Tuorla Observatory News on 7 February 2006

The colours of the Sun

Astronomers at Tuorla Observatory have made a new, indirect measurement of the "colours" of the Sun.

Stars have been known to have come in various colours since antiquity, with stars such as Betelgeuse being termed reddish, while Rigel appears blueish, 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 the star's temperature, in the same manner that a metal glows first red and finally blue-white as it is heated up. 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.

Over many decades, astronomers have put together systems of light filters for measuring the properties of stars just from their colours; physical properties, such as temperature, surface gravity, chemical composition and intrinsic luminosity.

One major problem has been to measure the colours of the Sun in the same manner. It's a nice quandary that the Sun is so bright and large on the sky, making the same measurements with telescopes designed to detect extremely faint, point-like stars, is next to impossible. One way around this is to find stars which are as very similar in other properties to the Sun, and infer from them the colours of the Sun. This is the technique used by Johan Holmberg, Laura Portinari and Chris Flynn at Tuorla Observatory. The researchers use the surface temperature of the stars and the Sun to infer the colours.

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 the Sun. This breaks through the old impasse to using this technique for measuring the Sun's colours.

The colours of the Sun in the Johnson/Cousins, Tycho, Strömgren, 2MASS, and SDSS systems.

The study has been accepted for publication in the Monthly Notices of the Royal Astronomical Society. Preprint here.

Measuring the colours, or distribution of energy in various wavelength passbands, is easy for stars, but next to impossible for the Sun. Too big and too bright, the Sun overwhelms the sensitive instruments installed around the globe on astronomical telescopes. One way around the impasse is to find stars as similar to the Sun as possible and use their properties to infer the Sun's colours. Image of the Sun from NASA.

This page was last modified by  Rami Rekola  on  31/05/2006 11:32  astroweb@utu.fi