Observations with the Hopkins Ultraviolet Telescope (HUT) of the most massive star currently known have revealed new features of its hot outer layers, which are being blown away from the star at speeds of up to 2300 miles per second due to its extreme luminous energy output. These features in turn provide information about physical characteristics of the star, such as its temperature, luminosity, chemical composition, age, and mass, or total amount of matter it contains.
The results were presented today at the meeting of the American Astronomical Society in Pittsburgh, PA. An international collaboration consisting of Drs. Nolan R. Walborn and Knox S. Long from the Space Telescope Science Institute in Baltimore, MD, and Drs. Rolf-Peter Kudritzki and Daniel J. Lennon from Munich University, Germany, reported observations of the stellar record holder with the Hopkins Ultraviolet Telescope, which was operated from the space shuttle Endeavour last March. The HUT data show previously unseen features of the "stellar wind" of HDE 269810 in the Large Magellanic Cloud (LMC).
Analysis of earlier data from the Hubble Space Telescope (HST) and the European Southern Observatory in Chile indicates that this star may be 190 times as massive as our Sun, the largest value to date. Its parent stellar system or galaxy, the LMC, is a relatively small satellite of our giant Milky Way Galaxy, at a distance of 170,000 light-years from us. (A light-year is the distance which light travels in one year at a speed of 186,000 miles per second, or about 6 trillion miles.)
The Hopkins Ultraviolet Telescope is able to probe shorter ultraviolet wavelengths of light than the HST, and so it can observe spectral features not accessible to previous instruments. For instance, one of these features is produced by oxygen atoms from which five electrons have been removed; it is formed in very-high-temperature regions of the star's expanding outer layers. This expansion ejects the material from the star and it is governed by the star's mass, luminosity, and other parameters, which can be derived from the observations. Further analysis of the newly observed features will refine and confirm the extreme characteristics of HDE 269810.
Massive stars have relatively short lifetimes, only a few million years compared to ten billion years for the Sun, because they burn their nuclear fuel more rapidly. They are very important components of the Universe, however, because their nuclear reactions synthesize most of the heavier chemical elements such as the iron in our bridges and our blood. These newly made elements are blasted out into space in the violent supernova explosions with which massive stars end their lives, and they mix with other interstellar material which may form new generations of stars. The material which makes up the Sun and Earth has been enriched with heavy elements made in massive stars which lived and died before our solar system formed.
Because of the large distance of the LMC, it is likely that some massive stars we observe there today have already exploded, but the events are still on the way to us across the intervening distance at the speed of light. The nearest supernova to us seen since the invention of the telescope was observed in the LMC in 1987; of course, the event actually occurred there 170,000 years earlier. The initial mass of the Supernova 1987A progenitor star was only 20 times that of the Sun. Perhaps HDE 269810 will have an even more spectacular demise, although some theories suggest that a star of such extreme mass may collapse completely into a black hole and simply disappear from sight when its nuclear fuel is exhausted, without any accompanying outburst of light and matter.
This work was supported by the National Aeronautics and Space Administration.
Dr. Nolan R. Walborn (410-338-4915)