from Hubble Show the Changing Faces of Infant Stars

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 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 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 (540,000 kilometers per hour), 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.

Credits: John Krist (Space
Telescope Science Institute
), Karl Stapelfeldt (NASA Jet Propulsion
Laboratory), Jeff Hester (Arizona State University), Chris Burrows
(European Space Agency/Space Telescope
Science Institute



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.

Alan Watson (Universidad Nacional Autonoma de Mexico, Mexico), Karl
Stapelfeldt (NASA Jet Propulsion Laboratory), John Krist (Space
Telescope Science Institute
), and Chris Burrows (European Space
Agency/ Space Telescope Science Institute)