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Academia
Fermi project could alter
how we view galaxy
By Gary Ruderman
Special to the
Tribune
Published March 11,
2002
The field guide to the universe is slowly
taking shape at the Fermi National Accelerator Laboratory near
Batavia.
Hundreds of gigabytes of digital pictures are delivered
weekly to Fermilab for compilation that, over the next three years, will
deliver the most comprehensive map of the hundreds of millions of galaxies
that make up what astronomer Carl Sagan called the "cosmos."
Like
the human genome project used microscopes to look deep within us, the
Sloan Digital Sky Survey uses telescopes to look toward the universe to
discover our place.
The Sloan survey's results are as immense as
the cosmos.
The main telescope, in the mountains of south-central
New Mexico, is picking up light from 14 billion years ago--a time when the
universe was just 6 percent of its current age. Focusing on one-quarter of
the sky in what is known as the northern galactic hemisphere, the Sloan
survey (www.sdss.org) has compiled enough data on the heavens in the last
36 months to fill 1,000 laptop computers, each with a 10 gigabyte hard
drive, and it's just 35 percent of the way through its
mission.
"This is what we call industrial astronomy," said Chris
Stoughton, the head of data processing and distribution at Fermilab. "In
the old days you'd take 10 pictures a night and write a paper" on the
findings. "This was one-by-one, hand astronomy.
"Sloan produces 600
pictures through five different filters every hour. That's industrial
astronomy, and enough to keep an astronomer busy for a year."
And
while Sloan's work may not affect our daily lives, its discoveries are
shaking up the world of astronomy. In addition to locating quasars at the
beginning of time (about 15 billion years ago), Sloan scientists have
identified new star structures that could alter how we view the galaxy and
a new class of celestial objects called brown dwarfs.
Scientists
from Fermilab and Rensselaer Polytechnic Institute in New York recently
presented their findings on the discovery of new star structures in the
halo of the Milky Way. Isolated from the 5 million stars logged so far by
the Sloan study, the new stars appear to be clumped in a "puffier"
configuration rather than what we're used to seeing as a flatter,
spherical look of the galaxy.
Before the latest Sloan findings, the
first and only "cool" brown dwarf star, which has properties of a planet
and a star, was discovered in 1995. They're called "free floating"
structures because they don't orbit a star or planet. The most recent
discoveries are a mere 30 light-years away.
"They are still so new
to astronomy that they require a new vocabulary," said astronomer Tom
Geballe of the Gemini observatory in Hawaii. "The name `methane dwarf' has
emerged, because of the dramatic presence of bands of methane in their
spectra. Methane is characteristic of giant planets, like Jupiter, but it
never appears in normal stars--they are much too hot--or even in most
brown dwarfs."
World's most powerful camera
While there are
many areas of interest to the non-astronomer or astrophysicist, the camera
and the information it gathers are probably most interesting to the
technologist.
The digital camera built for the Sloan Survey is the
world's most powerful, according to Jim Gunn, professor of astrophysics at
Princeton University, a Sloan project scientist and builder of the camera.
The digital camera detects 7 out of 10 photons (particles of light without
mass) hitting the lens, netting an efficiency approaching 70 percent. The
efficiency of a standard film camera is about one-tenth of 1 percent, Gunn
explained.
What makes the Sloan camera so efficient is its use of
charge-coupled devices. These light-sensitive squares detect the intensity
of incoming photons and convert light into digital signals. Each detector
is rated at 4 megapixels, giving the 30-CCD array a whopping
120-megapixels (120 million pixels) sensitivity. The higher the number of
pixels, the greater the resolution.
Gunn said the array of CCDs
took four years to accumulate partially because of exacting specifications
that meant as few as 1 of 3 CCDs shipped from the manufacturer were
accepted. "They're very expensive and we have only a few spares," Gunn
said.
The CCDs were half of the telescope's $5 million cost, paid
for by the Japanese government's Monbukagakusho scholarship program. The
entire survey will cost around $85 million, with the largest private
donation of around $20 million coming from the Alfred P. Sloan
Foundation.
Another $42 million came from Fermilab through the U.S.
Department of Energy, the National Aeronautics and Space Administration
and the National Science Foundation. Other underwriters include Johns
Hopkins University, the University of Chicago, the University of
Washington, New Mexico State University, the U.S. Naval Observatory and
Germany's Max Planck Institutes.
The 700-pound camera, which took
seven years to build, is a meter-long, vacuum-sealed cylinder cooled by
liquid nitrogen to keep moisture out. In addition to the two mirrors found
in a reflector telescope, two lenses focus incoming light to the 30 CCDs
that make up the camera. Two spectrographs are mounted nearby.
A
fifth of the CCDs are receptors for ultraviolet light, and another
three-fifths captures green, red and near-infrared light. The remaining
CCDs concentrate on getting what's called far-infrared light, and that's
where amazing things happen.
That "far" category reaches back
almost to the beginning of time.
Overall, the Sloan telescope picks
up light from an immense number of galaxies and quasars, estimated by the
astronomers at 100 million. They range in age from as young as two-tenths
of the age of the universe to events that occurred just a billion years
after the universe was formed and whose light is just now reaching the
camera.
When a night of observation is over, the millions of bytes
of data are written to 20 gigabyte data storage tapes in a protocol known
as flexible image transport system. The findings of five CCDs are recorded
on one tape.
Each tape and a backup copy are sent overnight to
Fermilab, where they are transferred to a host of Linux servers. Stoughton
said the amount of data is small compared with Fermilab's other projects
but is the largest capacity project ever assembled in
astronomy.
Most images go unseen
Surprisingly, with all the
money and time spent in the quest for a road map of the celestial past,
"most of the pictures have never been looked at," Stoughton said.
Stoughton said that because of the immense amount of information seeing
any part of it would take a lifetime.
Instead, investigators are
writing programs to look for specifics like "low surface brightness
galaxies," which could be described as wide--not
bright--galaxies.
So with all the data that few have seen, and few
practical business applications, it seems to raise the question as to why
are they mapping the universe.
"It's hard to say why people should
study astronomy," said Gunn. "But in the scheme of human intellect, it is
important to know where we came from and what's likely to be in store for
us."
Copyright © 2002, Chicago Tribune
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