1	Astro 1050 Wed. Oct. 2, 2002 Today: Feedback Questionnaires Chapter 7 -- The Sun
2	Classification of stars O B A F G K M scheme Originally in order of H strength – A,B,etc Above order is for decreasing temperature Standard mnemonic: Oh, Be A Fine Girl (Guy), Kiss Me Use numbers for finer divisions: A0, A1, ... A9, F0, F1, ... F9, G0, G1, ...
3	Basic Properties of the Sun Radius: R = 6.96 ´ 10⁵ km (q=diameter/distance) Mass: M = 1.99 ´ 10³⁰ kg
4	Basic Properties of the Sun Surface Temperature 5800 K (from l_max or spectral type) Not all that hot by laboratory standards Central Temperature 15 ´ 10⁶ K (explained later) Central temperature IS very high Luminosity (L) 3.8 ´ 10²⁶ J/s ( L = sT⁴_surface ´ 4 p R²_sun) ( L = F_{at Earth} ´ 4 p R²_{1 AU}) Will be important for understanding energy generation in Sun
5	Physical State of material in Sun At these T’s, r’s, hydrogen will be a gas At high enough T, as pressure (P) increases and r increases, you never really get a “liquid”, just a very dense gas. H ionization? On outside, H mostly neutral (a small fraction is ionized) remember H ionized and Balmer lines gone only above 10,000 K Over most of interior, H completely ionized separate electrons (e^-) and protons (p⁺) Ionized gas called a “plasma” No discrete “surface” – just increasing r, T, P, and “opacity”
6	How we determine T, r, P vs depth From theory: Pressure (P) and density (r) must increase with depth Weight of overlying gas compresses lower material -- “Hydrostatic equilibrium” Temperature (T) must increase with depth Energy is flowing out of the sun – and it flows from hot to cold -- so hot inside Numerical modeling of details let us calculate T(r), r(r), P(r) From observations of “oscillations” or “solar seismology” The sun oscillates like a bell (or the air in an organ pipe) The frequency depends upon sound speed, and that depends upon T(r), r(r), P(r) Observations from the “Global Oscillation Network Group (GONG) telescopes. From interpreting the spectrum using Kirchoff’s laws In general the spectrum of the Sun looks like continuous emission This must come from a solid – or a very dense gas We see many dark absorption lines in the solar spectrum This must come from cooler gas between us and the hot dense gas
7	The “surface” of the Sun No discrete “surface” – just increase r, T, P, and “opacity” “Surface” or photosphere defined by depth from which visible photons can escape. This would also be depth to which visible photons could penetrate if photons were somehow directed back into the sun. Opacity depends on wavelength, so apparent “surface” will be at different depths for different wavelengths High opacity in absorption lines because these photons easily absorbed/emitted Won’t see very far in at these wavelengths. Low opacity in between absorption lines Can see in deeper at these wavelengths. Eventually r so high gas opaque at all wavelengths (just as in solid) “surface” high = cool = dark in lines; deep = hot = bright between lines This gives us way to “probe” to different depths in sun Same technique used by satellites to “probe” temperatures in Earth’s atmosphere
8	T,r dependence upon depth inside the sun
9	Complicated T dependence at the very edge We see emission lines at some wavelengths: Implies very THIN HOT overlying gas at top of atmosphere Gas is so thin it has trouble radiating heat away Sound waves or magnetic fields heat thin gas Chromosphere (“colored region” glows at a few wavelengths ) Corona (“crown” seen during solar eclipses ) Solar Wind ( escaping wind of tenuous gas )
10	Detailed structure of the outer photosphere CONVECTION: Granulation and Supergranulation Heat carried by actual motion of gas Different than radiative transport energy carried by photons dominates deeper in sun SUNSPOTS Darker (and cooler) regions of sun Strong magnetic fields limit convection Come and go in 11 (really 22) year cycle Magnetic energy releases cause “flares” Material ejected causes aurora