A105 Stars and Galaxies  Homework 6 due today  Next Week: Rooftop Session on Oct. 11 at 9 PM  Reading: 54.4, 55, 56.1, 57.3, 58, 59 Today’s APODAPOD

READING FOR NEXT WEEK Background material in 54, 56, 57: –54.4: magnitude system –56.1: types of binary stars –57.3: Stefan-Boltzman Law Key topics (read carefully!): –55: Temperatures and Compositions of Stars –58: The H-R Diagram –59: Stellar Evolution

Basic Facts radius 7 x 10 5 km about 100 x Earth’s radius mass = about 300,000 x Earth’s mass distance 1 AU, 8 light minutes 1.5 x 10 8 km about 100 x Sun’s diameter Temperature about 6000 Kelvin (10,000 F) at the surface about 15 million Kelvin inside Composition 90% of atoms are hydrogen 10% of atoms are helium The Sun

What we covered on Tuesday: Internal structure of the Sun Composition of the Sun Source of the Sun’s energy The Sun’s lifetime Balancing pressure, gravity, and temperature Helioseismology What neutrinos tell us about the Sun Today: The Sun’s Atmosphere

Recall the structure of the Sun…

Corona The Sun’s Atmosphere: Photosphere, Chromosphere, Corona, Solar Wind

Photosphere Umbra Penumbra Sunspots Active regions The Sun Corona Chromosphere three layers of the Sun’s atmosphere –photosphere –chromosphere –corona

Photosphere Limb Convection Supergranules Granulation Limb darkening Umbra Penumbra Sunspots Active regions Prominence Limb darkening: …when we look near the Sun’s limb we do not see as deeply into the photosphere

Photosphere The photosphere is opaque to visible light The density is only 1% the density of air Temperature decreases from inside the Sun to a minimum of 4400K just above the photosphere

Granulation: convection cells ~ 1000 km (600 mi) The difference in T from center to edge is about 300 K -> hot gas from lower levels rises upward, cools off, and falls back into the Sun Cells form and disappear in few minutes. The Solar Photosphere – the layer we see

The convection moves at 1400 km/h =900 mi/h Supergranules last about a day Supergranules 35,000 km in diameter

Absorption Lines in the Photosphere Photosphere nearly a perfect thermal radiator with T=5800 K *upper part of the photosphere is 4400K (cooler!)

Solar Atmosphere  Absorption lines from the photosphere and chromosphere  67 different elements in various stages of excitation and ionization  The spectrum gives us a picture of the physical conditions in the solar atmosphere

Sunspot grouping Umbra Penumbra Blotchy sunspots appear on the Sun’s “surface” Note also the “granulation” resulting from convection under the surface

Sunspots are low-temperature regions in the Sun’s photosphere Sometimes they are isolated but frequently in sunspost groups ~ diameter of Earth (lasting between hours or months)

T (umbra) = 4300 K T (penumbra) = 5000 K T (photosphere = 5800 K The brightness depends on the 4 th power of the temperature (energy flux  T 4 ) Photosphere

The number of sunspots varies in an 11 years cycle

Magnetic Carpet Chromosphere Spicules Prominence Above the photosphere, the temperature increases again from 4400 K to 25,000 K at the top of the chromosphere

The chromosphere is characterized by spikes of rising gas Above the photosphere, the gas density is much lower (10 -4 less than the photosphere) but gas is much warmer (25,000 K) Red color from the emission line of hot hydrogen (Using Hydrogen filter)

Spicules last about 15 minutes they rise at speeds of 20km/s 300,000 spicules cover the Sun at any one time Spicules: jets of rising gas Spicules extend upward from the photosphere into the chromosphere along the boundaries of supergranules dense jets of gas that shoot up from the chromosphere

Solar Wind Coronal Mass Ejection Corona Aurora Coronal Holes The corona, the outermost layer of the solar atmosphere, is made of very high- temperature gases at extremely low density The solar corona blends into the solar wind at great distances from the Sun

CORONA The corona extends for millions of kilometers out from the Sun a million times fainter than the photosphere...

In the narrow region between the chromosphere and the corona, the temperature rises abruptly to more than a million degrees! (BUT NOT MUCH HEAT!)

Activity in the corona includes coronal mass ejections and coronal holes

The Sun also produces huge flares that burst into space A solar flare is a brief eruption of hot, ionized gases from a sunspot group A coronal mass ejection is a much larger eruption that involves immense amounts of gas from the corona

The Solar Wind The Sun is “evaporating” constantly (ONLY 0.1% of its mass since its formation) Interact with objects in the solar system –Earth: cause aurorae –Comets: produces tails –Interacts with edge of solar system: heliopause Radiation reach Earth in 8 min, particles take a few days (v= 500 km/s) Coronal particles (mostly electrons and protons) are thrown with such velocity that they cannot be held by the Sun’s gravity

Photosphere Limb Convection Supergranules Granulation Limb darkening Umbra Penumbra Sunspots Active regions The Sun Magnetic Carpet Solar Wind Coronal Mass Ejection Corona Aurora Coronal Holes Chromosphere Spicules Prominence

The X-ray Sun coronal hole What causes all this violent activity on the Sun????? Rotation and Magnetic Fields! mass ejectio n

Sunspots show the Sun is rotating (Galileo!) The Sun does not rotate rigidly: the equatorial regions rotate faster (25 days) than the poles (36 days) --- Differential Rotation

The Sun rotates at different rates at different solar latitudes P equator =25 days P pole =36 days

The Sun’s differential rotation winds up the Sun’s magnetic field, storing energy. When the magnetic field suddenly unwinds, that energy is released.

The Solar Magnetic Field The differential rotation “wraps up” the magnetic field of the Sun Sunspots and other solar activity are caused by the twisted magnetic field of the Sun

Solar Activity Varies in an Eleven Year Cycle The Sun’s magnetic field takes 11 years to twist up and then reestablish itself The number of sunspots, as well as the number of violent events depends on the state of the magnetic field

Variations in Sunspot Activity Maunder Minimum

 Stars! Units (parts)  Homework 7 Due THURS. HAND IN HOMEWORK!