The Vanishing Star
8 December 1988
Reinhold Häfner, visiting astronomer at the ESO La Silla Observatory, got his life's surprise when the star on the screen in front of him suddenly was gone. All the other stars were still visible, but this particular one had simply vanished.
The mysterious, 17 magnitude star (in the constellation Ophiuchus) has the designation PG 1550+131 and was first observed at the Palomar Observatory in the mid-seventies. At that time, it was found to have an unusual blue colour. Later observations indicated that its brightness varies somewhat, and Dr. Häfner had therefore decided to have a closer look at it. He thought that it might belong to a relatively rare type of stars, known as "cataclysmic variables".
"Cataclysmic variables" are double (binary) stars in which one of the two components has already gone through its entire evolution and has now become a small, compact object. The other star in the system is still in the main phase of its life, burning hydrogen into helium. The two stars orbit around each other, and there is a steady stream of gas from the larger star to the smaller one. This phenomenon sometimes gives rise to an abrupt increase in brightness; hence the term cataclysmic variable".
Less than two dozen binary stars are known which are presently in the state that immediately precedes the "cataclysmic" phase. They are known as "pre-cataclysmic binaries". In this phase, there is not yet a gas stream from the larger component. However, due to scarcity of accurate data, relatively little is known about binary stars in this transitory phase, for instance about the sizes of the stars, their temperatures, masses, orbital periods, etc. But the exact nature of "pre-cataclysmic binaries" must be known in order to fully understand the violent processes during the unstable "cataclysmic" phase.
Thanks to the occurence of pronounced eclipses of PG 1550+131, it may now become possible to learn more about this important "missing link" in the evolution of binary stars. The analysis of exactly how the light changes during an eclipse, allows to determine the physical properties of the system, e.g. the size of the components, the size and shape of the orbit, the distribution of light on the surfaces of the components, their temperatures, etc. These numbers then place constraints on the corresponding values for "cataclysmic variables".
The eclipses in PG 1550+131 obviously happen when the smaller and brighter of the two stars in the system (the one which is more evolved) during its orbital motion passes behind the other star, as seen from the Earth.
The "depth" of the eclipse, i.e. the amount of light lost, is a record 99%, or even more . This means that the secondary component - whose light is the only light left during the eclipse - is more than 100 times fainter than the star which it eclipses. Moreover, the very short duration of the eclipse - another record - indicates that the faint star is also very small; it is termed a "red dwarf" and its temperature is "only" about 3000 degrees.
Contrarily, the brighter of the two stars is of the "white dwarf" type with a surface temperature of 18 000 degrees. Interestingly, that hemisphere of the fainter "red dwarf" which faces the "white dwarf" is heated to about 6000 degrees. The orbital period in the system is only 187 minutes and the distance between the two components is about 700,000 kilometres. That means that the entire binary system could be contained within the space filled by our Sun.
Immediately after the discovery of the eclipses, spectra were obtained of PG 1550+131 with the ESO 3.6 m telescope, and they support this interpretation.
The designation means ``Herbig-Haro type object no. 111''. The astronomers George Herbig (USA) and Guillermo Haro (Mexico) discovered the first objects of this class in the early 1950's, although the underlying astrophysical phenomena have only recently become more fully understood. On celestial photographs they are seen as small, bright nebulae, always in front of large interstellar clouds. The Herbig-Haro nebulae emit light mainly from excited hydrogen, nitrogen, oxygen and sulphur atoms.
More observations are planned for mid-1989, which will hopefully lead to a refinement of the photometric and spectroscopic data. In the meantime, Dr. Häfner has submitted the preliminary findings for publication in the European journal Astronomy & Astrophysics.
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