Comet Lulin made its appearance early last week, and is still making its way around the Sun. Astromers tell us that the Comet originated in the so-called "Oort cloud," which apparently begins about one light year from the Sun and may end around three light years, or 18 trillion miles, away. The Oort cloud contains billions of comet nuclei, which scientists say are left over from the initial formation of our solar system. Occasionally, gravitational pertubations from passing stars or gravitational "tides" in the Milky Way itself will dislodge one of these objects and send it hurtling toward the sun. In some cases these comets are "captured" by the inner solar system and remain in short period orbit, as in the case of the famous Halley's comet. One astronomer has estimated that the close approach of the star Alpha Centauri, to a distance of 3.1 light years several thousand years from now will cause 200,000 comets to leave the Oort Cloud and cross earth's orbit 20 million years from now (recall that these comets will begin over a light year away and will not accelerate to high speed until they approach the inner reaches of the solar system). (The author assumes that there are 5 trillion comets in the Oort cloud.) See Robert A.J. Matthews, The Close Approach of Stars in the Solar Neighborhood, 35 Quarterly Journal of the Royal Astronomical Society 1-9 (1994).
The bottom photo above comes from NASA's "photo of the day" series and was taken in Virginia's Shenandoah Valley (hence the trees). The top photo is courtesy of Joseph Brimacombe, who licensed the photo via Creative Commons.
For a map of the comet's whereabouts in the night sky over ther next several nights, see the following entry on Flickr.
