1Class 25The Structure of the Sun Basic structure of the Sun Hydrostatic equilibrium Surface phenomena2I : The structure of the Sun Core (R<0.25Rsun) Extremely hot (1.5x107 K at center) Extremely dense (1.6x105 kg/m3) Site of energy generation (thermonuclear fusion) More next class… Radiative zone (0.25Rsun→0.7Rsun) Energy transported outwards by radiative diffusion Essentially, photons “random walk” their way outwards Takes about 170,000 years for energy to leak out! Convective zone (0.7Rsun→Rsun) Temperature low enough (below 2x106K) that somecombined hydrogen nuclei form. Gas become much more effective at absorbing radiation…chokes radiative diffusion process Energy transport continues via convection Photosphere, chromosphere and corona More later…34II : Hydrostatic equilibrium What supports the Sun against collapsing in itsown gravitational field? Consider any slab of gas in the Sun… it is inequilibrium (i.e., zero net force) when This is the equation of hydrostatic equilibrium Quick note (beyond ASTR120 level)… the actualequation for a spherical object like the Sun isDifference in pressureforces on top andbottom of slabWeight ofgas in slab= -5III : Surface phenomena Outer regions of the Sun (above convectivezone) consists of three layers Bottom layer : Photosphere Visible surface of Sun Top of the convective zone; T=5800K Can see convective cells Next : Chromosphere Region of hotter gas Temperature increases with height Displays spicules Finally : the Corona Very hot gas (million K or more); emits X-rays Most likely heated by phenomena associated withmagnetic fields6789 Many important phenomena are driven bymagnetic fields… Sunspots Regions where magnetic fields pierce the photosphere Prominences, flares and coronal mass ejections… Explosive events associated with the unraveling of astressed, twisted magnetic field Can send bursts of high-energy particles into space…can be problem for satellites and astronauts Magnetic phenomena have an 11 year cycleassociated with the solar
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