Washington DC
ASTRONOMERS have discovered the Universe鈥檚 true colour鈥攊t鈥檚 somewhere
between pale turquoise and aquamarine. This finding is helping them trace the
history of star formation.
Ivan Baldry and Karl Glazebrook from Johns Hopkins University in Baltimore
identified the universal hue by bringing together light from over 200,000
galaxies within 2 billion light years of Earth.
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Combining the light gave a peak in the blue part of the optical spectrum,
from the large number of young stars burning hydrogen, and another in the red
part of the spectrum, which comes from the glow of older red giants burning
heavier elements.
Astronomers routinely use the sizes of the peaks in an individual galaxy鈥檚
spectum to work out how old its stars are, but this is the first time anyone has
calculated a spectrum for enough galaxies to be representative of the whole
Universe.
Baldry and Glazebrook calculated how their spectrum would look to the human
eye, and came up with a pale green. 鈥淚t鈥檚 not my favourite colour,鈥� admits
Glazebrook, 鈥渋t鈥檚 on the greenish side of white.鈥�
From the spectrum, Glazebrook concludes that star formation peaked between 1
and 2 million years after the big bang. At that time, the Universe was mainly
blue, as a result of the large number of young stars. But it鈥檚 now getting
greener, and will eventually turn red as stars age over the next 5 billion
years.