Presentation on theme: "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."— Presentation transcript:
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 Earths mass 109 times Earths diameter Consists entirely of gas (av. density = 1.4 g/cm 3 ) 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 Heat Flow 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
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 Photosphere Energy generated in the suns center must be transported outward. In the photosphere, this happens through Convection: Bubbles of hot gas rising up Cool gas sinking down 1000 km Bubbles 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 suns 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 Magnetic Carpet of the Corona Corona contains very low-density, very hot (1 million o K) 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: 10 7 tons/year ( 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 Suns 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 Suns 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 suns photosphere, flowing along magnetic loops Relatively cool gas (60,000 – 80,000 o K) May be seen as dark filaments against the bright background of the photosphere
Eruptive Prominences and Flares (Ultraviolet images) Extreme events (solar flares) can significantly influence Earths magnetic field structure and cause northern lights (aurora borealis).
Solar Flares A Solar flare is a large explosion on Suns 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/m 2.
The Solar Interior Energy transport: The radiation zone is relatively transparent the cooler convection zone is opaque