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Dense Cloud Breaks Rules of Star Formation
The Spitzer Space Telescope’s view of the Galactic Center. G0.253+0.016 is so dense that even infrared light cannot penetrate this cloud. The cloud is seen as a dark silhouette in the left part of the image. The Galactic Center Black Hole—also known as Sgr A*—is very close to G0.253+0.016.
Credit: Jens Kauffmann, California Institute of Technology

This Behind the Scenes article was provided to LiveScience in partnership with the National Science Foundation.

Researchers are discovering new information about a dark bean-shaped cloud in the center of our galaxy. The cloud, G0.253+0.016, is unusually dense — so dense, that it blocks infrared light, which usually penetrates even the densest parts of clouds in space.

Dense clouds usually produce massive stars, yet this particular cloud, which spans 30 light years in length, has minimal star formation.

"This is surprising, since many clouds of lower mass and density form a much larger number of stars," explained Jens Kauffmann, a senior postdoctoral scholar in astrophysics at the California Institute of Technology. "In principle, the cloud contains enough mass to build about 200,000 stars like our sun."

Kauffmann along with postdoctoral scholar Thushara Pillai of the California Institute of Technology and astrophysicist Qizhou Zhang of the Harvard-Smithsonian Center for Astrophysics, are seeking to learn more about this mysterious cloud. They hope to better understand star formation in our galaxy and in other galaxies, and the role of star formation in the early universe.

To form stars from dense gas, a cloud increases in density until it collapses due to gravity. As the cloud collapses, the densest gas further clumps up and eventually forms stars.

"Think of a house of cards — you can build it up and up, but at some point it will become so heavy that the cards can't keep up and they collapse," explained Kauffmann. "In the case of star formation, clouds do something very similar: they collapse under their own weight and finally form stars."

In most cases, "the denser the cloud, the more prone it is to collapse and form stars," said Pillai.

For example, the Orion Nebula is relatively dense and is a huge star-forming region. The high-density bean-shaped cloud is 25 times denser than Orion, but almost completely starless.

To learn why, the researchers observed it with high powered radio telescopes: the Submillimeter Array (SMA), a collection of eight radio telescopes on top of Mauna Kea in Hawaii; and the Combined Array for Research in Millimeter-wave Astronomy (CARMA), a collection of 23 radio telescopes located in the Inyo Mountains of California.

Using the SMA and CARMA, the researchers measured the density and the velocity of the gas within the cloud. The researchers found that gravity is barely holding the cloud together and they also observed that the cloud is "extremely turbulent," said Pillai. The turbulent motion prevents the dense gas from settling, and as a result, there are fewer pockets of dense gas to form stars. In this unstable environment, it's not clear whether young massive star clusters can form. Over time, the turbulence could actually tear the cloud apart.

Infrared images of the cloud G0.253+0.016, obtained using the Spitzer Space Telescope, show a very dense and massive cloud obscuring the stars located behind it. New observations obtained using the Submillimeter Array (SMA) reveal just few star-forming “cores” of dense gas, and these cores have a relatively low mass. Data from the Combined Array for Research in Millimeter-wave Astronomy (CARMA) gives a more global picture of the cloud. This shows that the cloud is highly turbulent, which possibly suppresses the formation of stars.
Infrared images of the cloud G0.253+0.016, obtained using the Spitzer Space Telescope, show a very dense and massive cloud obscuring the stars located behind it. New observations obtained using the Submillimeter Array (SMA) reveal just few star-forming “cores” of dense gas, and these cores have a relatively low mass. Data from the Combined Array for Research in Millimeter-wave Astronomy (CARMA) gives a more global picture of the cloud. This shows that the cloud is highly turbulent, which possibly suppresses the formation of stars.
Credit: Jens Kauffmann, California Institute of Technology

Kauffmann explained that turbulence in the cloud functions similarly to sand in a glass of water — the sand acting as the gas in the cloud:

"Imagine a glass of water and mix sand into it ... the sand will settle at the bottom. Now, stir it up, which creates turbulence … the sand is lifted up and doesn't settle right away. The sand will only settle once the turbulence goes away. Now, our cloud is unusually turbulent. Like the sand in the glass of water, the turbulent dense gas may never clump up to form stars."

Currently, the researchers are trying to find out what is causing the turbulence. In other words, they are seeking to identify the "spoon" stirring the sand.

From these observations, the researchers determined that the environment within the cloud wasn't conducive to forming stars, with the exception of one particular section.

"There are signs of star formation in one particular location in G0.253," said Zhang.

Zhang explained that the next question was whether or not the cloud could form more stars in the future.

To explore this question, the researchers used the data from the telescopes to consider how the cloud will evolve over time. They hypothesized another possible fate for the cloud — to be ripped apart by tidal forces, or unequal gravitational forces exerted between the cloud and the center of the galaxy.

Ultimately, determining the future of the cloud is challenging. It is even possible that this cloud could collide with others or fall into the black hole in the center of the galaxy.

"It's very difficult to predict this all," said Kauffmann. "We only know how it looks at the moment. We do not have the means to access how [the cloud] will look in one hundred thousand years."

Currently, the National Science Foundation-funded researchers are using the SMA and CARMA to study a half-dozen clouds in the center of the Milky Way similar to G0.253+0.016. In 2013, the researchers will also have the most advanced radio telescope in the world to assist them in their studies, the Atacama Large Millimeter/Submillimeter Array (ALMA), which consists of 66 radio telescopes and is located in the Atacama Desert in Chile.

They hope to further their understanding of star formation by studying these clouds and their potential to produce massive stars in similar environments.

Editor's Note: The researchers depicted in Behind the Scenes articles have been supported by the National Science Foundation, the federal agency charged with funding basic research and education across all fields of science and engineering. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. See the Behind the Scenes Archive.