Time-lapse movies made from a series of pictures taken by NASA's Hubble Space Telescope are showing astronomers that young stars and their surroundings can change dramatically in just weeks or months. As with most children, a picture of these youngsters taken today won't look the same as one snapped a few months from now. The movies show jets of gas plowing into space at hundreds of thousands of miles per hour and moving shadows billions of miles in size.

The Hubble Space Telescope in orbit around Earth. Photo: NASA TV/Spaceflight Now

The young star systems featured in the movies, XZ Tauri and HH 30, reside about 450 light-years from Earth in the Taurus-Auriga molecular cloud, one of the nearest stellar nurseries to our planet. Both systems are probably less than a million years old, making them relative newborns, given that stars typically live for billions of years. Hubble's Wide Field and Planetary Camera 2 first observed them in 1995. Those views were so intriguing that additional images were taken in 1998, 1999, and 2000. The pictures were then combined into movies that document startling activity in the early stages of a star's life.

Stars form in clouds of gas and dust that collect into a swirling disk. Outflows of gas, like the bubbles and jets seen in these images, occur when some of the material feeding the infant star from the surrounding disk is diverted away by the star's magnetic field and accelerated out its magnetic poles. These outflows are often squeezed into narrow jets that can extend many light-years away from the star. Such outflows are a common and natural result of stellar birth.

XZ Tauri

These images taken with the Hubble Space Telescope's Wide Field and Planetary Camera 2 reveal the evolution of bubbles of glowing gas being blown out from the young binary star system XZ Tauri. Credit: NASA, John Krist (STScI), Karl Stapelfeldt (JPL), Jeff Hester (ASU), Chris Burrows (ESA/STScI). WATCH THE MOVIE

XZ Tauri is a young system with two stars orbiting each other. The pair is separated by about 4 billion miles (6 billion kilometers), about the distance from the Sun to the planet Pluto in our own solar system. Hubble astronomers were surprised to discover a bubble of hot, glowing gas extending nearly 60 billion miles (96 billion kilometers) from this young star system. The bubble's temperature is over 17,500 degrees Fahrenheit (9,700 degrees Celsius). The bubble appears much broader than the narrow jets seen in other young stars, but it is caused by the same process - the ejection of gas from a star. However, the Hubble images do not show the disk that feeds the outflow process - or even which star in the binary is the outflow source. Additional observations should help to point this out.

The movie shows that the outer edge of the bubble moves away from the binary system at a speed greater than 300,000 miles per hour (150 kilometers per second), which is typical for stellar jets. This rate and the size of the bubble indicate that it is only about 30 years old, a mere blink of an eye in the life of a star. Sideways expansion of the bubble indicates that it has a strong internal pressure. A second bubble appears halfway up the waist of the first, indicating that new ejections may occur sporadically. Occasionally, bright, compact clumps of gas appear and then disperse within the bubble.

Perhaps the most interesting aspect of the bubble is the change in its appearance between 1995 and 1998. In the first picture, its edge and interior appeared equally bright; in 1998, the edge became distinctly brighter. Astronomers theorize that the gas around the bubble's edges has cooled, allowing it to glow more strongly as hydrogen and sulfur atoms recombine with electrons. Continued expansion of the bubble should cause the entire structure to fade from view - until XZ Tauri sends another eruption of hot gas into its surroundings.

HH 30

T These images of HH 30 show changes over only a five-year period in the disk and jets of this newborn star, which is about half a million years old. Credit: NASA, Alan Watson (Universidad Nacional Autonoma de Mexico), Karl Stapelfeldt (JPL), John Krist and Chris Burrows (STScI). WATCH THE MOVIE

Hubble observations of HH 30 show a pair of thin jets streaming away from the center of a dusty disk. The disk, which is over 40 billion miles (64 billion kilometers) in diameter, is seen almost edge-on. Like a thin, dark cloud moving in front of the Sun, the disk blocks any direct view of its central star. All that is seen are the top and bottom sides of the dusty disk reflecting light from the star, like the "silver lining" of a cloud. The jets reveal the hidden star's location. Astronomers are interested in the disk because it is probably similar to the one from which the Sun and the planets in our solar system formed.

HH 30's disk and jet show dramatic changes in the six years covered by the time-lapse movie. The jets are easiest to explain: as in XZ Tauri, material is being ejected along the magnetic poles of the star at speeds of between 200,000 and 600,000 miles per hour (320,000 and 960,000 kilometers per hour). Every few months a compact clump of gas, called a knot, is ejected, and may eventually merge with other clumps downstream. However, astronomers aren't sure why the knots in the upper jet are moving only about half as fast as in the fainter, lower one.

The changes in the disk are quite peculiar: patterns of light appear to be moving around within it. Astronomers believe this effect is similar to distant clouds being illuminated by the beam from a lighthouse: As the light rotates, the clouds seem to brighten and then fade. In the case of HH 30, the lighthouse is the star and the inner part of its disk, which throws bright rays and casts dark shadows on the outer part of the disk. This "lighthouse" in HH 30 appears to be rotating between once every few days and once a year. Astronomers hope more observations will narrow down that cycle and thus show whether the light patterns are shadows cast by material in the disk or beams of light from hot spots on the star.