Astonishing Astronomy 101With Doctor Bones (Don R. Mueller,

Ph.D.)

EducatorEntertainer

JU

G G LE

RPLANETARY

Scientist

ScienceExplorer

Chapter 16 – The Sun

The Sun

• A huge gas ball in the center of the solar system.

• Releases the equivalent of 100 billion atomic bombs every second.

• Exists in a delicate balance of gravity and pressure.

• The mass of the Sun as compared to that of the Earth is around 300,000 times larger.

The Sun is a G-type main-sequence star: specifically a spectral type G2 (luminosity class V) star.

Properties of the SunProperty Value Method of DeterminationDistance (AU) 1 AU=150 million kilometers or 93

million milesTriangulation or Radar

Radius (km) 7 105 From angular size and distance

Mass (kg) 2 1030 From Kepler’s third law

Average density (kg/L)

1.4 From radius and mass

Surface temperature (K)

About 5800 Color-temperature relation -Wien’s law

Central temperature (K)

15 million Indirectly from need to balance internal pressure and gravity

Composition%

71% hydrogen, 27% helium and 2% elements such as carbon and iron

Spectra of gases in surface layers

Luminosity (W) 4 1026 From amount of energy reaching Earth and inverse square law

The Photosphere

• The photosphere is the visible “surface” of our star.

• Not really a surface, as the Sun is gaseous throughout.

• Photosphere is only 500 km thick.

• Average temperature is 5780 K.

Energy Transport in the Sun

• Below the photosphere is the convection zone.– Energy is transported from deep within the Sun by

convection.• Energy in the convection zone comes from the

radiative zone.– Energy from the core is transported outward via

radiation: the transfer of photons.– The energy produced by fusion in the core travels

through many layers to the solar photosphere before being liberated as sunlight.

– It likely takes more than 100,000 years for a single photon to escape the Sun’s grasp.

A delicate balance of gravity and pressure

• The immense mass of the Sun generates an enormous gravitational force.

• Gravity pulls all of the Sun’s matter toward its center.

• The crushing force produces very high temperatures and pressures within the Sun’s interior.

• This balance of gravity and pressure allows a star like the sun to live on for billions of years.

The Solar Interior

The Solar Interior

The Solar Thermostat

• Stars such as the Sun, can be seen as having a built-in thermostat.

• Gravity pulls in and pressure pushes out.

• When the temperature falls, the pressure decreases and gravity pulls more mass toward the center.

• This inward-falling mass increases the temperature and pressure, thus restoring the balance (equilibrium).

The Ideal Gas Law

Ideal Gas Equation: pV = nRT

p - pressure V - volume n - moles R - gas constant

T - temperature R = 0.0821 L · atm mol-1 · K-1

The Solar Atmosphere• Regions of the Sun above

the photosphere are called the Sun’s atmosphere.

• Just above the photosphere lies the chromosphere.

• Usually invisible, but it can be seen during eclipses.

• Above the chromosphere is the Corona:

Extremely high temperatures (more than 1 million K).

Rapidly expanding gas forms the solar wind.

A Very Active Star• The surface and atmosphere of the

Sun are extremely active.

• Solar wind streams out of coronal holes: regions of low magnetic field.

• Active regions send arcades of plasma shooting from the surface. These are regions of high magnetic fields.

• Coronal mass ejections send large quantities of mass out into space.

• Dark regions on the Sun’s photosphere are called Sunspots:

(Caused by high magnetic flux)

The Sun’s Energy

• The Sun’s energy comes from fusion – the merging of hydrogen nuclei into helium.

• The reaction releases little energy, but it happens readily.

• A helium nucleus has less mass than the four protons (hydrogen nuclei) that fuse.

• This difference in mass is converted into energy:

E = mc2 E: Energy in Joules

m: Mass in kg c: Speed of light in m/s

The Sun like other stars is a ball of gas held together by gravity and generating light via nuclear fusion reactions: Converting Hydrogen (H) to Helium (He).

Nuclear fusion in the Sun is approximately:98 - 99% Proton-Proton (P-P) fusion and 1 - 2% Carbon-Nitrogen-Oxygen (CNO) fusion.

High temperatures in the Sun’s core suggest that nuclei are moving fast and the high pressure in the core is “pushing” nuclei closer together. The high speed nuclei collide and fuse via the proton-proton chain.

Proton-Proton (P-P) Fusion is the primary fusion process fueling the Sun and other stars with core temperatures approaching 15 million degrees Kelvin. The net result of this process is the fusion of four H atoms (protons) into one He atom (alpha particle, which has two protons and two neutrons) along with generating two positrons, two neutrinos and the release of energy.

The Carbon-Nitrogen-Oxygen (CNO) cycle is the other known fusion reaction employed by the Sun to convert hydrogen to helium. the other being the proton–proton (P-P) chain reaction. However, unlike the P-P chain fusion reaction, the CNO cycle is a catalytic cycle.

The Proton-Proton (P-P) Chain:

The nuclear fusion of H to He.

The basic steps for the proposed

mechanism of the P-P Chain are

illustrated in the following slide.

Steps 1 & 2: Proton fusion forms Deuterium

Step 3: Deuterium-proton fusion forms Helium-3

Steps 4 & 5: Helium-3 fusion & Helium-4 (α-particle) formation

The Carbon-Nitrogen-Oxygen (CNO) cycle, which converts hydrogen to helium, is thought to occur in a 6-step sequence of reactions.

The “players” in the CNO cycle are isotopes of Carbon, Nitrogen and Oxygen along with a cast of characters that include those found in the diagram below:

The details for this proposed nuclear fusion mechanism are illustrate in the following 6 slides.

1

2

34

5

6

1

Step 1: A mass-12 Carbon isotope captures a proton (H) and emits a gamma-ray (γ) thus producing a mass-13 isotope of Nitrogen.

Step 2: An unstable Nitrogen-13 isotope, beta decays to a mass-13 Carbon isotope with a half-life of ~ 10 minutes.

2Beta decay (β) is a form of radioactive decay whereby a proton is transformed into a neutron or vice versa within the atom’s nucleus.

The consequence of this transformation is that the nucleus emits a beta particle (i.e., electron or positron).

Step 3: A mass-13 Carbon isotope captures a proton and emits a gamma-ray (γ) to become a mass-14 isotope of Nitrogen.

3

Step 4: A mass-14 Nitrogen isotope captures a proton and emits a gamma-ray (γ) becoming a mass-15 isotope of Oxygen.

4

Step 5: The mass-15 Oxygen, beta decays, to form mass-15 Nitrogen.

5 Notice in this decay: that the O-15 isotope emits both a positron and a neutrino as a proton is transformed into a neutron.

Step 6: A mass-15 Nitrogen isotope captures a proton, emits an alpha-particle (helium) and closes the catalytic cycle: recreating Carbon-12.

6

Neutrinos• One product of the proton-proton chain is the neutrino:

• A very low mass, very high-energy particle.

• Neutrinos pass through matter easily and are hard to detect.

• Neutrino measurements on Earth confirm our models of fusion in the Sun’s core. Neutrino Detector

Sunspots

Sunspots can be many times larger than the Earth.

They contain intense magnetic fields, as evidenced by the Zeeman effect.

Sunspots are localized cool regions within the photosphere of the Sun:

Discovered by the great Italian physicist Galileo.

20,000 km

Sunspot’s Magnetic Field

• The intense magnetic fields found in sunspots suppress particle motion.

• Solar ions cannot leave these regions of high magnetic field and the field lines are “frozen” to the plasma.

• Trapped plasma keeps hot material from surfacing below the sunspot, thus keeping it cool.

Magnetic Field

ChargedParticlesSpiraling

Prominences• Magnetic fields have their “feet” in the sunspots in the photosphere.

• These loops are relatively unstable and can quickly release vast amounts of plasma into space.

• Prominences are large loops of glowing solar plasma, trapped by magnetic fields.

• Coronal Mass Ejections.

Magneticfield

Cool gasin field

Pair ofSunspots

Solar Flares • These are huge eruptions of hot gas and radiation in the photosphere.

• They can travel from the Sun to Earth in less than an hour.

• Cause damage to satellites, spacecraft and humans in space.

• The study of coronal mass ejections and solar flares is called “space weather.”

A Coronal Mass Ejection or “CME”

The Aurora• When material from CME’s

reaches Earth, it interacts with the Earth’s magnetic field, colliding with the ionospheric particles.

• These collisions excite the ionospheric oxygen, which causes photon emission.

• We see these emitted photons as the Aurora or “Northern Lights.”

The Solar Cycle

• Sunspot numbers increase and decrease periodically. Every 11 years or so, sunspot numbers peak: The Solar Maximum.

• Around 5.5 years after the Solar Maximum, the sunspot number is at its lowest level: The Solar Minimum.

Solar activity (CMEs, flares, etc.) peaks with the sunspot number.

Differential Rotation and the Babcock Cycle

• Different parts of the sun rotate at different speeds:– The equator rotates faster than the poles.– Solar magnetic fields get twisted as time evolves.

Magnetic field at start of the Babcock Cycle.

Differential rotation begins to twist the magnetic field below the surface.

Subsurface magnetic fields begins to coil on themselves.

The coiling continues. Only a single field line is shown here.

Babcock cycle - the regenerative process of the Sun's magnetic field.

Subsurface magnetic field is now coiled.

The coils form kinks, which break through the surface.

Spot Spot

Magnetic Field

Hot rising gas blocked

Hot rising gas blocked

Sunspots