NEW HORIZONS CROSSES THE ORBIT OF NEPTUNE
NASA
The Pluto-bound New Horizons spacecraft has crossed the orbit of Neptune. That is its last major crossing en route to becoming the first probe to make a close encounter with Pluto, which is expected to occur on 2015 July 14. The piano-sized spacecraft, which was launched in 2006, reached Neptune's orbit -- nearly 2750 million miles from the Earth -- in a record eight years and eight months. New Horizons' milestone matches precisely the 25th anniversary of the historic encounter of the Voyager 2 spacecraft with Neptune in 1989. Voyager's visit to the Neptune system revealed previously unseen features of Neptune itself, such as the Great Dark Spot, a massive storm similar to, but not as long-lived as, Jupiter's Great Red Spot. Voyager also, for the first time, obtained clear images of Neptune's ring system. Some researchers feel that the 1989 Neptune fly-by may have offered a preview of what is to come next year. Some scientists suggest that Neptune's major satellite Triton, with its icy surface, bright poles, varied landforms and cryo-volcanoes, is a Pluto-like object that Neptune captured into orbit. Scientists recently re-worked Voyager's observations of Triton and used them to construct an improved global colour map -- further whetting appetites for a Pluto close-up. There is a lot of speculation over whether Pluto will look like Triton. Like Voyagers 1 and 2, New Horizons is also on a path toward potential discoveries in the Kuiper Belt, which is a disc-shaped region of icy objects past the orbit of Neptune, and further parts of the outer Solar System.
NEW CATALOGUE OF STELLAR COMPOSITIONS
Arizona State University
A larger catalogue of stellar compositions than any previous ones has been produced. It is called after Hypatia, who was one of the first female astronomers and lived about 350 AD in Alexandria. The catalogue is a compilation of spectroscopic abundance data from 84 literature sources for 50 elements in 3,058 roughly solar-type (F, G and K) stars within 160 parsecs. Vizier, a data base provided by the Stellar Data Centre in Strasbourg, was a good starting point, but is not quite exhaustive. The Hypatia catalogue has a number of applications. It tends to show that the compositions of nearby stars are not as uniform as was thought. It has been known that stars with Jupiter-like planets tend to have high metal abundances, but in another example of its application the new catalogue makes it easier to study other elements systematically to see if there are relation-ships between the presence of a planet (gaseous or terrestrial) and the various elemental abundances.
EVIDENCE FOR SUPERNOVAE NEAR THE EARTH
NASA
About once every 50 years, a massive star explodes somewhere in the Milky Way. The resulting blast puts out more energy in a split second than the Sun emits in a million years. At its peak, a supernova can outshine the entire Milky Way, and it seems obvious that a supernova explosion too near the Earth would be undesirable. Yet there is some evidence that, about 10 million years ago, a 'nearby' cluster of supernovae went off. We know that, because the explosions blew an enormous bubble in the interstellar medium, and we are inside it. Astronomers call it the 'Local Bubble'. It is peanut-shaped, about 100 parsecs long, and filled with almost nothing. Gas inside the bubble is very thin (0.001 atoms per cubic centimetre) and very hot (roughly a million degrees), a sharp departure from ordinary interstellar material. The Local Bubble was discovered gradually in the 1970s and 1980s. Optical and radio astronomers looked for interstellar gas in our part of the Galaxy, but did not find much. Meanwhile, X-ray astronomers were getting their first look at the sky from sounding rockets and orbiting satellites, which revealed a million-degree X-ray glow coming from all directions. It seemed to add up to the Earth being inside a bubble of hot gas blown by exploding stars.
Within the last decade, however, some scientists have disagreed with the supernova interpretation, suggesting that much or all of the soft-X-ray diffuse background is instead a result of 'charge exchange'. Charge exchange happens when the electrically-charged solar wind comes into contact with neutral gas. The solar wind can steal electrons from the gas, resulting in an X-ray glow that looks a lot like the glow from an old supernova. Charge exchange has been observed many times in comets. So, is the X-ray glow that fills the sky a sign of peaceful charge exchange in the Solar System or evidence of great explosions in the past? To find out, researchers developed an X-ray detector that could distinguish between the two possibilities. The device was named DXL, for Diffuse X-ray emission from the Local Galaxy. DXL was launched on a sounding rocket from White Sands in New Mexico, reaching a peak altitude of 160 miles and spending five minutes above the atmosphere. That was all it needed to measure the amount of charge-exchange X-rays inside the Solar System. The results indicated that only about 40% of the soft-X-ray background originates within the Solar System. The rest must come from a Local Bubble of hot gas, the relic of ancient supernovae outside the Solar System. Obviously, the supernovae were not close enough to exterminate life on Earth, but they were close enough to wrap the Solar System in a bubble of hot gas that persists millions of years later.
DISTANCE TO PLEIADES SETTLED
National Radio Astronomy Observatory
Astronomers have used a worldwide network of radio telescopes to resolve a controversy over the distance to the Pleiades -- a controversy that posed a potential challenge to scientists' basic understanding of how stars form and evolve. The new work shows that the measurement made by the Hipparcos satellite was inaccurate. The Pleiades include hundreds of young, hot stars formed about 100 million years ago. As a nearby example of such young clusters, the Pleiades have served as a key for refining scientists' understanding of how such clusters form. Moreover, astronomers have used the measured physical characteristics of Pleiades stars as a tool for estimating the distances of other, more distant, clusters. Until the 1990s, the consensus was that the Pleiades are about 430 light-years away. However, the satellite Hipparcos, which was launched in 1989 and over four years of operation measured distances to 118,000 stars, found a distance of 'only' about 390 light-years. That may not seem like a huge percentage difference, but it did not fit the physical characteristics of the Pleiades stars according to our general understanding of how stars form and evolve.
In an effort to solve the problem, astronomers used a global network of radio telescopes to make a new determination of the parallax of the Pleiades stars. They obtained a distance of 443 light-years, accurate, the astronomers said, to 1%. That is close enough to the pre-Hipparcos distance that the standard scientific models of star formation accurately represent the stars in the Pleiades. The question now is what happened to Hipparcos? The cause of its error in measuring the distance to the Pleiades is unknown.
PEBBLE-SIZE PARTICLES MAY INITIATE PLANET FORMATION
National Radio Astronomy Observatory
Rocky planets like the Earth start out as microscopic bits of dust, so theories suggest. Astronomers using the Green Bank radio telescope (GBT) have discovered that filaments of star-forming gas near the Orion Nebula may contain lots of pebble-size particles -- planetary building blocks 100 to 1000 times larger than the dust grains typically found around proto-stars. If confirmed, such rocky material may well represent a new, mid-size class of interstellar particles that could help to initiate planet formation. The new observations extend across the northern portion of the Orion Molecular Cloud complex, a star-forming region that includes the Orion Nebula. The star-forming material in the section studied by the GBT, called OMC-2/3, has condensed into long, dust-rich filaments. The filaments are dotted with many dense knots known as cores. Some of the cores are just starting to coalesce while others have begun to form proto-stars -- the first early concentrations of dust and gas along the path to star formation. Astronomers speculate that in the next 100,000 to 1 million years, that region will probably evolve into a new star cluster. The OMC-2/3 region is located approximately 1,500 light-years from us and is roughly 10 light-years long.
From earlier maps of that region made with the IRAM 30-m radio telescope in Spain, the astronomers expected to find a certain brightness to the dust emission when they observed the filaments at slightly longer wavelengths with the GBT. Instead, they discovered that the area was shining much more brightly than expected at mm wavelengths. That implies that the material there has properties differing from those expected for normal interstellar dust. In particular, since the particles are more efficient than expected at emitting at millimetre wavelengths, the grains are very likely to be at least a millimetre, possibly as large as a centimetre, across. Though tiny compared to even the most modest of asteroids, dust grains on the order of a few millimetres to a centimetre are extraordinarily large for such young star-forming regions. Owing to the unique environment in the Orion Molecular Cloud, the researchers propose two intriguing theories for their origin. The first is that the filaments themselves helped the dust grains grow to such unusual proportions. Those regions, compared to molecular clouds in general, have lower temperatures, higher densities, and lower velocities -- all of which would encourage grain growth. The second idea is that the rocky particles originally grew inside a previous generation of cores or perhaps even proto-planetary discs. The material could then have escaped back into the surrounding molecular cloud rather than becoming part of the original newly forming star system.
RADIOACTIVE COBALT IN SUPERNOVA EXPLOSION
Moscow Institute of Physics and Technology
A group of astrophysicists has detected the formation of radioactive cobalt during a supernova explosion, lending credence to a theory of such explosions. The team reported an analysis of data collected with the INTEGRAL gamma-ray orbital telescope, which they used to detect the radioactive isotope cobalt-56. Cobalt-56 has a half-life of just 77 days, so it does not exist in normal conditions. However, during a supernova explosion, that short-lived isotope is produced in large quantities. Radiating cobalt was recorded in the supernova 2014J, 11 million light-years away. Astrophysicists had not obtained similar spectra before; the reason was the rarity of explosions at such a distance -- 11 million light-years is a long way on the Galactic scale but on an intergalactic scale it is a relatively short distance. There are several hundred galaxies within a radius of ten million light-years; supernovae produce explosions of the relevant type (type Ia) once every few centuries in a galaxy. For example, a type-Ia supernova last exploded in the Milky Way in 1606.
SN 2014J was discovered in the galaxy M82 last January by astronomers from University College London. Several observatories, including INTEGRAL, started observations immediately. Russian researchers spent a million seconds of their quota for the use of the INTEGRAL telescope to study the supernova. In addition to the spectra, they obtained data on how its brightness changed over time. According to a theory that was developed earlier, during an explosion of the Ia type the remnants of the star barely radiate in the gamma-ray range in the initial weeks. The star's shell is opaque in that region of the spectrum; a supernova begins to produce gamma radiation only after the outer layer becomes sufficiently rarefied. By that time, radioactive nickel-56, with a half-life of 10 days, synthesized during the explosion, transforms into radioactive cobalt-56, whose spectral lines at energies of 847 and 1237 keV were detected by INTEGRAL. The data also allowed the researchers to assess how much radioactive cobalt was produced during the explosion -- the equivalent of about 60% of the Sun's mass. Over time, cobalt-56 turns into the most common isotope of iron, iron-56. That is the most common isotope because it can be obtained from nickel formed during supernova explosions (nickel turns into cobalt, and cobalt turns into iron). Thus, the new results back up simulations of supernova explosions and also suggest that our planet includes matter that has gone through thermonuclear explosions of an astronomical scale.
Topic: Early September Astronomy Bulletin (Read 1897 times)