Unit 7:

The Sun: A Star Close Up
 
 

OVERVIEW

The structure of the Sun is described. It is considered in three parts: the solar interior, the solar atmosphere, and the solar wind. Details on how the various parts are further divided into zones are given (see KEY WORDS). The processes of energy transport in the interior of a star are discussed, including earthly examples. Radiative and convective energy transport occur in the Sun; conductive energy transport can occur in very dense stars. The difference between heat and temperature is reviewed. Weather phenomena on the Sun is discussed. A discussion is given of how the gravitational effects associated with the Sun were used to verify Einstein's General Theory of Relativity and reject Newton's Theory of Gravity.

LEARNING OBJECTIVES

At the end of this unit you should be able to:

1. Describe the overall structure of the Sun: its interior, its atmosphere, and its wind.

2. Describe the zones in the Sun's interior (the energy generating core, the radiative zone, and the convective zone), how energy is transported or moved through a star's interior (by radiation, convection, or conduction), and how energy is transported through the Sun's interior in particular.

3. Describe some earthly examples of how energy is transported from one place to another.

4. Discuss the difference between heat and temperature.

5. Describe the zones in the Sun's atmosphere (the photosphere, the chromosphere, and the corona).

6. Describe weather phenomena in the Sun's atmosphere (see KEY WORDS) and the relation to the solar magnetic cycle.

 7. Describe the solar wind and how it can be detected.

8. Discuss how observations of the gravitational effects of the Sun led to the acceptance of Einstein's General Theory of Relativity (GTR) over Newton's Theory of Gravity.

 KEY WORDS solar interior

radiative zone

convective zone

radiation

convection

conduction

solar core

density

temperature

pressure

gamma rays

solar atmosphere

photosphere

chromosphere

corona

granulation

coronal holes

sunspots

umbra

penumbra

sunspot cycle

11 year cycle

reverse in polarity of the magnetic field

22 year cycle

flares

plages

filaments

prominences

heat versus temperature

solar oscillations

solar seismology

solar wind

comet's tail and solar wind

the Sun and the General Theory of Relativity (GTR)

precession of Mercury's orbit

gravitational deflection of star light

GTR versus the Special Theory of Relativity (STR).

¶ gravitational lens

¶ length contraction

¶ time dilation

 WRITTEN NOTES

THE STRUCTURE OF THE SUN

Since the Sun is the nearest star, we can learn a lot about the structure of all stars and how they work by studying the Sun in detail.

The structure of the Sun or any star can be broken up into component parts. Different physical processes dominate in these various parts. In our discussion we will consider the structure in three broad regions, but each of these regions can be broken up into further subcomponents or areas of study:

1. The Solar (or Stellar) Interior. This region consists of an inner "radiative zone" (which includes the Sun's energy generating core) and an outer "convective zone."

2. The Solar (or Stellar) Atmosphere. This region consists of the inner photosphere (where the light we see comes from), the chromosphere, and the outer corona. In addition to these global zones, there are many local phenomenon:

i. granulation caused by convection

ii. coronal holes

iii. sunspots (umbra & penumbra) and their 11 year cycle

iv. flares

v. plages

vi. filaments, and

vii. prominences.

 3. The Solar (or Stellar) Wind. This "wind" extends out into the solar system and affects the Earth, the tails of comets, etc. Note that the photosphere of the Sun (which is the part of the Sun's atmosphere that defines its visible size) is 1.4 million kilometers (1.4 x 106 km) in diameter. This is 109 times the diameter of the Earth. The Sun has a mass which is 333,000 times the mass of the earth. 1. THE SOLAR INTERIOR Energy (electromagnetic radiation) generated by nuclear reactions in the center of the Sun takes 30,000 years to make its way to the Sun's surface (photosphere) because it is absorbed and emitted by the Sun's matter so often.

During this absorption and emission, the wavelengths of the radiation are changed. Gamma rays generated in the Sun's very hot core eventually become visible photons emitted from the cooler material in the photosphere.

Because the properties of the radiation coming from the Sun's interior are altered by absorption and emission, it is very difficult to directly study what is going on in the Sun's interior.

Consequently, most of our knowledge of the Sun's interior structure comes from theoretical calculations. We know that the density, temperature, and pressure of the Sun's matter increases as one moves from the Sun's surface toward its core. The temperature at the Sun's core is calculated to be 15 million degrees Kelvin.

However, there are two methods of making observations which tell us about the Sun's interior structure:

1. We observe solar oscillations ("solar seismology").

2. We observe solar neutrinos generated in nuclear reactions (Chapter 24). Neutrinos are not absorbed by the Sun's matter and those generated in the core escape the Sun in 2 seconds.

 How is Energy Transported (Moved) from the Sun's Core to its Photosphere? In stars (and in our everyday experiences) we find that energy can be moved in one of three ways: 1. Energy transport by radiation occurs when electromagnetic radiation itself moves from one place to another. Energy transport is by radiation in the Sun's core and inner 70% (in radius) of its interior. [Earthly examples: when the Sun's light warms the ground or our skin; when we are warmed by a fire; when a microwave cooks something; when we broil something in an oven there is partial radiative heating.] 2. Energy transport by convection occurs when matter moves energy from one place to another. Energy transport is by convection in the outer 30% (in radius) of the Sun's interior. [Earthly examples: when hot air rises; when there is forced air heating in a house; when a shady region outdoors gets warm.] 3. Energy transport by conduction occurs when energetic atoms communicate their agitation to nearby cooler atoms by collisions. In very dense stars (like white dwarfs) energy transport is by conduction. [Earthly examples: when a metal spoon placed in a hot liquid heats up; when any solid heats up.] In summary the Sun's interior consists of two parts: 1. The Radiative Zone. This zone encompasses 70% (in radius) of the Sun's interior and includes the core. The core encompasses 30% of the Sun's interior. Nuclear energy is generated only in the core. Energy transport throughout this zone is by radiation.

2. The Convective Zone. This zone encompasses the remaining outer 30% (in radius) of the Sun's interior. No energy is generated here and energy transport is by convection.

 A Note on the Concepts of Heat and Temperature When something has energy it has the ability to do the work to make things happen.

In physics we often talk about potential energy (stored energy) or kinetic energy (energy of motion). These concepts are incorporated into theories that make accurate predictions.

Heat is one form of energy.

When we discuss the temperature of a gas in astronomy, we are referring to how fast the atoms, ions, or molecules that make up the gas are moving.

It is possible for a gas to have a very high temperature because its atoms are moving randomly very quickly. However if the gas is tenuous (not dense), there may be very little heat energy associated with it.

 2. THE SOLAR ATMOSPHERE The properties of the Sun's atmosphere are nothing like the properties of the planets' atmospheres.

The boundary between the Sun's interior and atmosphere is defined to be the photosphere, and the photosphere is considered to be part of the Sun's atmosphere.

There are three global zones to the Sun's atmosphere: the photosphere, the chromosphere, and the corona.

 The Photosphere The photosphere is the surface of the Sun that we see.

As radiation from the Sun's interior makes its way toward the surface, it is absorbed for the last time in the photosphere and then is emitted into space.

Therefore, observations of the Sun reveal a great deal about its photosphere.

We have used Wien's Law, the Stefan-Boltzmann Law, and Planck's Law to make deductions about the characteristics (e.g. color) and amount of electromagnetic radiation coming from the photosphere.

The photosphere is found to be approximately 330 kilometers deep and it has an average temperature of 5,800 degrees Kelvin.

The photosphere radiates the spectrum of a G type main sequence star.

 The Chromosphere The chromosphere is a 10,000 kilometer (1 x 104 km) deep region which lies above the photosphere.

It is not a shell which encloses the Sun (like the photosphere) but is a series of jets 700 km across and 7,000 km long which shoot up off the photosphere. These jets are called spicules and they have lifetimes of 5 to 15 minutes.

 Organized convective cells which are rising and falling in the chromosphere can be seen. Their motions can be studied spectroscopically by observing Doppler shifts. The convective cells are called supergranulation cells (30,000 km across) and they contain hundreds of smaller cells called granules.

Chromospheric matter has a temperature of about 15,000 degrees Kelvin, however it does not contain much heat energy because it is so tenuous.

 The Corona The corona is the outermost part of the Sun's atmosphere and it extends into the solar wind throughout the solar system.

Gas in parts of the corona have a temperature of 2 million degrees Kelvin, however the corona also does not contain much heat energy because it is tenuous.

The hot parts of the corona emit X-rays, while the cooler parts (called coronal holes) do not.

The Corona can be directly observed during a total solar eclipse.

 Sunspots and Other Local Phenomenon

Associated with Magnetic Fields

Strong magnetic fields are associated with many parts of the Sun.

When the Sun's magnetic field activity is strong the Sun is said to be active.

Since the magnetic fields vary in strength with an 11 year cycle (the cycle is actually 22 years if the reverse in polarity of the magnetic field is considered), many local phenomenon on the Sun grow stronger then weaker over this same time period.

The best way to observe the Sun's 11 year magnetic field cycle is to count the number of sunspots. When there are more sunspots, magnetic activity is stronger.

Some of the local phenomenon affected by magnetic fields are: sunspots, plages, filaments, and prominences.

 1. Sunspots. Sunspots are magnetic storms on the Sun. The typical size of a sunspot is about the size of the Earth. A sunspot looks like a darker region on the Sun because its temperature is lower (thus, according to the Stefan-Boltzmann Law, it emits less energy than the surrounding higher temperature regions). The dark central region of a sunspot is called the umbra while the region around the umbra which is not quite as dark is called the penumbra. [Note that by studying the motion of sunspots on the Sun we find that the Sun rotates once every 25 days at the equator, but once every 28 days at higher solar latitudes. This differential rotation helps create the solar magnetic fields.] 2. Flares. A flare is a rapid brightening of a small area of the surface of the Sun. It is associated with visible emission lines, x-ray emission, and radio emission.

3. Plages. Plages are bright regions near sunspots which emit strong hydrogen spectral lines.

4. Filaments and Prominences. Filaments are string-like structures that may extend up to 100,000 km across the surface of the Sun. In hydrogen spectral lines they are seen as dark features because they absorb the light from the background photosphere. However, when we see a filament protrude off the edge of the Sun (called a prominence), emission lines from them are observed.

 3. THE SOLAR WIND The solar corona and ions associated with flares expand out into space to form the solar wind.

However, the solar wind is not like a normal wind as it has only about 5 particles per cubic cm near Earth.

The presence of the solar wind is known from observations of comets' tails. Since the solar wind emanates from the Sun, a comet's tail is seen to always point away from the Sun because the solar wind is pushing the tail in that direction.

The solar wind has a pronounced effect on the Earth, especially when the Sun is in an active state. Charged particles (ions) from flares reach the Earth in a few hours and cause the aurora (charged particles trapped in the magnetic field emanating from the Earth's poles). Extremely energetic flares may cause disruptions in radio transmission and even power black outs.

 The Solar Constant The so-called solar constant is a measure of the amount of the Sun's energy which reaches the Earth (Note: this is not a constant!).

When the number of sunspots on the Sun is large, about 0.2% less energy reaches the Sun than is normal.

Observers are interested in determining if there are long term changes in the solar constant.

 The Sun and Einstein's General Theory of Relativity In the late 1910s and early 1920s scientists realized that Einstein's theory of gravity (called the General Theory of Relativity) made better predictions in the presence of strong gravitational fields than did Newton's theory.

Two observations made in the presence of the Sun's gravitational field led to this realization:

1. Einstein's theory accounted for Mercury's orbit accurately, while Newton's theory resulted in a positional error of 43 arcsec per 100 years.

2. Einstein's theory accounted for the gravitational deflection of star light. This deflection can be observed during a total solar eclipse. (Note that although light has zero rest mass, it is still affected by gravity because mass and energy are equivalent according to Einstein's Special Theory of Relativity.)

[A third prediction of the General Theory of Relativity was the gravitational redshift of light. We will not cover this yet.] What is Einstein's Special Theory of Relativity (STR)? Proposed in 1905, the STR is based on the fact that the speed of light is the same for all observers regardless of their motion.  No object that has mass can move at or above the speed of light.

Because of this, if we watch an experiment moving near the speed of light we see length contraction, time dilation, and other effects that defy common sense.  Length contraction is the fact that you will see an object moving relative to you as being shorter than it is when it is at rest relative to you.  Time dilation is the fact that you will see a clock moving relative to you to be running more slowly than it is when it is at rest relative to you.  These are not simply optical illusions; they are absolutely real.  The faster the object is moving (relative to you), the shorter you see it to be, and the more slowly you see its time to run.  As a clock moves faster and faster, you see it run more and more slowly as it approaches the speed of light, time coming almost to a complete stop when it gets close to light speed.

That energy and mass are related (E = mc2 where c is the speed of light) results from the STR.

 What is Einstein's General Theory of Relativity (GTR)? Proposed in 1916, the GTR incorporates the ideas of the STR into curved spaces.

Conceptually, mass  causes space to curve.  This curvature is what creates the force of gravity.  Think of the analogy of space as a thin rubber sheet, and stars and galaxies are marbles placed on that sheet, sinking down and causing the sheet to bend.  The more massive an object is, the more space curves around it.

¶ Light always travels in a straight line, but when space itself is curved, the meaning of a "straight line" has to be specified:  A straight line is one in which the light beam turns neither to the right nor the left as it moves.  Thus on a spherical surface like the Earth, the equator is a "straight line," because if you walked along it, you would never turn to one side or the other.  Meridian lines (lines of longitude) are also straight lines in this sense, but latitude lines are not, except for the equator.  On the surface of the Earth, straight lines are called "great circle" routes, and they are used as airplane routes because they are the shortest distance between two places on the Earth.

¶ Because mass curves space, and light always travels in straight lines (as we have defined them) through the curved space, then massive obects can "bend" light beams around them.  This was first discovered with starlight bending around the Sun in the early part of this century.  A larger mass will bend light even more.  If a distant source of light, such as a quasar, is behind a very massive galaxy, then it is possible for the galaxy to bend the quasar's light so much that we see two complete images of the quasar.  This is called a gravitational lens.  The galaxy bending the light is the "lens" in this case.  Depending on the alignment, you can wind up with two quasar images, three, four (called an "Einstein Cross", or in the case of perfect alignment, an entire ring surounding the lens (an "Einstein Ring").

 READING ASSIGNMENT

Text:  Chapter 14, Chapter S4 (but in S4, you are only responsible for the material we covered in class)
 

HOMEWORK

No homework for this lesson.