The universe is twice as bright as it appears, astronomers
now suggest.
The light bulb went on when they calculated that dust blocks
about the half the light emitted from stars and galaxies.
Astronomers have known about interstellar dust for a while,
but they haven't been able to quantify just how much light it blocks. Now a
team of researchers has studied a catalogue of galaxies and found that dust
shields roughly 50 percent of their light.
"I was shocked by the sheer scale of the effect,"
said Simon Driver, an astronomer from the University of St. Andrews in Scotland
who led the study. "Most people just kind of said, 'We suspect dust is a
minor problem.' I spent much of my career working on deep images from Hubble
and I've always ignored dust almost entirely."
The result will likely cause many astronomers to revise
their calculations of the intrinsic brightness of many celestial objects,
Driver said. Until now, many astronomers thought stars and galaxies were really
about 10 percent brighter in optical light than they appeared because of dust.
If the new findings are true, it turns out that objects in the sky are about
twice as bright than they appear.
"This is a strong, clear-cut result," Driver told
SPACE.com. "We've really got to take dust seriously and we've got to
make large adjustments to our magnitude calculations." (A magnitude scale
is used to define brightness of celestial objects.)
The astronomers detailed their findings in the May 10 issue
of the Astrophysical Journal Letters.
Interstellar
dust isn't exactly the same thing that coats our bookshelves and covers our
TV screens. It's made up of lumps of carbon and silicates that form dust grains
only a few thousandths of a millimeter long. It hangs out in galaxies, but
generally steers clear of the space between them.
To calculate dust's effect, the researchers analyzed data
from the Millennium Galaxy Catalogue, a collection of images of 10,000 galaxies
compiled by Driver and his team using the Isaac Newton Telescope on La Palma
and others.
They counted the number of galaxies in the catalogue that
were directly facing us, and compared it to the number that were tilted 90
degrees away from us. Without dust, they reasoned, they should see just about
equal numbers of galaxies in each orientation. But with dust, they would likely
find fewer edge-on than face-on galaxies. Since dust lies in the disks of
spiral galaxies, and not the dense central bulge,
when we view galaxies from the side we are looking through thicker layers of
dust, so we should see less light. In fact, the researchers counted about 70
percent fewer edge-on galaxies than face-on galaxies.
They used this discrepancy to quantify dust's effect by
combing their counts with a model of dust distribution in galaxies developed by
Cristina Popescu of the University of Central Lancashire and Richard Tuffs of
the Max Plank Institute for Nuclear Physics.
"It's been a revelation to many people in the
community, but there are small groups that had a suspicion this was
coming," Driver said. "I wouldn't be surprised if there's a
refinement of the result, but I think the result is basically here to
stay."
The research was funded by the Science and Technology
Facilities Council, the Australian Research Council, the Max-Planck Society and
a Livesey award from the University of Central Lancashire.
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