24 August 2009

Danish star research achieves stardom

The astronomical catalogue, the Tycho-2 catalogue, which contains the positions, motions and luminosity of the brightest 2,5 million stars in the sky, is one of the most used astronometric catalogues in the world. The scientific work was lead by astronomer Erik Høg of the Niels Bohr Institute.

A square degree of the Tycho-2 catalogue

A square degree of the Tycho-2 catalogue. The
two brightest stars are Alcor and Mizar located at the bend in the handle of the Big Dipper

An article was written in the scientific journal, Astronomy and Astrophysics, to announce the catalogue and this article has now had the special honour of being reprinted in a special issue. The journal is turning 40 years old and an issue with the 40 most cited articles has just been published to mark the anniversary. In the 40 years 50.000 articles have been published and among the 40 most cited is the article: The Tycho-2 Catalogue of the 2.5 million brightest stars, of which Erik Høg is the lead author. The article has received 600 citations since publication in the year 2000 and citations continue to be received with the same speed. The Tycho-2 catalogue itself receives an average of 230.000 inquiries a month.

Erik Høg, Emeritus associate professor, has worked with astrometry since working with the meridian circle at Brorfelde Observatory in 1953. Astrometry is the branch of astronomy that measures the positions of stars and other celestial bodies. If measurements are taken very accurately over a number of years then it is possible to calculate the motions of the stars and their distances from the Earth. He developed photoelectric photometry where the individual photons (light particles) are counted through a lattice with photoelectric cells placed behind it. Erik Høg also developed the lattice, through which the light is measured. It was a very sensitive method which meant that much fainter stars could be measured than was previously possible.

Space technology to star positions

Illustration of the Hipparcos satellite

The Hipparcos satellite was launched by ESA in 1989.

In 1975 he was encouraged to take part in the Hipparcos project, which began as a French project to take astrometrical measurements from space. But he was sceptical. ”I did not believe in it, I thought it was completely unrealistic”, explains Erik Høg.

But the project went from being a national project to being a European project under ESA and he was told that he did not have to deal with the original project, but only think about how space technology could be used to measure the positions of stars. ”So a weight fell from my heart, for now I could think freely”, he explained and that is why he agreed to participate in the project.

The lattice that Erik Høg proposed.

The lattice that Erik Høg proposed.
A star runs from one side to the
other while the photons passing
through the slits are registered. 

In the course of just six weeks he designed a new satellite. It was developed and adopted in 1980. A little later he thought that there was some data in the satellite that would not be sent down to Earth. ”We must have the ENTIRE signal down and it should be called the Tycho experiment”, suggested Erik Høg, and it meant that instead of measurements for 100.000 stars, we got 1 million stars. The satellite was launched in 1989 and the measurements continued until 1993.

After the end of the project, Erik Høg, together with two colleagues from the Niels Bohr Institute, C. Fabricius and V.V. Makarov and along with partners from Germany and the US, has been able to, with the help of modern computers, use the records from the Tycho experiment to produce the Tycho-2 catalogue with 2.5 million stars, all in our galaxy, the Milky Way. The catalogue is heavily used in studies of the dynamics and structure of the Milky Way as well as position reference for the large digital Sky Surveys.