatory is centered around a tank that contains 1000 tons of water. Occasionally, a neutrino entering the tank interacts with one or another of the particles already there. Such interactions create flashes of light, called Cerenkov radiation. Some 9600 light detectors sense this light. The numerous silver protrusions are the back sides of the light detectors prior to their being wired and connected to electronics in the lab, seen at the bottom of the photograph. (Ernest Orlando Lawrence/Berkeley National Laboratory)

Solar Granulation in the photosphere can be seen in movies taken by the SOHO cameras This granulation shows that convection is occurring under the surface of the Sun. On average these granules are about the size of a large state like Texas (up to 1000 miles across).

The atmosphere of the Sun The outer layers are all parts of the Sun’s atmosphere: Corona Transition zone Chromosphere The Photosphere is the “surface” of the Sun; it emits the light that we see. The Convection and Radiation Zones are named for how energy is transported in the interior of the Sun.

Above the granular photosphere is the chromosphere.

The Solar Corona is most obvious during a total solar eclipse.

Solar Atmospheric Temperature So much energy is flowing through this region and the density is so low that the temperature of these regions is very high. All of this energy causes gas to “boil” off into space, or causes gas to be “pushed” off the surface of the Sun. This gas is called the Solar Wind.

Coronal Hole, seen in X-ray images by Yohkoh. (link)

Solar Spectrum This absorption spectra tells us what elements are in the Sun’s chromosphere and most likely in the rest of the Sun, except in the core.

Sunspots Sunspots are cooler regions of the Sun’s surface

Sunspots are also regions of intense magnetic fields. Sunspots, Up Close Sunspots are also regions of intense magnetic fields. Just like regular magnets, sunspots come in pairs one is a “North pole” and one is a “South pole” Dark region – umbra (4300 K) Brighter region – penumbra (5000 K) Granules – (5800 K)

Sunspot Magnetism

Sunspots behave somewhat like a horseshoe magnet, causing a magnetic field above the photosphere.

Above the sunspot, the magnet field causes the hot gas of the corona to concentrate along the field lines, seen here is a photo in the ultraviolet.

The “active Sun” refers to times when there are lots of sunspots and the surface of the Sun is very active in other ways. Many Prominences, Flares, and Coronal Mass Ejections can be seen. Also during this time the corona becomes larger and more irregularly shaped

Sunspot Cycle Both the number and the location of the sunspots change during the Sunspot or Solar Cycle which lasts 11 years

Maunder Minimum The solar cycle has varied a lot in the past and we still do not know all of the details of how it works.

Coronal Mass Ejection Coronal Mass Ejection events (CME) is when an eruption on the surface of the Sun ejects large amounts of gas into space. These events are larger than solar flares and are less frequent.

Solar Prominences are huge outbursts of hot gases that blow out into space, then often the gas cools and falls back into the Sun.

are loops of hot gas that rise from the surface of the sun. They are Solar Prominences Solar Prominences are loops of hot gas that rise from the surface of the sun. They are shaped by the magnetic fields of the Sun. SOHO website: http://sohowww.nascom.nasa.gov/ For Mar. 28, 2014, there is a movie of 8 CMEs in five days: https://sohowww.nascom.nasa.gov/pickoftheweek/old/28mar2014/

Solar Flares are much more rapid than prominences.

A Solar flare on Nov. 11, 2003. SOHO obtained numerous images of the active Sun in fall 2003.

Some vocabulary related to CMEs magnetopause - the boundary between the solar wind and the region protected by the Earth's magnetic field magnetotail - the portion of the Earth's magnetic field that flows away from Earth in the direction away from the Sun and provides the largest source of particles that cause the aurora bow shock - similar to the type of wave that builds up in front of the bow of a boat Carrington event of 1859 - a strong CME hit Earth and caused sparking of telegraph equipment due to electric effects in the long wires strung between cities of that era. For more on this see Wikipedia https://en.wikipedia.org/wiki/Solar_storm_of_1859 Space Weather Prediction Center http://www.swpc.noaa.gov/ SpaceWeather.com has links to several related sites

Videos about CMEs and solar flares The difference between flares and CMEs is shown here: http://www.nasa.gov/content/goddard/the-difference-between-flares-and-cmes/ The strong CME of July 2012 is described in a video at http://www.nasa.gov/goddard/mapping-the-journey-of-a-giant-coronal-mass-ejection/ which mentions coronagraphs, which block light from the Sun’s photosphere to allow taking pictures of surrounding space. Another video is at https://www.youtube.com/watch?v=7ukQhycKOFw Simulations of the effect of a CME on the Earth's magnetic field is shown in a video at (see the second video on this page) http://www.nasa.gov/content/goddard/how-nasa-watches-cmes/ or on YouTube at https://www.youtube.com/watch?v=cLLq6plMjU0 Carrington event of 1859 - a strong CME hit Earth and caused sparking of telegraph equipment due to electric effects in the long wires strung between cities of that era. For more on this see Wikipedia https://en.wikipedia.org/wiki/Solar_storm_of_1859

Exam # 3 will be on Thursday, April 4 (this week). It will cover Ch Exam # 3 will be on Thursday, April 4 (this week). It will cover Ch. 11-16 in the OpenStax textbook and have a format similar to the previous exams. In the OpenStax book, we have covered these sections: 11.1-3, 12.1-5, 13.1-4, 14.1-4, 15.1-4, 16.1-3. I will review for about half an hour before starting the exam.