Transcript Lecture 1 - X-ray and Observational Astronomy Group

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
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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
The Sun and the Stars
Reference material
• Astronomy and Astrophysics (Zeilik and Gregory),
4th edition
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
slow – involves weak interaction
5x109
yrs
1H
+ 1H 2H + e+ + e +0.42 MeV
e- + e+ 2 + 1.02MeV
1s
3x105 yrs
2H
+
3He
1H
3He
+3He
x2
+ +5.49MeV
4He
+
21H

26.7 MeV
+12.86MeV
(PPI)
Net result
41H 4He + 2e+ +2e + 2
2e+ + 2e-  4
MHe = 4 MH x 0.993 (0.7% lost)
E = m c2
2e fly out with no interaction (very,very small x-section for interaction)
6  undergo multiple scatterings, reach photosphere after 105 years
Dr Matt Burleigh
The Sun and the Stars
proton-proton chain
1H
+ 1H 2H + e+ + e
2H
+ 1H 3He + 
(cycle ends here for low-mass stars!)
69%
3He
31%
+ 3He  4He + 21H
3He
+ 4He 
7Be
+
(106 yrs)
(PPI, T~1.0-1.4x107 K)
99.7%
(140 days)
7Be
+ e-  7Li +e
(10 mins)
7Li
+ 1H  24He
(PPII, T~1.4-2.3x107K)
0.3%
7Be
+ 1H  8B + 
8B
8Be
(66 years)
 8Be + e+ + e (0.9secs)
 24He
(9.7x10-17 secs)
(PPIII, T>2.3x107K)
Dr Matt Burleigh
The Sun and the Stars
Structure
Corona
(T~2x106 K)
Photosphere
(T~5800K)
Convection zone
(T~2x106 K)
Chromosphere
Thermonuclear Core
(T~1.5x107K)
Radiative zone
(T~107 K)
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
The Sun and the Stars
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.
sunspot
granules
umbra
Penumbra
-
Strongly magnetic
regions 2000K
cooler than rest of
surface.
Faculae
- Bright areas of
concentrated
magnetic field
each granule is ~ 1000 km across
Granules are transient
Granules are tops of convection cells
Dr Matt Burleigh
Dr Matt Burleigh
The Sun and the Stars
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.
H observations of the sun
Plages –
bright patches
in H light
surrounding
sunspots
Filaments
- Dark areas
chromospheric network
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 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.
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
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.
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
Dr Matt Burleigh