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Chapter 1 Our Place in the Universe 1 What is our place in the universe? 2 Star ! (Usually:) A large, glowing ball of gas that generates heat and light through nuclear fusion. ! The energy generated in a star is in equilibrium with the gravity of the star. ! As we will see later, there are end stages of stars (White Dwarfs, Neutron Stars) that are not balls of gas, and that are not supported by nuclear energy. 3 Planet Mars Neptune ! A moderately large celestial body that orbits a star. Mars Neptune ! A planet may be rocky, icy, or gaseous in composition. ! A planet is massive enough to be approximately spherical under its own gravity. ! A planet is not massive enough to have significant nuclear fusion; it shines by reflected light. ! A planet has cleared the neighborhood around its orbit. (IAU 2006) ... Pluto is no longer considered a planet but a dwarf planet, along with Eris and Ceres. 4 Moon (or Satellite) ! A celestial body that orbits a planet. Ganymede (orbits Jupiter) Earth and its Moon 5 Asteroid ! A relatively small and rocky object that orbits a star. ! Gravity is too small to form a sphere. Ida Asteroid Ida (with its moon Dactyl) NASA: http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-asteroids.html Asteroid Itokawa 6 Comet ! A relatively small and icy object that orbits a star. ! Comets are very dim and cold during most of their orbit far away from the Sun. They get their spectacular tails only when they get heated up close to the Sun. ! The plasma tail points away from the Sun, being carried by the Solar Wind. 7 Solar System (Planetary System) ! A star and all the material that orbits it, including its planets and moons. 8 Nebula ! An interstellar cloud of gas and/or dust. Eagle Nebula, HST/NASA 9 Galaxy ! A great island of stars in space, all held together by gravity and orbiting a common center M31, The Great Galaxy in Andromeda 10 Universe ! The total of all matter and energy, including everything within and between all galaxies. Video on the Hubble Deep Field http://www.deepastronomy.com/hubble-deep-field.html 11.

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Perspective in Search of Planets and Life Around Other Stars
Proc. Natl. Acad. Sci. USA Vol. 96, pp. 5353–5355, May 1999 Perspective In search of planets and life around other stars Jonathan I. Lunine* Reparto di Planetologia, Consiglio Nazionale delle Ricerche, Istituto di Astrofisica Spaziale, Rome I-00133, Italy The discovery of over a dozen low-mass companions to nearby stars has intensified scientific and public interest in a longer term search for habitable planets like our own. However, the nature of the detected companions, and in particular whether they resemble Jupiter in properties and origin, remains undetermined. The first detection of a planet orbiting around another star like Table 1. Jovian-mass planets around main-sequence stars our Sun came in 1995, after decades of null results and false Star’s Semi-major Period, alarms. As of this writing, 18 objects have been found with masses Star mass axis, AU days Eccentricity* Mass† potentially less than 12 times that of Jupiter, but none less than 40% of Jupiter’s mass, around main-sequence stars (1) (see Table HD187123 1.00 0.042 3.097 0.03 0.57 1). In addition, several planetary-mass bodies have been detected HD75289 1.05 0.046 3.5097 0.00 0.42 orbiting pulsars. Although these are interesting as well, the exotic Tau Boo 1.20 0.047 3.3126 0.00 3.66 environments around neutron stars make it unlikely that their 51 Peg 0.98 0.051 4.2308 0.01 0.44 companions are abodes for life. It is the success in finding planets Ups And 1.10 0.054 4.62 0.15 0.61 around Sun-like stars that provides a technological stepping stone HD217107 0.96 0.072 7.11 0.14 1.28 ‡ to one of the great aspirations of humanity: to find a world like 55 Cnc 0.90 0.110 14.656 0.04 1.5–3 the Earth orbiting around a distant star.
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