Due to Sun s (and rest of solar system s) motion [Fig 16-3, relative_motion.avi]

Transcription

1 Chapter 6: Basic Properties of Stars Star Names Ancient Arabic, Greek or Latin names By constellation, ecreasing orer of brightness α alpha, β beta, γ gamma... Stellar istances Pre-telescope Observations + Ptolemy s moel + celestial sphere Stars lie beyon most istant (known) planet, Saturn Brightness ecreases with istance (assume all stars, incluing sun, have same brightness) Parallax [fig6., parallax.avi,6_.mov, Parallax_Nav.swf] All stellar parallaxes are smaller than arc secon. Parsec: istance which woul have exactly arc secon of parallax, about 3.6 light years = 06,000 AU (parsec) = /p (") Light Year = 9.46x0 km = 5.86x0 mi Groun base parallax sensitivity ~.0, nearest star has p ~.75 Hipparcos satellite: parallax to.00 The Motions of Stars positions of stars change very slowly largest ~ 0 per year = egree every 350 years nearby stars move as much as per year [Fig 6-] Constellations change little over 000 s of years Due to Sun s (an rest of solar system s) motion [Fig 6-3, relative_motion.avi] stars being approache by sun appear to iverge receing stars appear to converge

2 The Brightness of Stars Apparent Brightness: brightness as seen from Earth. Stellar Magnitues Ptolemy s star catalog with first magnitue = brightest to sixth magnitue = just visible Moern (calibrate) scale: ifference of 5 magnitues = factor of 00 in brightness, ifference of magnitue ~ factor of.5 in brightness. M M =.5 log L L [table 6.] Herschel's metho for measuring relative brightness. [fig 6.4] Some Apparent Magnitues [ table 6.] Sun -6.5 Jupiter -3 Full Moon -.5 Sirius -.4 Venus -4 Polaris.0 Uranus 5.5 Neptune 7.8 Nake eye limit 6.5 Binocular Limit 0 Large Telescope limit 4 Hubble Limit 9 3 Absolute Magnitue: a measure of the output power of a star apparent magnitue epens upon actual brightness (energy output) an istance. absolute magnitue: brightness star woul have at a istance of 0pc M abs = M 5 log 0 luminosity = total raiate energy output from star luminosity function = relative number of stars for each absolute magnitue (each value of luminosity) [Figure 6-5] 4

3 Stellar Spectra (light from stars) Kirchoff s Laws [Figure 6-6] Hot soli, liqui or ense gas prouces a continuous spectrum. (Thermal Raiation) Thin gas (against a cooler backgroun) prouces a bright line, or emission line spectrum. Thin gas (against a hotter backgroun) prouces a ark line, or absorption line spectrum. For a particular gas, emission line spectra are the same as absorption line spectra! Stellar Spectra reveal composition of stars [Spectroscopy_Nav.swf] 5 Atomic Structure Electrons orbit nucleus [Fig 6-7] Electromagnetic forces only certain orbits allowe (Quantum Mechanics) [Figure 6-8] only certain energies are possible Energy Levels [Figure 6-9] + conservation of energy spectral lines [6_6.mov, Bohr_Nav.swf] material absorption or emission of light correspons to absorption or emission of photons by atom or molecule The spectra of a material is a fingerprint of that material the etails epens upon temperature, pressure etc. 6

4 Spectral Classification of Stars Original classification scheme base upon appearance of Balmer series of hyrogen absorption lines. [Figure 6.9-, Spectroscopy_Nav.swf] Moern classification scheme base on reorganization an reorering of original scheme base upon temperature O B A F G K M correspons to ecreasing temperature subclasses (0-9) (ecreasing temperature) Sun = G Temperature affects the spectrum increasing temperature there are increasing number of atoms in N = state, until T ~ 0,000 K, fewer atoms in N = state (more are ionize). 7 Luminosity Class Luminosity = total power output classes: I Supergiants III Giants V Dwarfs Sun: GV Chemical Abunance [Fig 6.4,stellar_spectroscopy_in.swf, fig 6.5 (note scale)] Mostly Hyrogen Astronomers sometimes refer abunance of elements heavier than Hyrogen as metallic Doppler shift of Spectra irect measurement of raial velocities [Figure 6-6] rotation causes line broaening [Figure 6-7, 8] 8

5 Hertzsprung-Russell (H-R) Diagrams [Fig 6.9,0 ] Plot of Luminosities (or Absolute Magnitue) versus Temperatures (or Spectral Class) Clustering correspons to long-lasting evolutionary stages Stellar Masses Binary Stars yiel masses (Newton s & Kepler s Laws) M + M M M = = P 3 Mass-Luminosity Relation for Main Sequence Stars [Figure 6-] L=M 3.5 9