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The Sun and the Stars The Sun and the Stars

The Sun and the Stars - PowerPoint Presentation

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The Sun and the Stars - PPT Presentation

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

sun stars surface field stars sun field surface lines photosphere magnetic layer coronal zone solar convection corona unit 3he star temperature gas

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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+ +2e + 2 MHe = 4 MH x 0.993 (0.7% lost)2e+ + 2e-  4 E = m c22e 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 cellsSlide20
Slide21

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 surfaceSlide24
Slide25

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