The Sun and the Stars Stellar Lifecycle Starbirth Young Stars Globular Clusters Star Death Star Death Star Death Star Death Aims and Objectives To introduce you to the properties of t he Sun and the stars and their evolution ID: 533203
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Slide1
The Sun and the Stars
The Sun and the StarsSlide2
Stellar LifecycleSlide3
StarbirthSlide4
Young StarsSlide5
Globular ClustersSlide6
Star DeathSlide7
Star DeathSlide8
Star DeathSlide9
Star DeathSlide10
Aims and ObjectivesTo introduce you to the properties of the Sun and the stars, and their evolution
This will be fairly descriptive: in the second and third year you will learn the detailed physics underlying our understanding Slide11
Lifecycles of StarsUnit 1 – The Sun
Structure,limb darkening, magnetic field & solar cycleUnit 2 – StarsClassification from spectra, measuring distances, masses and radii, the Hertzsprung-Russell diagramUnit 3 – Stellar evolutionThe life cycles of stars of different massesUnit 4 - Stellar StructureEnergy generation, hydrostatic equilibriumUnit 5 – Stars of special interestBinary stars, pulsating stars, stellar remnantsSlide12
Reference material
Astronomy and Astrophysics (Zeilik and Gregory), 4th edition
The Sun and the StarsSlide13
The Sun and the Stars
Sun:
centre of our solar system. Our nearest star (closer by a factor of 2x10
5
),
therefore the best understood.
Vital statistics
:
Distance
150x10
6
km = 1 AU (astronomical unit) = 8 light-minutes
Radius
6.95x10
5
km = 1R
⊙
Apparent diameter
½ degree
Mass
1.99x10
30
kg = 1 M
⊙
Luminosity
3.8x10
26
W (J/s) = 3.8x10
33
erg/s = 1 L
⊙
Effective temperature
T
eff
= 5770 K
Composition
(by mass) 74% Hydrogen, 24% Helium, 2% other elements (metals)
Age
~ 4.5x10
9
yearsSlide14
The Sun and the Stars
Energy generation
(solar):
Not chemical, gravitational (10
7
years)
X
Nuclear Fusion
:
proton-proton chain
1
H +
1
H
2
H + e
+
+
e
+0.42 MeV
e
-
+ e
+
2 + 1.02MeV 2H + 1H 3He + +5.49MeV3He +3He 4He + 21H +12.86MeV (PPI)Net result 41H 4He + 2e+ +2e + 2 MHe = 4 MH x 0.993 (0.7% lost)2e+ + 2e- 4 E = m c22e fly out with no interaction (very,very small x-section for interaction)6 undergo multiple scatterings, reach photosphere after 105 years
x2
26.7 MeV
slow – involves weak interaction
5x109 yrs
1 s
3x10
5
yrsSlide15
1H + 1H
2H + e+ + e 2H + 1H 3He + (cycle ends here for low-mass stars!) 69% 31% 3He + 3He 4He + 21H 3He + 4He 7Be + (10
6
yrs)
(PPI, T~1.0-1.4x10
7
K)
(140 days)
7
Be + e
-
7
Li +
e
7
Be +
1
H
8
B +
(66 years)
(10 mins) 7Li + 1H 24He 8B 8Be + e+ + e (0.9secs) (PPII, T~1.4-2.3x107K) 8Be 24He (9.7x10-17 secs) (PPIII, T>2.3x107K) The Sun and the Stars
99.7%
0.3%
proton-proton chainSlide16
The Sun and the Stars
Structure
Corona
(T~2x10
6
K)
Photosphere
(T~5800K)
Convection zone
(T~2x10
6
K)
Radiative zone
(T~10
7
K)
Thermonuclear Core
(T~1.5x10
7
K)
ChromosphereSlide17
The Sun and the Stars
Core
:
Sight of thermonuclear reactions, extends out to 0.25R
⊙
T~1.5x10
7
K,
~150 g/cm
3
(10x
gold
).
Nuclear burning almost completely shut off beyond
0.25R
⊙
Radiative zone
:
Extends from 0.25-0.7R
⊙
(the tachocline or interface layer).
Energy transported by radiation.
Photons scatter many times, take approx 10
5
years to reach interface layer.
Density falls from 20 g/cm
3
to 0.2 g/cm
3
, T falls from 7x106 K to 2x106 K over same distance.Tachocline (interface layer) :Lies between radiative zone and convection zone. Thought to be site of magnetic field generation. Shearing (ie. changes in fluid velocities) in this layer can stretch and enhance magnetic field lines.Slide18
Convection zone
: 0.7R⊙ – to photosphere (visible surface). At base, T~2x106K , cool enough for heavier elements (e.g. C,N,O,Ca, Fe) to hang onto some of their electrons.
Increased opacity, makes it harder for photons to get through. Trapped heat makes the fluid unstable and it starts to convect (boil).
Convection carries heat rapidly to surface.
Material expands and cools as it rises.
Temperature at surface (photosphere) ~ 5770 K,
~2x10
-7
g/cm
3.Slide19
Photosphere: The visible surface of the sun. Layer ~ 100 km thick at top of convection zone. Surface has non-uniform intensity (limb darkened).
Photosphere exhibits a number of surface features, dark sunspots, bright faculae, and granules.
The Sun and the Stars
sunspot
umbra
Penumbra
Strongly magnetic regions 2000K cooler than rest of surface.
Faculae
-
Bright areas of concentrated magnetic field
granules
each granule is ~ 1000 km across
Granules are transient
Granules are tops of convection cellsSlide20Slide21
Chromosphere
:Irregular layer above photosphere, T~20,000 K. At this temperature, gas emits strongly in H (6563 A, 1A=10-10m). When observed through H filter, sun displays new features, the chromospheric network, filaments, plages, prominences and spicules.
The Sun and the Stars
H
observations of the sun
Plages –
bright patches in H
light surrounding sunspots
chromospheric network
Filaments
-
Dark areas in H
light. Cool, dense clouds suspended above surface by magnetic field.
Prominences
-
Filaments seen at Sun’s limb
Spicules
-
small, jet like eruptions seen as dark streaks in H
light. Last a few minutes. Slide22
The Sun and the Stars
Transition region:
Separates hot corona from chromosphere.
Heat flows from corona into chromosphere, producing thin layer where temperature changes very rapidly (T~10
6
K – 20,000K).
Hydrogen is completely ionised, emission dominated by highly ionised lines of C, O and Si (CIV,OIV,SiIV).
These are ultraviolet lines and can only be seen from space by satellite observatories.
SOHO (Solar Heliospheric Observatory)
observation of the transition region
CIV emission.Slide23
The Sun and the Stars
Corona
:
Suns outer atmosphere, visible only during total eclipse or with use of a
coronographic disc (an opaque disc which blocks the light from the photosphere).
T> 1x10
6
K. Elements H, He, C,N,O completely stripped. Emission dominated by heavy trace elements e.g. Ca and Fe. The corona displays a number of features including; streamers, plumes, coronal loops and holes.
Plume –
streamers at poles associated with open field lines
coronal loop –
associated with closed field lines between sunspots
coronal hole –
associated with open field lines. Seen in Xrays.
Coronal gas hot enough to emit low energy X-rays
X-ray images show irregular gas distribution
Large loop structures
hot gas trapped in magnetic loops
Dark regions (gas less hot and dense) coronal holes
Holes correspond to magnetic field lines that do not reconnect with the surfaceSlide24Slide25
The Sun and the Stars
Solar wind :
Solar gravity is insufficient to retain high temperature coronal gas
Gas is a plasma (ionized but electrically neutral on a large scale)
Material from outer corona blows off into space , usually along open field-lines
(e.g. coronal holes), but also following solar flares.
Mass loss ~10
-13
M
sun
/yr
Wind accelerates as it expands: 300km/s at 30R
sun
400km/s at 1 AU
Proton/electron energy ~10
3
eV
Density at Earth ~(0.4-8.0)x10
6
m
-3Slide26
The Sun and the Stars
Putting it all together