Physics 101 - Astronomy - Spring 2019
Class notes for day 20, April 2, 2019
This material is from Ch. 16, about the energy source of the Sun, the star that is closest to us. The PowerPoint slides are in day20sun2.pptx. I showed some video clips which are listed in the PowerPoint and also below.
We started by reviewing some 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, like on Jupiter).
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 degrees Celsius).
The energy travels outward through various zones.
We continued the lecture by starting with the core and explaining how all the
energy is generated.
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, which travel without being
absorbed through the radiative zone, which is transparent to these X-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, about
the size of states in the USA.
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. See the PowerPoint for a schematic graphic.
The particles that participate in this process are:
Proton – the nucleus of a hydrogen atom, an elementary particle with a positive
electrical charge.
Neutron – another elementary particle, found in the nucleus of heavier elements.
A neutron is electrically neutral.
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.
Positron – the antiparticle to the electron, with the same mass but a positive
charge.
Deuteron – nucleus of deuterium, consists of a proton and a neutron bound
together.
Helium-3 – has 2 protons and 1 neutron in its nucleus.
Helium-4 – has 2 protons and 2 neutrons in its nucleus.
Neutrino – a very light particle with no electrical charge and the ability to
penetrate through ordinary matter easily.
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+ + nu + energy (nu represents neutrino)
The next slide shows several other reactions. See the PowerPoint for the 3 stage
process shown in your textbook.
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.
These solar neutrinos can be detected at a neutrino detector near Kamioka,
Japan, called the Super-KamiokaNDE. (Kamioka Neutrino Detection Experiment).
This detects neutrinos from the core of the Sun.
Another Solar Neutrino Experiment is 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.
For a discussion of the difference between CMEs and solar flares, we need some
vocabulary words:
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
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.
At a conference in 2014, several scientists presented more information about the 2012 CME, described in this video: 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
http://www.nasa.gov/content/goddard/how-nasa-watches-cmes/
or on YouTube at
https://www.youtube.com/watch?v=cLLq6plMjU0
The Carrington event of 1859 was an extremely strong flare, and a strong CME
that 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
This could happen again and will be very destructive of our electrical power
system.
Exam # 3 will be on Thursday, April 4. That’s this week.
It will cover Ch. 11-16 and have a format similar to the previous exams. In the OpenStax book, see the review sections of the chapters which
correspond to this material. The corresponding sections are 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.