CHAPTER 10: The Sun: Our Extraordinary Ordinary Star

What do you know about the Sun? Brightest object in our Solar system Center of our S.S. Hot Large (largest object in the solar system) Star Can cause blindness Fusion at the core Plasma state Solar Wind Has been worshipped as Ra Gives us light Planets orbit the Sun Layered Rotate Average sized star 5 by old, 5 by left

THE SUN

Overview The Sun is a huge ball of hot gas Size Age Primary component is hydrogen Secondary component is helium tiny amounts of other elements are recognized by spectral analysis of surface gases Size 100 Earths across in diameter one million Earths would fit inside Contains 99.85% of the mass in the solar system If the Sun was as close to the Earth as the Moon, it would consume 2/3 of the sky. Age Estimated at 5 billion years old It will be another 5 billion years until it begins to die

BIRTH Rotating nebular dusts and gases condense and flatten, producing a protoplanetary disk and protosun at the center. Temperature of the protosun continues to rise as more matter collapses inward. If the temperature at the core of the protosun rises above 10 million Kelvin, then fusion of hydrogen into helium nuclei occurs. The Sun becomes a star at this time.

ANATOMY OF THE SUN Internal Structure--The Sun has three internal layers Deep, dense, hot core (+ 10 million Kelvin) A radiative zone A convective zone

THE SOLAR INTERIOR

Internal Structure Thermonuclear Core (25 % of radius) Radiative zone (55 % of radius) Convective zone (20% of radius) Photons created in the core take 170,000 years to make the journey through the radiative zone. Hot gas rises and falls in the convective zone, eventually reaching the surface of the Sun.

The Core Fusion at the core The crushing, immense weight of the outer layers compresses the gas and increases its temperature When the temperature reaches 10 million Kelvin, fusion of hydrogen into helium begins Fusion of hydrogen into helium releases energy at the expense of mass

The Sun is powered by thermonuclear fusion, which converts hydrogen into helium.

Radiative Zone It takes about 170,000 years for the energy released from the core to travel through the radiative layer. Photons created at the core carry the energy through this layer

Convective Zone This energy heats up the gases in the this zone and delivers its energy to the Sun’s surface by convection (hot, less dense gases rise and cooler, more dense gases sink)

Why does the Sun stay a constant size? Outward pressure due to hydrogen fusion is balanced by the inward pressure of the overlying gases (hydrostatic equilibrium).

The Sun’s interior is held stable by a balance between pressure forces and gravity, in a condition called hydrostatic equilibrium.

Outer layers of the Sun Photosphere (5800 K, low density, 0.01% of air at sea level) Chromosphere (4000 K – 10,000 K, less dense than photosphere) Corona (1 million K, extremely low density—10 trillion times less than air at sea level)

Photosphere Light energy (mostly in the form of visible light, and a smaller percentages of UV rays and infrared light, x-rays, gamma rays, microwaves and radio waves) is released from the Sun’s surface, which is 5800 Kelvin in temperature. The density of the photosphere (Sun’s surface) is 0.01% of the air that we breathe. Granulated surface represents the rising and falling hot gases Darker areas represent cooler temperatures

The bright visible surface of the Sun is called the photosphere. When looking at the Sun, the edges appear orange and darker than the central yellow region. This is known as limb darkening.

Upon closer inspection, the Sun has a marbled pattern called granulation, caused by the convection of gases just beneath the photosphere.

During an eclipse, sometimes you can see the layers of the Sun’s atmosphere just above the photosphere, which emits only certain wavelengths of light, resulting in a reddish appearance. We call this the sphere of color, or chromosphere.

The solar chromosphere is characterized by jets of gas extending upward called spicules.

Corona Hot, ionized extremely thin gas up to 1 million K caused by the Sun’s complex magnetic fields charged particles are moving so fast that they can escape the gravitational pull of the Sun. This is the solar wind. Sun ejects about a million tons of matter per second matter travels fast (2.9 x 106 km/h) 5 particles per cc by time it reaches the Earth (atm 6 x 1019 particles per cc)

THE SOLAR CORONA This x-ray image shows the million-degree gases. Seen in visible light during an eclipse. Bright areas are where the Sun’s magnetic field is so strong that it trap the super heated gases of the Corona. Darker areas represent coronal holes. This is where the solar wind originates (700 km/s).

The temperature of the solar gases increase with distance from the solar surface. Within the narrow transition region between the chromosphere and the corona, the temperature increases by 100 times.

Surface Features of the Sun Sunspots Plages Filament (a top view of a prominence) Prominences Solar Flares Coronal Mass Ejections All are due to changes in solar magnetic fields

Sunspots are regions of intense magnetic fields

For sunspots to form, the magnetic field lines of the Sun become intertwined after several rotations, creating regions of intense magnetic fields. Sunspots are produced at distortions along these field lines. Coronal loop of hot electrified gas can be 300,000 miles high and span 30 Earths. It takes the Sun 25 days to rotate at its equator, and 35 days to rotate at its poles. Its rotational speed is roughly 2 km/s.

Sunspots Overlapping sunspots Sunspots have two regions: the inner, darker umbra and the outer penumbra. Darker regions of the Sun are cooler than the brighter yellow regions.

The number of sunspots on the photosphere varies over an eleven-year cycle. Sunspot Maximum Sunspot Minimum Sunspot max to min to max = 22 years

Sunspots Sunspot cycle is 11 years from sunspot maximum to minimum (22 years for full cycle) 10,000 km across A sunspot develops at a place where the magnetic field pokes through the photosphere A plage is a bright spot associated with an emerging magnetic field that compresses and heats up gases. Differential rotation of the Sun leads to overlapping magnetic fields which leads to unstable conditions on the photosphere

Sunspots can be used to determine the rate of the sun’s rotation.

Prominances Arched volumes of hot gas pushed up by magnetic field. Upward of 50,000 K Almost always associated with sunspots

Solar Flares Violent, eruptive event Associated with sunspot activity Releases vast quantities of high energy particles and x-ray and uv rays. Powerful, will leave Sun’s surface quaking for an hour or more

Coronal Mass Ejections Largest ejection event 8 minutes for light 2-4 days for charged particles These ejections overwhelm the Van Allen Belts (earth’s magnetic field) and lead to dramatic aurorae and potential disruptions of communications.

Viewing the Sun with an H-Alpha filter reveals an active chromosphere during a sunspot maximum

Ionized gases trapped by magnetic fields form prominences that arc far above the solar surface. Sometimes these gases are ejected into space.

Violent eruptions called solar flares eject huge amounts of solar gases into space.

By following the trails of gases released during a solar flare, we can map the Sun’s global magnetic field.

Coronal Mass Ejections (CMEs) typically expel 2 trillion tons of matter at 400km per second. It reaches Earth two to four days later, and is fortunately deflected by our magnetic field. An x-ray view of a coronal mass ejection

Changes in Physical Properties of Solar Gases from the Solar Core to the Photosphere

A mystery involving undetected neutrinos produced in the Sun’s core prompted an investigation into the fundamental nature of these particles. Subsequent experiments showed that neutrinos can change as they travel through space.

During the sunspot cycle, the latitude at which sunspots appear changes. A plot of the latitude of appearing sunspots over time reveals that early in the sunspot cycle, they appear away from the equator, then slowly move toward the equator as the cycle progresses.

DEATH OF THE SUN The Sun does not have enough mass to explode as a supernova (low mass star). It is to become a red giant that will alternately expand and contract in response to variations in inward and outward pressure. At some point, the star will expand and then slowly release its outer gas layer. Up to 80 % mass loss. The carbon-oxygen core cools and is called white dwarf.

WHAT DID YOU THINK? How does the mass of the Sun compare with that of the rest of the solar system? The Sun contains 99.85% of the solar system’s mass. Are there stars nearer the Earth than the Sun? No, the Sun is our closest star. Does the Sun have a solid and liquid interior like the Earth? No, the Sun is composed of hot gases.

WHAT DID YOU THINK? What is the surface of the Sun like? The Sun has no solid surface, and no solid or liquids anywhere. The surface we see is composed of hot, churning gases. Does the Sun rotate? The Sun’s surface rotates differentially; once every 35 days near its poles, and once every 25 days near its equator. What makes the Sun shine? Thermonuclear fusion in the Sun’s core

Key Terms Cerenkov radiation chromosphere convective zone core (of the Sun) corona coronal hole coronal mass ejection filament granule helioseismology hydrogen fusion hydrostatic equilibrium limb (of the Sun) limb darkening magnetic dynamo neutrino photosphere plage plasma positron prominence radiative zone solar cycle solar flare solar luminosity solar model solar wind spicule sunspot sunspot maximum sunspot minimum supergranule thermonuclear fusion transition zone Zeeman effect

You will discover… why the Sun is a typical star how today’s technology has led to a new understanding of solar phenomena, from sunspots to the powerful ejections of matter that sometimes enter our atmosphere that some features of the Sun generated by its varying magnetic field occur in cycles how the Sun generates the energy that makes it shine new insights into the nature of matter from solar neutrinos

The Sun undergoes differential rotation. The rotation period of the Sun’s gases varies from 25 days in the equatorial region to 35 days near the solar poles.

WHAT DO YOU THINK? How does the mass of the Sun compare with that of the rest of the solar system? Are there stars nearer the Earth than the Sun? Does the Sun have a solid and liquid interior like the Earth? What is the surface of the Sun like? Does the Sun rotate? What makes the Sun shine?

Coronal holes are conduits for gases to flow out from the Sun