ASTR 1514 1st Edition Lecture 27The sun is divided into three layers: The photosphere, Chromosphere, The corona. The surface of the Sun- The Photosphere - Visible surface of the sun. - Where most of the light energy is emitted- Average temperature about 5770K, emits in the visual band. - Limb darkening – Because we look through les material at the edges, it appears darker. - The photosphere is an apparent surface – The location of the last photon scattering- Like fog or a cloud, you cannot stand on the photosphere- In the photosphere, cooler outer layers absorb some of the light from hotter, deeper layers.- This produces a complex absorption spectrum with more than 70 elements. - Sunspots- Dark areas in the surface of the sun that come and go immediately. - The gas in this area is cooler so it appears darkerThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Dark umbra with surrounding penumbra, caused by magnetic fields. - Sunspots start out relatively large and then fade away over a period of days to weeks- Granulation on the Photosphere- The convection layer of the sun observed from above.- Radiation transport in the sun occurs by radiation (in the middle) and convection (topmost layer)- Cooler gas is dimmer, so we see the granulation structure from convection- Chromosphere (second layer)- Thin layer of low density gas, located above the photosphere- Higher temperature than the photosphere - Gives off reddish emission – line spectrum. (Due to Hydrogen)- The temperature of the chromosphere is about 10,000K- Spicules – flame-like jets of gas in the chromosphere that emit hydrogen emission lines. - Corona (Third Layer)- Emits strongly in X-rays- The temperature of the corona is about 1 million K. - Above the Chromosphere- Can extend for several solar radii. - Does not smoothly cover the whole sun- Spots with coronal activity called active regions - Solar active region are associated with sunspots1. Sunspots have a high magnetic field associated with them2. Sunspots occur in pairs, with magnetic field of one polarity coming out of one spot, and going back into the other one3. Magnetic fields can trap hot plasma (ionized gas) – magnetic bottle – because ions don’t like to cross magnetic field lines. - The Corona is heated by particles accelerated by magnetic reconnection, resulting in solar flares. - Solar flares can become as big as 3% of the diameter of the sun. (much bigger than the Earth)- Coronal Hole – Places on the sun where corona is absent, possibly a place where magnetic field does not loop back to the surface. - Coronal Mass Ejection – Violent episodes of solar wind that can be very dangerous. Frequently come out of the coronal hole. Can strongly affect the rate of solar windsThe Solar Cycle- The sun has an 11 year solar activity cycle. - Location of sunspots changes- Number of sunspots changes- Number of flares and X-ray regions changes- Number of coronal mass ejections and associated phenomena, like aurora, changes. - Solar Maximum – most sunspots and activity- The Maunder Minimum – Lack of sunspots between 1645 and 1715. - Latitude of sunspots changes – “Butterfly Diagram” At the beginning of a cycle there are few spots, mostly at high altitudes. At the middle of the cycle (solar Max) the sun is covered with spots with a lot near the equator. - We think the reason the sun has a solar cycle is because of the differential rotation of the sun. The sun rotates faster at the equator than the poles. - Babcock Model for Solar activity – Proposes that the differential rotation of the sun wraps up the sun’s magnetic field inside it. Eventually the magnetic field is so wrapped up that magnetic recombination occurs, finally relaxing the magnetic field to a simple configuration