The Sun

Guiding Questions1. What is the source of the Sun’s energy?2. What is the internal structure of the Sun?3. How can we measure the properties of the Sun’s interior?4. How can we be sure that thermonuclear reactions are happening in the

Sun’s core?5. Does the Sun have a solid surface?6. Since the Sun is so bright, how is it possible to see its dim outer

atmosphere?7. Where does the solar wind come from?8. What are sunspots? Why do they appear dark?9. What is the connection between sunspots and the Sun’s magnetic field?10. What causes eruptions in the Sun’s atmosphere?

• The Sun’s luminosity (power output) is

L = 3.9 x 1026 watts (or joules per second)• The Sun is powered by thermonuclear fusion

reactions in the core, where hydrogen is converted into helium, releasing energy in a process called the proton-proton-chain.

• Einstein’s equation, E = mc2 describes how much energy, E, can be created from an amount of mass, m.

The Sun’s energy is generated by thermonuclear reactions in its core.

At extremely high temperatures and pressures, 4 Hydrogen atoms can combine to make 1 Helium atom

and release energy by E = mc2

4H He + energy

HYDROGEN FUSION

Thermonuclear reactions in the Sun’s core turn mass into energy.

A theoretical model of the Sun shows how energy gets from its center to its

surface.

Thermonuclear fusion can only occur at very high temperatures and

pressures.

Solar energy flows from the

(1) core

(2) radiative zone

(3) convective zone

Astronomers probe the solar interior using the Sun’s own vibrations.

Sections of the Sun’s surface quickly oscillate up on down.

• Exploring the Sun’s interior by studying its vibrations is called HELIOSEISMOLOGY.

• Because we can not actually “see” inside the Sun, helioseismology provides theoreticians with a way to check to be sure their models of the solar interior are correct.

The Sun’s surface vibrations reveal its internal structure and motions.

Neutrinos provide information about the Sun’s core - and have surprises of their own.

• Current models of the solar interior predict that 1038 neutrinos should be released every second by solar fusion.

• Neutrino () detectors on Earth measure captures by cleaning fluid:

Cl + Ar + e-

n + p + e-

• MYSTERY: Only 1/3 of the expected neutrinos from the Sun are being detected.

• Solution to this “solar neutrino problem”: neutrinos have mass, so they change identity enroute!

Outer Layers of the Sun’s Atmosphere

• Photosphere - the 5800 K “surface” we see.

• Chromosphere - the lower solar atmosphere, which rises to 25,000 K

• Corona - the very thin outer atmosphere at millions of degrees (T>106 K)

The photosphere is the lowest of three main layers in the

Sun’s atmosphere.

Granulation caused by convection

The chromosphere is characterized by spikes of rising gas.

• The chromosphere is the thin, pinkish layer of SPICULES just above the photosphere.

• Spectrum is dominated by H emission lines, suggesting it is quite tenuous.

• MYSTERY: The temperature is rises with height about the solar surface, though we would expect it to cool with increasing distance from the source!

• Solution = solar magnetic fields

• Most easily seen during an eclipse.

• Thin gas at millions of degrees

• The outflow of mass from the Sun is called the solar wind.

The corona ejects mass into space to form the solar wind.

Magnetic fields cause structure, heating, and energy outbursts from the Sun’s

atmosphere.

• Sunspots

• Hot atmosphere

• Flares and mass ejections

• Solar cycle

• Temperature changes on Earth

Sunspots are low-temperature regions in the photosphere.

The daily movement of

sunspots reveals that the Sun’s

rotation takes about 4 weeks.

The cyclical change in the latitude of sunspots also reveals that the Sun

experiences an 11-year solar cycle.

Extreme peaks or absences of sunspots may change Earth’s climate

Maunder Minimum ~ 1650

• Few sunspots

• Colder climate

• Famine in Europe

• Thames froze

Medieval Maximum ~ 1100

• Many sunspots

• Hotter climate

• Famine in N. America

• Ancient Pueblo people abandoned Chaco canyon

• Icemelt permitted Vikings to reach N. America

Sunspots are produced by a 22-year cycle in the Sun’s magnetic field.

• Charged particles, such as electrons, will move along magnetic field lines.

• The Sun’s positive pole is in the North for 11 years, then switches to the South for 11 years

This X-ray image of the Sun shows bright regions where gas follows magnetic field lines.

The sunspot cycle is partly due to the Sun’s differential rotation. This helps solar

magnetic fields twist up, intensify, emerge, cancel, then repeat the cycle.

The interior of the Sun rotates at slower than the equator and faster than the poles.

The radiative zone seems to rotate as a rigid sphere.

The Sun’s magnetic field also produces other forms of solar activity.

• The highly charged gases in the Sun’s outer atmosphere, follow loops in the Sun’s magnetic field.

Solar magnetic fields createplages and filaments / prominences.

Solar magnetic fields also create coronal holes, solar wind, flares, …

… and Coronal Mass Ejections, which can disrupt cell phone service

(among other things)

The Sun’s magnetic field affects all life on Earth,

not just cell phone users.

Guiding Questions

1. What is the source of the Sun’s energy?2. What is the internal structure of the Sun?3. How can we measure the properties of the Sun’s interior?4. How can we be sure that thermonuclear reactions are happening in the

Sun’s core?5. Does the Sun have a solid surface?6. Since the Sun is so bright, how is it possible to see its dim outer

atmosphere?7. Where does the solar wind come from?8. What are sunspots? Why do they appear dark?9. What is the connection between sunspots and the Sun’s magnetic field?10. What causes eruptions in the Sun’s atmosphere?