the sun

PTYS/ASTR 206 Sun3/1/07

The Sun


Announcements

• Reading Assignment– Review and finish reading Chapter 18– Optional reading

• March 2006 Scientific American: article by Gene Parker titled “Shielding Space Travelers”

• http://en.wikipedia.org/wiki/Solar_variability

• 3rd Homework due today; 4th homework now posted on course website

• Next study-group session is next Wednesday from 10:30AM-12:00Noon – in room 330.


The Sun: Our Star

• 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

• Temperature in the corona– 2 x 106 K


The Sun

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

other objects in the solar system combined (by a long shot)

• Diameter– 103 Earth Diameters

• Average Density– 1410 kg/m3

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

Helium, 1% other elements Sun seen in Xrays


The Sun’s appearance from the ground

• Using a Baader solar filter– Can see the sun’s visible

surface, or photosphere– This is how the Sun appears

when we simply cut down on its brightness with a filter (i.e. it is seen in “white light”)

– These types of filters are very Inexpensive

– Many “solar observing glasses” are made out of this material

– Used to see sunspots


The Sun’s appearance from the ground

• Hα filters– Observes the Sun at a particular

wavelength• The a line at 6563 angstroms (Å)

which is the strongest of the Sun’s absorption lines associated with the presence of hydrogen in its atmosphere

– Much more pricey– Observe the chromosphere– Used to see Prominences,

filaments, and sometimes flares

• Very impressive Sun seen with an Hα filter


SOLAR CORONA – SEEN DURING A TOTAL ECLIPSE


The Sun’s Appearance from Space

Solar Corona As seen in x-rays Solar Corona As with a coronagraph


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


Detecting Solar Neutrinos

• Solar Neutrinos are the only direct probe of the Sun’s interior that we have – hence, they are a valuable tool for understanding the Sun’s interior

• Underground detectors are used to avoid interference from cosmic rays

• For a long time, it was not clear why there were fewer neutrinos observed than predicted. We now know why this is.


What is the Sun made of?

• Mostly hydrogen and helium in the form of plasma

• A normal gas (i.e. not a plasma) only exists in the thin region of the photosphere and lower chromosphere– Sunspots (in the photosphere)

are cool enough that simple molecules can form

• But other than this small region, the Sun is almost entirely made of plasma!


The Structure of 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 from the edge of the core to about 0.71 solar radius– Here energy travels outward

through radiative diffusion

• The convective zone is the next layer and is a rather opaque gas– Here energy travels outward

primarily through convection


• The convection zone is just outside the radiative zone

• turbulent convective motions cause overturning (bubbling) motions inside the Sun.– Like a pot of boiling water – These are responsible for

the granulation pattern seen on the Sun’s surface.

Radiative zone


• These are convection cells that are about 1000 km wide

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

Solar Granulation


High-resolution images of granulation and a sunspot


Solar Oscillations

• Waves can propagate through the Sun causing a variety of vibrations– In some sense, they are like

seismic waves on Earth– Like sound waves– 5-minute oscillations

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

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


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


Limb Darkening

• The edges of the Sun’s photosphere 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


The Chromosphere

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

“Color Sphere”

• Spicules– Regions of rising gas

• Filaments – dark, thread-like features

• Plage – bright patches

surrounding sunspots


• Prominences are the 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

• Because of the high temperatures, the corona is best viewed in the X-ray part of the spectrum


X-Ray image of the Sun


Sunspots

• Existence known since 350 BC (Greece), 28 BC (China)

• Lower temperature than surrounding atmosphere

• Umbra (darkest part) and penumbra

• Associated with Intense magnetic fields

– Magnetic Fields are measured by using the Zeeman effect


Zeeman Effect: The splitting of spectral lines by a magnetic field


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”


These changes are caused by convection and the Sun’s differential rotation: The

Solar Dynamo


Next Class: Solar Variability and its Effect on Earth and its Inhabitants

the sun

Documents

suns center

suns corefusion

sunspotsthe suns appearance

xraysthe suns appearance

suns visible surface

suns interiorhydrogen

suns absorption lines

solar wind