LIFECYCLES OF STARS
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Transcript of LIFECYCLES OF STARS
M.R. Burleigh 2601/Unit 3
DEPARTMENT OF PHYSICS AND ASTRONOMY
LIFECYCLES OF STARSLIFECYCLES OF STARS
Option 2601Option 2601
M.R. Burleigh 2601/Unit 3
Stellar PhysicsStellar Physics
Observational properties of starsObservational properties of stars Stellar SpectraStellar Spectra The SunThe Sun Stellar StructureStellar Structure Stellar EvolutionStellar Evolution Stars of particular interestStars of particular interest
M.R. Burleigh 2601/Unit 3
DEPARTMENT OF PHYSICS AND ASTRONOMY
Unit 3Unit 3
The SunThe Sun
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The SunThe Sun
Basic physical parametersBasic physical parameters Structure of interior and atmosphereStructure of interior and atmosphere Surface featuresSurface features Magnetic fieldMagnetic field Solar activity, flares and pulsationsSolar activity, flares and pulsations Relationship to other starsRelationship to other stars
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Our nearest starOur nearest star
Nearest, therefore studied in most detailNearest, therefore studied in most detail Standard against which other stars are Standard against which other stars are
comparedcompared– Radius = 6.96x10Radius = 6.96x1055 km (~109R km (~109R))
– Mass = 1.99x10Mass = 1.99x103030 kg (~333,000M kg (~333,000M))
– Luminosity = 3.86x10Luminosity = 3.86x102626 W W– Spectral type/luminosity class = G2 VSpectral type/luminosity class = G2 V
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StructureStructure
Core – region of nuclear burningCore – region of nuclear burning Radiative zoneRadiative zone Convection zoneConvection zone PhotospherePhotosphere ChromosphereChromosphere CoronaCorona
Only regions directly observableOnly regions directly observable
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Solar interiorSolar interior
HelioseismologyHelioseismology Solar oscillations used to study the Solar oscillations used to study the
structure similar to seismology on Earthstructure similar to seismology on Earth Periods range from 5 min to 2h 40minPeriods range from 5 min to 2h 40min
– Detected by periodic changes in Doppler Detected by periodic changes in Doppler shifts of spectral linesshifts of spectral lines
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PhotospherePhotosphere
GranulationGranulation Base of photosphere is deepest Base of photosphere is deepest
region observable region observable Patchwork of granulesPatchwork of granules
– d~700 kmd~700 km– Transient (5-10mins)Transient (5-10mins)– Bright irregular formations Bright irregular formations
surrounded by darker lanessurrounded by darker lanes Top layer of convection zoneTop layer of convection zone
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PhotospherePhotosphere
Bright regions are rising hot gas Bright regions are rising hot gas (convection cells)(convection cells)
Dark regions are falling cooler gasDark regions are falling cooler gas
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SunspotsSunspots
Cooler regions (appear Cooler regions (appear darker) than surrounding darker) than surrounding photospherephotosphere
Temperatures ~3800K cf. Temperatures ~3800K cf. 5800K elsewhere5800K elsewhere
Associated with high Associated with high magnetic fieldsmagnetic fields
More later…More later…
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M.R. Burleigh 2601/Unit 3
Limb darkeningLimb darkening
Brightness of solar disk decreases from Brightness of solar disk decreases from centre to limb (edge of disk)centre to limb (edge of disk)
Arises because we see deeper hotter Arises because we see deeper hotter gas at centre, cooler layers at limbgas at centre, cooler layers at limb
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M.R. Burleigh 2601/Unit 3
Limb darkeningLimb darkening
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In a slab of thickness dx, density In a slab of thickness dx, density , fraction of , fraction of flux flux FF absorbed is: absorbed is:
dxFdF
xFxF exp0
Optical depth:Optical depth: dxd
Absorption in photosphereAbsorption in photosphere
Units of opacity Units of opacity κ are mκ are m22/kg (or cm/kg (or cm22/g)/g)
Main source of opacity in solar photosphere is HMain source of opacity in solar photosphere is H ¯̄
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Absorption linesAbsorption lines
Discussed in detail in last UnitDiscussed in detail in last Unit First mapped by Fraunhoffer (1787-First mapped by Fraunhoffer (1787-
1826)1826)
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Absorption linesAbsorption lines
Alphabetic designation… capital letters Alphabetic designation… capital letters for strong, lower case for weak linesfor strong, lower case for weak lines
Hence… Na D lines, CaII H & K, Mg bHence… Na D lines, CaII H & K, Mg b
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ChromosphereChromosphere
Spectrum contains emission features Spectrum contains emission features from highly excited/ionized species (e.g. from highly excited/ionized species (e.g. H Balmer, HeII)H Balmer, HeII)
High temperatures (see last lecture)High temperatures (see last lecture)
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ChromosphereChromosphere
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ChromosphereChromosphere
Photospheric continuum absorbed by Photospheric continuum absorbed by Chromospheric gasChromospheric gas
Absorption lines projected against solar Absorption lines projected against solar diskdisk
Emission lines seen against dark spaceEmission lines seen against dark space
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Chromospheric fine structureChromospheric fine structure
Some absorption lines have large Some absorption lines have large optical depth (e.g. Hoptical depth (e.g. H, CaII H & K), CaII H & K)
Monochromatic photos show large Monochromatic photos show large bright and dark patches… bright and dark patches… plagesplages and and filamentsfilaments
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Chromospheric fine structureChromospheric fine structure
Structure appears over whole diskStructure appears over whole disk Bright Bright networknetwork associated with associated with
magnetic fields at boundaries of magnetic fields at boundaries of supergranulessupergranules
Brightening (i.e. less absorption) of CaII Brightening (i.e. less absorption) of CaII K K increasing magnetic field strength increasing magnetic field strength
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Chromospheric fine structureChromospheric fine structure
See See spiculesspicules at the limb, jets of glowing at the limb, jets of glowing gas emerging at 20-25km/sgas emerging at 20-25km/s– 500-1500km across, 10000km high500-1500km across, 10000km high
Form a network following supergranule Form a network following supergranule boundariesboundaries
Probably play a significant role in mass Probably play a significant role in mass transport… chromosphere transport… chromosphere corona corona windwind
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Transition RegionTransition Region
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Transition RegionTransition Region
Gives rise to UV spectral featuresGives rise to UV spectral features– e.g. Lyman e.g. Lyman , CIII, NIII, OVI, CIII, NIII, OVI
Network continues through this regionNetwork continues through this region Disappears at ~1.6x10Disappears at ~1.6x1066K in MgX K in MgX
imagesimages
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Solar CoronaSolar Corona
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Solar CoronaSolar Corona
In visible light…In visible light… K Corona – dominates near the SunK Corona – dominates near the Sun
– Light scattered by (1-2)x10Light scattered by (1-2)x1066K electronsK electrons– Strongly affected by solar activityStrongly affected by solar activity
F Corona – visible at a few solar radiiF Corona – visible at a few solar radii– Light scattered from dustLight scattered from dust
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Solar CoronaSolar Corona
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Solar CoronaSolar Corona
Radio Corona: arises from free-free Radio Corona: arises from free-free transitions of free electrons & transitions of free electrons & atoms/ionsatoms/ions
Line emission: “forbidden” lines due to Line emission: “forbidden” lines due to high temperature & low densityhigh temperature & low density
EUV lines: e.g. FeVIII-XVI… high EUV lines: e.g. FeVIII-XVI… high ionization statesionization states
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Solar Corona Solar Corona
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Coronal Loops & HolesCoronal Loops & Holes
Coronal gas hot enough to emit low Coronal gas hot enough to emit low energy X-raysenergy X-rays
X-ray images show irregular gas X-ray images show irregular gas distributiondistribution
Large loop structures Large loop structures hot gas trapped hot gas trapped in magnetic loopsin magnetic loops
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Coronal Loops & HolesCoronal Loops & Holes
Dark regions (gas less hot and dense) Dark regions (gas less hot and dense) coronal holes coronal holes
Holes correspond to magnetic field lines Holes correspond to magnetic field lines that do not reconnect with the surfacethat do not reconnect with the surface
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Solar Corona Solar Corona
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Solar WindSolar Wind
Solar gravity is insufficient to retain high Solar gravity is insufficient to retain high temperature coronal gastemperature coronal gas
Gas is a plasma (ionized but electrically Gas is a plasma (ionized but electrically neutral on a large scale)neutral on a large scale)
Thermal conductivity high Thermal conductivity high high T high T prevails out to large distancesprevails out to large distances
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Solar WindSolar Wind
Wind accelerates as it expandsWind accelerates as it expands– 300km/s at 30R 300km/s at 30R 400km/s at 1 AU 400km/s at 1 AU
Proton/electron energy ~10Proton/electron energy ~1033eVeV Density at Earth ~(0.4-8.0)x10Density at Earth ~(0.4-8.0)x1066mm-3-3
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Solar WindSolar Wind
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Solar ActivitySolar Activity
We can easily observe transient We can easily observe transient phenomenaphenomena
These are manifestations of These are manifestations of Solar Solar ActivityActivity
Linked through solar rotation and Linked through solar rotation and magnetic fieldmagnetic field
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SunspotsSunspots
Field strengths (deduced from Zeeman effect) Field strengths (deduced from Zeeman effect) ~0.1T (up to 0.4T)~0.1T (up to 0.4T)
Fields may inhibit convective energy transportFields may inhibit convective energy transport Any given spot has an associated magnetic Any given spot has an associated magnetic
polaritypolarity May be paired with spot of opposite polarity May be paired with spot of opposite polarity
(or diffuse region no observed as a spot)(or diffuse region no observed as a spot)
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SunspotsSunspots
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SunspotsSunspots
Can measure solar rotation rate by Can measure solar rotation rate by following spotsfollowing spots
PPequatorequator~25d~25d
PP4040oo~27d~27d
PP7070oo~30d~30d
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SunspotsSunspots
Spot numbers vary with an 11 year Spot numbers vary with an 11 year cycle (except Maunder Minimum)cycle (except Maunder Minimum)
Spot latitudes vary during cycleSpot latitudes vary during cycle Spot lifetimes days to monthsSpot lifetimes days to months
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SunspotsSunspots
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SunspotsSunspots
Bipolar spot pair – preceding spot Bipolar spot pair – preceding spot always has same polarity through cyclealways has same polarity through cycle
Polarities are opposite in each Polarities are opposite in each hemispherehemisphere
Reverse at end of 11yr cycle – overall Reverse at end of 11yr cycle – overall 22 year cycle22 year cycle
Spots always follow constant latitudeSpots always follow constant latitude
M.R. Burleigh 2601/Unit 3
M.R. Burleigh 2601/Unit 3
Sunspot CycleSunspot Cycle
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Active RegionsActive Regions
As spot numbers increase so does solar As spot numbers increase so does solar activityactivity
Each sunspot group is associated with Each sunspot group is associated with an an active regionactive region several x 10 several x 1055km acrosskm across
Magnetic activity is concentrated in Magnetic activity is concentrated in thesethese
Usually bipolar (Usually bipolar (Bipolar Magnetic Bipolar Magnetic RegionsRegions – BMRs) – BMRs)
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Active Regions Active Regions
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Active RegionsActive Regions
Bright areas associated with BMRs in Bright areas associated with BMRs in various zonesvarious zones
In photosphere – In photosphere – faculaefaculae Chromosphere – Chromosphere – plagesplages Corona – Corona – streamersstreamers
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ProminencesProminences
Streams of chromospheric gas – dark Streams of chromospheric gas – dark when viewed against diskwhen viewed against disk
QuiescentQuiescent– Long lived (weeks) curtain-like gas along Long lived (weeks) curtain-like gas along
neutral line separating poles of BMRneutral line separating poles of BMR
ActiveActive– Few hours – loops closely associated with Few hours – loops closely associated with
solar flaressolar flares
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ProminencesProminences
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ProminencesProminences
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Solar FlaresSolar Flares
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Solar FlaresSolar Flares
Transient outbursts – probably originate Transient outbursts – probably originate by magnetic reconnectionby magnetic reconnection
Radiate from radio to Radiate from radio to -rays-rays Emit high energy particles (solar cosmic Emit high energy particles (solar cosmic
rays)rays)
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Solar FlaresSolar Flares
10000-30000km in size10000-30000km in size Brighten within 5 minBrighten within 5 min Decay in ~20min (up to 3hrs for largest)Decay in ~20min (up to 3hrs for largest) The larger the flare, more energetic and The larger the flare, more energetic and
longer livedlonger lived At solar max – small flares hourly, large At solar max – small flares hourly, large
flares monthlyflares monthly
M.R. Burleigh 2601/Unit 3
M.R. Burleigh 2601/Unit 3
M.R. Burleigh 2601/Unit 3
The SunThe Sun
Basic physical parametersBasic physical parameters Structure of interior and atmosphereStructure of interior and atmosphere Surface featuresSurface features Magnetic fieldMagnetic field Solar activity, flares and pulsationsSolar activity, flares and pulsations Relationship to other starsRelationship to other stars
M.R. Burleigh 2601/Unit 3
Next lecturesNext lectures
Tuesday 8Tuesday 8thth March March 10am and 1pm10am and 1pm LRBLRB No lecture Monday 7No lecture Monday 7thth March March
M.R. Burleigh 2601/Unit 3
DEPARTMENT OF PHYSICS AND ASTRONOMY
Unit 3Unit 3
The SunThe Sun
M.R. Burleigh 2601/Unit 3
DEPARTMENT OF PHYSICS AND ASTRONOMY
LIFECYCLES OF STARSLIFECYCLES OF STARS
Option 2601Option 2601