Astro 105: The Milky Way

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)

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

Internal Stucture of the Sun

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).

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
 
 

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

Electrons are unbound from nuclei in radiation zone

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

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.
 
 

Recall Kirchhoffs laws: Cold tenuous gas will absorb radiation that passes through it.

The photosphere is the last layer of the sun that "produces" 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 absorbtion 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 Chromosphere

The chromosphere can not usally be seen. Only during a solar eclipse can the inner regions of the sun's atmosphere be seen.

It has a reddish color which comes from the first Balmer lines of H

(Halpha)

Photo Of A Total Solar Eclipse - This photograph of a total solar eclipse shows the solar chromosphere, a few thousand kilometers above the Sun's surface.

The Chromosphere is a very violent place. Solar storms called Spicules are jets of hot material being tossed into the upper atmosphere

Solar spicules, short-lived narrow jets of gas that typically last mere minutes, can be seen sprouting up from the solar chromosphere in this Ha image of the Sun. The spicules are the thin, dark, spikelike regions. They appear dark against the face of the Sun because they are cooler than the solar photosphere.

The Corona

The corona is the very hot outer most layer of the sun. The temperature are almost as high as in the core but the densities are many orders of magnitide lower (no fusion!).
 
 

Spectal lines in the Corona indicate very high ionization. That means many electrons have been stripped of atoms. Before people knew about the high temps these lines were thought to come from a new element coronium.

The Solar Corona - When both the photosphere and the chromosphere are obscured by the Moon during a solar eclipse, the faint corona becomes visible. This photograph shows clearly the emission of radiation from the solar corona.
 
 

Gas Temperature Changes In The Lower Solar Atmosphere - The change of gas temperature in the lower solar atmosphere is dramatic. The minimum temperature marks the outer edge of the chromosphere. Beyond that, the temperature rises sharply in the transition zone, finally leveling off at over 1,000,000 K in the corona.

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.

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 Sun Seen In X-Ray Light - Images of X-ray emission from the Sun observed by the Skylab space station. These frames were taken at one-day intervals. Note the dark, boot-shaped coronal hole traveling from left to right, where the X-ray observations outline in dramatic detail the abnormally thin regions through which the high-speed solar wind streams forth.

The Violent Sun

The sun has its own powerful magnetic field. It is strong enough that the gas and the field get tied together .

Magnetic Activity on the sun produces a variety of violent behavior
 
 

Sunspots

Sunspots are regions where strong fields emerge from the solar surface. The spots appear dark becuase the gas is cooler inside the region of high magnetic field.

Dark Inner regions - The Umbra

Ligher Outer regions - The Penumbra

Sunspot Pair - This photograph of the entire Sun, taken during a period of maximum solar activity, shows several groups of sunspots. The largest spots in this image are over 20,000 km across-twice the diameter of the Earth. Typical sunspots are only about half this size

Sunspot The Size Of Earth - (a) An enlarged photograph of the largest pair of sunspots in Figure 16.15. Each spot consists of a cool, dark inner region called the umbra, surrounded by a warmer, brighter region called the penumbra. The spots appear dark because they are slightly cooler than the surrounding photosphere. (b) A high-resolution, true-color image of a single sunspot shows details of its structure as well as much surface granularity surrounding it. The spot is about the size of the Earth.

Sunspots come in pairs. Magnetic Field "Lines" link the pairs, rising out of one spot and diving down into another.

The orientation of the field changes from the Northern Hemisphere to the Southern

Magnetic Field Between Susnspot Pair - Sunspot pairs are linked by magnetic field lines. The Sun's magnetic field emerges from the surface through one member of the pair and reenters the Sun through the other.

Distribution Of Susnspot Pairs - The leading members of all sunspot pairs in the solar northern hemisphere have the same polarity-if the magnetic field lines are directed into the Sun in one leading spot, they are inwardly directed in all leading spots in that hemisphere. The same is true in the southern hemisphere, except that the polarities are always opposite to those in the north.
 
 
 
 

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 belived 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.)
 
 

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 amxium 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 latitute at which most spots appear changes as spots move from higher latitutes 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 cyle is a 22 year cycle where the polartity or direction of the magnetic field changes as well. These chages wust 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.

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

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

The size of these can 100,000 km across

Loop-Like Solar Prominence - The looplike structure of this prominence clearly reveals the magnetic field lines connecting the two members of a sunspot pair.

Unusually Large Solar Prominence - This image of a particularly large solar prominence was observed by ultraviolet detectors aboard the Skylab space station in 1979. (See also Figures 16.18 and 16.19.)

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.

Much more violent than a prominence, a solar flare is an explosion on the Sun's surface that sweeps across an active region in a matter of minutes, accelerating solar material to high speeds and blasting it into space. Visible here through a red Ha filter.

The Corona also changes in step with the solar cycle.

July 1995 Photo Of Solar Corona - Photograph of the solar corona during the July, 1991 eclipse, at the peak of the sunspot cycle. At these times, the corona is much less regular and much more extended than at sunspot minimum . Astronomers believe that coronal heating is caused by surface activity on the Sun. The changing shape and size of the corona are the direct result of variations in prominence and flare activity over the course of the solar cycle.

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.

The changing surface of the sun has a great effect on the Earth. Satillites get fried by particularly violent events called Coronal Mass Ejections where hugh blobs of plasma (electrons and protons) get driven into space towards us.

These are called Space Storms. As we rely more on space for telecommunications space weather will be more of an issue.

Note we are just heading into a new maximum!!

Thermonuclear Fusion

When two nuclei come together to form a third + energy

nucleus 1 + 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 complicted 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.

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?