M.R. Burleigh 2601/Unit 3 DEPARTMENT OF PHYSICS AND ASTRONOMY LIFECYCLES OF STARS Option 2601

M.R. Burleigh 2601/Unit 3 Stellar Physics  Observational properties of stars  Stellar Spectra  The Sun  Stellar Structure  Stellar Evolution  Stars of particular interest

M.R. Burleigh 2601/Unit 3 DEPARTMENT OF PHYSICS AND ASTRONOMY Unit 3 The Sun

M.R. Burleigh 2601/Unit 3 The Sun  Basic physical parameters  Structure of interior and atmosphere  Surface features  Magnetic field  Solar activity, flares and pulsations  Relationship to other stars

M.R. Burleigh 2601/Unit 3 Our nearest star  Nearest, therefore studied in most detail  Standard against which other stars are compared –Radius = 6.96x10 5 km (~109R  ) –Mass = 1.99x10 30 kg (~333,000M  ) –Luminosity = 3.86x10 26 W –Spectral type/luminosity class = G2 V

M.R. Burleigh 2601/Unit 3 Structure  Core – region of nuclear burning  Radiative zone  Convection zone  Photosphere  Chromosphere  Corona Only regions directly observable

M.R. Burleigh 2601/Unit 3

Solar interior Helioseismology  Solar oscillations used to study the structure similar to seismology on Earth  Periods range from 5 min to 2h 40min –Detected by periodic changes in Doppler shifts of spectral lines

M.R. Burleigh 2601/Unit 3

Photosphere Granulation  Base of photosphere is deepest region observable  Patchwork of granules –d~700 km –Transient (5-10mins) –Bright irregular formations surrounded by darker lanes  Top layer of convection zone

M.R. Burleigh 2601/Unit 3 Photosphere  Bright regions are rising hot gas (convection cells)  Dark regions are falling cooler gas

M.R. Burleigh 2601/Unit 3 Sunspots  Cooler regions (appear darker) than surrounding photosphere  Temperatures ~3800K cf. 5800K elsewhere  Associated with high magnetic fields  More later…

M.R. Burleigh 2601/Unit 3

Limb darkening  Brightness of solar disk decreases from centre to limb (edge of disk)  Arises because we see deeper hotter gas at centre, cooler layers at limb

M.R. Burleigh 2601/Unit 3

Limb darkening

M.R. Burleigh 2601/Unit 3 In a slab of thickness dx, density , fraction of flux F absorbed is: Optical depth: Absorption in photosphere Units of opacity κ are m 2 /kg (or cm 2 /g) Main source of opacity in solar photosphere is H ¯

M.R. Burleigh 2601/Unit 3 Absorption lines  Discussed in detail in last Unit  First mapped by Fraunhoffer ( )

M.R. Burleigh 2601/Unit 3 Absorption lines  Alphabetic designation… capital letters for strong, lower case for weak lines  Hence… Na D lines, CaII H & K, Mg b

M.R. Burleigh 2601/Unit 3 Chromosphere  Spectrum contains emission features from highly excited/ionized species (e.g. H Balmer, HeII)  High temperatures (see last lecture)

M.R. Burleigh 2601/Unit 3 Chromosphere

Chromosphere  Photospheric continuum absorbed by Chromospheric gas  Absorption lines projected against solar disk  Emission lines seen against dark space

M.R. Burleigh 2601/Unit 3 Chromospheric fine structure  Some absorption lines have large optical depth (e.g. H , CaII H & K)  Monochromatic photos show large bright and dark patches… plages and filaments

M.R. Burleigh 2601/Unit 3 Chromospheric fine structure  Structure appears over whole disk  Bright network associated with magnetic fields at boundaries of supergranules  Brightening (i.e. less absorption) of CaII K  increasing magnetic field strength

M.R. Burleigh 2601/Unit 3 Chromospheric fine structure  See spicules at the limb, jets of glowing gas emerging at 20-25km/s – km across, 10000km high  Form a network following supergranule boundaries  Probably play a significant role in mass transport… chromosphere  corona  wind

M.R. Burleigh 2601/Unit 3 Transition Region

M.R. Burleigh 2601/Unit 3 Transition Region  Gives rise to UV spectral features –e.g. Lyman , CIII, NIII, OVI  Network continues through this region  Disappears at ~1.6x10 6 K in MgX images

M.R. Burleigh 2601/Unit 3 Solar Corona

M.R. Burleigh 2601/Unit 3 Solar Corona  In visible light…  K Corona – dominates near the Sun –Light scattered by (1-2)x10 6 K electrons –Strongly affected by solar activity  F Corona – visible at a few solar radii –Light scattered from dust

M.R. Burleigh 2601/Unit 3 Solar Corona

M.R. Burleigh 2601/Unit 3 Solar Corona  Radio Corona: arises from free-free transitions of free electrons & atoms/ions  Line emission: “forbidden” lines due to high temperature & low density  EUV lines: e.g. FeVIII-XVI… high ionization states

M.R. Burleigh 2601/Unit 3 Solar Corona

M.R. Burleigh 2601/Unit 3 Coronal Loops & 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

M.R. Burleigh 2601/Unit 3 Coronal Loops & Holes  Dark regions (gas less hot and dense)  coronal holes  Holes correspond to magnetic field lines that do not reconnect with the surface

M.R. Burleigh 2601/Unit 3 Solar Corona

M.R. Burleigh 2601/Unit 3 Solar Wind  Solar gravity is insufficient to retain high temperature coronal gas  Gas is a plasma (ionized but electrically neutral on a large scale)  Thermal conductivity high  high T prevails out to large distances

M.R. Burleigh 2601/Unit 3 Solar Wind  Wind accelerates as it expands –300km/s at 30R  400km/s at 1 AU  Proton/electron energy ~10 3 eV  Density at Earth ~( )x10 6 m -3

M.R. Burleigh 2601/Unit 3 Solar Wind

M.R. Burleigh 2601/Unit 3 Solar Activity  We can easily observe transient phenomena  These are manifestations of Solar Activity  Linked through solar rotation and magnetic field

M.R. Burleigh 2601/Unit 3 Sunspots  Field strengths (deduced from Zeeman effect) ~0.1T (up to 0.4T)  Fields may inhibit convective energy transport  Any given spot has an associated magnetic polarity  May be paired with spot of opposite polarity (or diffuse region no observed as a spot)

M.R. Burleigh 2601/Unit 3 Sunspots

Sunspots  Can measure solar rotation rate by following spots  P equator ~25d  P 40 o ~27d  P 70 o ~30d

M.R. Burleigh 2601/Unit 3 Sunspots  Spot numbers vary with an 11 year cycle (except Maunder Minimum)  Spot latitudes vary during cycle  Spot lifetimes days to months

M.R. Burleigh 2601/Unit 3 Sunspots

Sunspots  Bipolar spot pair – preceding spot always has same polarity through cycle  Polarities are opposite in each hemisphere  Reverse at end of 11yr cycle – overall 22 year cycle  Spots always follow constant latitude

M.R. Burleigh 2601/Unit 3

Sunspot Cycle

M.R. Burleigh 2601/Unit 3

Active Regions  As spot numbers increase so does solar activity  Each sunspot group is associated with an active region several x 10 5 km across  Magnetic activity is concentrated in these  Usually bipolar (Bipolar Magnetic Regions – BMRs)

M.R. Burleigh 2601/Unit 3 Active Regions

M.R. Burleigh 2601/Unit 3 Active Regions  Bright areas associated with BMRs in various zones  In photosphere – faculae  Chromosphere – plages  Corona – streamers

M.R. Burleigh 2601/Unit 3 Prominences  Streams of chromospheric gas – dark when viewed against disk  Quiescent –Long lived (weeks) curtain-like gas along neutral line separating poles of BMR  Active –Few hours – loops closely associated with solar flares

M.R. Burleigh 2601/Unit 3 Prominences

Prominences

Solar Flares

M.R. Burleigh 2601/Unit 3 Solar Flares  Transient outbursts – probably originate by magnetic reconnection  Radiate from radio to  -rays  Emit high energy particles (solar cosmic rays)

M.R. Burleigh 2601/Unit 3 Solar Flares  km in size  Brighten within 5 min  Decay in ~20min (up to 3hrs for largest)  The larger the flare, more energetic and longer lived  At solar max – small flares hourly, large flares monthly

M.R. Burleigh 2601/Unit 3

The Sun  Basic physical parameters  Structure of interior and atmosphere  Surface features  Magnetic field  Solar activity, flares and pulsations  Relationship to other stars

M.R. Burleigh 2601/Unit 3 Next lectures  Tuesday 8 th March  10am and 1pm  LRB  No lecture Monday 7 th March

M.R. Burleigh 2601/Unit 3 DEPARTMENT OF PHYSICS AND ASTRONOMY Unit 3 The Sun

M.R. Burleigh 2601/Unit 3 DEPARTMENT OF PHYSICS AND ASTRONOMY LIFECYCLES OF STARS Option 2601