We think we know how stars live and die, but our picture of how stars form to begin with is incomplete. Although astronomers have discovered more than 1000 planets in other solar systems, we do not really know what conditions actually produce life.
The lives of massive stars end dramatically with powerful supernovae explosions, with core remnants as neutron stars or black holes. Low mass stars like our Sun, on the other hand, die relatively peacefully and over a much longer timescale. When hydrogen is completely burned, the core of such a star contracts until it becomes hot enough to initiate helium burning. In this stage, a thin layer of hydrogen burning may continue around the helium core. The envelope of the star begins expanding and the star becomes a red giant.
Young stellar objects (YSOs) are stars in the earliest stages of development. There are two principal kinds of YSOs: protostars and pre-main sequence (PMS) stars. YSOs are almost always found within or near interstellar gas and dust, most often embedded or partially embedded in molecular clouds. They are also intimately associated with the other manifestations of the star formation process such as bipolar outflows, Herbig-Haro objects, jets, water masers and circumstellar disks.
About one-tenth of the mass of our Galaxy is interstellar gas, about half of it atomic hydrogen and helium and most of the rest molecules-mainly molecular hydrogen but over a hundred other compounds as well. Because molecular hydrogen has no strong transitions at radio or millimeter wavelengths, the molecular component was largely unsuspected until about 35 years ago when the trace molecule carbon monoxide (CO) was first detected in space.
Massive stars (stars more massive than 8 times that of the Sun) are dominant players in the Galaxy. Despite their small number, they produce most of the visible light in the Galaxy. In their relatively short lives, they have great impact on the galactic environment by ionizing the interstellar medium via strong ultraviolet radiation, and alter the makeup of the interstellar medium through manufacturing heavy elements via supernovae explosion.
Embedded clusters are stellar clusters that are partially or fully embedded in interstellar gas and dust within molecular clouds. They consist of extremely young, recently formed or forming stars. Because they are immersed in significant amounts of interstellar dust they are typically invisible at optical wavelengths. They are best detected and studied in the infrared part of the electromagnetic spectrum. These systems are important laboratories for the study of star formation and early stellar evolution.
Dense cloud cores are condensations in star-forming molecular clouds. Their gas mass is typically a few stellar masses, and they are frequently observed to harbor protostars, the youngest known stars. They are widely believed to be the birth sites of stars, and their physical properties are believed to represent the "initial conditions" for star formation.
At least 1 in 10 nearby Sun-like stars hosts a giant planet. A massive effort is underway to find more exoplanets, determine their key properties, and associate demographic trends with models of their formation. Ultimately, the goal is to develop a robust theoretical framework grounded in this growing suite of empirical evidence that explains how different kinds of planets are made. That formation process is intimately tied to the initial conditions in the reservoirs of planet-building material - the disks around young stars.
Careful follow-up observations of nearby transiting planet systems have revolutionized our understanding of a whole new kind of planet: hot Jupiters. They have been used to reveal absorption by atmospheric atomic sodium (Charbonneau et al. 2002) and the presence of an extended hydrogen exosphere (Vidal-Madjar et al. 2003) in HD209458b, as well as to detect the thermal infrared emission from TrES-1, HD209458b, and HD189733b (Charbonneau et al. 2005; Deming et al. 2005; Deming et al. 2006). They have been used to investigate the spin-orbit alignment of HD209458b (Queloz et al.
Pre-main sequence low mass stars have X-ray emission 2-4 orders of magnitude greater than main sequence stars. This emission is thought to arise from enhanced solar--like activity including coronae and flares. The bulk of the pre-main sequence stars observed in X-rays are G, K and early M stars. Such bright emission can be used to identify young stars in the absence of other indication of youth such as a disk. The coronal enhancements are thought to come from a rotational dynamo.
Stars low and intermediate mass, up to about 8 solar masses, end their life as a white dwarf - the collapsed dead core of a red giant that has shed its out layers in a planetary nebula. The core is no longer able to sustain nuclear fusion reactions and resists complete gravitational collapse only through electron degeneracy pressure when it reaches a size similar to that of the Earth. The energy released by contraction, coupled with the initially very hot core temperature, means that white dwarfs are born hot - up to 200,000 K.