1

The Sun as Our Star

• Last class we talked about how the Sun compares to other stars in the sky

• Today's lecture will concentrate on the different layers of the Sun's interior and its atmosphere

• We will also talk about the behavior of the Sun over time and some of the cycles it goes through

Properties of the Sun

-3.85 x 1026 WLuminosity

~ 300 K for Earth

5780 KSurface Temperature

28 x Earth274 m/s2Surface Gravity

0.255 x Earth1.41 g/cm3Average Density

109 x Earth696,000 kmRadius

332,000 x Earth1.99 x 1030 kgMass

Solar Composition

0.14 %Iron

0.058 %Neon

0.076 %Magnesium

0.099 %Silicon

0.096 %Nitrogen

0.40 %Carbon

0.97 %Oxygen

27.1%Helium

71.0 %Hydrogen

Abundance (by mass)Element

2

The Solar Interior

• We already know that the core of the Sun is where fusion takes place

• There are two other main levels in the solar interior, the radiation zone and the convection zone

• _

The Solar Core

• The central core of the Sun is about 200,000 km in size

• _

• All fusion of hydrogen into helium takes place inside the core

Core

The Radiation Zone• The next layer out from the

core is the radiation zone (about 300,000 km thick)

• _

• In this layer, the energy produced in the core is transported by radiation (light)

• Gamma rays scatter repeatedly off atoms in this layer, losing energy and making their way to the surface

RadiationZone

3

The Convection Zone

• The convection layer is the final interior layer and is about 200,000 km thick

• _

• Here, the energy of the Sun is transported by the swirling motion of the heated gas

• Hot cells of gas rise to the surface, release their energy, cool, and drop down toward the interior

ConvectionZone

The Solar Atmosphere

• The outer layers of the Sun are comprised of the material we can actually see

• The 'surface' of the Sun is the point at which light is free to escape, without bumping into more and more atoms

• _

The Photosphere

• The photosphere is the first layer in the solar atmosphere and is usually thought of as the 'surface of the Sun' (only 500 km thick)

• _

• The photosphere can be viewed using a filter which blocks out the majority of the light from the Sun

• Images of the photosphere will show sunspots on the surface of the Sun

4

Features of the Photosphere

• Sunspots are features in the photosphere created by interactions with the Sun's magnetic field

• Just like a huge magnet, some bodies in the Solar System (like the Sun, Earth) will have a magnetic field

• Sunspots usually come in pairs, each member of the pair can be thought of a north and south component of a magnetic field line

• _

Umbra

Penumbra

Features of the Photosphere

• Close up pictures of the Sun's photosphere reveal granulation -the Sun's surface appear to be broken into small cells

• _

• Each cell represents a convective 'bubble' that has risen to the surface

• The bright interior is where hot material is coming to the surface, the darker exterior is where cooler material is sinking below

Features of the Photosphere

Solar Granulation

The Chromosphere

• The next layer of the solar atmosphere is the chromosphere, about 1,500 km thick

• _

• This region is not easily visible since the light of the photosphere is so much greater The Sun using an H-alpha filter

5

Features of the Chromosphere

• Close up images of the chromosphere reveal small jets of gas shooting up from the surface, called spicules

• On much larger scales, long stringy filaments of gas can also be seen in the chromosphere

• _

Spicules

Filaments

The Corona

• The outermost layer of the Sun's atmosphere is called the corona, and extends 100,000's of km away from the Sun's surface

• Surprisingly, the corona is much hotter than the other layers of the atmosphere, in the millions of Kelvin

• _ The corona become visible during a total solar eclipse

The Solar Wind

• _

• The solar wind is most dense near the Sun, and dissipates as you move further and further away

• The solar wind is easily detectable at the Earth, and is still strong at Jupiter and beyond

6

Solar Activity

• Very frequently, large amounts of gas will erupt from the surface of the Sun

• _

• A stream of material coming off the Sun is usually called a flare, while a loop of material that falls back to the Sun is called a prominence

Solar flare

SolarProminence

Coronal Mass Ejection

• Occasionally (a couple times a week), very large eruptions will explode off the Sun's surface into space

• These coronal mass ejections (CME's) travel very fast outward through the solar system

• A large CME directed toward the Earth can affect satellites, the space station, and electrical grids on the Earth's surface

SOHO

7

Solar Sunspot Cycle

• _

• During the 1600's, there was a lack of activity now known as the Maunder Minimum

• After that, it became quite apparent that sunspot activity follows an 11-year period

What Causes the Sunspot Cycle?• Observations of the Sun's

rotation have revealed that it does not rotate at the same speed in all places

• _

• At its equator, the Sun rotates significantly faster than at the poles

• The magnetic field of the Sun is tied to the rotating gas

Twisted Magnetic Fields• Differential rotation causes the magnetic field lines of the

Sun to become twisted

• The more twisted they become, the higher the amount of activity on the Sun's surface

8

The Sunspot Cycle

• Eventually, the magnetic field lines become so disrupted that the field disappears and then reforms

• Each time the magnetic field is reformed, the field lines are straight and solar activity decreases

• Also, the polarity of the Sun's poles change (the north magnetic pole becomes the south and vice versa)