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The Sun

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The Sun Our Star Chapter 8 Guidepost Summary of General Properties The Sun: applying black-body radiation laws What we want to know: Internal structure and ... – PowerPoint PPT presentation

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Title: The Sun


1
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2
The Sun Our Star
  • Chapter 8

3
Guidepost
The preceding chapter described how we can get
information from a spectrum. In this chapter, we
apply these techniques to the sun, to learn about
its complexities. This chapter gives us our
first close look at how scientists work, how they
use evidence and hypothesis to understand nature.
Here we will follow carefully developed logical
arguments to understand our sun. Most important,
this chapter gives us our first detailed look at
a star. The chapters that follow will discuss the
many kinds of stars that fill the heavens, but
this chapter shows us that each of them is both
complex and beautiful each is a sun.
4
Summary of General Properties
  • Average star
  • Spectral type G2
  • Only appears so bright because it is so close.
  • Absolute visual magnitude 4.83 (magnitude if
    it were at a distance of 10 pc or 32.6
    light years)
  • 109 times Earths diameter
  • 333,000 times Earths mass
  • Consists entirely of gas (av. density 1.4
    g/cm3)
  • Central temperature 15 million 0K
  • Surface temperature 5800 0K

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The Sun applying black-body radiation laws
What we see
Radius R 700,000 km Distance 1 AU 1.5x108
km Mass M 2x1030 kg
Yellow light ? 520 nm (G2) Maximum of the
black-body spectrum (Wiens law)
Surface temperature T ?5800 K (Class G2)
The Stefan-Boltzmann law
Total radiated power (luminosity) L ?T4 4?R2 ?
4?1026 W
Solar flux at Earth 1360 W/m2
7
What we want to know
  • Internal structure and composition
  • Source of energy
  • Lifetime
  • Origin of Suns activity and variability

8
The Composition of Stars
From the relative strength of absorption lines
one can infer the composition of stars.
9
Stars are gaseous spheres held together by
gravity
Mean density
Higher than the density of water!
Central density is 160 g/cm3 15 times higher
than the density of lead
Still, stars are gaseous due to very high
temperatures
Central temperature Tc ? 1.5?107 K
Matter is in the state of plasma (ionized gas)
Kinetic energy of particles is much larger than
their potential energy
10
What defines an internal structure?
Central temperature Tc ? 1.5?107 K
Surface temperature Tc ? 5800 K
Energy generated in the suns center must be
transported outward.
Heat transfer from the center to the surface!
Heat transfer determines the internal structure
11
Heat transfer mechanisms
  • Conduction
  • Convection
  • Radiation

12
Convection
13
Conduction, Convection, and Radiation
(SLIDESHOW MODE ONLY)
14
Internal Structure
Energy transport via convection
Sun
Energy transport via radiation
Flow of energy
Energy generation via nuclear fusion
Basically the same structure for all stars with
approx. 1 solar mass or less.
Temperature, density and pressure decreasing
15
Energy Transport in the Sun-like stars
Energy generated in the stars center must be
transported to the surface.
Outer layers (including photosphere) Convection
Inner layers Radiative energy transport
Bubbles of hot gas rising up
Cool gas sinking down
Gas particles of solar interior
g-rays
16
Convection
Convection is the most efficient way to transport
heat
Cool gas sinking down
Bubbles of hot gas rising up
1000 km
Bubbles last for 10 20 min.
17
Granulation
is the visible consequence of convection
18
The solar atmosphere
It takes 10,000 years for a photon emitted in the
core to reach the surface!
19
The Solar Atmosphere
Heat Flow
Temp. incr. inward
Solar interior
20
The Photosphere
  • Apparent surface layer of the sun
  • Depth 500 km
  • Temperature 5800 oK
  • Absorbs and re-emits radiation produced in the
    solar interior

The solar corona
21
The Chromosphere
  • Region of suns atmosphere just above the
    photosphere.
  • Visible, UV, and X-ray lines from highly ionized
    gases
  • Temperature increases gradually from 4500 oK
    to 10,000 oK, then jumps to 1 million oK

Filaments
Transition region
Chromospheric structures visible in Ha emission
(filtergram)
22
The Chromosphere (2)
Spicules Filaments of cooler gas from the
photosphere, rising up into the chromosphere.
Visible in Ha emission.
Each one lasting about 5 15 min.
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The Magnetic Carpet of the Corona
  • Corona contains very low-density, very hot (1
    million oK) gas
  • Coronal gas is heated through motions of
    magnetic fields anchored in the photosphere below
    (magnetic carpet). Precise mechanism is unknown.

Computer model of the magnetic carpet
25
Changing Face of the Sun
Solar Activity, seen in soft X-rays
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Sun Spots
Cooler regions of the photosphere (T 4200 K).
Only appear dark against the bright sun. Would
still be brighter than the full moon when placed
on the night sky!
28
Sun Spots (2)
Active Regions
Visible
Ultraviolet
29
Sun Spots (3)
Magnetic field in sun spots is about 1000 times
stronger than average.
Magnetic North Poles
Magnetic South Poles
? Sun Spots are related to magnetic activity on
the photosphere
30
Magnetic Field Lines
Magnetic North Pole
Magnetic South Pole
Magnetic Field Lines
31
Magnetic Loops
Magnetic field lines
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33
Zeeman effect
34
The Suns Magnetic Dynamo
The sun rotates faster at the equator than near
the poles.
This differential rotation might be responsible
for magnetic activity of the sun.
35
The Suns Magnetic Cycle
After 11 years, the magnetic field pattern
becomes so complex that the field structure is
re-arranged.
? New magnetic field structure is similar to the
original one, but reversed!
? New 11-year cycle starts with reversed
magnetic-field orientation
36
11-year period of solar activity
37
The Solar Cycle
After 11 years, North/South order of
leading/trailing sun spots is reversed
11-year cycle
Reversal of magnetic polarity
gt Total solar cycle 22 years
38
The Maunder Minimum
The sun spot number also fluctuates on much
longer time scales
Historical data indicate a very quiet phase of
the sun, 1650 1700 The Maunder Minimum
39
The Little Ice Age unusually cold winters in the
XVII-XVIII cen.
40
Prominences
Relatively cool gas (60,000 80,000 oK)
May be seen as dark filaments against the bright
background of the photosphere
Looped Prominences gas ejected from the suns
photosphere, flowing along magnetic loops
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43
Eruptive Prominences
(Ultraviolet images)
Extreme events (solar flares) can significantly
influence Earths magnetic field structure and
cause northern lights (aurora borealis).
44
Solar Flares
5 minutes
Sound waves produced by a solar flare
45
Coronal Holes
X-ray images of the sun reveal coronal holes.
These arise at the foot points of open field
lines and are the origin of the solar wind.
46
Coronal holes
47
The Solar Wind
Constant flow of particles from the sun.
Velocity 300 800 km/s
  • Sun is constantly losing mass
  • 107 tons/year
  • ( 10-14 of its mass per year)

48
Solar wind creates a hot, rarefied plasma bubble
in space
49
Solar wind hits the Earths magnetosphere
50
Coronal mass ejections
51
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53
Aurora
54
Summary solar activity
  • It is driven by the Suns magnetic field
  • The magnetic field is generated by the
    differential rotation of the Sun
  • Active regions are associated with sunspots that
    carry a strong magnetic field ( 1000 G)
  • Prominence is a solar plasma trapped in magnetic
    field arches above the active region
  • Violent eruptive phenomena solar flares
    (equivalent to billions of H-bombs!) and coronal
    mass ejections
  • Ejected plasma shakes the Earths magnetic field
    and causes the magnetic storm
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