LIFECYCLES OF STARS

M.R. Burleigh 2601/Unit 3

DEPARTMENT OF PHYSICS AND ASTRONOMY

LIFECYCLES OF STARSLIFECYCLES OF STARS

Option 2601Option 2601

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

Unit 3Unit 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

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

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

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

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

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

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…

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

Limb darkeningLimb darkening

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:

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 ¯̄

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)

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

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)

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

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

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

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

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

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

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

Solar Corona Solar Corona

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

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

Solar Corona Solar Corona

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

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

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

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)

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

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

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

Sunspot CycleSunspot Cycle

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)

Active Regions Active Regions

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

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

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)

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

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

Next lecturesNext lectures

Tuesday 8Tuesday 8thth March March 10am and 1pm10am and 1pm LRBLRB No lecture Monday 7No lecture Monday 7thth March March

Unit 3Unit 3

The SunThe Sun

LIFECYCLES OF STARSLIFECYCLES OF STARS

Option 2601Option 2601

LIFECYCLES OF STARS

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