The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/a96/disk11/95_24.txt

Text File | 1996-01-19 | 22KB | 430 lines

                                          EMBARGOED UNTIL:  2:00 P.M. (EDT)
                                                               June 6, 1995

CONTACT:  Don Savage
          NASA Headquarters, Washington, DC
          (Phone:  [phone])

          Tammy Jones
          Goddard Space Flight Center, Greenbelt, MD
          (Phone:  [phone])

          Ray Villard
          Space Telescope Science Institute
          (Phone:  [phone])


                           HUBBLE OBSERVES THE FIRE
                        AND FURY OF A STELLAR BIRTH

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.

                                * * * * * *
The Space Telescope Science Institute is operated by the Association
of Universities for Research in Astronomy, Inc. (AURA) for NASA,
under contract with the Goddard Space Flight Center, Greenbelt, MD.
The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency (ESA).

Image files in GIF and JPEG format may be accessed on Internet via
anonymous ftp from ftp.stsci.edu in /pubinfo:
                                          GIF             JPEG
     PRC95-24a      HH30/HH34/HH47      gif/JetDisk3   jpeg/JetDisk3
     PRC95-24b      HH30 Jet Motion     gif/HH30       jpeg/HH30
     PRC95-24c      HH1/HH2 Details     gif/HH1-2      jpeg/HH1-2
     PRC95-24d      HH47 Jet Detail     gif/HH47       jpeg/HH47

The same images are available via World Wide Web from URL
http://www.stsci.edu/Latest.html, or via links in
http://www.stsci.edu/public.html.


                            SCIENCE BACKGROUND

                           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.


                                       EMBARGOED UNTIL:  2:00 P.M. (EDT)
                                                            JUNE 6, 1995

                                        PHOTO RELEASE NO.:  STSCI-PRC95-24a


                  HUBBLE VIEWS OF THREE STELLAR JETS

These NASA Hubble Space Telescope views of gaseous jets from
three newly forming stars  show a new level of detail in the star
formation process, and are helping to solve decade-old  questions
about the secrets of star birth.  Jets are a common "exhaust product"
of the dynamics of  star formation.  They are blasted away from a disk
of gas and dust falling onto an embryonic star.

[upper left] - This view of a protostellar object called HH-30 reveals an
edge-on disk of dust encircling a  newly forming star.   Light from the
forming star illuminates the top and bottom surfaces of  the disk,
making them visible, while the star itself is  hidden behind the
densest parts of the  disk.  The reddish jet emanates from the
inner region of the disk, and possibly directly from the star itself.
Hubble's detailed view shows, for the first time, that the jet expands
for several billion miles from the star, but then stays confined to a
narrow beam.  The protostar is 450 light-years away in the
constellation Taurus.

Credit:  C.  Burrows (STScI & ESA), the WFPC 2 Investigation
              Definition Team, and NASA

[upper right] - This view of a different and more distant jet in object
HH-34 shows a remarkable beaded structure.  Once thought to be a
hydrodynamic effect (similar to shock diamonds in a jet aircraft
exhaust), this structure is actually produced by a machine-gun-like
blast of "bullets" of dense gas ejected from the star at speeds of
one-half million miles per hour.  This structure suggests the star goes
through episodic "fits" of construction where chunks of material fall
onto the star from a surrounding disk.  The protostar is 1,500 light-
years away and in the vicinity of the Orion Nebula, a nearby star
birth region.

Credit:  J. Hester (Arizona State University), the WFPC 2 Investigation
         Definition Team, and NASA

[bottom] - This view of a three trillion mile-long jet called HH-47 reveals
a very complicated jet pattern that indicates the star (hidden inside a
dust cloud near the left edge of the image) might be wobbling, possibly
caused by the gravitational pull of a companion star.  Hubble's detailed
view shows that the jet has burrowed a cavity through the dense gas
cloud and now travels at high speed into interstellar space.  Shock
waves form when the jet collides with interstellar gas, causing the jet
to glow.  The white filaments on the left reflect light from the obscured
newborn star.  The HH-47 system is 1,500 light-years away, and lies at
the edge of the Gum Nebula, possibly an ancient supernova remnant
which can be seen from Earth's southern hemisphere.

Credit: J.  Morse/STScI, and NASA

The scale in the bottom left corner of each picture represents 93 billion miles,
or 1,000 times the distance between Earth and the Sun.  All images were taken
with the Wide Field  Planetary Camera 2 in visible light.  The HH designation
stands for "Herbig-Haro" object -- the name for  bright patches of nebulosity
which appear to be moving away from  associated protostars.


                    EMBARGOED UNTIL:  2:00 P.M. (EDT) JUNE 6, 1995

                    PHOTO RELEASE NO.:  STSCI-PRC95-24b


               MOTION OF JETS FROM AN EMBRYONIC STAR (HH-30)

This NASA Hubble Space Telescope image reveals unprecedented
detail in a newly forming star called HH-30.  Exposures taken a year
apart show the motion of high speed blobs of gas (arrows) that are
being ejected from the star at a half-million miles per hour.

The jets emanate from the center of a dark disk of dust which encircles
the star and hides it from view.  Presumably the disk feeds material onto
the star, and some of it is superheated and squirts out along the star's
spin axis.  The presence of the blobs suggests that the star formation
process is fitful and episodic, as chunks of material fall onto the
newborn star.

For the first time, Hubble Space Telescope shows the accretion disk
which is about the size of our solar system, around a forming star.
The top and bottom surfaces of the disk can be seen directly in this
view, which visually confirms the conventional accretion disk theory
for star formation.   When the star becomes hot enough it will stop
accreting material and blow away much of the disk -- but perhaps not
before planets have formed around the star.  The generally accepted
theory for the creation of our solar system is that it formed from a disk,
and that the orbits of the planet are the "skeletal" remnant of the disk.
It also explains why the planets all orbit the Sun in the same direction
and roughly the same plane.  The disk can be seen to "flare" away
from the star.  (It is thicker at larger distances from the star.)  This
behavior can be understood because it takes material farther out in
the disk longer to settle to the disk midplane.  The flaring has been
conjectured in order to explain details of the spectra of such objects,
but never directly observed before on these scales.

The picture was taken with the Wide Field Planetary Camera 2.   HH-30
lies 450 light-years away in the constellation Taurus.

Credit:  C.  Burrows (STScI & ESA), the WFPC 2 Investigation
         Definition Team,  and NASA
Co-investigators:  K.  Stapelfeldt (JPL), A Watson (Lowell Observatory)


                                       EMBARGOED UNTIL:  2:00 P.M. (EDT)
                                                            JUNE 6, 1995

                                       PHOTO RELEASE NO.:  STSCI-PRC95-24c


                   PAIR OF JETS FROM A YOUNG STAR (HH1/HH2)

This NASA Hubble Space Telescope image reveals new secrets of star
birth as revealed in a pair of eerie spectacular jet of gas the star has
ejected by a young star.

[top] - Tip to tip, this jet spans slightly more than a light-year.  The
fountainhead of this structure -- the young star -- lies midway between
the jet, and is hidden from view behind a dark cloud of dust.   The nearly
symmetrical blobs of gas at either end are where the jet has slammed into
interstellar  gas.

[bottom left] - A close-up of a region near the star reveals a string of
glowing clumps of gas, ejected by the star in machine-gun like burst
fashion.  This provides new clues to the dynamics of the star formation
process.  The jets are ejected from a whirlpool of gas and dust orbiting
the young star.

[bottom right] - This arrowhead structure is a classic bowshock pattern
produced when high-speed material encounters a slower-speed medium.
Young stellar jets were discovered 20 years ago, in part due to visible-
light observations of bright patches of nebulosity (called Herbig-Haro
objects), which appear to be moving away from associated protostars.

The picture was taken with the Wide Field Planetary Camera 2.
HH-1/ HH-2 lies 1,500 light-years away in the constellation Orion.

Credit: J.  Hester (Arizona State University), the WFPC 2 Investigation
        Definition Team, and NASA




                                 EMBARGOED UNTIL:  2:00 P.M. (EDT)
                                                      JUNE 6, 1995

                                 PHOTO RELEASE NO.:  STSCI-PRC95-24d


                    WIGGLING JET FROM A WOBBLING STAR (HH-47)

This NASA Hubble Space Telescope image reveals new secrets of star
birth as recorded in a spectacular jet of gas the star has ejected.

[center] - Resembling the vertebrae of an imaginary space alien, this
one-half light-year long jet of gas has burst out of a dark cloud of gas
and dust which hides the newly forming star located in the lower left
corner of the image.

[upper left] - An enlargement of a portion of the jet near the star shows
the complicated interactions that take place when the ejected gas collides
with the interstellar medium.  The apparent changes in direction might
be produced by wobbling of the star, as it feels the gravitational tug of
an unseen companion star or instability mechanisms.

[lower right] - A  massive clump of jet material collides with upstream
gas and creates a bow-shaped shock wave, like a boat speeding across a
lake.  Through this process the jet sweeps out a cavity around the star
and may thereby restrict how much material is available to fall onto the
star as part of the gravitational accretion process.

The images used to make this picture were taken with the Wide Field
Planetary Camera 2 on March 26 and 29, 1994.  HH-47 lies about 1,500
light-years away in the constellation Vela.  The star is forming in a
dense gas cloud at the edge of the Gum Nebula.

Credit: J.  Morse (STScI),  and NASA
Co-investigators:  B. Reipurth (European Southern Observ.),
S. Heathcote (Cerro Tololo Inter-American Observ.), P. Hartigan
(Rice Univ.), J. Bally (Univ. of Colorado), R. Schwartz (Univ. of
Missouri), J. Stone (Univ. of Maryland).

The insets show portions of the jet that were computer enhanced by
A. Boden  and D. Redding (JPL) and J. Mo and R. Hanisch (STScI).