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THE SUN. The star we see by day. Goals. Summarize the overall properties of the Sun. What are the different parts of the Sun? Where does the light we see come from? The scientific method: solar neutrinos. The Sun, Our Star. The Sun is an average star.

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THE SUN

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The sun

THE SUN

The star we see by day


Goals

Goals

  • Summarize the overall properties of the Sun.

  • What are the different parts of the Sun?

  • Where does the light we see come from?

  • The scientific method: solar neutrinos.


The sun our star

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.


Vital statistics

Vital Statistics

  • Radius = 100 x Earth (696,000 km)

  • Mass = 300,000 x Earth (1.99 x 1030 kg)

  • Surface temp = 5,800 K

  • Core temp = 15,000,000 K

  • Luminosity = 4 x 1026 Watts

  • Solar “Day” =

    • 24.9 Earth days (equator)

    • 29.8 Earth days (poles)


Structure

Structure

  • ‘Surface’

    • Photosphere

  • ‘Atmosphere’

    • Chromosphere

    • Transistion zone

    • Corona

    • Solar wind

  • ‘Interior’

    • Convection zone

    • Radiation zone

    • Core


Interior properties

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


Do you see the light

Do you see the light?

  • Everything in the solar system reflects light.

  • Everything also absorbs light and heats up producing blackbody radiation.

  • Q: Where does this light come from?

  • A: The Sun.

  • But where does the Sun’s light come from?


In the core

Density = 20 x density of Iron

Temperature = 15,000,000 K

Hydrogen atoms fuse together.

Create Helium atoms.

In The Core


Nuclear fusion

Nuclear Fusion

  • 4H  He

  • The mass of 4 H nuclei (4 protons):

    4 x (1.6726 x10-27 kg) = 6.690 x 10-27 kg

  • The mass of He nuclei: = 6.643 x 10-27 kg

  • Where does the extra 4.7 x 10-29 kg go?

  • ENERGY!  E = mc2

  • E = (4.7 x 10-29 kg ) x (3.0 x 108 m/s)2

  • E = hc/l l = 4.6 x 10-14 m (gamma rays)

  • So: 4H  He + light!


The radiation zone

The Radiation Zone

  • This region is transparent to light.

  • Why?

    • At the temperatures near the core all atoms are ionized.

    • Electrons float freely from nuclei

    • If light wave hits atom, no electron to absorb it.

  • So: Light and atoms don’t interact.

  • Energy is passed from core, through this region, and towards surface by radiation.


The convection zone

The Convection Zone

  • This region is totally opaque to light.

  • Why?

    • Closer to surface, the temperature is cooler.

    • Atoms are no longer ionized.

    • Electrons around nuclei can absorb light from below.

  • No light from core ever reaches the surface!

  • But where does the energy in the light go?

  • Energy instead makes it to the surface by convection.


Convection

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

Solar Cross-Section

  • Progressively smaller convection cells carry the energy towards surface.

  • See tops of these cells as granules.


The photosphere

The Photosphere

  • This is the origin of the 5,800 K blackbody radiation we see.

  • Why?

    • At the photosphere, the density is so low that the gas is again transparent to light.

    • The hot convection cell tops radiate energy as a function of their temperature (5800 K).

      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.


The solar atmosphere

HOT

You

COOL

The Solar Atmosphere

  • Above the photosphere:

    • Thin cool gas

    • Mostly transparent to light again

  • Unlike radiative zone, here atoms not totally ionized.

  • Therefore, there are electrons in atoms able to absorb light.

  • Absorption lines in solar spectrum are from these layers in the atmosphere.


The chromosphere

The Chromosphere

  • Hydrogen most common.

  • Brightest hydrogen line – Ha.

  • Chromosphere = color


H a sun

Ha Sun

Photo by Big Bear Solar Observatory


Prominences

Prominences


Corona

Corona

  • Magnetic activity carry energy up to the Transition Zone.

  • 10,000 km above photosphere.

    • Temperature climbs to 1,000,000 K

    • Remember photosphere is only 5800 K

  • The hot, low density, gas at this altitude emits the radiation we see as the Corona.

    • But corona very faint compared to photosphere.


Solar wind

Solar Wind

  • At and above the corona:

  • Gas is very hot

  • Very energetic

  • Like steam above our boiling pot of water, the gas ‘evaporates’.

  • Wind passes out through Coronal Holes

  • Solar Wind 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.


Aurorae

Aurorae

  • The solar wind

    passes out

    through the

    Solar System.

  • Consists of electrons, protons and other charged particles stripped from the Sun’s surface.

  • When charged particles and magnetic fields interact: light!


Solar cycle

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.


Magnetic fields and sunspots

Magnetic fields and Sunspots

  • At kinks, disruption in convection cells.

  • Sunspots form.


Sunspots

  • 11-year sunspot cycle.

  • Center – Umbra: 4500 K

  • Edge – Penumbra: 5500 K

  • Photosphere: 5800 K

Sunspots


Magnetic fields and sunspots1

Magnetic fields and Sunspots

  • Where magnetic fields “pop out” of Sun, form sunspots.

  • Sunspots come in pairs.


Solar neutrino problem

Solar Neutrino Problem

  • We observe:

    • Sun’s luminosity (total light radiated).

  • We hypothesize:

    • 4H  He + light + neutrinos

  • We can test:

    • Observe number neutrinos reaching Earth

  • Does or test agree with hypothesis?

  • No


What to do

What to Do?

  • For 30 years:

    • Theorists certain of nuclear reaction.

    • Observers positive of observations.

    • Detected only 1/3 the hypothesized neutrinos.

  • What to do?


Neutrino flavors

Neutrino Flavors

  • 3 types of neutrinos

    • Electron neutrino

    • Tau neutrino

    • Muon neutrino

  • Nuclear reactions produce only electron neutrino.

  • Previous detectors only detected electron neutrinos.


Neutrino fluctuations

Neutrino Fluctuations

  • New detector (2002) gives number of all three flavors.

  • Total number agrees with number predicted in core of Sun.

  • Conclusion:

    • Nuclear hypothesis is correct.

    • Neutrinos change flavor.

    • Neutrinos have mass (used to be thought massless).

  • Problem solved  new science discovered.


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