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Young stars blamed for space cloud ripples

Young, massive stars have been caught setting off ripples through a giant space cloud in the same way that wind drives waves on the ocean.
Image: Infrared images of stars
Top: near-infrared image of the Orion nebula. The massive stars are in the bright region. Bottom: Zoom on the region of the waves shown at mid-infrared (green), and radio wavelengths (red). The mid-infrared component shows the emission of warm small dust particles, while the radio emission comes from the cold gas. Each inset corresponds to a different velocity of the gas, observed with the IRAM 30m radio-telescope.NASA-JPL & Caltech, ESO-VISTA
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Young, massive stars have been caught setting off ripples through a giant space cloud in the same way that wind drives waves on the ocean.

The intermittent ripples were seen in a molecular cloud associated with the Orion nebula, a well-known stellar nursery within the constellation Orion.

A new study led by Olivier Bernof Leiden University in the Netherlands found that intense radiation emitted by nearby stars more massive than our sun was at the root of the process. The research is detailed in the Aug. 19 issue of the journal Nature.

Bern and his colleagues think the ripples are a signature of the high-velocity "wind" from the stars that once blew across the cloud surface. (Image of the massive stars and rippling cloud)

"We think it happened less than 1 million years ago, which is somewhat recent in astronomical scales," Bern told

Massive stars make their mark
The Orion cloud's young, massive stars emit radiation that interacts with the surrounding molecular gas and dust in their birth-cloud. Astronomers had thought this interaction could compress or fragment the cloud, but until now, there had been no clear evidence of such occurrences.

Bern and his colleagues observed the ripples at the surface of the Orion cloud and determined that the process is similar to a familiar one on Earth.

"In simple terms, it is the same type of ripples that you can see at the surface of the ocean when the wind is blowing over the water," Bernsaid. "So what we think is happening is that the massive stars that are nearby, they blow winds of ionized gas, and this wind is running over the interstellar clouds and making these ripples in a similar way."

When massive stars form, they produce very energetic ultraviolet light, Bernexplained. This light heats the surrounding gas to temperatures of over 1,800 degrees Fahrenheit (1,000 degrees Celsius). Scientists call the cosmic ripple effect the KelvinHelmholtz instability.

Studying the ripples
The rippling effect, said Bern can best be understood by comparing it to processes seen on Earth.

"The ripples on the ocean occur when you have two fluids and a difference in velocity between these fluids," Bernsaid. "This produces the instability. You can see it on the ocean, and you can see it in the atmosphere at the surface of clouds. When clouds in the sky get ripples, it is because of this same mechanism of instability."

Bernand his colleagues combined data from two different telescopes to make their observations. Using NASA's infrared Spitzer Space Telescope, the researchers traced the surface of the Orion molecular cloud, where the dust is heated to very high temperatures.

These data were combined with findings from a second radio telescope, which probed the inside of the cloud, where the gas is very cold. In some regions, temperatures dip to below minus 320 degrees Fahrenheit.

What this tells us
The results of the study will help researchers understand more about the feedback that massive stars have on their surrounding environment.

"It allows us to understand the turbulent nature of the interstellar medium," Bernsaid. "This is important when we want to understand how new stars can then form from these interstellar clouds."

The rippling effect also will help researchers better understand interstellar chemistry and some of the detailed processes that shape the interstellar medium. In particular, the ripples in the Orion molecular cloud act as relics of past activity in the region, helping to shed light on its history.

"We can see these ripples as a footprint of past activity of the massive stars," Bernsaid. "This is useful to understand what happened in the past in this region."