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 Betelguise being termed red-ish,
while Rigel appears as a 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 their
temperatures, 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 system. 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 from their colours
infer 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
Posted 7th February, 2006.
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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 |