The Structure and Evolution of Hot White Dwarfs are among the oldest objects in the galaxy and are the remnants of the earliest phases of star formation. No bigger than the Earth, they are formed when a star like our Sun ends its life. When white dwarfs are born the are incredibly hot (about 200,000 degrees C) and then cool down slowly over thousands of millions of years. Study of the distribution and physical characteristics of the white dwarf population is an essential part of understanding the early history of the galaxy. In addition, observing white dwarfs allows us to address a number of problems concerning the latter stages of stellar evolution, particularly those of mass loss and cessation of nuclear fusion during the post Red Giant phases, and study the behaviour of matter under conditions of extreme temperature and pressure which cannot be reproduced in a laboratory. As white dwarfs cool, from time-to-time they become unstable and begin to "ring", rather like an enormous church bell. We see this "ringing" as tiny variations in the brightness of the star which, if measured very accurately, can tell us how massive the star is, what it is made of and how fast it is cooling. The Whole Earth Telescope, actually a world-wide network, is run as a single astronomical instrument with many operators. The collaboration includes scientists from around the globe in data acquisition, reduction, analysis, and theoretical interpretation. The Whole Earth Telescope complements the work on stellar atmospheres by studying pulsation modes to determine their interior structure. The ability to resolve complex oscillations relies on having long, comparatively unbroken observations stretching over a few weeks. The Life and Death of Stars - Main sequence stars are stars, like our Sun, that fuse hydrogen atoms together to make helium atoms in their cores. For a given chemical composition and stellar age, a stars' luminosity, the total energy radiated by the star per unit time, depends only on its mass. Stars that are ten times more massive than the Sun are over a thousand times more luminous than the Sun. However, we should not be too embarrassed by the Sun's low luminosity: it is ten times brighter than a star half its mass. The more massive a main sequence star, the brighter and bluer it is. For example, Sirius, the dog star, located to the lower left of the constellation Orion, is more massive than the Sun, and is noticeably bluer. On the other hand, Proxima Centauri, our nearest neighbor, is less massive than the Sun, and is thus redder and less luminous. Lives and Deaths of Stars - Stars live for a very long time compared to human lifetimes. Your great, great grandparents saw the same stars as you will see tonight (if it's clear). Our lifetimes are measured in years. Star lifetimes are measured in millions of years. Even though star timescales are enormous, it is possible to know how stars are born, live, and die. White Dwarfs form as the outer layers of a low-mass red giant star puff out to make a planetary nebula. Since the lower mass stars make the white dwarfs, this type of remnant is the most common endpoint for stellar evolution. If the remaining mass of the core is less than 1.4 solar masses, the pressure from the degenerate electrons (called electron degeneracy pressure) is enough to prevent further collapse. Fix: Chapter 17 The Sun Fix: Chapter 18 The Formation of Stars and Planets Fix: Chapter 19 The Evolution of Stars Fix: Chapter 20 White Dwarfs, Neutron Stars, and Black Holes White Dwarfs Cool Photometry of M57 Field Stars   by Brian Skiff Globular Clusters     © Copyright 2007 - Samuel J. Wormley   by swormley1@mchsi.com