Learning Objectives. wavelengths of light do we use to see each of them? mass ejections? Which are the most violent?

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1 Our Beacon: The Sun

2 Learning Objectives! What are the outer layers of the Sun, in order? What wavelengths of light do we use to see each of them?! Why does limb darkening tell us the inner Sun is hotter?! What are granules? Spicules? Prominences? Coronal mass ejections? Which are the most violent?! What features of the spectrum of the Sun tell us how hot it is? What features tell us what the Sun is made of?! What is differential rotation? Could the Earth display it?! Why are sunspots dark in color? How are sunspots related to the Sun s corona and magnetic field?! What are solar maximum and minimum? Why is the Sun s magnetic field on about an 11 (or 22) year cycle?

3 The Outer Layers of the Sun Corona (x-rays) Chromosphere (ultraviolet) Photosphere (optical)

4 Structure of the Outer Layers

5 The Photosphere! The apparent surface of the Sun!Ionized atoms make the gas highly opaque! Most of the Sun s light we see comes from the photosphere! Temperature is about 5800 K!Hotter as you go deeper into the Sun

6 Limb Darkening! The Sun s photosphere is less bright around the edge (the limb )! This is because the top of the photosphere is cooler than the base and thus appears dimmer

7 Granules! The photosphere is not smooth! It is covered in granules! Each is about 700 km across! Each granule survives ~15 minutes before dissipating! The photosphere is seething and bubbling like a pot of boiling water

8 Important bad drawing The Spectrum of the Sun

9 Red Hot or White Hot?

10 Solar Composition! From spectral absorption lines, we can determine the Sun s composition!92% Hydrogen!8% Helium!Less than 0.1% other stuff

11 The Chromosphere! A thin (sparse) gas layer above the photosphere! Hot over 10,000 K! Produces very little radiation it s too sparse! Only seen during eclipses or with special instruments! The element Helium was first discovered in the chromosphere! Home to huge jets of fast-moving hot gas (72,000 kph) called spicules. 300,000 at any given time

12 The Corona! The Sun s outer atmosphere! Visible only by blocking light from the photosphere! Heated by magnetic activity to temperatures of about 2 million K! Bright as the Full Moon in the optical (but drowned out by the photosphere) Solar corona in the optical

13 The Corona! Temperatures of about 2 million Kelvin, which is hot enough to produce X-rays! Very high-energy particles in the corona can escape the Sun s gravity (this is called Solar Corona in X-rays Solar Corona in X-rays the solar wind )! Solar wind is mostly hydrogen, helium and electrons! Travels as fast as 2.9 x 106 km per hour

14 ! Sunspots are dark spots on the photosphere! They tend to appear in pairs and groups! Their sizes range from 1,500 to 50,000 km Sunspots

15 ! The motion of sunspots reveals the Sun s rotation! The Sun spins about once every 25 days at its equator! At its poles, it spins once every ~35 days! This is called differential rotation Sunspot Motion

16 Sunspots and the Outer Layers Photosphere (optical) Chromosphere (ultraviolet) Corona (x-rays)

17 What Causes Sunspots?! Magnetic field loops popping through the photosphere! Powerful magnetic activity shuts down convection! ~5,000 times stronger than the Earth s field! Gas cools off (to only about 4500 K)! Thus sunspots appear darker than the rest of the photosphere

18 The Sunspot Cycle! The number of sunspots on the photosphere varies! It varies from a minimum ( solar minimum ) to the next minimum about every 11 years! Sun s magnetic field then flips North-to-South. So, the full magnetic cycle is about 22 years

19 The Magnetic Cycle

20 Magnetic Activity on the Sun

21 ! Ropes of gas trapped in magnetic loops! Almost always associated with sunspots! Gas can reach temperatures of 50,000 K Prominences

22 Solar Flares

23 Coronal Mass Ejections! Huge bubbles of gas ejected from the Sun! 2 trillion tons of hot ionized gas hurled into the Solar System! Can catastrophically affect satellites! 2 or 3 per day near solar maximum!1 per week, normally

24 Next Time Set the Controls for the Heart of the Sun