The Sun and the Stars - PowerPoint Presentation

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The Sun and the Stars

The Sun and the StarsSlide2

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 centreii) The temperature of the sun decreases with increasing distance (at least through the photosphere)Slide3
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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

SoSlide5

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.Slide6

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) Rotation2) Convection3) Electrically conducting materialStart 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.Slide7

Sunspots

dark patches that appear on photosphere – appear dark as they are cooler ~3800KComponents – dark umbra, lighter surrounding penumbra site of intense magnetic field (B~0.1T, c.f. 0.001T at top of coronal loop) appear in pairs with opposite polarity (or diffuse region not observed as a spot) linked to solar cycleIntense 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 daysSlide8

The Sun and the Stars

The sunspot cycle:

Daily observations began at Zurich observatory in1749, and with the addition of other observatories, ~continuously from 1849.

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 1645-1715, there were very few sunspots observed (Maunder minimum). Coincided with the mini Ice-age in Northern Europe.

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The Sun and the Stars

The sunspot cycle:

Maunder minimum (1645-1715). Coincided with the mini Ice-age in Northern Europe.

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Active RegionsAs spot numbers increase so does solar activity

Each sunspot group is associated with an active region several x 105km acrossMagnetic activity is concentrated in theseUsually bipolar (Bipolar Magnetic Regions – BMRs)

Bright areas associated with BMRs in various zones

In photosphere –

faculae

Chromosphere –

plages

Corona –

streamersSlide12

ProminencesStreams of chromospheric

gas – dark when viewed against diskQuiescentLong lived (weeks) curtain-like gas along neutral line separating poles of BMRActiveFew hours – loops closely associated with solar flaresSlide13

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 flaresSize ~ 10,000 – 300,000 kmDuration ~ rise to maximum ~ 5min – decay time 20 min (small flare)

large flares ~ hours

Temp ~ several million K

energy up to 10

30 J for largest flaresSlide14

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 ionosphereCosmic rays damage satellites and would be lethal to unprotected astronauts(implications for manned mission to mars)For a great NASA movie on flares, see:http://upload.wikimedia.org/wikipedia/commons/5/56/X-class_flares.ogvSlide15
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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

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radio emission

and cosmic raysSlide17

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 outwardRadio emission and cosmic raysReconnection point moves outwards