The Ghosts of Dead Stars

Although many stars like our sun can remain stable for billions of years, more massive stars can race through their entire life cycles in a relatively short 10 million years or so, ending in a cataclysmic explosion called a supernova that literally tears the aging star apart. A supernova remnant is the expanding gaseous nebula created by these titanic explosions. Supernova remnants are of interest to many areas of astrophysics. Young supernova remnants (less than about 1000 years after the SN explosion) still show large enrichments from the heavy elements that were created deep inside the star before it exploded. Observations of the light from these objects provides our only direct test of the process of nucleosynthesis, whereby lighter elements are fused into heavier elements in the centers of stars.

However, some supernova remnants remain visible for tens of thousands of years before finally blending back into the general interstellar medium, or tenuous gas in the regions between the stars. As a supernova shock wave travels outward from the point of the explosion, it sweeps up the gas of the interstellar medium into a roughly spherical, expanding shell. This gas "glows" because of heating from particle collisions. The light given off from these objects spans the entire electromagnetic spectrum from X-rays and ultraviolet light through the optical, infrared and radio. Analysis of this light provides information on the chemical composition and physical conditions (temperature, density, and variations thereof) in these normally invisible regions of interstellar space. Hence, supernova remnants can be used as probes of the structure of the interstellar medium, as well as providing information on the physics of shock waves in conditions much different from those that can be created in a laboratory on earth. Shock waves from supernovas play an important role in energizing the interstellar medium and by compressing clouds of gas and dust in the interstellar medium may be responsible for starting new cycles of star formation. (Click here for an image)

HUT's combination of a sensitive detector, good spectral resolution, and far ultraviolet spectral coverage will provide a unique new capability for studying supernova remnants. The cooling gas behind shock waves in supernova remnants gives off emission lines from many elements and temperature zones in the shock. The primary HUT spectral bandpass (830 - 1860 angstroms) contains many important emission lines, some of which sample shock temperatures as high as 300,000 degrees. This temperature is intermediate between X-ray temperatures and previously sampled UV/optical emission temperatures. As such, HUT will provide unique information on the structure of and physical conditions in supernova remnant shock waves. HUT will observe several different supernova remnants to sample the conditions in shock waves of various velocities and interstellar environments. Also, HUT observations will permit more accurate chemical element determinations to aid our understanding of nucleosynthesis and the chemical make-up of the interstellar medium. In combination with data at other wavelengths, HUT spectra will contribute to a much better physical understanding of these "ghosts" of dead stars.

William P. Blair