dwarf remnant (see Figure 2.8). For all these stars, some newly created elements are ejected from the surface in stellar winds before the final collapse. A major goal of stellar astrophysics is to understand the various mechanisms of mass loss and how they contribute to the continually increasing abundance of heavier elements in the universe. Many of the recommended new facilities will make strong contributions to the necessary investigations: ALMA and CARMA by studying the chemistry of the outflows, GSMT by acquiring spatially resolved spectra, and Constellation-X by observing the newly formed elements in supernova ejecta.

If a white dwarf has a closely orbiting companion star, it may accrete matter from the companion and become a supernova itself. Such supernovae (called Type Ia) have luminosities that can be calibrated, so that they can be used as standard candles. This means that their apparent brightness can be converted to distance. By measuring the distances and redshifts of many supernovae, it is possible to probe the geometry of the universe (is it flat or curved?) and determine how its expansion rate is changing with time. One of the major goals of stellar research during this decade will be to understand Type Ia supernovae both observationally and theoretically in order to calibrate their luminosities. LSST will aid in discovering large numbers of supernovae, and both NGST and GSMT will enable detailed study of their spectra even when they are at high redshifts.

Stars that are reborn as compact objects have such strong gravitational fields at their surfaces that they radiate high-energy photons when material falls on them, thus making them observable in the x-ray region of the spectrum. Neutron stars and white dwarfs also radiate the thermal energy stored in them at birth, and if they are magnetized and spinning, they can accelerate particles that also radiate. These objects provide laboratories in which matter can be studied under extreme conditions that cannot be duplicated on Earth. For example, the past decade saw the discovery of the theoretically predicted “magnetars,” which are neutron stars with magnetic fields 100 times that of normal neutron stars and a billion times that of the largest static fields in the laboratory. One of the major goals of Constellation-X is to image gas indirectly as it accretes onto a black hole, by studying how its spectrum evolves with time. Another goal is to measure accurately how the radius of a neutron star depends on its mass, which will tell researchers about the properties of matter at nuclear densities.

Gamma-ray bursts are mysterious phenomena discovered by satel-