Astro 105: The Milky Way

Lecture VII: The Sun

"The Universe is not made of Atoms it is made of Stories"

Muriel Rukheyser

The last month we have spend looking at the history of astronomy/astrophysics and some philosophical issues surrounding science as a whole.

What we learned was a few terms in the language of physics and astrophysics.

Keplers Laws
Newtons Laws
Gravitation
Escape Velocity
Electromagnetic Spectrum
Blackbody Radiation
Emission and Absorption Lines
Quantum Mechanics

These are knowledge in and of themselves but astronomers use them as the key to scientific story telling.

These descriptions we will learn are STORIES! Its what people do and its how we make sense of the world.

Now we begin exploring the storyline for the Milky Way in earnest.

Our goal for the next month is to look at what "lives" in the Milky Way and to tell the story of its ecology.

This is the modern view, the contemporary story of our home galaxy.

Is it stable? How much will it change over the next few hundred/thousand/million years?

We begin with stars.

We begin with our star, Sol, the Sun.

Sun-fact # 1: Each square centimeter of the solar surface emits as much light as a 6000 Watt lamp!

SOHO IMAGE WEBSITE

The Structure of the Sun

The Sun is a gigantic ball of gas which is supported against its own gravity (weight) by pressure generated in thermonuclear reactions in its core.

Mass of Sun: M = 1.99x10³⁰ kg
Radius of Sun: R = 6.9x10⁸ m
Age: t = 4.5 to 5 billion years
Composition: H 74%, He 25% Other (Metals) 1%
(In astronomy everything bigger than He is called a metal)

From it's center to its outer layers the character of the sun changes dramatically.

Six Main Regions Of The Sun - Six main regions of the Sun, not drawn to scale, with physical dimensions labeled.

density of water = 1 g/cm³ = 1000 kg/m³

Region	Inner radius (km)	Temperature (K)	density (kg/m3)	Properties
Core	0	1.5x10⁷	150,000	Energy generated by Fusion
Radiation Zone	200,000	7,000,000	15,000	Energy Transported by radiation
Convection Zone	500,000	2,000,000	150	Energy Transported by convection
Photosphere	696,000	5800	2x10^-4	Radiation can escape (Part of sun we see)
Chromosphere	696,500	4500	5x10^-6	Cool lower atmosphere
Transition Zone	698,000	8000	2x10^-10	Rapid T increase
Corona	706,000	1,000,000	1x10^-12	Hot low density upper atmosphere
Solar Wind	10,000,000	2,000,000	1x10^-23	Flow of gas into space

Internal Structure of the Sun

By watching how waves bounce around on the Sun we can tell what is it like inside. This is called heliosiesmology.

GONG (Global Oscillations Network)

Computer Model Of Seismic Vibrations On Sun - The Sun has been found to vibrate in a very complex way. By observing the motion of the solar surface, scientists can determine the wavelength and the frequencies of the individual waves and deduce information about the solar interior not obtainable by other means. The alternating patches represent gas moving down (red) and up (blue).

From these studies we get a picture of the inside of the sun.

Theoretical Model Of Solar Interior - Theoretically modeled profiles of density (b) and temperature (c) for the interior of the Sun, presented for perspective in (a). All three parts describe a cross-sectional cut through the center of the Sun.

The conditions in the sun are incredible.

Ultra-high temperatures and densities.
Particles move around very fast and collide.

This is what allows nuclear Fusion to occur in the core.

Fusion = simple nuclei slamming together to make new bigger nuclei

More too it than this, (see below)

Energy (light) generated in the core makes its way out in the radiative zone.

Electrons are unbound from nuclei in radiation zone (ionization)

In the convective zone the electrons can combine with nuclei to make atoms. (recombination)

Energy is absorbed by atoms. The absorbed energy sets solar gas moving like water in a pot.

Convection = circular boiling motions

Physical Transport Of Energy From Sun's Interior - Physical transport of energy in the Sun's convection zone. We can visualize the upper interior as a boiling, seething sea of gas. Each convective loop is about 1000 km across. The convective cells are arranged in tiers with cells of progressively smaller size as the surface is neared. (This is a highly simplified diagram; there are many different cell sizes, and they are not so neatly arranged.)

Convection Movies

The convection occurs in "cells". On the surface of the sun these cells produce regions of light and dark (hot and cold)
This is what produces the granulation of the solar surface

The Granulated Photosphere - Skylab photograph of the granulated solar photosphere. Typical solar granules are comparable in size to the Earth's continents. The bright portions of the image are regions where hot material is upwelling from below. The dark regions correspond to cooler gas that is sinking back down into the interior.

Sunspots
The sun has its own powerful magnetic field which is generated in the convection zone. It is strong enough that the gas and the field get tied together .

Sunspots are areas on the Sun where the magnetic field is much higher than elsewhere. This increases the `magnetic pressure', which in turn decreases the normal atmospheric pressure. And because of that, the temperature in the solar spots is lower than in the surrounding area.

Sunspots are dark because the are relatively cold
(Blackbody Radiation again)

The Solar Atmosphere

Recall Kirchhoffs laws: Cold tenuous gas will absorb radiation that passes through it.
The photosphere is the last layer of the sun that "produces" the optical
radiation.
The light streaming up from the inner layers of the sun (blackbody) has to pass through the cooler less dense outer layers.
This is where absorption lines are formed.

Formation Of Solar Absorption Lines - Formation of solar absorption lines. Photons with energies well away from any atomic transition can escape from relatively deep in the photosphere, but those with energies close to a transition are more likely to be reabsorbed before escaping, so the ones we see on Earth tend to come from higher, cooler levels in the solar atmosphere. The inset shows a close-up tracing of two of the thousands of solar absorption lines, those produced by calcium at about 395 nm.

The Solar Spectrum - A detailed spectrum of our Sun in a portion of the visible domain shows thousands of spectral lines, which indicate the presence of some 67 different elements in various stages of excitation and ionization in the lower solar atmosphere.

The Corona
T = 1 million degrees

Corona Movie

Why is the temperature so high in the corona?
Not sure yet. It could magnetic waves which are driven up from lower regions
which are dissipated (like friction) heating the gas
The Solar Wind
The Sun pumps out a continual stream of particles. These particles form the solar wind which interacts with all the planets.
The Earths magnetic field intercepts the solar wind. This is the origin of aurora.

Aurora_Movie_Big

Without the earth' magnetic field we would be fried!

X-Rays show the Corona quite well as well as its structure.

The solar wind flows out from holes in the Corona.

The Solar Cycle
Sunspots come and go.
Overall there is a pattern where at some times the sun will have more spots then at other times.
The cycle repeats itself ever 11 years,
(11 years from maximum to maximum).

Sunspot Cycle In 20th Century - This graph presents the annual number of sunspots throughout the twentieth century, showing the 5-year average of the annual data to make long-term trends more evident. The (roughly) 11-year solar cycle is clearly visible. At the time of solar minimum, hardly any sunspots are seen. About 4 years later, at solar maximum, as many as 100-200 spots are observed per year.

In the midst of a cycle the latitude at which most spots appear changes as spots move from higher latitudes to lower ones

Sunspot Location At Latitude Vs Activity - Sunspots cluster at high latitudes when solar activity is at a minimum. They appear at lower latitudes as the number of sunspots peaks. Finally, they are prominent near the Sun's equator as solar minimum is again approached. The most recent solar maximum occurred in 1990.

On top of the 11 year cycle is a 22 year cycle where the polarity or direction of the magnetic field changes as well.

These changes must be related to the solar dynamo.

The solar cycle also has dips in it.

In the 1600s the number of spots decreased in what is called a Maunder minimum.
During this time the winters in Europe were recorded to be much colder producing a "little ice age".
Is there a connection?

Sunspot Activity Over 400 Years - This graph plots the number of sunspots occurring each year. Note the approximate 11-year "periodicity" and the absence of spots during the late seventeenth century.

Origin of Solar Magnetic Field

One of the greatest problems of Solar Physics is Understanding how the Sun gets its Magnetic Field.
This called the Solar-Dynamo Problem.
It is believed that the differential rotation of the Sun plays a crucial role.

Differential Rotation On The Sun Twisting Magnetic Fields - This diagram illustrates how the Sun's differential rotation wraps and distorts the solar magnetic field. Occasionally, the field lines burst out of the surface and loop through the lower atmosphere, thereby creating a sunspot pair. The underlying pattern of the solar field lines explains the observed pattern of sunspot polarities. (If the loop happens to occur on the limb of the Sun and is seen against the blackness of space, we see a phenomenon called a prominence, described in a later section.)

Dynamo layers are actually deep below solar surface.

EARTH’S DYNAMO

Above on Earth, magnetic field generated in Dynamo via molten iron core. The flowing liquid iron provides the currents which generate the magnetic field. On the sun it’s the flowing ionzed gas – called PLASMA – which provides the currents.

The Active Sun

The surface of the sun undergoes period outbursts of enormous power.

Magnetic Activity on the sun produces a variety of violent behavior

First there are prominences: Great Arcs of gas and magnetic fields with the power of many atomic bombs.

The size of these can 100,000 km across

Unusually Large Solar Prominence - This image of a particularly large solar prominence was observed by ultraviolet detectors aboard the Skylab space station in 1979.

Flares are even more violent rupturing the surface of the sun in just minutes.

Temperatures in the flares can get as high as 100,000,000 degrees K.

effect of flare
.

Overview Of Energetic Events On Sun's Surface - This summary piece illustrates many of the salient features of our Sun-from prominences in its upper atmosphere, to spicules and flares in its lower atmosphere, to granules and spots on the surface.
Solar activity changes in step with the solar cycle.

Flare Movie 1
Flare Movie 2
Flare Movie 3

The strongest Events on the Sun are called Coronal Mass Ejections (CMEs)

CME Movie 1

The changing surface of the sun has a great effect on the Earth.

SPACE WEATHER

SpaceWeatherSite

Solar wind (Coronal Mass Ejection) interacting with Earth’s magnetic field.

Movie Page of CMEs

CME anim 1
CME anim 2

Satellites get fried by Coronal Mass Ejections.
Powerlines get overloaded.
Astronauts can be in big trouble.

As we rely more on space for telecommunications space weather will be more of an issue.
Note we are in a new maximum!!

Powering The Sun

Thermonuclear Fusion
When two nuclei come together to form a third + energy

nucleus1 + nucleus 2 = nucleus 3 + energy
Nucleus 3 is lighter than nucleus 1 + nucleus 2
E = mc²
^{Basic reactions: the
proton-proton chain}

Proton Collision - Two protons collide violently, initiating the chain of nuclear fusion that powers the Sun.
In the full chain is pretty complicated with He being the end result.
During the reactions particles called neutrinos are produced.
Neutrinos are very "light" and hardly interact with matter at all.
They go flying right out of the sun

Proton-Proton Chain Collision - Diagram of the entire proton-proton chain. A total of six protons (and two electrons) are converted into two protons, one helium-4 nucleus, and two neutrinos. The two leftover protons are available as fuel for new proton-proton reactions, so the net effect is that four protons are fused to form one helium-4 nucleus. Energy, in the form of gamma rays, is produced in each reaction.

Efficiency of Solar Energy Production

Note that 1 kg converted into energy will produce E = mc² worth of energy with m = 1 kg

But 1 km worth H fused in He does produce E = (1 kg)c² worth of energy.

Only .7% of the H “rest mass” is released as energy. This is the binding energy given up when the free protons convert and bind into a nucleus.

One major issue is we don't detect the number of neutrinos on Earth we expect from theory.
This is called the Solar Neutrino Problem
Why are we wrong?