The universe is full of dust, and it's not all on terrestrial bookshelves. In space, entire planetary systems (a billion miles wide), star clusters (a million billion miles wide) and even galaxies (a billion billion miles wide) are enshrouded in dust. That's a lot of dust, and a big problem for astronomers trying to see through it.
Complicating matters more, the dust itself is invisible, disguising dynamic celestial systems as dark, seemingly empty swaths of sky.
To pierce the dust, on Aug. 25 four years ago, NASA -- in conjunction with Cornell, the University of Arizona, the Smithsonian Astrophysical Observatory and Ball Aerospace -- launched the $800 million Spitzer Space Telescope, an infrared telescope orbiting the sun that will remain operational until its essential liquid helium coolant runs out in early 2009. Spitzer is the last of NASA's four Great Observatories (including the Hubble Space Telescope, the Compton Gamma Ray Observatory and the Chandra X-ray Observatory).
Jim Houck, the Kenneth A. Wallace Professor of Astronomy at Cornell and leader of Cornell's Spitzer team, an early proponent of Spitzer's infrared spectrometer instrument, believes the orbiting telescope's greatest accomplishments are yet to be revealed: "Right now everyone is working hard on individual projects; connecting those requires time for reflection." Houck, who plans to throw a picnic on Spitzer's anniversary, notes that the immediate goal is accomplishing enough to inform the next infrared mission, the James Webb Space Telescope, in 2013.
By looking in the infrared -- light made of "colors" below the red end of the visible spectrum -- Spitzer sees what optical telescopes like Hubble can't. That's important because dust transmits infrared starlight but absorbs visible starlight. The absorption then heats the dust, making it glow in the infrared. Even where dust is thin, such cooler objects as exosolar planets, brown dwarfs (often called failed stars) and giant molecular clouds shine more brightly in the infrared. Many molecules in space, including organics, also have unique signatures in the infrared.
Although not the first infrared space telescope, Spitzer is 100 times more sensitive than its predecessors: If they are compared to feather dusters, Spitzer is the power dust-buster of the universe.
"Spitzer does things that were simply impossible before," said Vassilis Charmandaris, associate professor of physics at the University of Crete and former Cornell research associate. "It did more than improve on previous infrared telescopes: It opened something completely new, similar to what the rovers did for Mars."
"Without Spitzer we would have a much less complete understanding of the infrared universe, which means a much less complete understanding of dust," added Greg Sloan, a Cornell astronomy research associate who uses Spitzer to study how old supergiant stars (the source of all dust) spew dust into their surroundings. Before the first generations of stars grew old, the universe was filled with just hydrogen and helium gas. "Dust acts as a repository for heavier elements and allows processes that can't occur with gas alone," explained Sloan. "It was dust grains that first accreted into planets: no dust, no Earth; no Earth, no life. The dust is pretty important."
For similar reasons, regions lacking dust -- particularly blue compact dwarf galaxies (BCDs) -- draw the attention of Houck and his team. BCDs, named for their bluish tint and smallness, resemble young galaxies that may have lived just a few hundred million years after the big bang, before there was much dust. Because they are nearby, explained Houck, BCDs serve as "laboratories" for studying how early galaxies evolved into large dusty ones we see today, including our own Milky Way. Before Spitzer, only four BCDs were studied in the infrared -- it's now over 60.
After several billion years of growth, these galaxies may merge into ultra-luminous infrared galaxies (ULIRGs) through colossal collisions, studied by Cornell research associates Henrik Spoon and Duncan Farrah. These collisions convert extraordinary amounts of dust into new stars, producing the light of a trillion suns. Once rarely observed, Spitzer has now cataloged tens of thousands of ULIRGs. They may signpost the peak of star formation in the universe. "The universe now is fairly quiet and placid, but 6 billion years ago incredibly violent ULIRGs dominated -- we aren't sure why things changed," said Farrah.
Certainly, Spitzer is unveiling the universe's life cycle in unprecedented fashion. "I'm seeing things as a complete ecosystem," said Sloan. "How do you get from primordial hydrogen and helium to small galaxies to big galaxies and stuff you can make life from? The dust makes it happen."
Graduate student Thomas Oberst is a science writer intern at the Cornell Chronicle.