ASTR Fall Semester Joel E. Tohline, Alumni Professor Office: 247 Nicholson Hall [Slides from Lecture07]

Chapter 16: Our Star, the Sun

Chapter Overview The Sun’s Surface & Atmosphere –16-5: Why the gaseous Sun appears to have a sharp outer edge –16-6: Why the upper regions of the solar atmosphere have an emission spectrum –16-7: The relationship between the Sun’s corona and the solar wind –16-8: The nature of sunspots –16-9: The connection between sunspots and the Sun’s magnetic field –16-10: How magnetic reconnection can power immense solar eruptions

Chapter Overview The Sun’s Interior –16-1: The source of the Sun’s heat and light –16-2: How scientists model the Sun’s internal structure –16-3: How the Sun’s vibrations reveal what lies beneath its glowing surface –16-4: How scientists are able to probe the Sun’s energy-generating core

Chapter Overview The Sun’s Interior –16-1: The source of the Sun’s heat and light –16-2: How scientists model the Sun’s internal structure –16-3: How the Sun’s vibrations reveal what lies beneath its glowing surface –16-4: How scientists are able to probe the Sun’s energy-generating core This is the textbook material on which I will focus.

Chapter Overview The Sun’s Interior –16-1: The source of the Sun’s heat and light –16-2: How scientists model the Sun’s internal structure –16-3: How the Sun’s vibrations reveal what lies beneath its glowing surface –16-4: How scientists are able to probe the Sun’s energy-generating core This is the textbook material on which I will focus, but first, let’s skim through the material in sections 16-5 through

16-5: The Sun’s Photosphere Figure 16-7

Granulation of the Photosphere Figure 16-9

Granulation of the Photosphere Figure 16-9 A high-resolution photograph of the Sun’s surface reveals a blotchy pattern called granulation.

Granulation of the Photosphere Figure 16-9 A high-resolution photograph of the Sun’s surface reveals a blotchy pattern called granulation; this is evidence of heat convection (the surface is boiling).

16-8: Sunspots (low-temperature regions in the photosphere) Figure 16-7

16-8: Sunspots (Tracking the Sun’s Rotation) Figure 16-17

16-8: Sunspots (Tracking the Sun’s Rotation) Figure The Sun rotates once in about 4 weeks.

16-8: Sunspots (The Sunspot Cycle) Figure 16-18

16-8: Sunspots (The Sunspot Cycle) Figure The number of sunspots on the Sun varies with a period of about 11 years; most recent maximum in year 2000.

16-6: The Sun’s Chromosphere Figure 16-11

16-7: The Solar Corona (visible light) Figure 16-13

16-7: The Solar Corona (ultraviolet light) Figure 16-15

16-10: Coronal Prominences Figures & 16-28

16-9: Sun’s Magnetic Field The Sun contains a magnetic field with a fairly complex structure “Coronal loops” (see Fig ) and “prominences” often outline the magnetic field’s complex structure Sunspots appear to be associated with regions on the Sun’s surface where the magnetic field is especially strong The north and south magnetic poles of the Sun reverse every 11 years!

16-9: Sun’s Magnetic Field The Sun contains a magnetic field with a fairly complex structure “Coronal loops” (see Fig ) and “prominences” often outline the magnetic field’s complex structure Sunspots appear to be associated with regions on the Sun’s surface where the magnetic field is especially strong The north and south magnetic poles of the Sun reverse every 11 years! NOTE ( see §9-4 ): The Earth’s own magnetic field reverses direction on an irregular schedule ranging from tens of thousands to hundreds of thousands of years!

Relevance to Other Stars… If the Sun is a “typical” star, think about how all these surface phenomena may be relevant to our studies of all other stars –Intrinsic brightness can be variable –Mass of a star may decrease over time –Magnetic fields may be important

Chapter Overview The Sun’s Interior –16-1: The source of the Sun’s heat and light –16-2: How scientists model the Sun’s internal structure –16-3: How the Sun’s vibrations reveal what lies beneath its glowing surface –16-4: How scientists are able to probe the Sun’s energy-generating core This is the textbook material on which I will focus.

Sun’s Internal Structure Figure 16-4

Modeling the Sun’s Interior 1.Hydrostatic Equilibrium 2.Thermal Equilibrium 3.Energy from nuclear fusion (E = mc 2 )

Modeling the Sun’s Interior Hydrostatic Equilibrium –Gas pressure force (directed outward) balances force of gravity (directed inward) throughout the interior –If not balanced, Sun’s structure should change appreciably in a matter of hours!

Modeling the Sun’s Interior Hydrostatic Equilibrium –Gas pressure force (directed outward) balances force of gravity (directed inward) throughout the interior –If not balanced, Sun’s structure should change appreciably in a matter of hours!

Modeling the Sun’s Interior Thermal Equilibrium –Sun is steadily losing energy at its surface (it’s shining!); it is trying to “cool off” –Heat from the Sun’s interior slowly diffuses toward the surface –This lost heat can be replenished by slow gravitational contraction (whenever a gas is compressed, its temperature rises); this is referred to as “Kelvin-Helmholtz contraction” ( see §16-1 ) –If Kelvin-Helmholtz contraction is responsible for keeping the Sun’s interior hot, the Sun’s structure should change appreciably on a time scale of ~ 10 million years

Modeling the Sun’s Interior Thermal Equilibrium –Sun is steadily losing energy at its surface (it’s shining!); it is trying to “cool off” –Heat from the Sun’s interior slowly diffuses toward the surface –This lost heat can be replenished by slow gravitational contraction (whenever a gas is compressed, its temperature rises); this is referred to as “Kelvin-Helmholtz contraction” ( see §16-1 ) –If Kelvin-Helmholtz contraction is responsible for keeping the Sun’s interior hot, the Sun’s structure should change appreciably on a time scale of ~ 10 million years

Modeling the Sun’s Interior Thermal Equilibrium –Sun is steadily losing energy at its surface (it’s shining!); it is trying to “cool off” –Heat from the Sun’s interior slowly diffuses toward the surface –This lost heat can be replenished by slow gravitational contraction (whenever a gas is compressed, its temperature rises); this is referred to as “Kelvin-Helmholtz contraction” ( see §16-1 ) –If Kelvin-Helmholtz contraction is responsible for keeping the Sun’s interior hot, the Sun’s structure should change appreciably on a time scale of ~ 10 million years

Modeling the Sun’s Interior Thermal Equilibrium –Sun is steadily losing energy at its surface (it’s shining!); it is trying to “cool off” –Heat from the Sun’s interior slowly diffuses toward the surface –This lost heat can be replenished by slow gravitational contraction (whenever a gas is compressed, its temperature rises); this is referred to as “Kelvin-Helmholtz contraction” ( see §16-1 ) –If Kelvin-Helmholtz contraction is responsible for keeping the Sun’s interior hot, the Sun’s structure should change appreciably on a time scale of ~ 10 million years

A Problem with Time Scales!