The Sun – Our Star

General Properties Average star Spectral type G2 Only appears so bright because it is so close. Absolute visual magnitude = 4.83 (magnitude if it were at a distance of 32.6 light years) 109 times Earth’s diameter 333,000 times Earth’s mass Consists entirely of gas (av. density = 1.4 g/cm3) Central temperature = 15 million K 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 Only visible during solar eclipses Apparent surface of the sun Heat Flow Temp. incr. inward Solar interior

The Photosphere Apparent surface layer of the sun Depth ≈ 500 km Temperature ≈ 5800 K Highly opaque (H- ions) Absorbs and re-emits radiation produced in the solar interior The Photosphere ·       The photosphere is where the Sun becomes opaque to our view. ·       It is a little over 400 km deep. ·       The photosphere is a very narrow layer. If the Sun was the size of a bowling ball, the photosphere would be thinner than one layer of one-ply tissue wrapped around it. ·       The temperature of the photosphere is ~ 5800 K in the middle of the layer. ·       At this temperature, some of the hydrogen atoms are ionized (lose an electron). With loose electrons about, other hydrogen atoms pick up extra electrons, becoming negative ions. These extra electrons are easily knocked off if hit by a photon. In the process the photon is absorbed. All of the photons coming from beneath the photosphere are absorbed in this region and then are re-emitted. ·       Although the photosphere looks opaque, it is really a very low density gas. The density at mid-photosphere is about 0.1 percent the air density at sea level. This is a rather good vacuum. The solar corona

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 ·       Good photographs of the photosphere show that it is not uniform, but is mottled by a pattern of bright cells called granulation. ·       Each granule is approximately 1000 km in diameter (~ the size of Texas) and is separated from its neighbors by a dark boundary. ·       Centers of granules are hotter, rising gas (bright), while the edges are cooler, sinking gas (dark). These are the tops of convection cells. ·       Recent spectrographic studies show another kind of granulation. Supergranules are regions about 30,000 km in diameter (2.3 times the diameter of the Earth) and include about 300 granules. These are regions of slowly rising currents lasting a day or two.

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

Energy Transport in the Photosphere Energy generated in the sun’s center must be transported outward. In the photosphere, this happens through Convection: Cool gas sinking down Bubbles of hot gas rising up ≈ 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 sun’s atmosphere just above the photosphere. Temperature increases gradually from ≈ 4500 K to ≈ 10,000 K, then jumps to ≈ 1 million K ·       The chromosphere is a layer of nearly invisible gas above the photosphere. It is about 1000 times fainter than the photosphere, so we only see it during a solar eclipse. ·       It is about 2000 to 3000 km thick below the spicules. (Some books include the spicules as part of the chromosphere, making it closer to 10,000 km thick.) ·       The term "chromosphere" comes from the Greek word "chroma" meaning color. The chromosphere is pink. ·       It is a hot, low density gas. ·       The density decreases as we go up, but the temperature increases from ~ 4500 K (at the photosphere) to ~ 10,000 K at the upper levels. ·       Observations of the chromosphere led to the discovery of helium in 1868. The Transition Region ·       This lies above the chromosphere. Some books include it as part of the chromosphere. ·       The temperature increases from ~ 10,000 K at the top of the chromosphere to nearly 1,000,000 K at the corona. ·       It varies in thickness, but is considered a thin layer. ·       It contains spicules. These are jet-like spikes of gas originating in the chromosphere and rising through the transition region at velocities of 30 km/s. Transition region

The Chromosphere (2) Spicules: Filaments of cooler gas from the photosphere, rising up into the chromosphere. Visible in Ha emission. ·       Spicules are flamelike structures 100 to 1000 km in diameter that rise up to 20,000 km above the photosphere and last from 5 to 15 minutes. ·       Spicules appear to be cooler regions (~ 10,000 K) rising into the hot transition region. Spicules spring up around the edges of supergranules. Perhaps they are channels through which energy flows from below the photosphere to the corona Each one lasting about 5 – 15 min.

Spicules on the Solar Limb

The Corona The Corona · The outermost layer of the sun's atmosphere. ·       First observed during solar eclipses. ·       Named for the Greek word for "crown." ·       We have been able to trace the corona out to 30 solar radii. ·       It is very low in density. At the bottom it has about 109 atoms per cubic centimeter. Compare this to the 1019 molecules per cubic centimeter at sea level in Earth's atmosphere. ·       The corona is very hot, over 1,000,000 K. This causes it to give off a continuous spectrum with bright lines on top of it. ·       How can it be this hot? Unsure. Thought to be linked to the magnetic field of the sun. The magnetic field stores energy and carries it to the chromosphere and corona where it is converted to electrical currents. These heat the gases of the Sun's outer atmosphere. Precise method for this is not understood.

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) The Solar Wind ·       A stream of charged particles from the Sun's atmosphere into space. ·       Particles flow outward at about 400 km/s. ·       Gases in the corona are so hot and moving so fast that they cannot be held by solar gravity and escape into space. ·       This was discovered by it's effect on the ion tails of comets. ·       Charged particles of the solar wind can be caught in Earth's magnetic field lines. They follow the lines down to our upper atmosphere and strike the atoms and molucules of air causing a glow called an aurora at the northern and southern magnetic poles of the planet.

Cooler regions of the photosphere (T ≈ 4240 K). Sun Spots Sunspots ·       Sunspots are dark, cool areas on the Sun's surface (photosphere). ·       The center of the spot is darker and called the umbra. The outer border is called the penumbra. ·       The average spot is twice the diameter of the Earth and may last a week or so. ·       Sunspots tend to form in groups that may contain up to 100 individual spots and may last up to 2 months or more. ·       The center of a large sunspot may be ~ 4240 K, which is cooler than the average 5800 K of the photosphere. ·       Watching sunspot motion gives us the rotation period of the sun. ·       The sun has differential rotation. It's period is ~ 25 days at the equator and ~ 36 days at latitude 80. 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

Reversal of magnetic polarity The Solar Cycle After 11 years, North/South order of leading/trailing sun spots is reversed 11-year cycle ·       Sunspots have a cycle that has been clearly established. The total number of sunspots hits a maximum every 11.1 years. This is related to the 22 year magnetic cycle of the sun. ·       The overall polarity of the sunspot pairs reverses every 11 years. So a complete cycle takes 22 years. Reversal of magnetic polarity => Total solar cycle = 22 years

The Solar Cycle (2) The Sun has an 11-year sunspot cycle, during which sunspot numbers rise, fall, and then rise again:

Maunder Butterfly Diagram 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 The sun rotates faster at the equator than near the poles. ·       Every 11 years the magnetic field lines wind so tight that they snap and the magnetic poles of the sun are reversed. Then they start again. Therefore full cycle will take 22 years. This differential rotation might be responsible for magnetic activity of the sun.

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 ·       The magnetic field lines wind around the sun as the center spins faster than the poles. As it winds tighter, you get loops and snags in the line. A loop that leaves the surface and returns gives you a pair of sunspots where the line leaves and returns.

Sun Spots (3) Magnetic field in sun spots is about 1000 times stronger than average. Magnetic North Poles ·       Sunspots occur in magnetic pairs. One spot would be a magnetic north pole, while the other is a south pole. The pole of the leading spot is reversed as we go from N to S hemisphere. Magnetic South Poles In sun spots, magnetic field lines emerge out of the photosphere.

The sun spot number also fluctuates on much longer time scales: The Maunder Minimum The sun spot number also fluctuates on much longer time scales: Solar Energy ·       The sun puts out a fairly constant amount of energy. Variations of only 0.01% have been measured. ·       The solar constant is 1360 Joules per square meter per second. ·       A change in the solar constant of as little as 1% could change the average temperature of Earth by 1-2 C. For comparison, during the last ice age the average temperature on Earth was about 5 C cooler than it is now. Surprisingly little variation could drastically change our climate. There was a period of cold weather between 1500 and 1850 known as the Little Ice Age. This may have been caused by fluctuations in the solar constant. During the time between 1645 and 1715, there was an era of few sunspots known as the Maunder Minimum. Reports of total solar eclipses during this time make no mention of the corona or chromosphere, and almost no record of auroral displays. This seems to have been a period of reduced solar activity. Historical data indicate a very quiet phase of the sun, ~ 1650 – 1700: The Maunder Minimum

Prominences Areas around sunspots are active; large eruptions may occur in photosphere. Solar prominence is large sheet of ejected gas:

Relatively cool gas (60,000 – 80,000 oK) Prominences Relatively cool gas (60,000 – 80,000 oK) May be seen as dark filaments against the bright background of the photosphere ·       Prominences are red, flame-like protuberances rising above the sun and reaching high into the corona. ·       Some are graceful loops that remain for hours or days. Others move upward or have arches that move back and forth. ·       Prominences originate near regions of sunspot activity and lie of the boundary between regions of opposite magnetic polarity. Eruptive prominences result from sudden changes in the magnetic field. Looped Prominences: gas ejected from the sun’s photosphere, flowing along magnetic loops

Prominences                                                                                                                                                  This prominence on July 24, 1999 was particularly large and looping, extending over 35 Earths out from the sun. Erupting prominences, when directed toward Earth, can affect communications, navigation systems,and even power grids, while producing auroras visible in the night sky.

Eruptive Prominences and Flares (Ultraviolet images) Extreme events (solar flares) can significantly influence Earth’s magnetic field structure and cause northern lights (aurora borealis). ·       There are rare eruptive prominences that send matter upward at speeds up to 1300 km/s. ·       Flares are rapid eruptions. They last 5 to 10 minutes. ·       Flares occur when magnetic fields pointing in opposite directions release energy by interacting with and destroying each other - like a stretched rubber band releases energy when it breaks.

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                                                                                                                                                  The sun's magnetic field and releases of plasma directly affect the Earth and the rest of the solar system. Solar wind shapes the Earth's magnetosphere, and magnetic storms are illustrated here as approaching Earth. These storms, which occur frequently, can disrupt communications and navigational equipment, damage satellites and even cause blackouts. The white lines represent the solar wind; the purple line is the bow shock line; and the blue lines surrounding Earth represent its protective magnetosphere. The magnetic cloud of plasma can extend to 30 million miles or 50 million km wide by the time it reaches 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