Weekly Notes Archive 1

By combining red, green, and blue images taken at 0:41 UT April 25th, astronomers assembled this highly detailed view of the two red spots and their turbulent surroundings. The dark band protruding from upper left is the camera’s occulting finger. North is up. NASA, ESA, I. de Pater and M. Wong (Univ. of California, Berkeley).

Last April two teams of astronomers used the Hubble Space Telescope to obtain our sharpest views of Jupiter's long-lasting, Earth-size storm: "Red Spot Jr." Also known as Oval BA, in February the storm stunned observers by suddenly and mysteriously changing color from white to orange-red. The oval is now nearly identical in hue to the planet's famous Great Red Spot (GRS).

The first group, led by Amy Simon-Miller (NASA/Goddard Space Flight Center), used the high-resolution channel of Hubble's Advanced Camera for Surveys (ACS) to capture Red Spot Jr. in near-ultraviolet to near-infrared light on April 8th. The second group, spearheaded jointly by Imke de Pater and Philip Marcus (both with the University of California, Berkeley), observed with the ACS's high-resolution and wide-field channels at visible and near-infrared wavelengths on April 16th, 24th, and 25th.

The Hubble images, nearly as detailed as those obtained during the Voyager flybys in 1979, show swirling cloud formations within and around the new spot, including the storm's light "collar," which is currently quite prominent in Red Spot Jr. but darker around the Great Red Spot.

Many compounds of sulfur, phosphorus, hydrogen, and carbon have been postulated over the years that would account for the Great Red Spot's coloration, but these are usually ruled out based on spectral observations — they are either the wrong color or are produced under the wrong conditions.

One of the most popular theories is that phosphine, PH3, a colorless, flammable, poisonous gas, is being dredged up by the storms from deep in the Jovian atmosphere to high altitudes where it is broken down by ultraviolet photons from the Sun. Subsequent chemical reactions eventually lead to the formation of red phosphorus, P4. "Unfortunately, P4 generally seems to be the wrong shade of red!" says Simon-Miller.

This close-up view of Jupiter's Red Spot Jr. (left of center), which is about the same size as Earth, was taken with Hubble's Advanced Camera for Surveys (ACS) on April 8th at 2:33 Universal Time through blue and red filters. Note the prominent light "collar" around the spot. Part of the Great Red Spot can be seen at right. North is up. NASA, ESA, and A. Simon-Miller (NASA/GSFC).

Red Spot Jr. lies in the South Temperate Belt, following behind the Great Red Spot by approximately an hour of Jupiter's rotation. They should pass each other sometime this July. The upstart spot formed in 1998–2000, when three smaller white ovals — known as BC, DE, and FA — collided and merged to form Oval BA. A similar merger centuries ago may have given birth to the Great Red Spot, which is roughly twice as large as Red Spot Jr.

The Great Red Spot is arguably the most powerful storm in the solar system. Using data from the Galileo spacecraft in 1997, Simon-Miller clocked wind velocities of up to 650 kilometers per hour (400 mph) in the GRS, compared to a maximum of 430 km per hour for Oval BC (before the merger). "These are much higher winds than in a terrestrial hurricane or cyclone," notes Simon-Miller. "On Earth, these low-pressure systems rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. On Jupiter, these [spots] are high-pressure systems, or anticyclones, rotating the opposite way."

Scientists have no idea why there are no similar-size storms occurring on the planet's northern hemisphere. "We really don't understand everything there is to know about generating these kinds of storms," she adds.

The reddening of Red Spot Jr. could indicate that the storm is intensifying, though its diameter has so far remained largely unchanged. Measurements by Simon-Miller of her team's HST image give a long-axis dimension of 13,480 km for Oval BA and 20,740 km for the Great Red Spot. It's anyone's guess how long Red Spot Jr. will remain red and whether it will grow or shrink in the coming months.

Hubble took this false-color portrait of Jupiter at 18:42 UT April 16th through near-infrared and blue/cyan filters. It shows Red Spot Jr. about to cross the central meridian; the Great Red Spot is to its upper right. The orange color traces high-altitude methane haze over the planet's Equatorial Zone, the two red spots, and the polar regions. North is up. NASA, ESA, I. de Pater and M. Wong (Univ. of California, Berkeley).

Joining de Pater and Marcus's team as co-investigator is Christopher Go, an accomplished amateur planetary imager from Cebu, Philippines, and a member of the Astronomical League of the Philippines. It was Go who discovered the color change last February 24th; he alerted observers and astrophotographers worldwide through the Association of Lunar and Planetary Observers' Jupiter Section. He has set up a special Web site for compiling ground-based images of Red Spot Jr.

"I'm quite excited working with Chris on this," says de Pater. "I'm really happy to have invited him. He's a tremendous help in the effort; pretty crucial, I'd say."

"It's all very exciting," exclaims Go. "Never in my wildest dreams did I imagine myself being part of an HST team!"

With good seeing, both the GRS and Red Spot Jr. should be visible in backyard telescopes 6 inches or larger under moderate to high magnifications. For a list of all times when the Great Red Spot crosses Jupiter's central meridian as seen from Earth, see our Red Spot calculator. Add an hour to get the approximate transit times of Red Spot Jr.

As Red Spot Jr. drifts slowly eastward and the Great Red Spot westward, the two are expected to pass each other in longitude around July 10th, according to Hans-Jörg Mettig of JUPOS, the Database for Object Positions on Jupiter. But since the spots do not move uniformly, the actual passage date could be off by a few days.

Philippine-based astrophotographer Christopher Go regularly images the planets from his balcony in Cebu. He currently uses a Celestron C11 Schmidt-Cassegrain telescope on an Astro-Physics AP900GTO mount and an Imaging Source DMK21BF04 monochrome Firewire CCD video camera. Go credits his success to the superb seeing at his site, about 575 km southeast of Manila.

"We don't know what will happen," says de Pater. "Will the small one be shred to pieces? Will it be gobbled up? It's probably still too far away from the GRS for that to happen. Or will [Oval BA] simply pass the GRS and continue its way?"

She adds, "The outer regions of the spots will interact, we think, but beyond that, it's hard to predict what might happen."

Check SkyandTelescope.com for the latest updates. And keep watch for yourself.

In recent years, astronomers have discovered a handful of dwarf galaxies right next door in the extended halo of the Milky Way, obscured by the gas and dust of our galactic plane. Now, astronomers sifting through data from the Sloan Digital Sky Survey (SDSS) have announced the discovery of two sparse, low-luminosity Milky Way neighbors. Both dwarf satellite galaxies are invisible in most telescopes.

Selected by color and magnitude, stars from the Sloan Digital Sky Survey photometric database clump together to reveal a faint Milky Way companion galaxy in the direction of Boötes. Vasily Belokurov/SDSS collaboration

Daniel Zucker (Cambridge University, England) discovered the first one while poring over the survey data. He noticed an overdensity of distant stars in the constellation Canes Venatici (CVn). Comprised of predominantly old stars, the CVn dwarf is located about 640,000 light-years away. "Its distance makes it one of the most remote of the Milky Way's companion galaxies," says Zucker. With a diameter of more than 5,000 light-years, it is also one of the largest Milky Way dwarf spheroidal satellites.

Only hours after the announcement, Zucker's fellow SDSS researcher and Cambridge colleague, Vasily Belokurov, announced his own discovery of an even fainter dwarf galaxy in Boötes. Dubbed "Boo," the ultra-faint galaxy shines with a luminosity of only 10,000 Suns. At a distance of about 196,000 light-years, its absolute magnitude is –5.7, which makes it the least luminous galaxy known. The previous record holder was discovered in 2005 in Ursa Major.

Until now, astronomers had identified 10 dwarf companions to the Milky Way, not including the Large and Small Magellanic Clouds. The dwarfs have remained hidden because they are thinly scattered, obscured by dust, and sometimes spread across tens of degrees of sky. Sorting out these loose clusters by color and magnitude from Milky Way foreground stars is a painstaking task.

Moreover, astronomers have a hard time classifying these clumps. Companion galaxies are typically in various stages of being torn asunder by their revolutions around the Milky Way. Debate continues on whether an overdensity of stars in Canis Major, for example, is part of the Milky Way's warped outer disk or part of a larger, mostly dissolved dwarf galaxy.

In some cases, a stream of stars may be all that's left. In March, Carl Grillmair (Caltech) and Roberta Johnson (California State University, Long Beach) reported in Astrophysical Journal Letters their detection of a "river of stars" streaming across 45° of the northern sky. The narrow stream is approximately 76,000 light-years away, forming a giant arc across the Milky Way's disk. The feature appears to be a result of gravitational tides stripping the globular cluster NGC 5466.

Boo, too, appears to be a distorted structure that has likely been battered by the Milky Way's gravitational tides. Because of the dwarf's extent and apparent dynamics, the survey team ruled out the possibility that Boo is the remains of a globular cluster. The structure and motion of satellite galaxies such as CVn and Boo help us better understand the properties of dark matter and galaxy interactions. "One of the biggest surprises of the SDSS is the galactic science that is being produced," says SDSS survey colleague Don Schneider (Penn State University).

As astonishing as it seems that galaxies so nearby have gone undetected until now, astronomers agree that the Boo and CVn discoveries are just the tip of the iceberg. Zucker, Belokurov, and their colleagues report in papers submitted to Astrophysical Journal Letters that the discovery of such faint galaxies in proximity to us indicates that many more remain to be found.

"Between the Ursa Major dwarf and these new objects, the field is truly being revolutionized," says colleague Beth Willman (New York University), adding that several more large sky surveys will enable more discoveries.

The longevity of NASA's Mars Exploration Rovers, Spirit and Opportunity, is becoming the stuff of legend. Mission scientists had hoped that the wheeled robots would each last 90 sols (Martian days) on the surface and perhaps drive as far as 600 meters (2,000 feet). As of early May, both rovers had passed the 800-sol mark; Spirit had traveled 6.8 kilometers (4.2 miles) and Opportunity 7.5 km. Combined, the twin craft have shot more than 150,000 images and analyzed many dozens of rocks. Most importantly, the rovers, reporting from opposite sides of the planet, have confirmed suspicions that ancient Mars was indeed wet.

In January 2006 Opportunity imaged these wavy layers within a rock dubbed Overgaard, found near the edge of Erebus Crater in Meridiani Planum. The centimeter-size ripples are festoons — sedimentary features formed when waves lap against a beach. NASA/JPL/Caltech/Cornell Univ.

The rovers are showing their age, however. Spirit's right front wheel has seized up completely, and as the rover limps backward across the landscape, it carves a deep trench in its wake. Mobility is typically limited to less than 10 meters a day. Opportunity's instrument arm has a balky shoulder joint, and engineers have had to develop a new way to analyze rocks with a limited range of arm motion.

Recently Spirit, at a latitude of 15° south, spent many days hobbling to a sunny spot for the long, cold winter. Because the Martian southern hemisphere is tipped away from the Sun, when the Red Planet is near the far point of its eccentric orbit, only 70% as much sunlight falls on the rover's solar cells in winter as in summer. To conserve precious energy, the rover team has parked the craft on a 10° slope facing the Sun. "We're probably going to sit there for the Martian winter," says panoramic camera lead scientist Jim Bell (Cornell University). "We're basically acting like a lander for the next six months." While Spirit huddles for warmth, the team plans to analyze reachable rock and soil targets in great detail, monitor the atmosphere, and shoot an enormous 360° mosaic using all of the camera's filters.

Meanwhile, since Opportunity is driving in Meridiani Planum, close to the equator, it receives more sunlight and has no need to find slopes to perch up against in order to survive. With its six good wheels, the rover traversing up to 50 meters per day toward Victoria crater. As of early May the 800-meter-wide depression was only 1.3 km away. Arrival is expected sometime in June or July. Once there, the rover will spend the winter scouting around the edge and looking for rock outcrops to examine. Controllers may even plot a path inside for next summer.

But how long will the rovers survive? Right now there is no real answer. By winter's end they will have lived 10 times longer than their "expiration dates." "We are heading into our second Martian winter and we think we'll survive," says Bell. The science team is resigned that someday a critical part will break on each rover and both will fall silent. "But until that happens we are going to just keep driving until the wheels fall off," says Bell.

Astronomers say they have spotted a cloud of alcohol in deep space that measures 463-billion kilometres across, a finding that could shed light on how giant stars are formed from primordial gas.

The vast bridge-shaped cloud of methyl alcohol has been spotted in a region of our galaxy, the Milky Way, that is called W3(OH), where stars are being formed by the gravitational collapse of concentrations of gas and dust, the discoverers said in a press release.

Methanol, an organic (carbon-based) molecule, is a cousin of ethanol, which is found in alcoholic beverages. Methanol is not suitable for human consumption.

The cloud was spotted by astronomers based at Britain's Jodrell Bank Observatory led by Lisa Harvey-Smith. Their work was to be presented on Tuesday at a meeting in Leicester, central England, of the Royal Astronomical Society (RAS).

In 2004, methanol, also called methyl alcohol, was spotted for the first time in one of the disk-like clusters that form around nascent stars.

That discovery opened up a new area of debate in astrophysics, challenging the conventional view that interstellar chemistry could not provide the conditions for creating complex molecules, as they would be ripped apart by ultraviolet radiation from stars and other tough conditions.

Around 130 organic molecules have also been identified so far in outer space, fuelling speculation that these complex molecules may have helped to sow the seeds for life on the fledgling Earth

The supermassive black holes that dot outer space are the "most fuel efficient engines" in the universe, according to the findings of a new US study that used a powerful NASA X-ray observatory to observe nine vast black holes.

Researchers used the National Aeronautics and Space Administration's Chandra X-ray Observatory to study nine supermassive black holes at the centres of elliptical galaxies to reach their findings.

The black holes, which were .2 to three billion times the mass of the sun, are relatively old and generate much less radiation than quasars, the fast growing black holes seen in the early universe.

The researchers said the Chandra findings show that most of the energy released by matter falling toward a supermassive black hole is in the form of high-energy jets travelling at near the speed of light away from a black hole.

"Just as with cars, it's critical to know the fuel efficiency of black holes," said lead author Steve Allen of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University and the Stanford Linear Accelerator Centre.

The researchers believe their work could be crucial to understanding how the high-energy jets can be launched from magnetised disks of gas near the black hole's event horizon, the distance from a black hole within which nothing, even light, can escape.

The authors said they were surprised to discover that the black holes are all producing much more energy in jets of high-energy particles than in visible light or X-rays.

Some of the gas first drawn to the black holes may be blown away by the energetic activity before it gets too close to a black hole, but a significant amount must eventually approach the event horizon, where it is used with high efficiency to power the jets, the authors said.

Their findings will be published in the upcoming issue of the Monthly Notices of the Royal Astronomical Society.

Hubble's stunning view of nearby dust clouds
SPACE TELESCOPE SCIENCE INSTITUTE NEWS RELEASE
Posted: April 4, 2006

Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
Download larger image version click on picture

The yearly ritual of spring cleaning clears a house of dust as well as dust "bunnies," those pesky dust balls that frolic under beds and behind furniture. NASA's Hubble Space Telescope has photographed similar dense knots of dust and gas in our Milky Way Galaxy. This cosmic dust, however, is not a nuisance. It is a concentration of elements that are responsible for the formation of stars in our galaxy and throughout the universe.

These opaque, dark knots of gas and dust are called "Bok globules," and they are absorbing light in the center of the nearby emission nebula and star-forming region, NGC 281. The globules are named after astronomer Bart Bok, who proposed their existence in the 1940's.

Bok hypothesized that giant molecular clouds, on the order of hundreds of light-years in size, can become perturbed and form small pockets where the dust and gas are highly concentrated. These small pockets become gravitationally bound and accumulate dust and gas from the surrounding area. If they can capture enough mass, they have the potential of creating stars in their cores; however, not all Bok globules will form stars. Some will dissipate before they can collapse to form stars. That may be what's happening to the globules seen here in NGC 281.

Near the globules are bright blue stars, members of the young open cluster IC 1590. The cluster is made up of a few hundred stars. The cluster's core, off the image towards the top, is a tight grouping of extremely hot, massive stars with an immense stellar wind. The stars emit visible and ultraviolet light that energizes the surrounding hydrogen gas in NGC 281. This gas then becomes super heated in a process called ionization, and it glows pink in the image.

The Bok globules in NGC 281 are located very close to the center of the IC 1590 cluster. The exquisite resolution of these Hubble observations shows the jagged structure of the dust clouds as if they are being stripped apart from the outside. The heavy fracturing of the globules may appear beautifully serene but is in fact evident of the harsh, violent environment created by the nearby massive stars.

The Bok globules in NGC 281 are visually striking nonetheless. They are silhouetted against the luminous pink hydrogen gas of the emission nebula, creating a stark visual contrast. The dust knots are opaque in visual light. Conversely, the nebulous gas surrounding the globules is transparent and allows light from background stars and even background galaxies to shine through.

These images were taken with Hubble's Advanced Camera for Surveys in October 2005. The hydrogen-emission image that clearly shows the outline of the dark globules was combined with images taken in red, blue, and green light in order to help establish the true color of the stars in the field. NGC 281 is located nearly 9,500 light-years away in the direction of the constellation Cassiopeia.

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA). The Space Telescope Science Institute in Baltimore conducts Hubble science operations. The Institute is operated for NASA by the Association of Universities for Research in Astronomy, Inc., Washington.

Two supermassive black holes spirling to collision
UNIVERSITY OF VIRGINIA NEWS RELEASE
Posted: April 6, 2006

A pair of supermassive black holes in the distant universe are intertwined and spiraling toward a merger that will create a single super-supermassive black hole capable of swallowing billions of stars, according to a new study by astronomers at the University of Virginia, Bonn University and the U.S. Naval Research Laboratory.

The study appears in the April 6, 2006 issue of the journal Astronomy & Astrophysics.

Black holes are among the oldest regions of the universe and hold clues to understanding the formation of the universe and its destiny. Though astronomers have theorized that coupled black holes exist, and that black holes sometimes merge and form supermassive black holes, the new study provides further evidence that this in fact occurs.

"The two key questions about supermassive black holes are: Where do they come from and how do they grow over time?" said Craig Sarazin, the W.H. Vanderbilt Professor of Astronomy at the University of Virginia and co-author of the study. "The birth, care and nurturing of supermassive black holes is a very active area of study in astronomy."

Supermassive black holes are areas in space that are so dense and massive they contain up to billions of stars and continually suck in more stars, further building their mass and gravitational pull. Even light cannot escape the pull of gravity in a black hole. The area appears as it is described: a black hole in space.

"Black holes are the ultimate garbage disposals," Sarazin said. "The material they swallow disappears without any trace, except for the gravity of the black hole."

Sarazin and his colleagues used NASA's Chandra X-ray Observatory to glean their results. Black holes are detectable because they produce large amounts of X-ray emission, similar to the radiation used for medical diagnosis. This high-energy radiation is invisible to our eyes, but can be seen with X-ray telescopes.

"There is no way to determine how a black hole was created or what kinds of things it has swallowed just by looking at the resulting black hole," Sarazin said. "You have to catch the black hole when it is sitting down to dinner or

"The question was: Is this pair of supermassive black holes an old married couple, or just strangers passing in the night?" Sarazin said. "We now know that they are coupled, but more like the mating of black widow spiders. One of the black holes invariably will eat the other."

NASA is interested in helping astronomers better understand the formation of supermassive black holes and is currently planning to build an array of three space satellites called LISA (Laser Interferometry Space Antenna) to detect gravity waves from merging black holes.

"Obviously, astronomers would like to be certain that this process of supermassive black hole mergers really does occur, so that LISA will have something to detect," Sarazin said.

In recent years, astronomers have discovered that every large galaxy in the present day universe likely has a supermassive black hole.

That, essentially, is what the Sarazin team has accomplished. They focused their observations on the center of a cluster of galaxies named Abell 400 where astronomers had previously suggested that a pair of supermassive black holes might be colliding. The two holes seemed to be relatively close together, but there was no proof that they were bound to one another or merging.

The Milky Way's own supermassive black hole has swallowed as much material as four million suns. The biggest galaxies contain black holes that have swallowed many billions of stars worth of material.

In some cases, two galaxies containing supermassive black holes collide and merge together, and eventually the two supermassive black holes fall into the center of the merged larger galaxy, and spiral together. Ultimately, they merge into one even larger hole.

Sarazin's team found that the two merging supermassive black holes in Abell 400 appear to be swallowing gas from their host galaxy, and each is ejecting a pair of oppositely-directed jets of radio-emitting plasma. As the supermassive black holes fall through the gas in the cluster Abell 400, jets of radio-emitting plasma are swept back behind them.

"The jets are similar to the contrails produced by planes as they fly through the air on Earth," Sarazin said. "From the contrails, we can determine where the planes have been, and in which direction they are going. What we see is that the jets are bent together and intertwined, which indicates that the pair of supermassive black holes are bound and moving together."

Sarazin's co-authors are Daniel Hudson and Thomas Reiprich of the University of Bonn, and Tracy Clarke of the U.S. Naval Research Laboratory. The work was begun when Reiprich and Clarke were post-doctoral fellows working with Professor Sarazin at the University of Virginia.