SAO Research Highlights

Each edition of the SAO e-zine will feature a research highlight from one of the SAO teaching staff, either one of our instructors or project supervisors. This issue, Dr Chris Flynn, an SAO instructor and project supervisor, will discuss his work on new, indirect measurements of the "colours" of the Sun.

Stars have been known to come in various colours since antiquity, with stars such as Betelguese being termed "red-ish", while Rigel appears as "blue-ish", although stars are such pin-prick sources of light, that the colours are difficult to perceive in all but the brightest of them. It was the introduction of photography and spectroscopy into astronomy, over 100 years ago, which showed that star colour is closely related to stellar temperature. The surfaces of stars range in temperature from a few thousand degrees (red) to a few 100,000 degrees celsius (blue-white).

Nowadays, stellar colour is measured by comparing how bright a star appears when viewed through a red and a blue filter. The redder, or cooler, the star, the greater the amount of red light it will emit compared to blue. Conversely, hotter stars emit relatively more blue light than red. The "colour" of a star is defined (more or less) as the ratio of the amounts of red and blue light emitted.

One major problem has been to measure the colour of the Sun in the same system as used for stars. The Sun is so bright and large on the sky, making the same measurements with telescopes designed to detect faint, point-like stars, next to impossible. One way around this is to find stars which are very similar in other properties to the Sun, and from their colours infer the colours of the Sun. This is the technique used by Johan Holmberg, Laura Portinari and myself at Tuorla Observatory, who use the surface temperature of the stars and the Sun to infer the Sun's colours.

A high resolution version of the spectrum of our Sun. The image mimics an echelle spectrum, with wavelength increasing from left to right along each strip, and from bottom to top. Each of the 50 slices covers 60 angstroms, for a complete spectrum across the visual range from 4000 to 7000 angstroms. Credit: N.A. Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF.

An age-old problem with this technique has always been to make sure that the temperature of the stars and of the Sun are measured consistently in the same scale - as different approaches can be used to define and measure stellar temperatures. To do this, the researchers have used the tremendous advances being made in the last two years at the European Southern Observatory's Very Large Telescope, which has been making direct measurements of the surface temperatures of stars using interferometry. The technique shows that surface temperature can be measured consistently for faint stars and for the Sun. This breaks through the old impasse to using this technique for measuring the Sun's colours.

The colours of the Sun have a wide range of uses, because so many stellar and galactic properties are compared to the Sun as the best understood star.

Dr Chris Flynn graduated from Mount Stromlo and Siding Spring Observatories in 1989, and held postdoctoral positions in Germany (Heidelberg), Denmark (Niels Bohr Institute and NORDITA) and the USA (Princeton and Ohio) before getting a permanent position in space based optical astronomy at Tuorla Observatory in Finland in 1999. He served as the Observatory's director in 2000-2001, before taking a 5 year posting as an Academy of Finland Senior Researcher. His research interests include: dark matter, galactic chemical evolution, low mass stars, Helium in the cosmos. Chris renovates old wooden Finnish buildings in his spare time!