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Photometry Filters

Percentage of light passing through filter (1.0 = 100%). Photometry Filters. Color Index: Difference between two filtered magnitudes. For example, B-V. Star: An object that during some part of its lifetime derives

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Photometry Filters

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  1. Percentage of light passing through filter (1.0 = 100%) Photometry Filters Color Index: Difference between two filtered magnitudes. For example, B-V.

  2. Star: An object that during some part of its lifetime derives 100% of its energy from the fusion of hydrogen nuclei (protons) to helium. Must have > 7% M_Sun. Brown dwarf: An object intermediate in size between a planet and a star (~1 -> 7% M_Sun). May briefly fuse deuterium. Planet: An object with a mass less than about 1% M_Sun. No fusion process occurs in them, and they shine primarily by reflected light.

  3. The Brightness of Stars Apparent Brightness: How bright an object appears in the sky. A measure of the observed light received from a star or other object at the Earth. Luminosity: The total energy radiated into space each second by a star or other object. Star luminosities range from 0.0001 L_Sun up to 1,000,000 L_Sun. Standard Candle: An astronomical object of known luminosity.

  4. Spectroscopy: The Key to the Universe Fig 16-7 From detailed analysis of a star’s spectrum, one can determine: • Radial velocity of star: Doppler shifting of spectral lines. • Stellar rotation rate: Doppler broadening of spectral lines. • Abundances of elements in star’s atmosphere: Careful spectral analysis combined with knowledge of stellar temperatures and pressures. Nearly all stars ~75% H, ~25% He, + small amount of heavier elements

  5. For “Normal” stars: OBAFGKM 40,000K -> 2,000K Most stars ~75% H, ~25% He Typically up to ~200 km/s Up to ~few hundred km/s >100 M_Sun down to ~0.1 M_Sun ~25 R_Sun -> ~0.5 R_Sun Table 17-1

  6. Astronomical Photometry Measuring Brightness: The Magnitude Scale Fig 16-3 Higher numbers = fainter object. A change of 5 magnitudes signifies a factor of 100 change in brightness. A change of 1 magnitude signifies a factor of ~2.5 change in brightness. Faintest object visible to naked eye = +6 mag.

  7. Characteristics of Stars on the H-R Diagram • H-R Diagram: A plot of luminosity vs. surface temperature for stars. • Main Sequence: Location on the H-R diagram where the majority of stars (roughly 90%) fall. Stars here are actively fusing hydrogen into helium in their cores. • Giant, supergiant: Star with large luminosity but relatively low surface temperature, indicating a very large radius. • White Dwarf: Star with a mass roughly that of the sun, but with a volume only 1 millionth that of the sun (about the size of Earth). Can be hot but, due to extremely small size, are not very luminous. Very high density and surface gravity.

  8. Parallax Fig 18-3, p.401 Units: arcseconds. There are 60 arcsec in 1 arcminute, and 60 arcminutes in a degree. Thus, there are 3600 arcsec in a degree. 360 degrees in a circle. (Full moon subtends 1/2 degree in sky.) Parallax: Defined to be one-half the angle that a a star shifts when seen from opposite sides of Earth’s orbit.

  9. Determining Stellar Distances with Parallax A parallax of 1 arcsec indicates a distance of 3.26 LY. Define: 1 parsec (pc) = 3.26 LY. D = distance (in parsec) p = parallax (in arcsec) Example: A star has a parallax of 0.1 arcsec. How far away is it in parsec? In LY?

  10. Distances in the Universe • Earth to Moon: ~1 light second. • Earth to Sun: ~8 light minutes. • Sun to Pluto: ~5 light hours. • Sun to nearest star: ~4 light years (LY). • Sun to center of Milky Way Galaxy (MW) : ~30,000 LY. • Visible diameter of MW: ~100,000 LY. • MW to nearest ‘dwarf’ galaxy: ~75,000 LY. • MW to nearest major galaxy (Andromeda): ~2 million LY. • Distance to edge of visible universe: ~14 billion LY.

  11. Measuring Distances in Astronomy • Parallax: Ground-based telescopes measure distances out to 60 LY. Space-based telescopes (Hipparchos) reach out to 300 LY. Most accurate, and least controversial, technique. • Cepheid Period-Luminosity Relation: Excellent extragalactic distance indicator. Provides accurate distances out to 100 million LY. Two Other (less important) Techniques • RR Lyrae stars as standard candles: Good out to ~2 Mly. • Spectroscopic parallax: In principle, can be used out to ~20 million LY for brightest stars, BUT not very accurate.

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