million degrees) through optical and ultraviolet observations, but the light levels from these regions are very, very low (they were only recently discovered). 2) Faint Nebulae. The following are all types of objects which have been of intense interest to astronomers for the past century: the remnants of supernova explosions (once the interior of stars); material ejected from thermonuclear detonations on the surfaces of white dwarfs; material lifted off the surface of a dying star by its own light and acoustic noise; and material ionized by energetic photons of very hot stars. Not all of these objects are faint, but most of the interesting ones have important properties that require low background levels in order to be studied. 3) Stellar Winds and Mass Loss. Stars are formed from the reservoir of dust and gas in our galaxy. It is easy to show that this reservoir is being depleted at a rate so large that, without replenishment, star formation would be completely arrested in only a billion years (the galaxy is thought to be many billions of years old). It seems certain that replenishment occurs, but it is not clear how. Recent studies have shown that many stars continuously lose their mass through a stellar "wind." Studies of the very faint wind and how the material in the wind finds it way back into the interstellar reservoir are at the limit of modern techniques. 4) Structure of the Galaxy. We now understand the galaxy to consist of spiral arms of gas, dust, and young stars. Outside and between the spiral arms, the structure of our galaxy is poorly understood—partly because the stellar constituents are cool and faint for the most part. Even though billions of stars are involved, their light and problems of foreground dust make studies of these objects very difficult. Studies of stars outside the galaxy in a surrounding "halo" and in extremely old "globular" clusters that appear to have been formed at about the same time as the galaxy are important to our understanding the general problem of galaxy formation and evolution.
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