Dr Matt Burleigh The Sun and the Stars

Dr Matt Burleigh The Sun and the Stars Limb darkening The surface of the sun does not have uniform brightness, it appears bright toward the centre, darker towards the edges, and the colour is redder, an effect known as Limb darkening - why? Ans. i) The density of the sun decreases with increasing distance from the centre ii) The temperature of the sun decreases with increasing distance (at least through the photosphere)

Dr Matt Burleigh

The Sun and the Stars When we look at the sun, we can only see to a depth at which the radiation escapes unhindered. Optical depth The optical depth is a measure of the transparency of a medium to the incident radiation. Consider an incident radiation flux F on a slab of gas with density , and thickness dx. Let the opacity (the amount of absorption at a given wavelength) of the gas be . Part of the incident radiation is absorbed by the gas and so, In a uniform medium So that the flux diminishes exponentially with increasing depth Astronomers define the quantity as the optical depth, where Note is dimensionless So

Dr Matt Burleigh Limiting cases: The Sun and the Stars optically thin (transparent) optically thick (opaque) An optical depth of unity represents the thickness of absorbing gas from which a fraction 1/e photons can escape. In Astronomy the photosphere of a star is defined as the surface where the optical depth is 2/3. Each photon emitted from this surface has an average of less than 1 scattering before escaping. Photosphere emits a continuous spectrum which is a close approximation to a blackbody – optically thick thermal emission The photosphere cools with increasing radius. Cool gas scatters incident photons by absorption and re-emission) producing dark lines at discrete energies. Although the temperature rises through chromosphere its optical depth is v. small,  effectively transparent to optical photons (optically thin) excited atoms, ions in chromosphere produce an emission-line spectrum.

Dr Matt Burleigh The Sun and the Stars The solar magnetic field For the sun or planet to have a magnetic field require a dynamo Dynamo conditions: 1) Rotation 2) Convection 3) Electrically conducting material Start with bipolar field. Differential solar rotation (faster at equator than at poles), stretches and enhances magnetic field lines (stretched field lines store energy), poloidal field becomes toroidal Field. Rising convection zones, twists toroidal field into poloidal field and so on. Sunspots occur where concentrated magnetic field lines erupt from photosphere. They occur in pairs, and appear dark because they are cooler than the surrounding area.

Dr Matt Burleigh Sunspots dark patches that appear on photosphere – appear dark as they are cooler ~3800K Components – dark umbra, lighter surrounding penumbra site of intense magnetic field (B~0.1T, c.f T at top of coronal loop) appear in pairs with opposite polarity (or diffuse region not observed as a spot) linked to solar cycle Intense B-field interferes with convection  convection interrupted  cooler The Sun and the Stars Sunspots appear at fixed latitudes Sunspots indicate that suns outer layers rotate differentially (Galileo) P (equator) ~ 25 days P (40deg) ~ 27 days

Dr Matt Burleigh The Sun and the Stars The sunspot cycle: Daily observations began at Zurich observatory in1749, and with the addition of other observatories, ~continuously from SSN = 10(Ng) + Ns (since each group contains roughly 10 sunspots) The number of sunspots is seen to vary on an  11 year cycle. We are currently around solar maximum Each new cycle starts with the appearance of sunspots at high latitudes forming 2 bands either side of the equator. As cycle progresses bands form at lower and lower latitudes (butterfly diagram, left). Note polarity of magnetic field reverses at the start of each new cycle. From , there were very few sunspots observed (Maunder minimum). Coincided with the mini Ice- age in Northern Europe.

Dr Matt Burleigh The Sun and the Stars The sunspot cycle: Maunder minimum ( ). Coincided with the mini Ice-age in Northern Europe.

Dr Matt Burleigh

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) Bright areas associated with BMRs in various zones In photosphere – faculae Chromosphere – plages Corona – streamers

Dr Matt Burleigh 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

Dr Matt Burleigh The Sun and the Stars Solar flares – closely associated with sunspot activity, activity related to solar cycle – v. few at solar minimum, solar maximum rate : hourly for small flares every few hours for large flares Size ~ 10,000 – 300,000 km Duration ~ rise to maximum ~ 5min – decay time 20 min (small flare) large flares ~ hours Temp ~ several million K energy up to J for largest flares

Dr Matt Burleigh Effects – radiate energy across whole of the E-M spectrum radio-gamma-rays eject highly energetic particles (cosmic rays) High energy photons disrupt terrestrial communications by disturbing ionosphere Cosmic rays damage satellites and would be lethal to unprotected astronauts (implications for manned mission to mars) For a great NASA movie on flares, see:

Dr Matt Burleigh

The Sun and the Stars (1) Pre-flare (2) Magnetic reconnection Joule heating produces X-ray, EUV and radio emission Current flows to photosphere  ribbon flare (seen in H-alpha) (3) Reconnection point moves outwards accelerated particles  radio emission and cosmic rays

Dr Matt Burleigh The Sun and the Stars Formation : Disturbance at base of magnetic field Energy released at top of loop where B-field reconnects Electric currents oppose reconnection Joule heating raises plasma temperature  X-rays UV and radio emission Currents heat gas at photosphere –ribbon flare Particles simultaneously accelerated outward  Radio emission and cosmic rays Reconnection point moves outwards