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+ + νe +0.42 MeV e- + e+ →2γ + 1.02MeV 2H + 1H →3He +γ +5.49MeV

3He +3He →4He + 21H +12.86MeV (PPI) Net result 41H →4He + 2e+ +2νe + 2γ MHe = 4 MH x 0.993 (0.7% lost) 2e+ + 2e- → 4γ E = Δm c2

2νe fly out with no interaction (very,very small x-section for interaction) 6 γ 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+ + νe

2H + 1H →3He + γ (cycle ends here for low-mass stars!)

69% 31%

3He + 3He → 4He + 21H 3He + 4He → 7Be + γ (106 yrs)

(PPI, T~1.0-1.4x107 K)

(140 days) 7Be + e- → 7Li +νe 7Be + 1H → 8B + γ (66 years)

(10 mins) 7Li + 1H → 24He 8B → 8Be + e+ + νe (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, ρ~150 g/cm3 (≈10x ρgold). 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, ρ~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

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 cells

Dr Matt Burleigh

Dr Matt Burleigh

Chromosphere: Irregular layer above photosphere, T~20,000 K. At this temperature, gas emits strongly in Hα (6563 A, 1A=10-10m). When observed through Hα filter, sun displays new features, the chromospheric network, filaments, plages, prominences and spicules.

The Sun and the Stars

Hα observations of the sun

Plages – bright patches in Hα light surrounding sunspots

chromospheric network

Filaments - Dark areas in Hα 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 Hα 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 region CIV emission.

Dr Matt Burleigh

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> 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-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 surface

Dr Matt Burleigh

Dr Matt Burleigh

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 Msun/yr Wind accelerates as it expands: 300km/s at 30Rsun ⇒ 400km/s at 1 AU Proton/electron energy ~103eV Density at Earth ~(0.4-8.0)x106m-3

Dr Matt Burleigh

The Sun and the Stars

Putting it all together