The Most Massive Stars

The heaviest stars contain between one and two hundred times the matter of the Sun, yet their lifetimes are much shorter: just a few million years compared to ten billion years for "our" star. That is because they have higher temperatures, so that the nuclear reactions which power them proceed much faster. Also, they produce so much energy that their outer layers are continually being blown off in what are called "stellar winds". The winds and consequent loss of material have significant effects on the life histories of the massive stars. The massive stars are an important component of the Universe, because their nuclear reactions create most of the heavier chemical elements. These nuclear products are blasted out into space in the final supernova explosions with which the massive stars end their lifetimes. These elements enrich the mostly hydrogen and helium content of interstellar gas and dust clouds, out of which new generations of stars may form. For instance, the material from which the Sun and planets of our solar system formed was enriched by the nuclear products of several prior generations of massive stars. We ourselves depend on that process for our existence: the iron in our blood was made inside massive stars which lived and died before the solar system formed. Hence, it is important to study the structure and life histories of massive stars in order to understand the Universe and biological life as well.

The winds and life histories of massive stars depend on their own initial content of chemical elements heavier than hydrogen and helium. Fortunately, the Magellanic Clouds, two nearby, small satellite galaxies of "our own" giant Milky Way Galaxy, have different, lower contents of the heavy elements than our Galaxy. Hence, by studying massive stars in the Magellanic Clouds, we can test the effects of different chemical compositions on their winds and life histories. Signatures of the winds can be seen directly in their short-wavelength (ultraviolet) light, when it is spread into a spectrum by a prism or grating as in the Hopkins Ultraviolet Telescope, because of the wavelength shifts in certain spectral features caused by the expansion speed. HUT is able to observe at even shorter wavelengths than the Hubble Space Telescope or the International Ultraviolet Explorer. Many important spectral features which show the effects of stellar winds are found at these shorter wavelengths, including one produced by oxygen atoms from which five electrons have been removed, corresponding to very high temperatures. Relatively few observations of very massive stars at these shorter wavelengths exist, and HUT is the first instrument capable of obtaining them for a sample in the Magellanic Clouds. In fact, one star in the HUT program, located in the Large Magellanic Cloud, is the current candidate for the most massive star known, with nearly two hundred times the matter of the Sun. It will be very interesting to see and study its far ultraviolet spectrum for the first time.