Sun Notes

The General Properties of the Sun The sun is an average star with average brightness. It only looks bright because it’s so close. It is 109 times the diameter of the Earth. It contains 99% of the mass of the solar system. It is made of entirely of gas with a core temperature of 15,000,000 K It is a main sequence star of spectral G2. It’s considered a yellow star. Light from it takes 8.32 minutes to get to Earth. It is 26,000 light years from the center of the Milky Way. It has an orbital period of 220 million years.

The Solar Interior The sun’s interior consists of 3 layers: Core: where the hydrogen fusion (where the sun gets its energy from) happens. Radiative zone: where energy from the core is carried to toward the surface as light. Convective zone: where energy is carried further toward the surface as heat.

Energy Production in the Interior Energy in the sun (and all other stars) is created using nuclear fusion. Nuclear fusion: the combination of 2 or more lighter atomic nuclei to produce heavier ones. Fusion has the ability to produce enough energy to create atoms on the Periodic Table up to iron. Once it tries fusing iron, fusion stops. The energy to create atoms heavier than iron is gained via nuclear fission. Fusion takes advantage of E = mc2. Matter (hydrogen) gets converted to helium and ENERGY. The sun needs to transform 5 million tons of mass into energy every second to counter its own gravity. The heavier the element being fused, the more energy created, the higher the repulsive force…the bigger the star.

RADIATION PRESSURE FROM The Sun’s interior is held stable by a balance between radiation pressure forces and gravity, in a condition called hydrostatic equilibrium. GRAVITY – pulls in RADIATION PRESSURE FROM HYDROGEN FUSION – pushes out

The Solar Atmosphere The atmosphere of the sun is only visible to us during solar eclipses. It is broken up into 3 layers: Photosphere: the apparent surface of the sun. This is where the light we see comes from. Chromosphere: the reddish layer just above the photosphere. Corona: the biggest and hottest layer. Visible Photosphere Ultraviolet Corona Chromosphere

Bubbles of hot gas rising up The Photosphere The apparent surface of the sun. It absorbs radiation from the interior and re-emits it into space. It is only 500 km deep . It has a temperature of 5800 K. Energy from the core must be transported outward. This is done through convection. Hot gas bubbles up from the interior, where they cool and sink back down. The bubbles last 10-20 minutes. The visible result of convection is granulation. It creates a grainy-like appearance to the photosphere. Real picture of granules Bubbles of hot gas rising up Cool gas sinking down ≈ 1000 km

The Chromosphere Found just above the photosphere. Emits only certain wavelengths of visible light, resulting in a reddish appearance. Characterized by jets of gas rising from the photosphere called spicules. Spicules last about 5-15 minutes. The temperature rises slowly from 5800 K to 10,000 K…then jumps to 1,000,000 K. Where this jump occurs is called the transition region. Transition region

The Corona The outer atmosphere of the sun. It’s best seen during a total solar eclipse. Has a very low density, but a VERY high temperature (1,000,000 K). It’s the biggest and hottest layer of the solar atmosphere. Gas in the corona is heated through the motions of magnetic fields in the photosphere below. The sun is constantly losing mass. The mass being lost is called the solar wind. It refers to the stream of charged particles that originate from coronal holes. These are areas in the corona that appear darker because there is less gas there.

The Sun and Magnetism The sun demonstrates differential rotation. It rotates faster at its equator than its poles. 25 day period at the equator, 35 at the pole. The result is that the magnetic field lines become intertwined, creating regions of intense magnetic fields. The magnetic field creates different features: Sunspots, prominences, solar flares, and coronal mass ejections (CMEs)

Sunspots Cooler regions of the photosphere that appear dark. They have a temperature of about 4240 K. They only appear dark against the bright photosphere. They would still be brighter than the full moon if placed in the night sky. Sunspots are related to magnetic field activity in the photosphere. The magnetic field in sunspots is about 1000 times stronger than average. Magnetic field lines emerge from sun spots. The number of sunspots varies in a 11 year cycle.

The Solar Cycle After 11 years, the magnetic field pattern becomes so complex that the field structure is re-arranged. The number of sunspots reaches a peak. The new magnetic field structure is similar to the original, just reversed. When the field reverses, a new, 11-year cycle begins. The number of sunspots also varies on much linger time scales. The Maunder Minimum is a period of very low sunspot activity between 1650-1700.

Prominences Phenomena in the corona in which gas appears to “erupt” from the corona and loop back into the sun. They consist of relatively cool gas (60,000-80,000 K) They are affected by magnetic field lines.

Solar Flares Sometimes a prominence is ejected out into space, creating a solar flare. Strong enough flares increase the push of the solar wind, increasing the intensity and breadth of the auroras on Earth. Solar Flare

Coronal Mass Ejections (CMEs) Occur when immense (2 trillion tons) quantities of solar material are ejected into space at immense (4 million mph) speeds. They are created when magnetic field lines, which normally snake around each other, connect, essentially short circuiting the system. An x-ray view of a coronal mass ejection It reaches Earth two to four days later, and is fortunately deflected by our magnetic field.