Lecture #15: Plan Telescopes (cont d) Effects of Earth s Atmosphere Extrasolar planets = Exoplanets
Collecting Area Light bucket : the bigger the area of the telescope s mirror or lens, the more photons it collects Collecting Area ~ (Diameter of telescope) 2
Collecting Area Example: Eye (D = 5 mm) vs Keck (D = 10 m) Collecting power (Keck) / Collecting power (eye) = D(Keck) 2 / D(eye) 2 = [D(Keck) / D(eye)] 2 = [10 / 5 x 10-3 ] 2 = [2 x 10 3 ] 2 = 4 x 10 6 Keck is 4,000,000 x more powerful than our eye!
Resolving Power (review) The bigger the size of the telescope, the better it is at discerning fine details Resolving power ~ (Diameter of telescope) / λ
Radio Telescopes are big! (radio! longer λ, so larger D is needed to get the same resolving power as optical telescopes)
Radio Telescopes: Interferometry (Combine the signal from an array of radio telescopes to increase effective D)
Effects of Earth s Atmosphere 1. Absorption: light is absorbed by atmosphere 2. Scattering: light is redirected by atmosphere 3. Refraction: light is bent by atmosphere
Atmospheric Absorption Oxygen (O 2 ) & ozone (O 3 ) absorb UV and X rays Water and carbon dioxide absorb far-infrared Electrons in upper atmosphere screen out long radio wavelengths Atmosphere is transparent to visible and short radio waves (and some windows in near-infrared)
Atmospheric Scattering Redirection of light in random directions when it strikes atoms, molecules or dust particles The amount of scattering is larger in blue than in red Consequences: 1. Sky is bright 2. Sky is blue 3. Sunset / sunrise are red
Refraction Bending of light when it moves from one medium to another
Example: Air to Water
Atmospheric Refraction: Consequences 1. Lingering (~2 min), elliptical sunsets Horizon (apparent position) (real position, below horizon)
Atmospheric Refraction: Consequences
Atmospheric Refraction: Consequences Green flash!
Atmospheric Refraction: Consequences 2. Scintillation ( twinkling ) of stars
Atmospheric Refraction: Consequences 3. Dispersion ( splitting ) of light into its different colors Flashing colors in twinkling stars
High Altitude Ground based Observatories or Space based Observatories
Lecture #15: Plan Telescopes (cont d) Effects of Earth s Atmosphere Extrasolar planets = Exoplanets
Direct Views of Exoplanets First thought: just get a picture! But angle is tiny and star is much brighter than the planet Current state-of-the-art: Detection of Jupiter-like planets around stars Future: Earth-like planet ~1010 times fainter than a Sunlike star!
Direct Views of Exoplanets Fomalhaut b: 0.5-2 Jupiter masses ~115 A.U. from star ~18 A.U. closer than the debris disk
Indirect Method: Spatial Wobbles Next possibility: orbital motion of (unseen) planet makes the star wobble (by Newton s 3 rd law) Expected stellar wobble << 1 arcsecond Requires excellent image quality Only works for the very nearest systems
Indirect Method: Kinematic Wobbles Measure the Doppler shift to tell us how fast the star is moving towards or away from us.
Indirect Method: Kinematic Wobbles The planet orbits the star But by Newton s 3 rd law, the star must move too We observe the star, not the planet Tiny motion shifts spectra back and forth From the radial velocity & period, one can infer the (minimum) mass of the unseen planet!
Indirect Method: Eclipses Another possibility: maybe planet passes in front of the star! Partial eclipse: reduces light during transit Need special orientation for an eclipse to occur
Example: Spitzer Space Telescope Note the vertical axis: not easy to see eclipse!
Properties of Exoplanets Broad range of masses, sizes, and orbital periods: - Most are more massive and bigger than Earth - Many are more massive and bigger than Jupiter
Properties of Exoplanets Jupiter Many are on a tight (< 1 AU) short-period orbit around their star
Exoplanet around nearest star might be in Habitable Zone! (The habitable zone is the region where H 2 O is a liquid)
Question #8 Name one of the effects of Earth s atmosphere on astronomical observations