Hubble Telescope observations of a pair of star clusters have provided new insights into how stars might have formed many billions of years ago in the early universe. The pair of clusters are 166,000 light-years away in the Large Magellanic Cloud (LMC) in the southern constellation Doradus. The clusters are unusually close together for being distinct and separate objects according to Hubble astronomers.
Previously, such detailed stellar population studies were restricted to nearby star-forming regions within the plane of our Milky Way Galaxy. However, Hubble's high-quality images extend these stellar studies one hundred times farther into the universe, out to the distance of a neighboring galaxy. Because the LMC lies outside of our Milky Way Galaxy, it is a natural laboratory for studying the birth and evolution of stars. The stars in the LMC have few heavy elements, thus their composition is more primordial--like the stars that first formed in the early universe.
A preliminary assessment of the HST observations indicates that these compact clusters contained many more massive stars than expected. "If this were also the case billions of years ago, it would have altered drastically the early history of the universe," says Dr. Nino Panagia of the Space Telescope Science Institute and the European Space Agency.
Panagia and R. Gilmozzi (also of STScI/ESA) and co-investigators utilized HST's unique capabilities--ultraviolet sensitivity, ability to see faint stars, and high resolution--to identify three separate populations in this concentration of nearly 10,000 stars. Previous observations with groundbased telescopes resolved less than 1000 stars in this region. About 60% of the stars belong to the dense cluster called NGC 1850, which is estimated to be 50 million years old. A loose distribution of extremely hot massive stars in the same region are only about 4 million years old and represent about 20% of the stars in the image. (The remainder are field stars in the LMC.) The significant difference between the two cluster ages suggests these are two separate star groups that lie along the same line of sight. The younger, more open cluster probably lies 200 light-years beyond the older cluster, says Panagia. He emphasizes that if it were in the foreground, then dust in the younger cluster would obscure stars in the older cluster.
Because having two well-defined star populations separated by such a small gap of space is unusual, this juxtaposition suggests that they might be linked in an evolutionary sense. A possible scenario is that an expanding "bubble" of hot gas, from more than 1000 supernova explosions in the older cluster, might have triggered the birth of the younger cluster. This would have happened when the bubble expanded across space for 45 million years before plowing into a wall of cool gas and dust. The shock front then caused the gas to contract, precipitating a new episode of star formation. The massive, hot stars are destined to explode in a few million years, and thus create yet a new expanding bubble of gas.
This composite image is assembled from exposures taken in ultraviolet, visible, and near-infrared light. Yellow stars correspond to Main Sequence stars (like our Sun) with average surface temperatures of 6000 Kelvin; red stars are cool giants and supergiants (3500 K); white stars are hot young stars (25,000 K or more) that are bright in ultraviolet.
Credit: R. Gilmozzi, Space Telescope Science Institute/European Space Agency; Shawn Ewald, JPL; and NASA
The International Astronomical Union (IAU) has approved the name Dactyl for the tiny moon discovered this year in orbit around the asteroid Ida by the Galileo spacecraft. The IAU also approved names for surface features on the asteroid Gaspra, which became the first asteroid ever visited by a spacecraft when Galileo flew by it on October 29, 1991.
Dactyl is the first natural satellite of an asteroid ever discovered and photographed. The tiny moon, about one mile (1.5 kilometers) across, was discovered in images returned by Galileo after its flyby of the asteroid on August 28, 1993. The discovery was confirmed in March 1994 by members of Galileo's imaging and infrared science teams who recommended the new name to the IAU, which is responsible for the formal naming of solar system bodies.
The name is derived from the Dactyli, a group of mythological beings who lived on Mount Ida, where the infant Zeus was hidden--and raised, in some accounts--by the nymph Ida and protected by the Dactyli. Other mythological accounts say that the Dactyli were Ida's children by Zeus.
Three regions on Gaspra were named for scientists associated with the asteroid. Neujmin Regio was named for G. Neujmin, the Ukrainian astronomer who discovered the asteroid in 1916. Yeates Regio honors the late Dr. Clayne M. Yeates, who was Galileo Science Manager and Science and Mission Design Manager until his death in 1991. Dunne Regio was named in honor of the late Dr. James A. Dunne, who served as Galileo Science and Mission Design Manager until late 1992.
Galileo continues on its way to Jupiter where it will send a probe into the atmosphere on December 7, 1995, and then go into orbit for a two- year scientific tour of the planet, its satellites, and its magnetosphere.
Fullerenes found in samples from Sudbury impact Structure
Fullerenes thought to be of extraterrestrial origin have been found in shock-produced impact breccias at the Sudbury impact structure in Ontario, Canada, by researchers at Scripps Institution of Oceanography, Argonne National Laboratories, and NASA Ames Research Center.
The huge crater, formed almost two billion years ago by an asteroid or comet and strongly deformed by subsequent geological activity, contains the largest deposit of natural fullerenes found on Earth to date. "For the first time, we can point to carbon in an impact crater and say that it is probably extraterrestrial," Theodore Bunch, a NASA Ames Research Center scientist said. Fullerenes, usually made of 60 carbon atoms (sometimes 70) arranged like a soccer ball-shaped cage, are the rarest form of elemental carbon that occurs naturally on Earth. Diamond and graphite are other, more common, forms.
Bunch collected rock samples from three sites in the crater, which is 110 miles (164 kilometers) in diameter. Laser analysis was performed by Luann Becker at the Argonne National Laboratories. Bunch said the molecules, also known as buckyballs, probably formed during the impact event by cannibalizing other carbon forms or organic compounds contained in the comet.
According to Bunch, heat from the impact may also have stripped carbon from the abundant carbon dioxide scientists think saturated Earth's early atmosphere. The object was a comet rather than an asteroid, Bunch suggests, because of the large amounts of carbon found in the impact deposits. He estimates that the comet was 10 miles (15 kilometers) in diameter and contained 20-30% carbon.
The fullerenes in rock samples from the Sudbury impact site range between 1 and 10 parts per million. The abundance of sedimentary carbon in the Sudbury impact target rocks is less than 1%, an insignificant carbon source for the fullerenes. "The startling thing is that not only were the fullerenes there, but they were there in an amount that is really extraordinary," said Jeff Bada, a professor at Scripps Institution of Oceanography.
Bunch, with another research team, recently found fullerenes in a tiny crater on the Long Duration Exposure Facility (LDEF) spacecraft that had orbited Earth for almost six years. It is unclear whether the fullerenes came from a carboneaceous micrometeorite or were formed by the high- speed collision creating the crater.
The large ball-shaped carbon molecules are thought to form in red giant or carbon stars that are nearing the end of their stellar lives. They were discovered in 1985 on Earth by accident when scientists heated carbon vapor to temperatures exceeding 14,000 degrees Farenheit.
The first naturally occurring fullerenes on Earth were found in July, 1992, in carbon-rich rock in ancient sediments in Russia. They have also been found in Colorado, formed in melted rock where vegetation (carbon) was present when lightning struck the ground. But the amounts previously detected on Earth are much smaller than those discovered at the Sudbury site.
The Sudbury structure is the second largest impact crater on Earth. Only the Chicxulub crater, formed by the 65-million-year-old impact that led to mass extinctions including the dinosaurs, is larger. Fullerenes associated with Chicxulub could have come from the impactor itself or have been formed in the intense global forest fires ignited by the explosion.
A similar process, Bunch said, may have happened during Comet Shoemaker- Levy's fiery plunge into Jupiter's stratosphere, burning carbon compounds in the jovian atmosphere. Carbon freed from the jovian atmosphere (and the comet) could have combined into soot and possibly some fullerenes, he said, forming the mysterious dark spots visible after the impacts.
HUBBLE TRACKS ROTATION OF URANUS
New Hubble Space Telescope images of Uranus reveal the motion of a pair of bright clouds in the planet's southern hemisphere and a high altitude haze that forms a cap above the planet's south pole.
The images were obtained on August 14, 1994, when Uranus was 1.7 billion miles (2.8 billion kilometers) from Earth. Atmospheric details have been seen before only by the Voyager 2 flyby in 1986. Since then, detailed observations of Uranus's atmospheric features have not been possible because the planet is at the resolution limit of groundbased telescopes.
Hubble's Wide Field Planetary Camera 2 observed Uranus through a filter that is sensitive to light reflected by high altitude clouds. This allows us to see a high-altitude haze over Uranus' south polar region, along with the pair of high-altitude clouds or plume-type features, 2500 and 1800 miles (4300 and 3100 kilometers) across, respectively. The sequence of images shows how the clouds (labeled A and B) rotate with the planet during the three hours that elapsed between the first two observations (left and center) and the five hours that elapsed between the second pair (center and right). Some cloud motion might be due to high-altitude winds on the planet.
By tracking the motion of high-altitude clouds, astronomers can make new measurements of Uranus' rotation period. Based on Voyager observations, Uranus is thought to complete one rotation every 7 hours, 14 minutes. One of the four gas giant planets of our solar system, Uranus appears largely featureless. Unlike other planets, its south pole points toward the Sun during part of the planet's 84-year orbit.
Credit: Kenneth Seidelmann, U.S. Naval Observatory, and NASA
Slight temperature differences in the two comet-forming regions of the solar system cause the water ice that largely makes up comets to form in different ways, researchers from NASA's Ames Research Center say. "We predict that comets from the Kuiper belt and Oort cloud contain structurally different forms of water ice," Peter Jenniskens said. Jenniskens, with David Blake, published their results in the August 5 issue of Science.
Comets are thought to be pristine chunks of debris left over from the solar system's formation about 5 billion years ago. They are made of more than 40% water ice and come from exceedingly cold regions of the solar system. According to Jenniskens, two populations of comets exist, based on their present location in the solar system and where scientists think they originated.
Some comets formed in the Kuiper belt, which is in the outer region of the solar system beyond Pluto's orbit. These comets, known as short- period comets, probably formed at temperatures colder than -370 F, he said. Oort cloud comets probably formed in the Neptune-Uranus region and were then expelled to much greater distances from the Sun. Oort cloud comets, known as long-period comets, were probably formed at temperatures warmer than -320 F, Jenniskens said.
Most Oort cloud comets come from a solar "sphere" around 30,000 astronomical units (AU) away, but scientists think the cloud itself extends almost halfway to the nearest star, Alpha Centauri, which is four light-years from the Sun. One AU equals the distance from the Earth to the Sun. There are about 60,000 AU in a light year.
Water vapor frozen onto the rocky grains which coalesced to form comets is frozen like a glassy film rather than a crystalline solid, Jenniskens said. This glassy ice has the same basic structure as liquid water. In this form, the water molecules are connected to each other by four strong hydrogen bonds in an open cage-like structure. At the very low temperatures of comet formation in the Kuiper belt_colder than -380 F-- some water molecules are trapped in the cages of this structure during freezing. But comets formed at slightly higher temperatures in the Oort cloud expelled the water from the cages. Kuiper belt comets, therefore, consist of a different form of water ice than Oort cloud comets.
Comet Shoemaker-Levy, which recently crashed into Jupiter, was most likely a lower-temperature, short-period comet formed in the Kuiper belt, Jenniskens said. Because short-period comets orbit the solar system faster and more often, Jupiter's gravity is more likely to capture them, he said.
These predictions are based on laboratory simulations of cometary ice formation under conditions thought to exist when the objects formed. The researchers discovered the water-trapping process of the lower temperature comets by simulating the freezing process in an Ames laboratory and observing with a transmission electron microscope.
The Magellan mission to Venus ended on October 12 as Earth-based tracking stations lost the spacecraft's radio signal at 10:02 Universal Time (3:02 a.m. PDT). The loss of signal, which had been expected, was caused by low power on the spacecraft exacerbated by Magellan's orientation as it performed a final experiment in the upper atmosphere. Magellan was expected to burn up in the planet's upper atmosphere within two days.
The spacecraft's thrusters were fired in four sequences on October 11 to lower its orbit into the thin upper atmosphere and set up the final experiment. Magellan gathered scientific data on the upper atmosphere and spacecraft aerodynamics during the final descent by orienting its solar panels in opposite directions like a windmill. The termination experiment was an extension of the windmill experiment performed in early September. It was carried out as the spacecraft was within weeks to days of the end of its life because of degradation of solar power output by the thermal stress of more than 15,000 orbits of Venus.
Launched in May 1989, Magellan entered Venus orbit in August 1990 and gathered data for over four years. It used radar to see through clouds enshrouding the planet to map 98% percent of the surface with an average resolution of better than 300 meters and compiled a high-resolution gravity field map for 95% of the planet. The gravity data will allow scientists to develop models for the planet's interior and evaluate them in light of surface features revealed by Magellan's radar imaging.
Magellan also performed the first aerobraking maneuver by dipping into the atmosphere to reshape its orbit. This technique is being used in designing the Mars Global Surveyor mission, enabling the spacecraft to enter Mars orbit in 1997 using less fuel, thus saving weight and cost.
"The Magellan mission to Venus has been successful beyond all expectations," said JPL Director Edward Stone. "It not only fulfilled its science and mission objectives, it also demonstrated innovative technologies for future missions."
