NASA's Hubble Space Telescope has provided a detailed look at the fitful, eruptive, and dynamic processes accompanying the final stages of a star's "construction."
Images from the orbiting observatory reveal new details that will require further refinement of star formation theories, according to several independent teams of astronomers that have used Hubble to observe different embryonic stars. The Hubble observations shed new light on one of modern astronomy's central questions: how do tenuous clouds of interstellar gas and dust make stars like our Sun?
"For the first time we are seeing a newborn star close up - at the scale of our solar system - and probing the inner workings," said Chris Burrows of the Space Telescope Science Institute,Baltimore, MD and the European Space Agency. "In doing so we will be able to create detailed models of star birth and gain a much better understanding of the formation of our Sun and planets."
The Hubble images provide a dramatically clear look at a collapsing circumstellar disk of dust and gas that builds the star and provides the ingredients for a planetary system, blowtorch-like jets of hot gas funneled from deep within several embryonic systems, and machine-gun like bursts of material fired from the stars at speeds of a half-million miles per hour.
The images offer clues to events that occurred in our solar system when the Sun was born 4.5 billion years ago. Astronomers commonly believe that Earth and the other eight planets condensed out of a circumstellar disk because they lie in the same plane and orbit the Sun in the same direction. According to this theory, when the Sun ignited it blew away the remaining disk, but not before the planets had formed.
"The Hubble images are opening up a whole new field of stellar research for astronomers and clearing up of a decade worth of uncertainty," added Jeff Hester of Arizona State University, Tempe, AZ. "Now we can look so close to a star that many details of star birth become clear immediately."
The key new details revealed by the new Hubble pictures:
- Jets originate from the star and the inner parts of the disk and become confined to a narrow beam within a few billion miles of their source. It's not known how the jets are focused, or collimated. One theory is that magnetic fields, generated by the star or disk, might constrain the jets.
- Stars shoot out clumps of gas that might provide insights into the nature of the disk collapsing onto the star. The beaded jet structure is a "ticker tape" recording of how clumps of material have, episodically, fallen onto the star. In one case, Hubble allowed astronomers to follow the motion of the blobs and measure their velocity.
- Jets "wiggle" along their multi-trillion-mile long paths, suggesting the gaseous fountains change their position and direction. The wiggles may result from the gravitational influence of one or more unseen protostellar companions.
More generally, Hester emphasizes: "Disks and jets are ubiquitous in the universe. They occur over a vast range of energies and physical scales, in a variety of phenomena." Gaining an understanding of these young circumstellar structures might shed light on similar activity in a wide array of astronomical phenomena: novae, black holes, radio galaxies and quasars.
"The Hubble pictures appear to exclude whole classes of models regarding jet formation and evolution," said Jon Morse of the Space Telescope Science Institute.
A disk appears to be a natural outcome when a slowly rotating cloud of gas collapses under the force of gravity – whether the gas is collapsing to form a star, or is falling onto a massive black hole. Material falling onto the star creates a jet when some of it is heated and blasted along a path that follows the star's rotation axis, like an axle through a wheel.
Jets may assist star formation by carrying away excess angular momentum that otherwise would prevent material from reaching the star. Jets also provide astronomers with a unique glimpse of the inner workings of the star and disk. "Not even the Hubble Telescope can watch as material makes it final plunge onto the surface of the forming star, but the new observations are still telling us much about that process," said Hester.
Burrows, Hester, Morse and their co-investigators independently observed several star birth sites in our galactic neighborhood. "All of these objects tell much the same story," Hester emphasized. "We are clearly seeing a process that is a crucial part of star formation, and not just the peculiarities of a few oddball objects."
The researchers all agree that the Hubble pictures generally confirm models of star formation but will send theorists back to the drawing board to explain the details. The researchers emphasize that future models of star formation will have to take into account why jets are ejected from such a well-defined region in the disk, why jets are collimated a few billion miles out from the star, and why gas in the jets is ejected quasi-periodically.
Changes are occurring so rapidly in the jets that Hubble will be able to follow their evolution of these objects over the next decade.
BACKGROUND INFORMATION: STELLAR DISKS AND JETS
Stellar jets are analogous to giant lawn sprinklers. Whether a sprinkler whirls, pulses or oscillates, it offers insights into how its tiny mechanism works. Likewise stellar jets, billions or trillions of miles long offer some clues to what's happening close into the star at scales of only millions of miles, which are below even Hubble's ability to resolve detail. Hubble's new findings address a number of outstanding questions:
Where Are Jets Made?
Hubble shows that a jet comes from close into a star rather than the surrounding disk of material. Material either at or near the star is heated and blasted into space, where it travels for billions of miles before colliding with interstellar material.
Why Are Jets So Narrow?
The Hubble pictures increase the mystery as to how jets are confined into a thin beam. The pictures tend to rule out the earlier notion that a disk was needed to form a nozzle for collimating the jets, much like a garden hose nozzle squeezes water to a narrow stream. One theoretical possibility is that magnetic fields in the disk might focus the gas into narrow beams, but there is as yet no direct observational evidence that magnetic fields are important.
What Causes a Jet's Beaded Structure?
Hubble is solving the puzzle of a unique beaded structure in the jets, first detected from the ground but never fully understood.
"Before the Hubble observations the emission knots were a mystery," said Jeff Hester. "Many astronomers thought that the knots were the result of interactions of the jet with the gas that the jet is passing through, while others thought that the knots were due to 'sputtering' of the central engine. We now know that the knots are the result of sputtering." Hester bases this conclusion on Hubble images which show the beads are real clumps of gas plowing through space like a string of motor boats. Competing theories, now disproved by Hubble, suggested a hydrodynamic effect such as shock-diamond patterns seen in the exhaust of a jet fighter.
What Do Jets Tell Us about Star Birth?
"The jet's clumpy structure is like a stockbroker's ticker tape; they represent a recorded history of events that occurred close to the star," said Jon Morse. "The spacing of the clumps in the jet reveals that variations are occurring on several time scales close to the star where the jet originates. Like a "put-put" motor, variations every 20 to 30 years create the strings of blobs we see," Morse concluded. "However, every few hundred years or so, a large amplitude variation generates a 'whopper' of a knot, which evolves into one of the major bow-shaped shock waves." Other Hubble views by Chris Burrows reveal new blobs may be ejected every few months. "If the circumstellar disk drives the jet then the clumpiness of the jet provides an indirect measure of irregularities in the disk."
Why Are Jets "Kinky"?
The Hubble pictures also show clear evidence that jets have unusual kinks along their path of motion. This might be evidence for a stellar companion or planetary system that pulls on the central star, causing it to wobble, which in turn causes the jet to change directions, like shaking a garden hose. The jet blast clears out material around the star, and perhaps determines how much gas finally collapses onto the star.
Star Formation
A star forms through the gravitational collapse of a vast cloud of interstellar hydrogen. According to theory, and confirmed by previous Hubble pictures, a dusty disk forms around the newborn star. As material falls onto the star, some of it can be heated and ejected along the star's spin axis as opposing jets. These jets of hot gas blaze for a relatively short period of the star's life, less than 100,000 years. However, that brief activity can predestine the star's evolution, since the final mass of a star determines its longevity, temperature, and ultimate fate. The jet might carry away a significant fraction of the material falling in toward the star, and, like a hose's water stream plowing into sand, sweeps out a cavity around the star that prevents additional gas from falling onto the circumstellar disk.
Historical Background
In the early 1950's, American astronomer George Herbig and Mexican astronomer Guillermo Haro independently catalogued several enigmatic "clots" of nebulosity near stars near the Orion nebula that have since been called Herbig-Haro objects. It is only in the last 20 years, however, that the true nature of these objects, and their role in the star formation process, has been revealed. Careful study showed that many of the Herbig-Haro objects represent portions of high-speed jets streaming away from nascent stars. Now there are nearly 300 Herbig-Haro objects identified by astronomers around the world, and the list is growing as new technologies and techniques are developed to probe the dusty depths of nearby stellar nurseries.
