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Stellar blasts caught on video

Astronomer Joe Liske tells the story behind the Hubble Space Telescope's observations of stellar jets in a "Hubblecast" presented by the European Space Agency's Hubble team.

Astronomers are using movies created from Hubble Space Telescope imagery to track the gassy burps belched out by young stars.

They say the moving pictures have already unraveled some of the mysteries surrounding Herbig-Haro objects, which are stars that send out colorful, blobby jets of glowing gas at supersonic speeds. The phenomenon is named after astronomers George Herbig and Guillermo Haro, who studied the outflows in the 1950s. Researchers still don't fully understand how the stars unleash such jets, but the new imagery has given them a better sense of the mechanism behind them.

The time-lapse movies were assembled from high-resolution still images collected by Hubble over the course of 14 years.

"For the first time, we can actually observe how these jets interact with their surroundings by watching these time-lapse movies," Rice University's Patrick Hartigan said today in a news release. "Those interactions tell us how young stars influence the environments out of which they form. With movies like these, we can now compare observations of jets with those produced by computer simulations and laboratory experiments to see which aspects of the interactions we understand and which we don't understand."

Hartigan is the leader of a research team that published an analysis of the time-lapse imagery in the July 20 issue of the Astrophysical Journal.

How the jets work
Herbig-Haro jets occur during a relatively brief phase of star formation, lasting about 100,000 years. When a star is born, its gravity pulls in still more material from the disk of gas and dust that swirls around it. That's when the stuff really hits the fan: As the star spins, it flings blobs of the in-falling gas back out into space. At first, those blobs may zoom outward in a tightly focused beam due to the star's strong magnetic field. But eventually they collide with each other, creating a cosmic traffic jam.

The process stops when the disk is emptied of its excess gas and dust, leaving behind planets and bits of cosmic flotsam and jetsam. Such a scenario may well have unfolded in our own solar system 4.5 billion years ago.

Hartigan and his colleagues focused on three stars where the Herbig-Haro jets are in full swing. One star, near the Orion Nebula, has opposing jets known as HH 1 and HH 2. Another star in the southern constellation Vela expels jets that are designated HH 46 and HH 47. The third star, in Orion, has a jet called HH 34. All three stars are about 1,350 light-years from Earth.

For each star, the astronomers collected Hubble imagery at three data points between 1994 and 2008. Then they fed the still pictures into a computer program that turned the pictures into smooth animations. That made it easier to analyze how different parts of the jets interacted as they were expelled. (Check out this webpage for the animated images.)

The movies confirm that blobs of material are not ejected in a continuous stream, but are belched out sporadically — apparently as the result of material falling onto the stars. The blobs move at different speeds, and when one blob plows into another, that creates a bow shock that heats up the gas. Bow shocks also occur when the blobs slam into concentrations of interstellar gas. Regions of the jets brighten and fade as the clumps of gas warm up and cool down.

Lessons from virtual nuclear blasts
"Taken together, our results paint a picture of jets as remarkably diverse objects that undergo highly structured interactions between material within the outflow, and between the jet and the surrounding gas," Hartigan said. "This contrasts with the bulk of the existing simulations, which depict jets as smooth systems."

To improve the fidelity of the simulations, Hartigan's team turned to experts in fluid dynamics from Los Alamos National Laboratory in New Mexico, the UK Atomic Weapons Establishment in Britain and General Atomics in San Diego, Calif., as well as computer specialists from the University of Rochester in New York. Those experts on simulated thermonuclear blasts helped the astronomers understand the interactions powered by Mother Nature's thermonuclear furnaces.

The astronomers are now conducting experiments at the Omega Laser Facility, which is housed at the University of Rochester.

"Our collaboration has exploited not just large laser facilities such as Omega, but also computer simulations that were developed for research into nuclear fusion," said Paula Rosen of the UK Atomic Weapons Establishment, a co-author of the paper. "Using these experimental methods has enabled us to identify aspects of the physics that the astronomers overlooked — it is exditing to know that what we do in the laboratory here on Earth can shed light on complex phenomena in stellar jets over 1,000 light-years away."

More oddities in the cosmic menagerie: 

In addition to Hartigan and Rosen, the authors of "Fluid Dynamics of Stellar Jets in Real Time: Third Epoch Hubble Space Telescope Images of HH 1, HH 34 and HH 37" include Adam Frank, John Foster, Bernie Wilde, Rob Coker, Melissa Douglas, Brent Blue and Freddy Hansen.

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