The universe is dustier than previously thought, which is why astronomers now suggest it is twice as bright as it appears.
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.)
Interstellar dust is 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.
Source: Xinhua/Agencies
|
Comment
Tell A Friend
Print Format
Save Article
http://english.people.com.cn/90001/90781/90876/6412633.pdf