Stars aren't shy about sending out birth announcements. They fire off energetic jets of glowing gas traveling at supersonic speeds in opposite directions through space.
Although astronomers for decades have looked at still pictures of stellar jets, they now can watch movies of them, thanks to NASA's Hubble Space Telescope.
A diverse team of scientists led by astronomer Patrick Hartigan of Rice University in Houston, Texas, has collected enough high-resolution Hubble images over a 14-year period to stitch together time-lapse movies of young jets ejected from three stars.
The moving pictures offer a unique view of stellar phenomena that move and change over just a few years. Most astronomical processes change over timescales that are much longer than a human lifetime.
The movies reveal the motion of the speedy outflows as they tear through their interstellar environments. Never-before-seen details in the jets' structure include knots of gas brightening and dimming over time and collisions between fast-moving and slow-moving material, creating glowing arrowhead features. These phenomena are providing clues about the final stages of a star's birth, offering a peek at how our Sun behaved 4.5 billion years ago.
"For the first time we can actually observe how these jets interact with their surroundings by watching these time-lapse movies," said Hartigan. "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 what aspects of the interactions we understand and what parts we don't understand."
Hartigan's team's results appeared in the July 20, 2011, issue of The Astrophysical Journal.
Jets are an active, short-lived phase of star formation, lasting only about 100,000 years. They are called Herbig-Haro (HH) objects, named in honor of George Herbig and Guillermo Haro, who studied the outflows in the 1950s. Astronomers don't know what role jets play in the star-formation process or exactly how the star unleashes them.
A star forms from a collapsing cloud of cold hydrogen gas. As the star grows, it gravitationally attracts more matter, creating a large spinning disk of gas and dust around it. Eventually, planets may arise within the disk as dust clumps together.
The disk material gradually spirals onto the star and escapes as high-velocity jets along the star's spin axis. The speedy jets may initially be confined to narrow beams by the star's powerful magnetic field. The jet phase stops when the disk runs out of material, usually a few million years after the star's birth.
Hartigan and his colleagues used the Wide Field Planetary Camera 2 to study jets HH 1, HH 2, HH 34, HH 46, and HH 47. HH 1-HH 2 and HH 46-HH 47 are pairs of jets emanating in opposite directions from single stars. Hubble followed the jets over three epochs: HH 1 and HH 2 in 1994, 1997, and 2007; HH 34 in 1994, 1998, and 2007; and HH 46 and HH 47 in 1994, 1999, and 2008. The jets are roughly 10 times the width of our solar system and zip along at more than 440,000 miles an hour (700,000 kilometers an hour).
All of the outflows are roughly 1,350 light-years from Earth. HH 34, HH 1, and HH 2 reside near the Orion Nebula, in the northern sky. HH 46 and HH 47 are in the southern constellation Vela.
Computer software wove together the years' worth of observations, generating movies that show continuous motion. The movies support previous observations revealing that the twin jets are not ejected in a steady stream, like water flowing from a garden hose. Instead, they are launched sporadically in clumps. The beaded-jet structure might be like a "ticker tape," recording how material episodically fell onto the star.
The movies show that the clumpy gas in the jets is moving at different speeds like traffic on a freeway. When fast-moving blobs "rear-end" slower gas, bow shocks arise as the material heats up. Bow shocks are glowing waves of material similar to waves produced by the bow of a ship plowing through water. In HH 2, for example, several bow shocks can be seen where several fast-moving clumps bunch up like cars in a traffic jam. In another jet, HH 34, a grouping of merged bow shocks reveals regions that brighten and fade over time as the heated material cools where the shocks intersect.
In other areas of the jets, bow shocks form from encounters with the surrounding dense gas cloud. In HH 1 a bow shock appears at the top of the jet as it grazes the edge of a dense gas cloud. New glowing knots of material also appear. These knots may represent gas from the cloud being swept up by the jet, just as a swift-flowing river pulls along mud from the shoreline.
The movies also provide evidence that the inherent clumpy nature of the jets begins near the newborn stars. In HH 34 Hartigan traced a glowing knot to within about 9 billion miles of the star.
"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 explained. "This contrasts with the bulk of the existing simulations, which depict jets as smooth systems."
The details revealed by Hubble were so complex that Hartigan consulted with experts in fluid dynamics from Los Alamos National Laboratory in New Mexico, the Atomic Weapons Establishment in England, and General Atomics in San Diego, Calif., as well as computer specialists from the University of Rochester in New York. Motivated by the Hubble results, Hartigan's team is now conducting laboratory experiments at the Omega Laser facility in New York to understand how supersonic jets interact with their environment.
"The fluid dynamicists immediately picked up on an aspect of the physics that astronomers typically overlook, and that led to a different interpretation for some of the features we were seeing," Hartigan explained. "The scientists from each discipline bring their own unique perspectives to the project, and having that range of expertise has proved invaluable for learning about this critical phase of stellar evolution."
Hartigan's research team consists of Adam Frank of the University of Rochester in New York; John Foster and Paula Rosen of the Atomic Weapons Establishment in Aldermaston, England; Bernie Wilde, Rob Coker, and Melissa Douglas of Los Alamos National Laboratory in New Mexico; and Brent Blue and Freddy Hansen of General Atomics in San Diego, Calif.
Rice University, Houston, Texas
Rice University, Houston, Texas