Astronomy The Final Review Part 2 of 2 Chapter 10 Measuring the Stars stereoscopic vision allows you to judge distance finger in front of nose close one eye then the other the finger appears to move moves a lot when at your nose moves less at arm s length if eyes were separated further we could judge further distances take pictures at different sides of Earth s orbit 6 months 2 AU between pictures parallax 1838 Henry Bessell found that 61 Cygni was 3 2 pc away stars apparent shift over a year twice the parallax parallax is inversely proportional to distance d parsecs 1 p arcsecs observational uncertainty we can t reliably measure distance few hundred pc using parallax Hipparchus classified stars by brightnesses 1 6 apparent magnitude m is brightness as it appears in the sky inversely proportional to the square of our distance luminosity L is intrinsic brightness need distance to determine B L 4 D absolute magnitude M brightness of a star if it was 10pc from us can be calculated from pulsation period if variable star most stars are less luminous than our sun Sun falls near the middle in 1912 Henrietta Leavitt found Cepheid s avg luminosity is proportional range though to pulsation period M m 5log10 d 10pc Calculating a star s profile Distance parallax d 1 p Luminosity need brightness and distance B L 4 Size Stefan Boltzmann L 4 r Boltzmann constant T 4 Mass total use Kepler s M1 M2 4 A 3 GP ratio know ratio of velocities or orbit size Temperature of surface Wien s Law T 2 900 peak wavelength stars don t have smooth continuous blackbody spectra dark bright at specific wavelengths 1913 Bohr model is incorrect electrons don t orbit they are in clouds like shelves in a bookcase can never be found BETWEEN shelves ground state is the lowest energy state here unless the atom absorbs a photon s energy which bumps the electron up to excited state when an atom ab sorbs a photon it emits one with the same energy but all absorbed photons travel in the same direction all emitted in random directions emission line if an electron decays the atom emits a photon in a ran dom direction color from the cloud forms an emission line absorption line if a photon s absorbed it missing from spectrum sharp dark line at the wavelength of this energy 3 states means 3 emission lines each element has matching spectra like a fingerprint strength of lines depends on of atoms present surface of star is smooth blackbody curve passing through atmos phere leaves absorption lines bright to faint Annie Jump Cannon classified spectral types Oh Be A Fine Gal Kiss Me O stars can be over 45 000K temp is so high electrons are stripped spectra appears featureless low temps can absorb light easier M stars low as 2800K Sun is G2 star around 5800K certain temps respond well to certain elements A stars absorb the most hydrogen at 10 000K so we can tell temp from spectra at lowest temps in stars atoms form molecules more than 90 of the atoms in a star s atmosphere are hydrogen rest is mostly helium binary stars when two stars orbit each other center of mass like the fulcrum of a see saw that lies at one focus the less massive star moves faster on a larger orbit 2 objects always on opposite sides of the stationary center of mass star 1 moves toward you blueshifted star 2 moves away redshifted period and separation total mass velocities or orbit sizes ratio of masses Visual can watch them orbit Spectroscopic stars too close together and far away see them only from spectral lines Eclipsing see a dip in brightness as one eclipses other must be edge on The H R Diagram Hertzprung and Russell plotted luminosity vs sur face temp hot blue stars on the left cool on the right plotted logarithmi cally Main sequence accounts for 90 of stars all burn hydrogen and leave helium ash 9 red giant 1 white dwarf we can use the luminosity temp graph to find radius star on upper right is cool but luminous so must be huge lower left is hot but not bright must be small mass of MS star determines all its other characteristics spectroscopic parallax if we know a star s L and m we can find its dis tance in core Chapter 11 Our Star The Sun Sun has burned slightly less than half of its total fuel luminosity of 3 85 x 10 26 watts per sec increasing with time mass is 1 99 x 10 30 hydrostatic equilibrium outward pressure always equals inward gravity conservation of energy energy radiated away equals energy produced nuclear fusion must be hot enough for 2 nuclei to get so close that they overcome strong nuclear force holding the nucleus together despite protons repelling each other and fuse E mc shows us that energy can be converted into mass and vice versa Proton Proton Chain how 4 Hydrogens make 1 Helium 1 2 Hydrogen nuclei fuse make a Deuterium nucleus Hydrogen iso tope and emit a positron will hit electron and give off Gammas and neu trino escapes freely Another proton hits the Deuterium they make a Helium isotope Gam One Helium isotope hits the other producing a regular Helium nu 2 3 mas released cleus gives off two protons some of the released energy escapes as neutrinos rest warms the core radiation works from the core out to 70 of the sun energy transfers from hotter to cooler regions via photons only efficient in core area of low opacity since photons emit in random directions the energy doesn t es cape quickly eventually piles up at radiative edge temp drops and convec tion takes over pockets of hot gas rise through low temp gas above them carrying energy outward to just below visible surface in outermost layers radiation takes over to space thermal energy takes 100 000 yrs to find its way from core to surface neutrinos only take 8 1 3 min speed of light 400 T neutrinos pass through your body as you read a sentence even travel through Earth at night Homestake experiment in S Dakota found only 1 3 of predicted of neutrinos solar neutrino problem they though neutrinos had zero mass but they have mass and oscillate among the electron muon and tau neu trinos while leaving the sun experiment only detected 1 of 3 forms SOHO Solar Heliospheric Observatory by NASA and European Space Agency moves lockstep with Earth 0 01 AU away between Earth and sun measures solar wind Solar Dynamics Observatory launched by NASA in 2010 studies solar magnetic field Photosphere 5800 K the apparent surface the sphere that light comes from density is low enough to be transparent limb darkening sun looks fainter at edges than center is b c we are looking through the photo sphere