FUSE What's Out There Between the Stars?

Galaxies are massive collections of stars, gas, and dust. Matter and energy are exchanged between these various components in a grand cycle that changes the chemical and physical properties of galaxies. Stars form from the interstellar material, synthesize chemical elements in their interiors, and return their products to the interstellar gas during their lives and in their death throes. All naturally occurring elements heavier than lithium are produced in stars. The carbon atoms that form the basis of life, the oxygen we breathe, and the silicon in the sand on our beaches were all formed deep inside of stars. The calcium in our teeth, the copper in our coins, and the iron in the steel frames of our cars were formed in massive stellar explosions called supernovae that occur as stars exhaust their nuclear fuel, explode, and reseed the interstellar gas for a new generation of stars.

Understanding how stars and the interstellar medium interact with each other is a major concern of astronomers. The energy produced by stars is shared with the interstellar medium as stellar winds sweep up gas and dust, and stellar explosions create "bubbles" filled with very tenuous, hot gas. This stellar activity can trigger interstellar gas clouds to collapse and form new stars and solar systems.

One of the major predictions of theories for these interactions is that some portion of the interstellar medium should be heated to very high temperatures by all this activity. In the hot gas, atoms are ionized - that is, the electrons that normally surround the atomic nuclei are stripped off the atoms. As the gas cools, some of the electrons reattach to the ions. One of the most important ions that astronomers can observe is a form of oxygen called O VI ("oxygen six"). It is a very good indicator of gas that has been heated to temperatures of one million degrees or more and is cooling as the recombining electrons and ions emit or absorb light.

FUSE is designed to make very sensitive measurements of O VI in the interstellar medium and the remnants of supernova explosions. One of the primary scientific objectives of the FUSE mission is to determine whether a large halo of hot gas surrounds our galaxy. By studying the distributions of O VI and many other atoms and ions, astronomers will be able to determine the composition of the interstellar gas, how well it is mixed, and which processes are effective in heating the gas. All of this information can be used to help us understand how galaxies evolve and form new generations of stars and planets.