solar science workshop 6/8/05 an introduction to the physics of our star: the sun distance from...

Solar Science Workshop6/8/05

An Introduction to the Physics of Our Star: The Sun

• Distance from Earth– 1 AU

• Travel time for Light to Earth– About 8 minutes

• Travel time for solar wind to 1 AU– A few days

• Mean surface temperature– 5800K

• Temperature in the Center– 1.55x107 K


The Sun

• Mass– 333,000 Earth Masses– More mass than all of the

other objects in the solar system combined

• Diameter– 218 Earth Diameters

• Average Density– 1410 kg/m3

• Composition (by mass)– 74% Hydrogen, 25%

Helium, 1% other elements

Sun seen in Xrays


The Sun Seen From Earth by Amateurs

• COMMON SENSE WARNING …

NEVER LOOK DIRECTLY AT THE SUN WITHOUT A PROPER FILTER !!– Looking directly at the Sun

without the proper filter WILL cause permanent eye damage

• Safest to use the method of projection– Project the image onto a

piece of paper– Can see sunspots, and

even “graph” them easily



• Using a Baader solar filter– Inexpensive– Many “solar

observing glasses” are made out of this material

– Used to see sunspots

– Similar to Mylar



• Hα filters– Much more pricey

• 100s-1000s of dollars

– Can see features in the solar chromosphere

– Used to see Prominences, filaments, flares

– Very impressive Sun seen with an Hα filter


How Old is the Sun ?

• Probably about the same age as Earth

• Has it been shining brightly the entire time?– The luminosity has

probably increased with time, but it has always been very bright

• What is the “fuel” that keeps it shining this long?


The Sun’s Energy Source is ThermonuclearFusion in its Core

• Proton-proton chain– Four hydrogen nuclei “fuse”

to form a single helium nucleus

– There is a slight loss of mass in this process which is converted to energy according to Einstein’s famous equation E = mc2

• Thermonuclear fusion occurs only at the very high temperatures at the Sun’s core

• Fusion should not be confused with fission !

• Will continue to heat the Sun for another 5 billion years


The Proton-Proton Chain


The Structure of the Sun

• The Interior– Core– Radiative zone– Convection zone

• The Surface and Atmosphere– Photosphere– Chromosphere– corona


The Sun’s Interior• Hydrogen fusion takes place

in a core extending from the Sun’s center to about 0.25 solar radius

• The radiative zone extends to about 0.71 solar radius– Here energy travels

outward through radiative diffusion

• The convective zone is a rather opaque gas– Here energy travels

outward primarily through convection


Where do the Neutrino’s and γ-ray photons go ?

• Neutrino’s exit the Sun, unimpeded– Can be used to probe the

solar interior– Early attempts at detecting

them found that their were about 3 times less seen at Earth than there should be

– THE SOLAR NEUTRINO PROBLEM !

• The gamma rays collide with matter and take millions of years to exit the Sun


Detecting Solar Neutrinos

• Underground detectors are used to avoid interference from cosmic rays


The Solution to the Solar Neutrino Problem

• Particle Physics gave us the answer– Solar neutrinos oscillate and the original

detectors could only see certain parts of the oscillations and not all of them

– New detectors were built to observe all neutrinos

– Two physicists won a Nobel Prize for their work

• Modern detectors are placed at different depths within the Earth to observe the actual oscillations

Raymond Davis Jr.

Masatoshi Koshiba


Solar Structure

• The standard solar model

– Theoretical model used to determine the physical properties of the Sun’s interior

– Assumes hydrostatic and thermal equilibrium


Solar Oscillations

• Waves can propagate through the Sun causing a variety of vibrations– Like sound waves

• These are used to infer pressures, densities, chemical compositions, and rotation rates within the Sun


Helioseismology

• The branch of science that studies solar oscillations is known as Helioseismology

• The movie shows evidence of seismic activity on the Sun as seen by the SOHO MDI experiment


The Convection Zone

• The convection zone is just outside the radiative zone. – Thickness is about 200,000 km

• Turbulent convective motions occur, similar to a pot of boiling water.

• Overturning (bubbling) motions inside the Sun are responsible for the granulation pattern seen on the Sun’s surface.


Convection Cells


• Convection cells that are about 1000 km wide

• These are part of the Sun’s atmosphere known as the Photosphere

Solar Granulation


Recent High-resolution Images of

granulation


“Flowers” and

Ribbons

• Produced by magnetic fields


The photosphere is the lowest of three main layers

in the Sun’s atmosphere

• The Sun’s atmosphere has three main layers: the photosphere, the chromosphere, and the corona

• Everything below the solar atmosphere is called the solar interior

• The visible surface of the Sun, the photosphere, is the lowest layer in the solar atmosphere

• The photosphere undergoes “differential” rotation


Limb Darkening

• The edges of the Sun appear darker than that seen “straight on”

• This is called limb darkening

• It is due to the fact that the temperature in the photosphere decreases with altitude


The Origin of Limb Darkening

• The light we see at the limb originated higher up in the atmosphere where it is cooler

– Thus it will be less bright there


Sunspots• Low temperature

regions– How do we know

this ?

• Darkest part is called the “umbra”

• Just outside the umbra is the penumbra

• Associated with Intense magnetic fields


Sunspots Often Come in Groups


The Chromosphere

• Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere

“Color Sphere”

• characterized by spikesof rising gas

• Spicules extend upward from the photosphere into the chromosphere along the boundaries of supergranules


Chromospheric Features:Plages and Filaments• Filaments are dark, thread-

like features seen in the red light of hydrogen (H-alpha). – Dense cooler material

suspended at high altitudes by magnetic fields

• Plage (the French word for beach) are bright patches surrounding sunspots that are best seen in H-alpha. – associated with

concentrations of magnetic fields


Chromospheric Features: Prominences

• Same as filaments, except that they are seen from the side rather than “straight on”


The Corona

• The outermost layer of the solar atmosphere, the corona, is made of very high-temperature gases at extremely low density

• The solar corona blends into the solar wind at great distances from the Sun


The 11-year Sunspot Cycle

Number of Sunspots versus time – they come and go every 11 years

Number of Sunspots versus latitude – forms a “butterfly pattern”


The Maunder Minimum

• Complete absence of sunspots for 50 years corresponds to a mini ice age

• There is a loose correlation between global man temperature and sunspots


These changes are caused by convection and the Sun’s

differential rotation: The Solar Dynamo


The Buildup of magnetic field energy must be released – how?

• Coronal Mass Ejections and Flares– Releases an enormous amount of

energy – A solar flare is a brief eruption of

hot, ionized gases from a sunspot group

– A coronal mass ejection is a much larger eruption that involves immense amounts of gas from the corona

• These storms can interact with the Earth and create huge geomagnetic storms

• They also accelerate particles to very high energies


Coronal loops expand from the surface of the Sun following a solar explosion (solar flare) on April 21, 2002

TRACE movie


The Halloween 2003 Flares were the Largest in Modern History

Perhaps the most extreme flare ever seen erupted on Nov. 4 ...

IMAGES: SOHO/NASA/ESA

... and then this coronal mass ejection was hurled into space.


Aurora The pattern of auroral light around the north and south magnetic poles is called the auroral oval. It expands and contracts over a period of hours and days, depending on geomagnetic activity.


Aurora in Tucson


Shock Waves in Space

• Analogy with sonic booms

• Can accelerate charged particles to very high energies

• Radiation Environment !

• Space Weather

solar science workshop 6/8/05 an introduction to the physics of our star: the sun distance from...

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