J. Brit. Astron. Assoc., 107, 2, 1997, p. 59

Hipparcos results change the distance scale of the Universe

Results from ESA's Hipparcos astrometric satellite, announced at a meeting of the Royal Astronomical Society in London on February 14, imply that the Universe may be about ten percent larger than was previously thought. This has profound implications for cosmology, and in particular the results suggest a resolution of the paradox whereby the Universe appeared to be younger than some of its oldest stars.

Hipparcos was launched by the European Space Agency in 1989 and operated until 1993. During these four years it observed and fixed the positions in the sky of 120,000 stars, to a precision of one thousandth of an arcsecond: the equivalent of observing a child standing on the surface of the Moon. Computations of the raw data are now complete and the positions are published as the Hipparcos Catalogue. A further million stars (the Tycho Catalogue) have been logged with slightly less precision, but still many times more accurate than previous surveys. Measurements of the changes in the apparent positions of stars as the Earth moves round its orbit (parallax) provide the most accurate and straightforward method of judging their distances.

The Hipparcos data were used by Prof. Michael Feast of the University of Cape Town to study the distances of Cepheid variable stars, the 'measuring rod' by which the size and scale of the Universe is gauged. Prof. Feast's results suggest that Cepheid stars in our own galaxy are about ten percent more luminous and further away than the previously accepted figures.

In 1912, Henrietta Leavitt discovered that the pulsating variables known as Cepheids follow a strict relationship whereby their pulse rate depends on their absolute luminosity. The distances to other galaxies can be judged by the apparent brightness of Cepheids seen varying in them – providing the distances to the local Cepheids (which calibrate the scale) are correctly estimated. Local Cepheids are typically 1000 to 2000 lightyears away, which even with the Hipparcos data is too far to measure directly using the parallax method. However, Michael Feast and his colleague Robin Catchpole at the RGO compared the new data for 26 local Cepheid stars, and arrived at consistent statistics that have redefined the period–luminosity relationship needed to judge their distances. When the new relationship is applied to existing data on Cepheids in other galaxies, the Hipparcos results increase their distances. For example, the Large Magellanic Cloud (LMC), previously thought to be at 163,000 lightyears, is found to be 175,000 lightyears away, an increase of ten percent.

Using the Cepheid results together with data from Mira and RR Lyrae type variable stars, Feast and Catchpole calculate the ages of globular clusters in our own galaxy, which contain some of the oldest stars in the Universe. They arrive at a figure of 11 billion years for the age of the oldest stars. Previous estimates had suggested they may be as old as 14 billion years, which seemed a nonsense, as it implied they were older than the Universe. Data from the Hubble Space Telescope recently suggested the Universe was 9 to 12 billion years old. Applying the latest Hipparcos results increases that range to 10–13 billion years. As Michael Feast said, 'If we can settle on an age of the Universe at, say, 12 billion years then everything will fit together nicely.'

Hazel McGee


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