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C2D: Before Planets and Life, There Are Stars and Disks

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The C2d Legacy Project captured this image of stars forming in a cosmic cloud in Perseus.
Credit: NASA/JPL-Caltech

What would a universe without stars look like?

Besides cold and dark, it would also look relatively empty and lifeless, as chemical elements like carbon, nitrogen, oxygen, and most others which are necessary to create planets and sustain life wouldn't exist. Almost all chemical elements other than hydrogen and helium are born inside stars and spread across the universe when stars die.

Despite their importance, much about the birth and early life of stars remains a mystery. That's why a team of astronomers on a Spitzer Legacy project called "From Molecular Cores to Planet-Forming Disks," abbreviated "c2d," are pointing all three instruments aboard NASA's Spitzer Space Telescope toward young stars forming in a variety of nearby cosmic clouds.

"The main goal of our project is to study the nature of star formation and the beginnings of planet formation," said Dr. Neal Evans, principal investigator of the c2d project.

Seeking Invisible Stars

Before c2d astronomers can learn about the birth and early life of stars, they must first identify a sample to study. Since star formation is an extremely dusty process, some astronomers note that recognizing infant stars may be easier said than done.

Stars are born from the collapse of dense gas and dust patches deep inside a cosmic cloud. For much of their early life, stars are visibly hidden behind the dust that created them. Although astronomers cannot see infant stars, they can detect their heat with infrared telescopes like Spitzer.

According to c2d co-investigator Dr. Lori Allen, identifying baby stars in a large cosmic cloud is very similar to looking down on the Earth at night and searching for small mid-western towns.

"When you're looking down on the Earth at night, big cities like Los Angeles and New York are easy to spot because their city lights are bright and concentrated, whereas small town lights are not, so they are harder to find," says Allen.

"When you are looking at a star-forming cloud, it is very much the same. The areas that contain large concentrations of stars are easier to see and study," she adds.

Allen notes that past infrared telescopes like, the Infrared Astronomical Satellite (IRAS), could map the entire sky but were only sensitive enough to see the "large cities" of stars. With the launch of Spitzer (the most sensitive infrared telescope ever flown) astronomers can see the "small towns," or regions were faint baby stars are just beginning to form.

"Thanks to Spitzer's superb sensitivity, we've greatly increased the number of young known stars in some of these clouds," says Evans.

Spitzer Studies Stellar and Planetary Beginnings

Once a small town of infant stars is identified, astronomers can gather details about how stars grow and develop by combining observations from all three instruments aboard Spitzer.

Currently astronomers do not know how a dense cloud patch collapses to form a star, how a baby star develops, or what initiates planet formation around a star. Because stars and planets form over thousands (sometimes millions) of years, scientists cannot learn about their formation by watching a single star or planet grow. Instead, they have to piece together the story of stellar and planetary development by taking snapshots of a variety of stars and planets at different life stages. In other words, by comparing an image of an "embryonic" star, an "infant" star, and a "toddler" star, astronomers can discern details about the "childhood" of all stars.

C2D team members obtain images of "stellar embryos" using Spitzer's Multiband Imaging Photometer (MIPS) instrument. In this development stage, the dense cloud patch is collapsing to form a star, and the embryonic orb is not massive enough to "light up" visibly by igniting nuclear fusion in its core. Visible-light telescopes and instruments cannot see a star in this phase, but Spitzer's MIPS instrument can. As the dense cloud patch collapses, it heats up slightly. MIPS' sensitive far-infrared eyes can detect this heat, allowing the instrument to capture never before seen pictures of stellar embryos.

Pictures of "stellar infants" are captured with Spitzer's Infrared Array Camera (IRAC). Stars in this stage are obscured from visible-light view by the cloud of gas and dust that created them. Eventually, some stars will develop winds strong enough to blow away their surrounding natal material and reveal themselves to the universe. Meanwhile, other stars will turn this birth material into planet-forming disks, astronomically known as "protoplanetary disks." Team members studying the early stages of planet formation also use IRAC to look at protoplanetary disks.

To gather insights into the environments where stellar embryos are forming, stellar infants are growing, and planets are developing, c2d astronomers turn to Spitzer's Infrared Spectrograph (IRS) instrument. According to Evans, IRS allows scientists to detect how much dust, ice, and other materials are present in the region. He notes that the instrument's superb sensitivity even allows astronomers to determine the kind of ice that is coating the dust grain, whether its carbon, hydrogen, nitrogen, or oxygen rich ice.

"Knowing that dust grains in a protoplanetary disk are coated with ice rich in molecules containing hydrogen, oxygen, carbon, and nitrogen, could be influential in helping us determine whether planets like Earth are common in the universe," said Evans. "All of these molecules are also necessary for the origin of life."

C2D Legacy

According to Allen, c2d's legacy to future astronomers lies in its unprecedented detailed infrared maps of approximately 100 dense star-forming regions, and five large nearby cosmic clouds, each located less than 1,000 light-years away.

In space, clouds absorb visible starlight from all the stars behind and inside of it. Thus, through a visible-light telescope lens, cosmic clouds look like big, dark, featureless blobs. With sensitive dust-piercing infrared telescopes like Spitzer, astronomers can see the heat emanating from stars inside and behind the cloud.

"While the great sensitivity of Spitzer allows c2d to find all the young objects in a cloud, it also captures background star-forming galaxies beyond the Milky Way," said Evans. "Because galaxies are so far away, they look like dots in our data and can be confused with stars. We actually spent a lot of time trying to weed galaxies out of our sample."

Similar to the way IRAS all-sky maps helped to point Spitzer in the direction of "small stellar towns," Allen notes that c2d maps created with Spitzer will point future, more sensitive infrared missions like the Herschel Space Observatory and the James Webb Space Telescope in the right direction, so that they may gain deeper insights into the early development of stars.

"Our maps may even help direct the Terrestrial Planet Finder mission toward terrestrial planets in other solar systems," she adds.

"My hope is for future astronomers to use our data in ways that we never thought possible," said Evans.



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