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VLT Observations Address the Age of the Universe
December 17, 1999The stream of important scientific results from ESOs Very Large Telescope (VLT) at Paranal (Chile) is increasing. Astronomers have had access to the first of the four 8.2-m telescopes since April 1999 and research articles based on observations with this new powerful facility are beginning to appear in larger numbers. The work reported here is one fine example.
Thanks to the enormous light gathering power and, not least, the excellent optical quality of ANTU, it has been possible for the first time to obtain detailed spectra of several very faint stars in the globular cluster NGC 6397. Earlier images with the Hubble Space Telescope (HST) hinted that they might be White Dwarf Stars (WDs), i.e. Earth-size, hot and extremely dense stars that represent the end products of the evolution of stars like the Sun.
important, have also permitted a first determination of the stellar spectral type, temperature and surface gravity, as well as the mass of four WDs in NGC 6397. In particular, the astronomers learned that the brightest of the four observed WDs has a mass of about 0.35 solar masses, much lower than the mean mass of 0.55 - 0.65 solar masses that is typical for local white dwarfs.
This means that the precise distance determination to a globular cluster, from a comparison of its WD stars to more nearby WDs with well-known masses and distances, will now be possible, provided the individual properties of the cluster WDs are well determined from such spectroscopic observations. The mass strongly influences the brightness of a WD and comparing a low-mass globular cluster WD to a more massive local WD would underestimate the distance to the globular cluster. As these stars are very faint, the spectroscopic observations required for their analysis are very difficult and can only be done with large telescopes like the VLT.
Accurate distances to the globular clusters allow to determine their age accurately. Milky Way globular clusters like NGC 6397 are believed to be the oldest stellar aggregates in our Galaxy. Their age therefore represents a lower limit to the age of the universe (the time since the Big Bang), currently estimated in the range of 13,000 - 16,000 million years. These VLT observations represent one crucial step towards improving both distance and age determinations.
European Southern Observatory (ESO)
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