Dr Matt Burleigh The Sun and the Stars The Sun and the Stars
Dr Matt Burleigh The Sun and the Stars. Dr Matt Burleigh Stellar Lifecycle.
Dec 13, 2015
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Dr Matt Burleigh
The Sun and the Stars
The Sun and the Stars
Dr Matt Burleigh
Stellar Lifecycle
Dr Matt Burleigh
Starbirth
Dr Matt Burleigh
Young Stars
Dr Matt Burleigh
Globular Clusters
Dr Matt Burleigh
Star Death
Dr Matt Burleigh
Star Death
Dr Matt Burleigh
Star Death
Dr Matt Burleigh
Star Death
Dr Matt Burleigh
Aims and Objectives
To 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
Dr Matt Burleigh
Lifecycles of Stars
Unit 1 – The Sun Structure,limb darkening, magnetic field & solar cycle
Unit 2 – Stars Classification from spectra, measuring distances, masses and
radii, the Hertzsprung-Russell diagram
Unit 3 – Stellar evolution The life cycles of stars of different masses
Unit 4 - Stellar Structure Energy generation, hydrostatic equilibrium
Unit 5 – Stars of special interest Binary stars, pulsating stars, stellar remnants
Dr Matt Burleigh
Reference material
• Astronomy and Astrophysics (Zeilik and Gregory), 4th edition
The Sun and the Stars
Dr Matt Burleigh
The Sun and the Stars
Sun: centre of our solar system. Our nearest star (closer by a factor of 2x105), therefore the best understood.
Vital statistics:
Distance 150x106 km = 1 AU (astronomical unit) = 8 light-minutes
Radius 6.95x105km = 1R⊙
Apparent diameter ½ degree
Mass 1.99x1030 kg = 1 M⊙
Luminosity 3.8x1026 W (J/s) = 3.8x1033 erg/s = 1 L⊙
Effective temperature Teff = 5770 K
Composition (by mass) 74% Hydrogen, 24% Helium, 2% other elements (metals) Age ~ 4.5x109 years
Dr Matt Burleigh
The Sun and the Stars
Energy generation (solar): Not chemical, gravitational (107 years) X
Nuclear Fusion : proton-proton chain
1H + 1H 2H + e+ + e +0.42 MeV
e- + e+ 2 + 1.02MeV
2H + 1H 3He + +5.49MeV
3He +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 c2
2e 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
3x105 yrs
Dr Matt Burleigh
1H + 1H 2H + e+ + e
2H + 1H 3He + (cycle ends here for low-mass stars!)
69% 31%
3He + 3He 4He + 21H 3He + 4He 7Be + (106 yrs)
(PPI, T~1.0-1.4x107 K)
(140 days) 7Be + e- 7Li +e 7Be + 1H 8B + (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 chain
Dr Matt Burleigh
The Sun and the Stars
Structure
Corona(T~2x106 K)
Photosphere(T~5800K)
Convection zone(T~2x106 K)
Radiative zone(T~107 K)
Thermonuclear Core(T~1.5x107K)
Chromosphere
Dr Matt Burleigh
The Sun and the Stars
Core :Sight of thermonuclear reactions, extends out to 0.25R⊙
T~1.5x107 K, ~150 g/cm3 (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 105 years to reach interface layer. Density falls from 20 g/cm3 to 0.2 g/cm3, 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.
Dr Matt Burleigh
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/cm3.
Dr Matt Burleigh
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
umbraPenumbra-Strongly magnetic regions 2000K cooler than rest of surface.
Faculae- Bright areas of concentrated magnetic field
granules
each granule is ~ 1000 km acrossGranules are transientGranules are tops of convection cells
Dr Matt Burleigh
Dr Matt Burleigh
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.
Dr Matt Burleigh
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~106K – 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 regionCIV emission.
Dr Matt Burleigh
The Sun and the Stars
Corona:Suns outer atmosphere, visible only during total eclipse or with use of acoronographic disc (an opaque disc which blocks the light from the photosphere).T> 1x106 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-raysX-ray images show irregular gas distributionLarge loop structures hot gas trapped in magnetic loopsDark regions (gas less hot and dense) coronal holesHoles correspond to magnetic field lines that do not reconnect with the surface
Dr Matt Burleigh
Dr Matt Burleigh
The Sun and the Stars
Solar wind :
Solar gravity is insufficient to retain high temperature coronal gasGas 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 Msun/yr
Wind accelerates as it expands: 300km/s at 30Rsun 400km/s at 1 AUProton/electron energy ~103eVDensity at Earth ~(0.4-8.0)x106m-3
Dr Matt Burleigh
The Sun and the Stars
Putting it all together
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