ASTR 113 – 003 Spring 2006 Lecture 02 Feb. 01, 2006 Review (Ch4-5): the Foundation Galaxy (Ch 25-27) Cosmology (Ch28-39) Introduction To Modern Astronomy II Star (Ch18-24) 1.Sun, Our star (Ch18) 2.Nature of Stars (Ch19) 3.Birth of Stars (Ch20) 4.Evolution of Stars (Ch21) 5.Death of Stars (Ch22) 6.Neutron Stars (Ch23) 7.Black Holes (Ch24) Extraterrestrial Life (Ch30)

Our Star, the Sun Chapter Eighteen ASTR 113 – 003 Spring 2006 Lecture 02 Feb. 01, 2006

Basic Facts Radius: 700,000 Km Distance to Earth: 1 AU = 1.5 X 10 8 km Light travel time: 8 minutes Angular size: 30 arcmin Effective Surface Temperature: 5800 K

Guiding Questions 1.What is the source of the Sun’s energy? 2.What is the internal structure of the Sun? 3.How can astronomers measure the properties of the Sun’s interior? 4.How can we be sure that thermonuclear reactions are happening in the Sun’s core? 5.Does the Sun have a solid surface? 6.Since the Sun is so bright, how is it possible to see its dim outer atmosphere? 7.Where does the solar wind come from? 8.What are sunspots? Why do they appear dark? 9.What is the connection between sunspots and the Sun’s magnetic field? 10.What causes eruptions in the Sun’s atmosphere?

The Sun’s energy is generated by thermonuclear reactions in its core Sun’s total energy output: watts Not chemical energy (only last 10,000 years) Not gravitational contraction (only last 25 million years) Energy from nuclear reaction –Corresponds to a reduction of mass according Einstein’s mass-energy equation E = mc 2

The Sun’s energy is generated by thermonuclear reactions in its core Thermonuclear fusion occurs at very high temperatures Hydrogen fusion occurs only at temperatures in excess of about 10 7 K In the Sun, hydrogen fusion occurs in the dense, hot core

Proton-Proton Chain Reaction The Sun’s energy is produced by hydrogen fusion, a sequence of thermonuclear reactions in which four hydrogen nuclei combine to produce a single helium nucleus; called proton-proton chain reaction

Proton-Proton Chain Reaction: Step 1

Proton-Proton Chain Reaction: Step 2

Proton-Proton Chain Reaction: Step 3

Proton-Proton Chain Reaction 4 H  He + energy + neutrinos Mass of 4 H > Mass of 1 He In every second, 600 million tons of hydrogen converts into helium to power the Sun At this rate, the Sun can continue the hydrogen burning for more than 6 billion years.

Theoretical Model of the Sun Using physical equations to calculate the distribution of temperature, density, and pressure along the radius of the Sun from the core to the surface. Based on physical conditions of 1.Hydrostatic equilibrium: no expansion, no contraction 2.Thermal equilibrium: no temperature change with time 3.Energy transportation 1.Conduction 2.Convection 3.Radiative diffusion

Hydrostatic Equilibrium

Theoretical Model of the Sun

From the center to the surface, luminosity increases, and mass increases

Theoretical Model of the Sun From the center to the surface, temperature decreases, and density decreases

Sun’s Internal Structure: three layers 1.Energy Core: Hydrogen fusion takes place, extending from the Sun’s center to about 0.25 solar radius 2.Radiative Zone: extending to about 0.71 solar radius –In this zone, energy travels outward through radiative diffusion 3.Convective Zone: an opaque zone at relatively low temperature and pressure –energy travels outward primarily through convection

Astronomers probe the solar interior using the Sun’s own vibrations Helioseismology is the study of how the Sun vibrates These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun

Neutrinos reveal information about the Sun’s core—and have surprises of their own Neutrinos emitted in thermonuclear reactions in the Sun’s core have been detected, but in smaller numbers than expected Recent neutrino experiments explain why this is so Nuclear reaction is indeed occurring in the Sun

The Sun’s Atmosphere The Sun atmosphere has three main layers: 1.Photosphere (400 Km thick, innermost) 2.Chromosphere (2000 Km thick, above photosphere) 3.Corona (millions of Km, extended) Everything below the solar atmosphere is called the solar interior

The photosphere is the lowest of three main layers in the Sun’s atmosphere The photosphere (sphere of light) is the visible surface of the Sun It is only 400 km thick because of its opaqueness; Photons emitted below 400 km can not escape. Temperature decreases upward

Photosphere: Limb Darkening Effect At the limb, one can not see as deeply as at the center The gas at higher altitude is less hot (or lower temperature), and thus emit less energy At the limb, appear dimmer

Convection in the photosphere produces granules Granulation is the direct evidence of convection Each granule is about 1000 km Granules from, disappear and reform in cycles lasting a few minutes

Super-granules in photosphere Very large convection cell Each super-granule is about 35,000 km Moves slowly, lasting about one day Hard to observe directly; better seen in Doppler images

The chromosphere (sphere of color) Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere Chromosphere is best seen in spectral emission lines, e.g., Hα line at nm (Hydrogen level 3-2 transition) Spicules, jets of rising gas, extend upward from the photosphere into the chromosphere along the boundaries of supergranules

The corona ejects mass into space to form the solar wind The Corona: the outermost layer of the Sun’s atmosphere Corona is made of very high-temperature gases at extremely low density It extends to several million Km Because of hot temperature, it expands into the outer space forming solar wind

Corona is directly seen in EUV light, which is sensitive to 1 million Kelvin plasma emission

The Sun’s Atmosphere

The Sun’s Magnetism The outer corona is much hotter than the inner chromosphere and photosphere The corona must be heated by a source other than the conduction or radiative diffusion from the underlying atmosphere, because the energy transfer of conduction and radiative diffusion is always from high temperature to low temperature The corona heating is related to the ubiquitous presence of magnetic field in the Sun’s atmosphere.

Sunspots are low-temperature regions in the photosphere Sunspots are shaped dark regions in the photosphere mIt appears dark because it is cooler (radiate less energy) Sunspot Umbra: core Sunspot Penumbra: the brighter border

Tracking the Sun’s Rotation with Sunspots

The average number of sunspots increases and decreases in a regular cycle of approximately 11 years, with reversed magnetic polarities from one 11-year cycle to the next The last solar maximum is in 2000 We are now in the declining phase of solar cycle 23 rd The next solar minimum is in year sunspot cycle or solar cycle

The Sun’s surface features vary in an 11-year cycle This is related to a 22-year cycle in which the surface magnetic field increases, decreases, and then increases again with the opposite polarity The average number of sunspots increases and decreases in a regular cycle of approximately 11 years, with reversed magnetic polarities from one 11- year cycle to the next Two such cycles make up the 22-year solar cycle

11 year-long Sunspot Cycle

The magnetic-dynamo model suggests that many features of the solar cycle are due to changes in the Sun’s magnetic field

These changes are caused by convection and the Sun’s differential rotation

Rotation of the Solar Interior

The Sun’s magnetic field also produces other forms of solar activity 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

Key Words 22-year solar cycle chromosphere CNO cycle conduction convection convective zone corona coronal hole coronal mass ejection differential rotation filament granulation granule helioseismology hydrogen fusion hydrostatic equilibrium limb darkening luminosity (of the Sun) magnetic-dynamo model magnetogram magnetic reconnection negative hydrogen ion neutrino neutrino oscillation photosphere plage plasma positron prominence