Hubble Unveils Colorful Star Birth Region on 100,000th Orbit

In commemoration of NASA's Hubble Space Telescope completing its 100,000th orbit during its
18th year of exploration and discovery, scientists aimed Hubble to take a snapshot of a dazzling region of celestial birth and renewal.

Hubble peered into a small portion of the Tarantula nebula near the star cluster NGC 2074. The region is a firestorm of raw stellar creation, perhaps triggered by a nearby supernova explosion. It lies about 170,000 light-years away and is one of the most active star-forming regions in our
local group of galaxies.

The image reveals dramatic ridges and valleys of dust, serpent-head "pillars of creation," and gaseous filaments glowing fiercely under torrential ultraviolet radiation. The region is on the edge of a dark molecular cloud that is an incubator for the birth of new stars.

The high-energy radiation blazing out from clusters of hot young stars is sculpting the wall of the nebula by slowly eroding it away. Another young cluster may be hidden beneath a circle of brilliant blue gas.

In this approximately 100-light-year-wide fantasy-like landscape, dark towers of dust rise above a glowing wall of gases on the surface of the molecular cloud. The seahorse-shaped pillar at lower, right is approximately 20 light-years long, roughly four times the distance between our sun and the nearest star, Alpha Centauri.

The region is in the Large Magellanic Cloud, a satellite of our Milky Way galaxy. It is a fascinating laboratory for observing star-formation regions and their evolution. Dwarf galaxies like the Large Magellanic Cloud are considered to be the primitive building blocks of larger galaxies.

"This morning, the greatest scientific instrument since Galileo's telescope has reached another great milestone -- its 100,000th orbit around the Earth," says Sen. Barbara Mikulski (D-Md.) chairwoman of the Commerce, Justice and Science Appropriations Subcommittee that funds NASA. "Hubble has given us amazing insight into the origins of our universe, and I'm so proud of the men and women at Goddard and the Space Telescope Science Institute for their contributions and dedication to these great discoveries. The entire world is looking forward to the Hubble servicing mission in October 2008, when Hubble will get new scientific instruments,
new batteries and new gyroscopes. The servicing mission will extend Hubble's life and give it a more powerful view of our universe. Hubble is the telescope that could, and its best years are ahead of it!"

NASA is preparing the fifth and final Hubble servicing mission. In October, shuttle astronauts will take new instruments, gyros, batteries, and other components to enable the telescope's continued success through the year 2013.

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Astronomers Find Grains of Sand Around Distant Stars

In a find that sheds light on how Earth-like planets may form, astronomers this week report finding the first evidence of small, sandy particles orbiting a newborn solar system at about the same distance as the Earth orbits the sun. The report will be published online this week by the journal Nature.

"Precisely how and when planets form is an open question," says study co-author Christopher Johns-Krull, assistant professor of physics and astronomy at Rice University. "We believe the disk-shaped clouds of dust around newly formed stars condense, forming microscopic grains of sand that eventually go on to become pebbles, boulders and whole planets."

In previous studies, astronomers have used infrared heat signals to identify microscopic dust particles around distant stars, but the method isn't precise enough to tell astronomers just how big they become, and whether the particles orbit near the star, like the Earth does the sun, or much further away at a distance more akin to Jupiter or Saturn.

In the new study, Johns-Krull and co-authors in the United States, Germany and Uzbekistan used reflected light from the sand itself to confirm the Earth-like orbit of grainy particles around a pair of stars called KH-15D in the constellation Monoceros. The stars are about 2,400 light years from Earth in the Cone Nebula, and they are only about 3 million years old, compared to the sun's 4.5 billion years.

"We were attracted to this system because it appears bright and dim at different times, which is odd," Johns-Krull says.

The researchers found that the Earth has a nearly edge-on view of KH-15D. From this perspective, the disk blocks one of the stars from view, but its twin has an eccentric orbit that causes it to rise above the disk at regular intervals.

"These eclipses let us study the system with the star there and with the star effectively not there," Johns-Krull says. "It's a very fortuitous arrangement because when the star is there all the time, it's so bright that we can't see the sand."

The team conducted both photometric and spectrographic analyses of data collected during the past 12 years from a dozen observatories, including the McDonald Observatory in west Texas, the Keck Observatory in Hawaii and the VLT on Mount Paranal in Chile.

"Because of how the light is being reflected there are opportunities to make observations about the chemical composition of these sand-like particles," says co-author William Herbst, an astronomer at Wesleyan University in Middletown, Conn. "That's very exciting because it opens up so many doors for new type of research on this disk."

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Physicists Test the Physics of Star Formation

The formation of stars and planets remains one of the big questions in astrophysical science. Currently, scientists do not understand the required conditions and the accretion, or matter collection process, involved in star and planet formation. There are contentious debates about whether hydrodynamic turbulence is responsible. The Magnetorotational Instability (MRI) experiment at the Princeton Plasma Physics Laboratory (PPPL) in collaboration with the Astrophysical Sciences Department of Princeton University is shedding light on this mystery.

Results published in Nature show that it is virtually impossible for hydrodynamic turbulence to generate sufficiently effective accretion to form stars and planets. The U.S. Department of Energy, NASA, and National Science Foundation are funding the work jointly.

"The Earth must have sufficient angular momentum so that it does not fall into the Sun under the influence of gravity. We also know that galaxies and solar systems have a preferred direction of rotation. Consequently, matter forming these systems must also have had net angular momentum, which must have been overcome by gravity for the matter to coalesce," says Hantao Ji, the lead author of the Nature paper. "The angular momentum prevents matter from falling into the star directly, so an accretion disk is formed, which consists of matter losing its angular momentum and swirling into the core of the star. For example, when our Sun was formed, the accretion process must have been very efficient in casting off angular momentum because most of the material comprising our solar system ended up in the Sun."

Since angular momentum cannot be created or destroyed, it must flow outward through the disk as the accreting mass flows inward. But how does this happen? Star formation occurs in deep space and therefore, while accretion disks are seen, the details of the accretion process cannot be discerned except in theoretical models and computer simulations. The Princeton project's primary mission is to test the plausibility of a 1991 theory that indicates the magnetorotational instability, a disruptive plasma process, plays a major role in accretion.

The elimination of hydrodynamic turbulence as a mechanism for accretion, makes it much more likely that magnetorotational instability is responsible. Matter in an accretion disk is composed of plasma, dust, and other materials. However, the MRI experiment does not use these materials. Ji and Jeremy Goodman, the primary collaborator from the Princeton University Astrophysics Department and also a co-author of the Nature paper, came up with a way to physically simulate an accretion disk with material "standing in" for the plasma, dust, and other materials. The system consists of two concentric cylinders, each 28 centimeters in length, free to rotate independently about a common axis.

The inner cylinder has a radius of 7.1 centimeters and is made of steel, and the outer cylinder has a radius of 20.3 centimeters and is made of plastic to allow visual inspection. The inner and outer cylinders rotate independently in the same direction, but at significantly different speeds, 1200 rpm and 160 rpm, respectively, as reported in the paper. What made this project a significant engineering challenge is the requirement to have two rotating disks at each end of the cylinders. The disks must be driven at different speeds by separate motors through six concentric pipes in order to achieve the required rotation patterns of the fluid.

For the experiments reported in Nature, the space between the cylinders was filled with water. Water cannot carry a significant electrical current or interact with a magnetic field and therefore cannot display magnetorotational instability, but according to nonmagnetic theories of accretion disks, should have become turbulent anyway with fast enough spinning. Future experiments are planned in which the space between the cylinders will be filled with a liquid metal chosen because it is easy to maintain and interacts with the magnetic field in ways similar to plasma. The researchers have chosen a mixture of 67 percent gallium, 20.5 percent indium and 12.5 percent tin. Future experiments will be conducted with and without a magnetic field parallel to the axis of the cylinders.

Computer simulations of the experiment predict that when a strong magnetic field is applied to the rotating liquid metal, magnetorotational instability will cause angular momentum to be transferred from the inner cylinder toward the outer cylinder, resulting in an increase in measured torque between the cylinders. This result would further support the hypothesis that magnetorotational instability is responsible for the transport of angular momentum in accretion disks, and hence for star formation. Accretion disks also form around massive black holes in the center of many galaxies and in binary star systems.

Results from the PPPL experiments will help astrophysicists better understand these phenomena. Understanding transport phenomena in plasmas is important for basic plasma physics in general, and for fusion plasmas in particular.

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