Astronomy Today 9th Ed.

Assigned Discussion 1-4,7-15

1. Refer to Figure 16.2 and Table 16.1 for help in answering this question.
  The regions are: 
a) core - where energy is generated through fusion of hydrogen into helium
   Tcore ~ 16 million K.
b) radiative zone - where energy is transferred outward primarily by
   radiation.
c) convective zone - where energy is transferred outward by convection 
   and radiation (and a little conduction).  The gas is more opaque in 
   this layer which makes radiation less effective.
d) photosphere - the surface of the Sun, that is the region where photons
   that reach our eyes last scattered.  It is roughly 500 km deep.
   Tsurf ~ 5800 K.
e) chromosphere - a colorful layer of low-density gas above the photosphere
f) transition zone - a region above the chromosphere in which the 
   temperature of the gas quickly increases to 1 million degrees.
g) corona - the outermost atmosphere, which blends into the solar wind

The radius of the core is 200,000 km, the radiative zone is 300,000 km 
thick, the convective zone is 200,000 km thick, the photosphere is 300 km 
thick, chromosphere is 3,000 km thick, and the corona extends about a few 
million km above the chromosphere. The photosphere is at a temperature of 
5800 K and the core is at a temperature of approximately 15 million K. 

2. Luminosity is a measure of the true brightness or total energy output 
of an object. For the Sun, it can be measured by experimentally determining
how much solar energy is received by one square meter at the distance of 
the Earth from the Sun. This is then multiplied by the surface area of a 
sphere whose radius is the semi-major axis of the Earth's orbit. 

3. Knowing basic facts about the Sun, like its total mass, total lum-
inosity, and the fact that it is made primarily of light gasses such as 
hydrogen and helium provide "boundary conditions" that the model must
satisfy.  In the lab, we also can figure out how such gasses behave under 
conditions of high pressure and temperature, allows astronomers to model 
the entire structure of the Sun. The model is a table of values of 
pressure, density, temperature, etc, as a function of depth.  These are
derived using the "structural equations" of astrophysics, one of which
is called "hydrostatic equilibrium".  
The model is successful if it successfully predicts the observed 
boundary conditions.  Helioseismology provides more constraints. 
Once the model "works", astronomers are then able to learn from the model 
about the properties in the interior of the Sun that are not directly
observable. 

4. Helioseismology is the study of waves that ripple across the surface of 
the Sun. There are standing wave patterns that are basically resonances
which depend on interior properties of the Sun.
Some of these waves travel from deep inside the Sun. 
Their appearance on the surface provides information about the interior 
of the Sun that cannot otherwise be observed, such as temperature, 
density, and rotation speed at different depths.  In a similar manner,
seismic waves from the Earth can tell us about the Earth's interior.

[ 5. The oscillations observed on the solar surface are similar to seismic 
waves observed on Earth, although they are different in origin. The patterns 
of the waves are influenced by the internal structure of the Sun. Models 
of the solar interior predict how the waves should behave; observed 
waves suggest how the models need to be modified until there is agreement 
between observations and models.  ]

[ 6. The solar radiation is first produced in the core of the Sun, largely 
in the form of gamma rays. Because the gas in the core is totally ionized, 
it is transparent to radiation and so the radiation passes through it 
freely. But as we get closer to the surface, the temperature drops, and 
more and more of the gas is not ionized or only partially ionized. Such a 
gas is opaque to radiation. At the outer edge of the radiation zone, 
all of the radiation has been absorbed by the gas. This heats the gas 
and it physically rises, while cooler gas from the surface falls. This 
is the region of convection. The energy is transported by convection to 
the photosphere. Here, the density of the gas is so low that radiation 
can freely escape into space, and travel in a straight line to Earth. ]

7. Virtually all the visible radiation we receive from the Sun comes from 
a thin layer called the photosphere. It is only about 500 km thick; a 
small fraction of the Sun's radius (1.3million km). The gas below the 
photosphere is too thick for light to escape, and the gas above is too 
thin to absorb and emit significant quantities of light.  Light can only 
escape from this narrow region, so the Sun has a very well-defined edge. 

8. Because the corona of the Sun is hot, some of the gas particles are 
traveling fast enough to escape the gravity of the Sun. The gas is mostly 
composed of the separated components of ionized hydrogen (protons, and 
electrons). This flow of high-speed particles away from the Sun is known 
as the solar wind. 

9. Activity in the Sun's magnetic field creates the cycle of sunspots seen 
on the Sun.  The solar magnetic field reverses itself every 11 years, so 
it takes 22 years (two sunspot cycles) to go through a single cycle of 
magnetic reversals. 

10. Sunspots, flares and prominances are caused by activity in the magnetic
field of the Sun. Sunspots are caused by kinks or loops of magnetic field 
extending through the lower atmosphere. These areas of concentrated 
magnetic field prevent hot material from rising up from the Sun's 
interior, so the section of the Sun underneath the knot cools off and 
darkens.  Flares, by contrast, are areas where large amounts of energy 
are released in a short amount of time (minutes).  Their origin is 
mysterious, but they are somehow connected to instabilities in the 
magnetic field.  Prominences are caused by material ejected from the 
Sun's surface that follows along huge loops of magnetic field that carry 
the luminous gas far above the solar surface. 

11. A coronal mass ejection is a cloud of ionized gas that travels 
quickly from the surface of the Sun to Earth, where it will be mostly 
captured by the Earth's magnetic field. Some of it can get through, 
however, and ionize the upper atmosphere.  This can affect radio 
communication. The particles are swept along by the magnetic field and 
can induce large currents in electrical power grids, knocking them out. 
Earth satellites are particularly vulnerable to these electrical and 
magnetic storms because of their delicate electronics and exposed 
position above the atmosphere. 

12. The Sun's energy output is fueled by nuclear fusion of hydrogen 
into helium. In the process that takes place in the core of the Sun, 
4 hydrogen atoms (really just protons) come together and fuse to form 
a heavier element, helium. In this process, a small amount of mass is 
lost.  That missing mass has been converted into energy.  According to 
Einstein's famous equation, E = mc^2, a small amount of mass can become a 
large amount of energy.

13. A total of 6 hydrogen atoms go into the proton-proton chain. What 
comes out is a helium nucleus, two neutrinos, two positrons (which are 
quickly annihilated by colliding with electrons), energy in the form of 
gamma rays, and two hydrogens. Thus, only 4 hydrogen atoms are consumed 
to make the helium. The mass of helium produced by the nuclear fusion is 
0.7% less than the mass of the four hydrogens that were fused to make it. 
This small amount of lost mass is converted into energy. 
The amount of energy is easily calculated from E = mc^2.

14. Neutrinos are produced in the proton-proton chain, which occurs in 
the core of the Sun. The neutrinos pass unimpeded through the Sun at 
nearly the speed of light. So neutrinos, in a sense, allow astronomers 
to directly observe the core of the Sun and the processes that occur 
there, almost as they happen. For a long time, astronomers were puzzled
because the Sun did not seem to be producing as many neutrinos as 
predicted.  There were two possible explanations for the low number of 
solar neutrinos received on Earth: either the Sun was under-producing 
neutrinos, or something was happening to the neutrinos as they traveled 
to Earth.  The first possibility was disturbing, as it would likely 
require the Sun's core to be cooling by 10%.  But what could alter a 
neutrino across the void of space?  Recently, we have discovered that 
there are different kinds of neutrinos, and that they can transform into 
each other during the trip to Earth through oscillations. By creating 
neutrino detectors that can detect all different kinds of neutrinos, we 
have confirmed that the Sun is producing all of the neutrinos we expect 
it to.

15. Light waves can take millions of years to fight their way out of the 
thick gasses in the Sun's interior, while neutrinos fly out in a straight 
line at almost the speed of light.  Therefore, if we have a means of 
detecting neutrinos, we would know within minutes if nuclear fusion in 
the Sun were to shut down.  If we only relied on visible light, however, 
it could take millions of years before the Sun would start to dim


Mult Choice. [Not assigned]
1. c
2. b
3. c
4. a
5. a
6. b
7. b
8. c
9. c
10. b