THE SUN
The star we see by day
The Sun, Our Star
• The Sun is an average star.
• From the Sun, we base our understanding of
all stars in the Universe.
• No solid surface.
Interior Properties
• Core = 20 x density of iron
• Surface = 10,000 x less dense
than air
• Average density = Jupiter
• Core = 15,000,000 K
• Surface = 5800 K
Scientific Method
• How do we know this stuff?
• Three examples:
– Fusion in the core (core temperature).
– Different zones in interior.
– Solar activity and Earth
1. The Core
• Scientific Method:
– Observations
– Make hypothesis (a model)
• Models make predictions
– Test predictions
• Compare results of predictions with observations
– Revise model if necessary.
Testing the Core
• Observe Sun’s:
–
–
–
–
Mass (how?)
Composition (how?)
Radius
Luminosity (total energy output)
• Use physics to make a model Sun.
• Predict:
– Surface temp/density (how do you test?)
– Surface Luminosity (how do you test?)
– Core temp/density  Fusion Rate  neutrino rate (test?)
In The Core
• Density = 20 x
density of Iron
• Temperature =
15,000,000 K
• Hydrogen
atoms fuse
together.
• Create Helium
atoms.
2. Helioseismology
• Continuous monitoring
of Sun.
– Ground based
observatories
– One spacecraft (SOHO)
• Surface of the Sun is
‘ringing’
• Sound waves cross the
the solar interior and
reflect off of the surface
(photosphere).
Solar Interior
• Core
– Only place with
fusion
• Radiation Zone
– Transparent
• Convections Zone
– Boiling hot
Convection
•
•
•
•
A pot of boiling water:
Hot material rises.
Cooler material sinks.
The energy from the
pot’s hot bottom is
physically carried by the
convection cells in the
water to the surface.
• Same for the Sun.
Solar Cross-Section
• Progressively smaller convection
cells carry the energy towards
surface.
• See tops of these cells as granules.
The Photosphere
• This is the origin of the 5,800 K thermal radiation
we see.
l = k/T = k/(5800 K)  l = 480 nm (visible light)
• This is the light we see.
• That’s why we see this as the “surface.”
3. Solar Activity and Earth
• Is there a connection between Solar Activity
and Earth’s Climate?
• Observation:
– Little Ice Age
– Maunder Minimum
What is Solar Activity?
•
•
•
•
•
•
•
Sunspots
Magnetic Fields
Coronal Mass Ejections
Solar Wind
Magnetic Storms
Aurora
Other effects?
•
•
•
•
11-year sunspot cycle.
Center – Umbra: 4500 K
Edge – Penumbra: 5500 K
Photosphere: 5800 K
Sunspots
Magnetic fields and Sunspots
• At kinks, disruption in convection cells.
• Sunspots form.
Magnetic fields and Sunspots
• Where
magnetic
fields “pop
out” of Sun,
form
sunspots.
• Sunspots
come in
pairs.
Prominences
Hot low density gas = emission lines
Stereo
Corona and Solar Wind
• Hot, low density, gas emits the radiation we
see as the Corona: 1,000,000 K
• Solar Wind: Like steam above our boiling pot
of water, the gas ‘evaporates’.
• Carries away a million tons of Sun’s mass
each second!
• Only 0.1% of total Sun’s mass in last 4.6
billion years.
Solar Cycle
• Increase in solar wind activity
- Coronal Mass Ejections
• Increase in Auroral displays on Earth
• Increase in disruptions on and around Earth.
Courtesy of SOHO/LASCO/EIT consortium.
Aurora
• The solar wind
passes out
through the
Solar System.
• Consists of electrons, protons and other charged particles
stripped from the Sun’s surface.
• Magnetic fields herd charged particles into atmosphere at
poles.
• Charged particles excite electrons in atoms.  Light!
2003 CME
Oklahoma 10/29/2003
Credit: E. Woldt