Dr. Anthony Picciano - Astronomy 100

Star Cycle - Birth and Death of a Star The star cycle starts with a nebula or large cloud of gas that collapses under its own gravity to form a star. A stable star such as the Sun steadily generates heat and light through nuclear fusion. Such a star is said to be in its main sequence. The main sequence phase ends when most of the hydrogen in its core has been converted to helium and fusion begins around the core thereby generating and expanding luminosity beyond the core, at which point it becomes a red giant. When the red giant begins to cool at its center, the red giant collapses to form a dwarf star, referred to as a white dwarf because it glows almost white as it cools. If a very large red giant begins to cool or attracts matter from a nearby star, a violent explosion or supernova can occur.

*Supernova* A supernova is the explosive death of a star, caused by the sudden onset of nuclear burning (Type I), or an enormous energetic shock wave(Type II). A supernova is one of the most energetic events in the universe and may temporarily outshine the rest of the galaxy in which it resides. This is a NASA Hubble Space Telescope image of the tattered debris of a star that exploded 3,000 years ago as a supernova. This supernova remnant, called N132D, lies 169,000 light-years away in the satellite galaxy, the Large Magellanic Cloud. A Hubble Wide Field Planetary Camera 2 image of the inner regions of the supernova remnant shows the complex collisions that take place as fast moving ejecta slam into cool, dense interstellar clouds. This level of detail in the expanding filaments could only be seen previously in much closer supernova remnants. Now, Hubble's capabilities extend the detailed study of supernovae out to the distance of a neighboring galaxy. Material thrown out from the interior of the exploded star at velocities of more than four million miles per hour (2,000 kilometers per second) plows into neighboring clouds to create luminescent shock fronts. The blue-green filaments in the image correspond to oxygen-rich gas ejected from the core of the star. The oxygen-rich filaments glow as they pass through a network of shock fronts reflected off dense interstellar clouds that surrounded the exploded star. These dense clouds, which appear as reddish filaments, also glow as the shock wave from the supernova crushes and heats the clouds. Supernova remnants provide a rare opportunity to observe directly the interiors of stars far more massive than our Sun. The precursor star to this remnant, which was located slightly below and left of center in the image, is estimated to have been 25 times the mass of our Sun. These stars "cook" heavier elements through nuclear fusion, including oxygen, nitrogen, carbon, iron etc., and the titanic supernova explosions scatter this material back into space where it is used to create new generations of stars. This is the mechanism by which the gas and dust that formed our solar system became enriched with the elements that sustain life on this planet. Hubble spectroscopic observations will be used to determine the exact chemical composition of this nuclear- processed material, and thereby test theories of stellar evolution. The image shows a region of the remnant 50 light-years across. The supernova explosion should have been visible from Earth's southern hemisphere around 1,000 B.C., but there are no known historical records that chronicle what would have appeared as a "new star" in the heavens. This "true color" picture was made by superposing images taken on 9-10 August 1994 in three of the strongest optical emission lines: singly ionized sulfur (red), doubly ionized oxygen (green), and singly ionized oxygen (blue). Photo credit: Jon A. Morse (STScI) and NASA

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