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.
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.
The ubiquity of rapidly expanding stellar winds from OB stars is one of the most unexpected and important discoveries of the early NASA space program. X-rays from O and B stars are thought mainly to arise from their massive supersonic winds, either in radiatively driven shocks, shocks from colliding winds in binaries, or perhaps from dissipation of energy in wind-embedded magnetic fields.
Surveys of the X-ray sky by the Einstein satellite in the late 1970's established normal stars as copious sources of X-rays. In the case of late-type stars like the Sun, X-rays are produced by a "corona" - a multi-million degree plasma confined by magnetic fields generated in the stellar interior and brought to the surface by their own buoyancy.
Planetary nebulae (PNe) are shells of gas and dust that have been ejected from a star during the process of its evolution from a hydrogen-burning main sequence star into a red giant and eventually into a white dwarf. The lifetimes of PNe are relatively short, on the order of 20,000 years, so they are relatively rare, with about 1,500 known in the Galaxy. The nebulae are approximately a light year in size and are illuminated and ionized by the hot central star. The ejected material is an important source of mass return and enrichment of the interstellar medium.
The outer "halo" region of the Milky Way, as with other galaxies, contains the vast majority of the mass of the Galaxy. Yet the exact mass and shape of the halo are not known. Unlike the Galaxy's spiral arms, which contain bright stars, the halo is mostly dark. The few stars in the halo are the oldest stars in the Galaxy, and include the globular clusters, but they can account for only a small fraction of the halo's mass. Understanding the exact nature of the Milky Way halo may help to explain the mysterious dark matter, which seems to pervade the universe.
We have developed a new technique using laser frequency combs to improve the accuracy and stability of wavelength calibration of astrophysical spectrographs by up to two orders of magnitude. This "astro-comb" will provide a key advance in the resolution of changes in astrophysical Doppler shifts and redshifts, and thus may allow the discovery of Earth-like planets and new measurements of astrophysical dynamics relevant to cosmology.
All stars vary in brightness. On geological time scales, main sequence stars become bright red giants and end their lives as dim white dwarfs, neutron stars, or black holes. As stars evolve, some pulsate regularly on time scales ranging from seconds to years. Other stars vary irregularly. SSP scientists study these variations to learn about the structure and life history of stars.
Cool stars like the Sun are surrounded by a corona, a million degree plasma extending far beyond the visible photosphere. The stellar wind is ionized gas ejected from the outer portions of the corona at speeds of hundreds of km/sec. SSP scientists are world leaders in applying spectroscopy to study the physical conditions of stellar coronae and winds and in developing theoretical models to explain these observations.
In the last two decades, new discoveries have improved our understanding of other planetary systems. By measuring the brightnesses and motions of nearby stars, astronomers have detected more than 200 planets in 170 planetary systems. Most planets are gas giants like Jupiter or Neptune. Others are icy super-Earths. SSP scientists use ground-based and space-based instruments to detect and to characterize exoplanets.