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Download AN INTRODUCTION TO ASTRONOMY Dr. Uri Griv Department of Physics, Ben-Gurion University
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BEN-GURION UNIVERSITY AN INTRODUCTION TO ASTRONOMY Dr. Uri Griv Department of Physics, Ben-Gurion University Tel.: 08-6428226 Email: [email protected] plays, the distant ringed world Uranus was last visited in 1986 by the Voyager 2 spacecraft. Astronomy Picture of the Day Tomorrow’s picture: x-ray galaxy Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. < | Archive | Index | Search | Calendar | Glossary | Education | About APOD | > 2003 January 15 Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA) NASA Technical Rep.: Jay Norris. Specific rights apply. A service of: LHEA at NASA / GSFC & NASA SEU Edu. Forum & Michigan Tech. U. Ringed Planet Uranus Credit: E. Lellouch, T. Encrenaz (Obs. Paris), J. Cuby , A. Jaunsen (ESO-Chile), VLT Antu, ESO Explanation: Yes it does look like Saturn, but Saturn is only one of four giant ringed planets in our Solar System. And while Saturn has the brightest rings, this system of rings and moons actually belongs to planet Uranus, imaged here in near-infrared light by the Antu telescope at the ESO Paranal Observatory in Chile. Since gas giant Uranus’ methane-laced atmosphere absorbs sunlight at near-infrared wavelengths the planet appears substantially darkened, improving the contrast between the otherwise relatively bright planet and the normally faint rings. In fact, the narrow Uranian rings are all but impossible to see in visible light with earthbound telescopes and were discovered only in 1977 as careful astronomers noticed the then unknown rings blocking light from background stars. The rings are thought to be younger than 100 million years and may be formed of debris from the collision of a small moon with a passing comet or asteroid-like object. With moons named for characters in Shakespeare’s The Sun as a Star • The matter in the interior is a plasma • The sizes of the particles (nuclei ∼ 10−13 ) • The plasma in the Sun behaves as a perfect gas: the total pressure P = nkT , where n = ρ/m is the number density • The central mass density ρs is about 110 times its average value 1.44 g cm−3 • The central number density ρs /mproton = 1 × 1026 cm−3 • The Sun’s temperature T = 6 × 103 − 1.5 × 107 K • The source of energy of the Sun → thermonuclear reactions 2 The Sun as a Star 3 • Earth orbits the Sun approximately in a circle with radius r = 1.5 × 1013 cm (1 AU) and period P = 1 year = 3.6 × 107 s. Hence, the speed v of the Earth’s orbital motion 2πr v= = 2.98 × 106 cm/s ≈ 30 km/s P • We may set F = ma in the form where a is the centripetal acceleration (magnitude v 2 /r) and F is the gravitational force (magnitude = GM⊙ m/r) to obtain rv 2 = 2 × 1033 g (1) M⊙ = G • The total energy E = mv 2 /2 + (−GM⊙ m/r) of the Earth’s orbital motion is equal to one-half of its potential energy (−GM⊙ m/r) 4 Gravitational (Jeans) Instability • The two-phase model of interstellar medium 5 Gravitational Instability 2 2 2 0 0 0 −2 −2 −2 −2 0 t=0.0 2 −2 0 t=0.5 2 2 2 2 0 0 0 −2 −2 −2 −2 0 t=1.5 2 −2 0 t=2.0 2 −2 0 t=1.0 2 −2 0 t=2.5 2 • A star is held together by gravitation 1 ∂P ∂Φ = ∂r ρ ∂r • Suggestion: Stars are formed by gravitational instability • Galaxies have also been formed by gravitational condensation from a pre-galactic medium ∂Φ 2 = rΩ ∂r 6 Gravitational Instability: Example • Cartwheel ring-type galaxy 7 Gravitational Instability: Example • M83 spiral galaxy 8 Gravitational Instability: Example • Local group of galaxies 9 Gravitational Instability: Example • Solar system 10 Gravitational Instability: Example • Extrasolar planetary system 55Cancri 11