the sun – our star

The Sun – Our Star

General Properties

• Average star

• Absolute visual magnitude = 4.83 (magnitude if it were at a distance of 32.6 light years)

• Central temperature = 15 million K

• 333,000 times Earth’s mass

• 109 times Earth’s diameter

• Consists entirely of gas (av. density = 1.4 g/cm3)

• Only appears so bright because it is so close.

• Spectral type G2

• Surface temperature = 5800 K

Physical Properties of the Sun

Interior structure of the Sun:

Outer layers are not to scale.

The core is where nuclear fusion takes place.

Very Important Warning:

Never look directly at the sun through

a telescope or binoculars!!!

This can cause permanent eye damage – even blindness.

Use a projection technique or a special sun viewing filter.

The Solar Atmosphere

Hea

t F

low

Solar interior

Temp. incr. inward

Only visible during solar eclipses

Apparent surface of the sun

• Apparent surface layer of the sun

The Photosphere

The solar corona

• Depth ≈ 500 km• Temperature ≈ 5800 K• Highly opaque (H- ions)

• Absorbs and re-emits radiation produced in the solar interior

Photosphere

WHY DOES OUR SUN APPEAR TO HAVE A WELL-DEFINED

SURFACE?

• The light we see from a star is radiated from a thin outer layer of gas called the photosphere.

• The gas inside the photosphere is opaque, that is, it is a plasma in which matter and radiation are strongly coupled.

• Gas outside the photosphere is transparent, that is, matter is neutral, hence uncoupled from radiation.

• The sharpness of this transition is why a star appears to have a surface, which is defined as the boundary between any two phases of matter.

Granulation… is the visible consequence of convection

The visible top layer of the convection zone is granulated, with areas of upwelling material surrounded by areas of sinking material:

Granulation

Energy Transport in the PhotosphereEnergy generated in the sun’s center must be transported outward.

In the photosphere, this happens through

Convection:

Bubbles of hot gas rising up

Cool gas sinking down

≈ 1000 kmBubbles last for ≈ 10 – 20 min.

Limb Darkening

The edges of the sun appear darker and slightly redder.

When we look at the limbs, we see light rays which must skim through the photosphere at a shallow angle to reach the Earth. They originate in the upper reaches of the photosphere, where the temperature is somewhat lower.

The Chromosphere• Region of sun’s atmosphere just above the photosphere.

• Temperature increases gradually from ≈ 4500 K to ≈ 10,000 K, then jumps to ≈ 1 million K

Transition region

The Chromosphere (2)

Spicules: Filaments of cooler gas from the photosphere, rising up into the chromosphere.

Visible in H emission.

Each one lasting about 5 – 15 min.

Spicules on the Solar Limb

The Corona

The Magnetic Carpet of the Corona• Corona contains very low-density, very hot (1 million oK) gas

• Coronal gas is heated through motions of magnetic fields anchored in the photosphere below (“magnetic carpet”)

Computer model of the magnetic carpet

The Solar Wind

Constant flow of particles from the sun.Velocity ≈ 300 – 800 km/s

Sun is constantly losing mass:

107 tons/year

(≈ 10-14 of its mass per year)

Sun Spots

Cooler regions of the photosphere (T ≈ 4240 K).

Only appear dark against the bright sun. Would still be

brighter than the full moon when placed on the night sky!

Sun Spots (2)

Active Regions

Visible

Ultraviolet

The Solar Cycle

11-year cycle

Reversal of magnetic polarity

After 11 years, North/South order of leading/trailing sun spots is reversed

=> Total solar cycle = 22 years

The Sun has an 11-year sunspot cycle, during which sunspot numbers rise, fall, and then rise again:

The Solar Cycle (2)

The Solar Cycle (3)Maunder Butterfly Diagram

Sun spot cycle starts out with spots at higher latitudes on the sun

Evolve to lower latitudes (towards the equator) throughout the cycle.

The Sun’s Magnetic Dynamo

This differential rotation might be responsible for magnetic activity of the sun.

The sun rotates faster at the equator than near the poles.

The Sun’s Magnetic Cycle

After 11 years, the magnetic field pattern becomes so

complex that the field structure is re-arranged.

New magnetic field structure is similar to the

original one, but reversed!

New 11-year cycle starts with reversed magnetic-field

orientation

Magnetic Loops

Magnetic field lines

Sun Spots (3)Magnetic field in sun spots is about 1000 times stronger than average.

In sun spots, magnetic field lines emerge out of the photosphere.

Magnetic North Poles

Magnetic South Poles

The Maunder Minimum

Historical data indicate a very quiet phase of the sun, ~ 1650 – 1700: The Maunder Minimum

The sun spot number also fluctuates on much longer time scales:

Prominences

Areas around sunspots are active; large eruptions may occur in photosphere.

Solar prominence is large sheet of ejected gas:

Prominences

Looped Prominences: gas ejected from the sun’s photosphere, flowing along magnetic loops

Relatively cool gas (60,000 – 80,000 oK)

May be seen as dark filaments against the bright background of

the photosphere

Prominences

Eruptive Prominences and Flares

(Ultraviolet images)

Extreme events (solar flares) can significantly influence Earth’s magnetic field structure and cause northern lights (aurora borealis).

Solar Flares

A Solar flare is a large explosion on Sun’s surface, emitting a similar amount of energy to a prominence, but in seconds or minutes rather than days or weeks:

Flares Affect Earth

Physical Properties of the Sun

Solar constant – amount of Sun's energy reaching Earth – is 1360 W/m2.

The Solar Interior

Energy transport:

The radiation zone is relatively transparent

the cooler convection zone is opaque