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Measuring the size of small things that shine in the sky at night Stellar Diameters ● Atmospheric Cherenkov Telescopes ● Cross-road ● Table top stars ● The future ● Stephan LeBohec, Micah Kohutek and Jamie Holder March 5th 2006 Galileo Galilei (1564-1642) Dialogue Concerning the Two Chief World Systems, 1632 Portrait of Galileo Galilei, by Ottavio Leoni (1578-1630). http://webexhibits.org/calendars/year-text-Galileo.html “I hung up a light rope in the direction of a star . . . and then by approaching and retreating from this cord placed between me and the star, I found the point where its width just hid the star from me. This done, I found the distance of my eye from the cord, which amounts to the same thing as one of the sides which includes the angle formed at my eye and extending over the breadth of the cord.” “And . . . the apparent diameter of a fixed star of the first magnitude is no more than 5 seconds.” d θ < 5” or 1mm at 40m D How far from the sun do we need to travel for it to appear no brighter than a bright star? From that distance, what would be its angular diameter? Mars d=227x106km r=3400km Earth d=150x106km Sir Isaac Newton, 1642-1727 Stars are 105 times further away than the sun π x 34002 LMARS = LSUN x = LSUN 4π x (227 x 106)2 2 x 1010 Stars are 105 smaller than the sun, or a few milli-arc-seconds Light behaves like a wave Visible light: wavelength ~500nm, frequency ~5x1014Hz. Electric field Magnetic field Light intensity ~ (Electric field )2 Huygens principle Christiaan Huygens 1629-1695 500 x 10-9 m / 1 x 10-3 m = 5 x 10-4 > 1 x 10-3 m / 40 m = 2.5x10-5 so Galileo could not have done the measurement he describes! Diffraction limits telescopes angular resolution William HERSCHELL 1738-1822. Thomas YOUNG 1773-1829. Visibility 1 Hippolyte FIZEAU 1819-1896. IMax - IMin Visibility = IMax + IMin Baseline Baseline = 1.22 λ / θ θ=1mas and λ=500nm ⇒ D=100m Michelson stellar interferometry 1890: Measure of one of Jupiter's satellite diameter 1920: with Pease, Measure of Betelgeuse diameter (47mas) Albert Michelson 1853-1931 Mount Wilson Hooker Telescope 1930: 50 foot interferometer 1932: The technique is abandoned for being too difficult with 20 foot Interferometer Cambridge Optical Aperture Synthesis Telescope Images of Capella (COAST) September 13th 1995 September 28th 1995 Images of Betelgeuse (COAST) 700nm (1997) 1290nm (1997) Colloquium by John Monnier on Thursday March 23rd at 4PM IOTA J, H, K ISI N KeckInterfero K meter Keck Aperture Masking J, H, K, L MIRA 1.2 R, I NPOI visible V, R, I PTI SUSI J,H,K B, V, R, I 7 0 6 10 30 50 10 85 85 2 3 5 0.5 30 5 9 30 300 7 5 86 5 110 640 VLTI near infrared J, H, K 12 46 130 VLTI mid infrared N 4 46 130 "dual-star" capable Used for interferometry a few weeks per year Used for interferometry a few weeks per year New Interferometers and Improvements to Existing Interferometers Limiting Maximum magnitude Minimum Waveband agnitude baseline (m) baseline (m) J, H, K 12 70 400 CHARA LBTI near J, H, K infrared MRO R, I, J, H, K VLTI near infrared using 4 ATs and J, H, K PRIMA VLTI near infrared using 3 UTs and PRIMA J, H, K >20 14 0 7 Comments 2005? 22 400 2006? 2008? 13 8 200 Operating every night 2007? 16 46 130 2007? SPACE INTERFEOMETRY MISSION . . . scheduled for launch in 2011 Cosmic Rays Protons, Nuclei, ?, ... Atmospheric Showers Electromagnetic . γ .e+ e- X0 Hadronic p. or Nucl. E +- E/2 E/4 E/8 E/16 E~80x106eV 1 atmosphere ~ 28 X0 1TeV (1012eV) 5ns ~100 photons/m2 130m 400 m 3.1011eV or 300 GeV gamma ray Http://www.mpihd.mpg.de/hfm/CosmicRay/ChLight/ChLat.html ~ 2o The Whipple observatory on Mount Hopkins MMT 10M 2200m a.s.l. The 10M Very High Energy Gamma Ray Telescope 7m 7m 10m 7m Atmospheric Showers Cherenkov Images p(?) gamma? muon Event rate ~ 30Hz ~ Gamma ray rate ~ 0.1Hz Undergrad seminar by David Kieda on March 23rd at 11:50AM Robert Hanbury Brown, 1916-2002 1950: Intensity Interferometry 1953: Radio measurements of CasA and CygA 1955: Search light measurement of Sirius 1963: Measurement of Vega from Narrabri (Australia) 1972: Operation of Narrabri interferometer stops 32 stars measured from Narrabri magnitudes < 2.5 0.41mas < diameters < 3.24mas 10 of them in the main sequence How does it work? D.C. Component “Wave noise” selected by A.C. coupling More generally: - c(d) is the Fourier transform of the light distribution across the source. - Measuring c(d) is measuring the source size and shape. Table top test bench Our first intensity interferometer Inside the black box The electronics Control and digitization Integrator Mixer Amplifiers Toward a Digital Intensity Interferometer Installation on VERITAS telescopes Analog optical fibers to central location Time delay Interferometric observation during full moon What science could we do? Stellar diameters Pulsating stars Stellar surface features Binary stars Extra solar planets Wolf-Rayet stars Be Stars Circum-stellar disks ... Km baseline, 1 mas VERITAS base lines