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Astronomers have measured the 3-D velocities of the Large Magellanic Cloud (shown here) and the Small Magellanic Cloud. They found surprisingly high speeds, which indicates that the Magellanic Clouds are not gravitationally bound to the Milky Way but instead are “just passing through.”

September 17, 2007
The Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) are two of the Milky Way’s closest neighboring galaxies. A stunning sight in the southern hemisphere, they were named after Ferdinand Magellan, who explored those waters in the 16th century. For hundreds of years, these galaxies were considered satellites of the Milky Way, gravitationally bound to our home galaxy. New research by Gurtina Besla of the Harvard-Smithsonian Center for Astrophysics and her colleagues shows that the Magellanic Clouds are recent arrivals on their first visit to the Milky Way’s neighborhood.

“We have known about the Clouds since the time of Magellan, and a single measurement has thrown out everything we thought we understood about their history and evolution,” says Besla.

Although they look like glowing clouds to the unaided eye, the LMC and SMC are both irregular galaxies. The Large Magellanic Cloud is located approximately 160,000 light-years from Earth. It’s about one-twentieth as large as our galaxy in diameter and holds about one-tenth as many stars. The Small Magellanic Cloud is located around 200,000 light-years from Earth. It’s about ten times smaller than its companion and a hundred times smaller than the Milky Way.

Earlier this year, CfA astronomers reported measuring the 3-D velocities of the Magellanic Clouds through space with greater accuracy than ever before. The velocities were anomalously high. Two explanations were proposed: 1) the Milky Way is more massive than previously thought, or 2) the Magellanic Clouds are not gravitationally bound to the Milky Way. Further analysis by Besla and her colleagues verified the second explanation. The parabolic orbit they calculated for the Clouds, based on the observed velocities, shows that both are on their first pass by the Milky Way.

This result carries several implications. For example, as a spiral galaxy the Milky Way has a large gaseous disk intermixed with billions of stars. That gaseous disk is known to be significantly warped, extending about 10,000 light-years above and below the galaxy’s plane. Astronomers theorized that gravitational tides due to previous passages of the Magellanic Clouds caused this warp. However, since the Clouds arrived only 1-3 billion years ago, they are not likely to be the source of the warp.

Another puzzle relates to the Magellanic Clouds themselves. A long trail of hydrogen gas called the Magellanic Stream extends behind the Clouds, spanning 100 degrees of the sky from the earth’s viewpoint. Some astronomers suggested that the Magellanic Stream formed due to tidal interactions between the Clouds and the Milky Way. Others believed that hydrogen was stripped from the Clouds by gas pressure as they plunged through the extremely tenuous gas surrounding our galaxy. A first-passage scenario rules out both scenarios.

“We’ve been left with a real mystery,” says Besla. “One answer has led to many more questions.”

Finally, the star-forming history of the Clouds themselves must be revisited. Rather than forming stars continuously like the Milky Way, the Magellanic Clouds have undergone several bursts of star formation followed by long quiet periods. Astronomers thought that the starbursts coincided with previous close passes by the Milky Way. This explanation no longer holds true. Instead, interactions between the SMC and LMC may be the primary force driving star formation in both galaxies.

In the future, Besla and her colleagues intend to focus on the origin of the Magellanic Stream, conducting N-body simulations to puzzle out possible formation mechanisms. Other astronomers will make direct observations and survey the Stream. The combined power of observational and theoretical research may answer the questions generated by the current work.

The paper describing this work has been accepted for publication in the Astrophysical Journal.

Strange planet-mass object found

In this artist depiction of the SWIFT J1756.9-2508 system, the foreground object is the planet-mass object. The pulsar, located at the upper right, is tidally distorting the companion into a teardrop-shaped object, and ripping gas from it. This material flows in a stream toward the pulsar and forms a disk around it. Eventually, enough gas builds up in the disk to produce an outburst bright enough to make the system visible from Earth.

Aurore Simonnet/Sonoma State University

September 14, 2007
Using NASA’s Swift and Rossi X-ray Timing Explorer (RXTE) satellites, astronomers have discovered one of the most bizarre planet-mass objects ever found.

The object’s minimum mass is only about 7 times the mass of Jupiter. But instead of orbiting a normal star, this low-mass body orbits a rapidly spinning pulsar. It orbits the pulsar every 54.7 minutes at an average distance of only about 230,000 miles (slightly less than the Earth-Moon distance).

“This object is merely the skeleton of a star,” says co-discoverer Craig Markwardt of NASA’s Goddard Space Flight Center in Greenbelt, Md. “The pulsar has eaten away the star’s outer envelope, and all the remains is its helium-rich core.”

Hans Krimm of NASA Goddard discovered the system on June 7, when Swift’s Burst Alert Telescope picked up an outburst of X rays and gamma rays in the direction of the galactic center. The source was named SWIFT J1756.9-2508 for its sky coordinates in the constellation Sagittarius.

RXTE began observing SWIFT J1756.9 on June 13 with its Proportional Counter Array (PCA). After analyzing the PCA data, Markwardt realized that the object was pulsing in X rays 182.07 times per second, which told him that it was a rapidly spinning pulsar. These so-called millisecond pulsars are neutron stars that spin hundreds of times per second, faster than a kitchen blender. Normally, the spin rate of neutron stars slows down as they age, but much like we can pull a string to “spin up” a top, gas spiraling onto a neutron star from its companion can maintain or even increase its fast spin.

In the case of SWIFT J1756.9-2508, Markwardt detected subtle modulations in the X-ray timing data that revealed a low-mass companion tugging the pulsar toward and away from Earth. His calculations show that the companion has a minimum mass about 7 times that of Jupiter. Because we don’t know the orbital inclination of the system, the companion’s actual mass is unknown, but it is extremely unlikely to exceed 30 Jupiters.

MIT astronomers led by Deepto Chakrabarty also observed the system with RXTE, before it faded to invisibility on June 21. Chakrabarty’s group reached identical conclusions, and the two teams have coauthored a paper that has been accepted for publication in the Astrophysical Journal Letters.

The system is only the eighth millisecond pulsar that is observed to be accreting mass from a companion. Only one other such system has a pulsar companion with such a low mass. The companion in this system, XTE J1807-294, also has a minimum mass of about 7 Jupiters. “Given that we don’t know the exact mass of either companion, ours could be the smallest,” says Krimm.

The system probably formed several billion years ago, when it consisted of a very massive star and a smaller star with perhaps 1 to
3 solar masses. The more massive star evolved quickly and exploded as a supernova, leaving behind the neutron star. The smaller star eventually started to puff up en route to becoming a red giant, and the two objects became embedded in the extended stellar envelope.
This drained orbital energy, causing the two stars to draw ever nearer, while simultaneously ejecting the envelope.

Today, the two objects are so close to each other than the neutron star’s powerful gravity produces a tidal bulge on its companion, siphoning off gas that flows into a disk that surrounds the neutron star. The flow eventually becomes unstable and dumps large quantities of gas onto the neutron star, causing an outburst like the one observed in June.

Evolution models by Christopher Deloye of Northwestern University suggest that the low-mass companion is helium dominated. “Despite its extremely low mass, the companion isn’t considered a planet because of its formation,” says Deloye. “It’s essentially a white dwarf that has been whittled down to a planetary mass.”

After billions of years, little remains of the companion star, and it remains unclear whether it will survive. “It’s been taking a beating, but that’s part of nature,” adds Krimm.

With an estimated distance of roughly 25,000 light-years, the system is normally too faint to be detected at any wavelength, and is only visible during an outburst. SWIFT J1756.9 has never been seen to erupt until this June, so as Markwardt points out, “We don’t know how long it will slumber before it wakes up again.”

Astronomy magazine podcast: Supernova 1987A

This Hubble telescope image shows the supernova’s triple-ring system, including the bright spots along the inner ring of gas surrounding the exploded star.

NASA, ESA, P. Challis and R. Kirshner (Harvard-Smithsonian Center for Astrophysics)

September 13, 2007
In this week’s show, Astronomy Senior Editor Rich Talcott looks back at Supernova 1987A and obstacles to our understanding of supernovae.

After you listen, e-mail us here and let us know what you think.

To subscribe to our podcast, click here.

Downloadable File(s)

Supernova 1987A (6620kB)

Stars going out in style

Hubble captured this four planetary nebulae: He 2-47 (top left), NGC 5315 (top right), IC 4593 (bottom left), and NGC 5307 (bottom right)

NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

September 13, 2007
The colorful, intricate shapes in these NASA Hubble Space Telescope images reveal how the glowing gas ejected by dying Sun-like stars evolves dramatically over time.

These gaseous clouds, called planetary nebulae, are created when stars in the last stages of life cast off their outer layers of material into space. Ultraviolet light from the remnant star makes the material glow. Planetary nebulae last for only 10,000 years, a fleeting episode in the 10-billion-year lifespan of Sun-like stars.

The name planetary nebula has nothing to do with planets. They got their name because their round shapes resembled planets when seen through the small telescopes of the eighteenth century.

The Hubble images show the evolution of planetary nebulae, revealing how they expand in size and change temperature over time. A young planetary nebula, such as He 2-47, at top, left, for example, is small and is dominated by relatively cool, glowing nitrogen gas. In the Hubble images, the red, green, and blue colors represent light emitted by nitrogen, hydrogen, and oxygen, respectively.

Over thousands of years, the clouds of gas expand away and the nebulae become larger. Energetic ultraviolet light from the star penetrates more deeply into the gas, causing the hydrogen and oxygen to glow more prominently, as seen near the center of NGC 5315. In the older nebulae, such as IC 4593, at bottom, left, and NGC 5307, at bottom, right, hydrogen and oxygen appear more extended in these regions, and red knots of nitrogen are still visible.

These four nebulae all lie in our Milky Way Galaxy. Their distances from Earth are all roughly the same, about 7,000 light-years. The snapshots were taken with Hubble’s Wide Field Planetary Camera 2 in February 2007. Like snowflakes, planetary nebulae show a wide variety of shapes, indicative of the complex processes that occur at the end of stellar life.

He 2-47, at top left, is dubbed the “starfish” because of its shape. The six lobes of gas and dust, which resemble the legs of a starfish, suggest that He 2-47 puffed off material at least three times in three different directions. Each time, the star fired off a narrow pair of opposite jets of gas. He 2-47 is in the southern constellation Carina.

NGC 5315, the chaotic-looking nebula at top right, reveals an x-shaped structure. This shape suggests that the star ejected material in two different outbursts in two distinct directions. Each outburst unleashed a pair of diametrically opposed outflows. NGC 5315 lies in the southern constellation Circinus.

IC 4593, at bottomleft, is in the northern constellation Hercules.

NGC 5307, at bottom right, displays a spiral pattern, which may have been caused by the dying star wobbling as it expelled jets of gas in different directions. NGC 5307 resides in the southern constellation Centaurus.

Small galaxies solve a big problem

The two large Keck telscopes can combine their light-gathering power through interferometry to probe the universe deeper than any other optical telescope system on Earth.

NASA/JPL

The two large Keck telscopes can combine their light-gathering power through interferometry to probe the universe deeper than any other optical telescope system on Earth.

NASA / JPL

September 12, 2007
In an attempt to resolve the “Missing Dwarf Galaxy” problem, two astronomers used the W. M. Keck Observatory in Hawaii to study a population of the darkest, most lightweight galaxies known, each containing 99% dark matter. The findings suggest the “Missing Dwarf Galaxy” problem is not as severe as previously thought, and may have been solved completely.

“It seems that very small, ultra-faint galaxies are far more plentiful than we thought,” said Marla Geha, co-author of the study and a Plaskett Institute of Astrophysics in Canada. “If you asked me last year whether galaxies this small and this dark existed, I would have said no. I’m astonished that so many tiny, dark matter-dominated galaxies have now been discovered.”

The Missing Dwarf Galaxy puzzle comes from a prediction of the “Cold Dark Matter” model, which explains the growth and evolution of the universe. It predicts large galaxies like the Milky Way should be surrounded by a swarm of up to several hundred smaller galaxies known as “dwarf galaxies.” However, until recently, only 11 such companions were known to be orbiting the Milky Way. To explain this large discrepancy, theorists suggested that while hundreds of dwarf galaxies near the Milky Way may indeed exist, the majority might have few, if any, stars. If so, the galaxies would be comprised almost entirely of dark matter-a mysterious type of matter that has gravitational effects on ordinary atoms, but which does not produce any light. But proving the existence of a large number of nearly invisible galaxies seemed problematic, until now.

Josh Simon, a Millikan Postdoctoral Scholar at the California Institute of Technology, and Geha used the 10-meter Keck II telescope with the DEIMOS spectrograph to conduct follow-up studies of eight new dwarf galaxies first discovered with the Sloan Digital Sky Survey. The results enabled the duo to calculate precisely the total mass of each galaxy. To their surprise, each system was among the smallest ever measured, more than 10,000 times smaller than the Milky Way.

This chart shows the distribution of newly discovered dwarf galaxies orbiting the Milky Way.

M. Geha

“The formation of such small galaxies is not very well understood from a theoretical perspective,” said Simon. “Explaining how stars form inside these remarkably tiny galaxies is difficult, and so it is hard to predict exactly how many dwarfs we should find near the Milky Way. Our work narrows the gap between the Cold Dark Matter theory and observations by significantly increasing the number of Milky Way dwarf galaxies and telling us more about the properties of these galaxies. We also now know that dwarf galaxies can be even smaller than we thought possible.”

Numerous, repeated measurements of 814 stars in the eight dwarf galaxies were obtained at W. M. Keck Observatory. The stars were found to be moving much slower than stars in any other known galaxy (about 4 to 7 km/sec.) For comparison, the Sun orbits the center of the Milky Way at a speed of about 220 km/sec. In all, the astronomers measured precise speeds for 18 to 214 stars in each galaxy, about three times more stars per galaxy than any previous study.

“This is a significant paper,” said Taft Armandroff, director of the W. M. Keck Observatory, whose own research includes the study of dwarf galaxies. “It is a compelling example of how large, ground-based telescopes can precisely measure the orbits of distant stars on the sky to just a few kilometers per second. I expect DEIMOS will soon tell us about the chemical composition of these stars to help us better understand how star formation takes place in such small galaxies.”

Some parameters of the Cold Dark Matter theory can now be updated to match observed conditions in the local universe. Based on the masses measured for the new dwarf galaxies, Simon and Geha concluded the fierce ultraviolet radiation given off by the first stars, which formed just a few hundred million years after the Big Bang, may have blown all of the hydrogen gas out of the dwarf galaxies forming at that time. The loss of gas prevented the galaxies from creating new stars, leaving them very faint, or in many cases completely dark. When this effect is included in theoretical models, the numbers of expected and observed dwarf galaxies agree.

“One of the implications of our results is that up to a few hundred completely dark galaxies really should exist in the Milky Way’s cosmic neighborhood,” said Geha. “If the Cold Dark Matter model is correct they have to be out there, and the next challenge for astronomers will be finding a way to detect their presence.”

Because the Sloan Digital Sky Survey only covered about 25 percent of the sky, future surveys of the remainder of the sky are expected to discover as many as 50 more dark matter dominated dwarf galaxies orbiting the Milky Way. Telescopes for one such survey, the Pan-STARRS project on Maui, are now under construction.

The paper, “Kinematics of the Ultra-Faint Milky Way Satellites: Solving the Missing Satellite Problem,” will be published in the November 10 issue of the Astrophysical Journal.

Mars rovers survive dust storms

NASA’s Mars Exploration Rover Opportunity used its navigation camera during its 1,278th martian sol (August 28, 2007) to capture this view form atop Victoria Crater.

NASA/JPL-Caltech

September 11, 2007
Two months after sky-darkening dust from severe storms nearly killed NASA’s Mars exploration rovers, the solar powered robots are awake and ready to continue their mission.

Opportunity’s planned descent into the giant Victoria Crater was delayed, but now the rover is preparing to drive into the 800-meter-diameter crater (half-mile-diameter) as early today.

Spirit, Opportunity’s rover twin, also survived the global dust storms. The rovers are 43 months into missions originally planned to last three months. On September 5, Spirit climbed onto its long-term destination called Home Plate, a plateau of layered bedrock bearing clues to an explosive mixture of lava and water.

“These rovers are tough. They faced dusty winds, power starvation and other challenges — and survived. Now they are back to doing groundbreaking field work on Mars. These spacecraft are amazing,” said Alan Stern, associate administrator of NASA’s Science Mission Directorate, Washington.

Victoria Crater contains an exposed layer of bright rocks that may preserve evidence of interaction between the Martian atmosphere and surface from millions of years ago, when the atmosphere might have been different from today’s. Victoria is the biggest crater Opportunity has visited.

Martian dust storms in July blocked so much sunlight that researchers grew concerned the rovers’ daily energy supplies could plunge too low for survival. Engineers at NASA’s Jet Propulsion Laboratory, Pasadena, California, put Opportunity onto a very low-energy regimen of no movement, few observations and reduced communication with Earth. Skies above both rovers remain dusty but have been clearing gradually since early August.

Dust from the sky has been falling onto both rovers’ solar panels, impeding their ability to collect energy from the sun. However, beneficial wind gusts removed some of the new buildup from Opportunity almost as soon as it accumulated.

Opportunity drove to the lip of Victoria Crater in late August and examined possible entry routes. This week, Opportunity has been driving about 40 meters (about 130 feet) toward its planned entry point. The route will provide better access to a top priority target inside the crater: a bright band of rocks about 12 meters (about 40 feet) from the rim. “We chose a point that gives us a straight path down, instead of driving cross-slope from our current location,” said Paolo Bellutta, a JPL rover driver plotting the route. “The rock surface on which Opportunity will be driving will provide good traction and control of its path into the crater.”

For its first foray into the crater, Opportunity will drive just far enough to get all six wheels in; it will then back out and assess slippage on the inner slope. “Opportunity might be ready for that first ‘toe dip’ into the crater as early as next week,” said JPL’s John Callas, rover project manager. “In addition to the drives to get to the entry point, we still need to conduct checkouts of two of Opportunity’s instruments before sending the rover into the crater.”

The rover team plans to assess if dust has impaired use of the microscopic imager. If that tool is working, the team will use it to observe whether a scanning mirror for the miniature thermal emission spectrometer (Mini-TES) can function accurately. This mirror is high on the rover’s camera mast. It reflects infrared light from the landscape to the spectrometer at the base of the mast, and it also can be positioned to close the hole in the mast as protection from dust. The last time the spectrometer was used, some aspects of the data suggested the instrument may have been viewing the inside of the mast instead of the Martian landscape.

“If the dust cover or mirror is no longer moving properly, we may have lost the ability to use that instrument on Opportunity,” said Steve Squyres of Cornell University, Ithaca, New York, principal investigator for the rovers’ science instruments. “It would be the first permanent loss of an instrument on either rover. But we’ll see.”

The instrument already has provided extensive valuable information about rocks and soils in the Meridiani region where Opportunity works. “Mini-TES has told us a lot about the rocks and soils at Meridiani, but we’ve learned that the differences among Meridiani rocks are often too subtle for it to distinguish,” Squyres said. “The same instrument on Spirit, at Gusev Crater, has a much more crucial role for us at this point in the mission because there is such diversity at Gusev.” Researchers will rely heavily on a different type of instrument, Opportunity’s alpha particle X-ray spectrometer, for analysis of rocks at the bright-band target layer in the crater.

October 2007: Our cosmic beginnings

September 11, 2007
WAUKESHA, WI – Today, observers see galaxies rushing away from each other in our expanding universe. But if you run cosmic evolution backward, everything draws together. The cosmos become denser, galaxies melt into primordial gas, which eventually breaks down into a sea of protons and neutrons.

Nuclear reactions between protons and neutrons in the universe’s first 3 minutes made the lightest elements. This find changed scientists’ minds about cosmic origins. In “How the Big Bang forged the first elements,” astrophysicist Adam Frank explains how Big Bang nucleosynthesis is spinning out a detailed picture of our cosmic beginnings.

“Big Bang nucleosynthesis shows us that a brief period of well-understood physics has consequences that trickle down 13.7 billion years to the universe we observe,” Frank writes.

Astronomy‘s October issue moves forward in time from the Big Bang to 1987 in “Supernova 1987A: 20 years later.” Astronomy Senior Editor Richard Talcott takes readers on a trip to February 23, 1987, when astronomers discovered the brightest supernova in nearly 400 years. The supernova taught astronomers a great deal about exploding stars – and points out a lot they still don’t know.

The October issue of Astronomy is available now on newsstands everywhere. More resources from Astronomy.com:

Astronomy

Astronomy‘s mission:
Astronomy promotes the science and hobby of astronomy through high-quality publications that engage, inform, entertain, and inspire. The October issue of Astronomy is available now on newsstands everywhere.

It’s all relative
When astronomers traced the history of 39 asteroids with closely-related orbits, they were surprised to discover the orbits converged as recently as 5.8 million years ago. The Karin cluster, probably created when a smaller asteroid slammed into a larger parent body, has since grown to 90 members. Planetary scientist Daniel Durda explains how the discovery was made in “Family ties reveal asteroid origins.”

The Moon and New York City
One night, sidewalk astronomer Jeffrey Jacobs offered passersby a free look at the Moon through a telescope. But would skeptical New Yorkers accept the gift? Phil Scott invites readers to “Come see the Moon!” in his story of street-corner astronomy in New York. “Where we live is not this small intersection in the Village,” writes Scott. “Where we really live is on a blue-and-white planet, orbited by a pitted gray moon, orbiting a yellow star, which orbits a spiral galaxy inside a vast universe.”

Deep-sky observing
Intricate gas clouds and sparkling stars await readers in the October issue. “Observe autumn’s best nebulae” teaches you how to spot photogenic, bright emission nebulae such as the Cepheus giants and the Bubble Nebula. With a little planning and a clear dark sky, some of the objects on author Richard Jakiel’s list may become your favorites, too.

Also in the October 2007 Astronomy:

“Illustrated: Mars in high def” – The Red Planet explodes with detail under Mars Reconnaissance Orbiter’s amazing cameras.

“Astronomy thrives in Texas” – Find your astronomical adventure in the Lone Star state.

“Sky-testing William Optics’ new refractors” – Sharp optics in two beautiful packages make these telescopes easy to look at and easier to look through.

“Experience Meade’s new eyepieces” – Choosing the right eyepiece is easy with Meade’s Series 5000 oculars.

The October 2007 Astronomy also includes Ask Astro, Astro news, Bob Berman’s strange universe, Glenn Chaple’s observing basics, Phil Harrington’s binocular universe, Stephen James O’Meara’s secret sky, The sky this month, New products, and Reader gallery.

Sky-event notice: Partial solar eclipse for some Southern Hemisphere observers

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WAUKESHA, WI – Grab your solar-observing gear and get ready for the last solar eclipse of 2007. On September 11, observers in parts of South America, Antarctica, and the South Atlantic will get the opportunity to witness a partial solar eclipse.

Experts at your disposal
Astronomy magazine editors are available to discuss this spectacle. To request an interview, please contact Matt Quandt at 262.798.6484 or mquandt@kalmbach.com.

An eclipse of the Sun, called a solar eclipse, occurs when the New Moon passes directly between Earth and the Sun. When the Moon’s shadow falls on Earth, people within that shadow see the Moon block a portion of the Sun’s light. The Moon’s shadow has two parts: a penumbra and an umbra. The penumbra is the Moon’s faint outer shadow. Observers within this shadow see a partial solar eclipse, with the Moon covering only part of the Sun’s disk.

The September 11 event begins at 10h25m46s Universal Time (UT), when the Moon’s penumbral shadow makes first contact with Earth. The eclipse ends at 14h36m33s UT as the shadow makes its last contact with Earth. At the point of greatest eclipse, which occurs at 12h31m21s UT, the Moon covers about 75 percent of the Sun’s disk.

How much of the Sun’s disk appears covered by the Moon depends on your observing location. Observers in southern Argentina and Chile will have the best view. In Palmer Station, Antarctica, the Moon will cover up to 70 percent of the Sun’s disk near noon. Most observers in Argentina will see about half of the Sun’s disk covered, while those in Brazil will see about one-fourth of the Sun obscured at greatest eclipse. For those in La Paz, Bolivia, Santiago, Chile, and Lima, Peru, the eclipse will already be in progress at sunrise.

Don’t forget to use proper eye protection when viewing a partial solar eclipse. Use a solar filter with your telescope, wear special solar-eclipse eyeglasses, or construct a pinhole camera and project the Sun’s image onto a piece of paper.

Partial solar eclipse for some Southern Hemisphere observers

A partial solar eclipse awaits observers throughout southern North America and western South America the afternoon of April 8.

John R. Foster

A partial solar eclipse awaits observers throughout southern North America and western South America the afternoon of April 8.

John R. Foster

September 10, 2007
Grab your solar-observing gear and get ready for the last solar eclipse of 2007. On September 11, observers in parts of South America, Antarctica, and the South Atlantic will get the opportunity to witness a partial solar eclipse.

The September 11 event begins at 10h25m46s Universal Time (UT), when the Moon’s penumbral shadow makes first contact with Earth. The eclipse ends at 14h36m33s UT as the shadow makes its last contact with Earth. At the point of greatest eclipse, which occurs at 12h31m21s UT, the Moon covers about 75 percent of the Sun’s disk.

How much of the Sun’s disk appears covered by the Moon depends on your observing location. Observers in southern Argentina and Chile will have the best view. In Palmer Station, Antarctica, the Moon will cover up to 70 percent of the Sun’s disk near noon. Most observers in Argentina will see about half of the Sun’s disk covered, while those in Brazil will see about one-fourth of the Sun obscured at greatest eclipse. For those in La Paz, Bolivia, Santiago, Chile, and Lima, Peru, the eclipse will already be in progress at sunrise.

Don’t forget to use proper eye protection when viewing a partial solar eclipse. Use a solar filter with your telescope, wear special solar-eclipse eyeglasses, or construct a pinhole camera and project the Sun’s image onto a piece of paper.

Astronomers find compact galaxies

The NASA/ESA Hubble Space Telescope and the NASA Spitzer Space Telescope have joined forces to discover nine of the smallest, faintest, most compact galaxies ever observed in the distant Universe. Blazing with the brilliance of millions of stars, each of the newly discovered galaxies is a hundred to a thousand times smaller than our Milky Way Galaxy.

NASA, ESA and N. Pirzkal (European Space Agency/STScI)

September 7, 2007
The NASA/ESA Hubble Space Telescope and the NASA Spitzer Space Telescope have joined forces to discover nine of the smallest, faintest, most compact galaxies ever observed in the distant universe. Blazing with the brilliance of millions of stars, each of the newly discovered galaxies is a hundred to a thousand times smaller than our Milky Way Galaxy.

The conventional model for galaxy evolution predicts that small galaxies in the early Universe evolved into the massive galaxies of today by coalescing. Nine Lego-like “building block” galaxies initially detected by Hubble likely contributed to the construction of the Universe as we know it. “These are among the lowest mass galaxies ever directly observed in the early Universe” says Nor Pirzkal of the European Space Agency/STScI.

Pirzkal was surprised to find that the galaxies’ estimated masses were so small. Hubble’s cousin observatory, NASA’s Spitzer Space Telescope was called upon to make precise determinations of their masses. The Spitzer observations confirmed that these galaxies are some of the smallest building blocks of the Universe.

These young galaxies offer important new insights into the Universe’s formative years, just one billion years after the Big Bang. Hubble detected sapphire blue stars residing within the nine pristine galaxies. The youthful stars are just a few million years old and are in the process of turning Big Bang elements (hydrogen and helium) into heavier elements. The stars have probably not yet begun to pollute the surrounding space with elemental products forged within their cores.

“While blue light seen by Hubble shows the presence of young stars, it is the absence of infrared light in the sensitive Spitzer images that was conclusive in showing that these are truly young galaxies without an earlier generation of stars,” says Sangeeta Malhotra of Arizona State University in Tempe, one of the investigators.

The galaxies were first identified by James Rhoads of Arizona State University and Chun Xu of the Shanghai Institute of Technical Physics in Shanghai, China. Three of the galaxies appear to be slightly disrupted — rather than being shaped like rounded blobs, they appear stretched into tadpole-like shapes. This is a sign that they may be interacting and merging with neighboring galaxies to form larger, cohesive structures.

The galaxies were observed in the Hubble Ultra Deep Field (HUDF) with Hubble’s Advanced Camera for Surveys and the Near Infrared Camera and Multi-Object Spectrometer as well as Spitzer’s Infrared Array Camera and the European Southern Observatory’s Infrared Spectrometer and Array Camera. Seeing and analysing such small galaxies at such a great distance is at the very limit of the capabilities of the most powerful telescopes. Images taken through different colour filters with the ACS were supplemented with exposures taken through a so-called grism which spreads the different colors emitted by the galaxies into short “trails”. The analysis of these trails allows the detection of emission from glowing hydrogen gas, giving both the distance and an estimate of the rate of star formation. These “grism spectra” — taken with Hubble and analyzed with software developed at the Space Telescope-European Coordinating Facility in Munich, Germany — can be obtained for objects that are significantly fainter than can be studied spectroscopically with any other current telescope.

Astronomy Day grand-prize winner

The Mignogna family and their new Meade telescope at the Carnegie Science Center.

Dan Malerbo

September 6, 2007
Astronomy magazine is proud to announce the Astronomy Day grand-prize winner! Diane Mignogna of Pitcairn, Pennsylvania, received a 10-inch LX200R Advanced Ritchey-Chrétien telescope, donated by Meade 4M. “We are so excited to have won,” Mignogna said. “I’m just thrilled.”

Astronomy magazine provided Astronomy Day events across the country with magazines and other prizes. In addition to the grand-prize telescope, Meade 4M also donated an ETX-80AT telescope for each participating venue to give away.

Observatories, planetaria, and museums celebrated National Astronomy Day April 21. But the Carnegie Science Center in Pittsburgh put its own spin on the year’s biggest star party.

“We celebrate earlier, and ours was an ‘Astronomy Weekend,'” said Dan Malerbo, education coordinator at Carnegie’s Henry Buhl, Jr. Planetarium and Observatory.

The observatory, in partnership with the Amateur Astronomers Association of Pittsburgh, gave astronomy-lovers the chance to share their passion with the astronomy-curious March 31 and April 1. Experts hosted observing sessions and discussions on the “demotion” of Pluto, space weather, and more. About 4,000 visitors also learned about telescope-making, the Dawn mission to the asteroid belt, and the latest astronomical news.

“The focus is to primarily let people see what astronomy goes on at the science center and in the community,” Malerbo said. “We have a very strong astronomy community.”

Mignogna attended Astronomy Weekend’s indoor party with a group of friends and family. As they were leaving the science center after attending events throughout the day, Mignogna’s 13-year-old daughter entered the grand-prize telescope drawing.

Exploring the stars has always been a family interest, Mignogna said, but she has never owned a telescope.

“When I was in college, I would sit in the moonlight in my bedroom and watch the Moon rise,” Mignogna said. Her children inherited their mother’s interest in the cosmos: Her daughter used to eat over star-chart placemats, and Mignogna remembers her 21-year-old son asking for his own telescope years ago.

Now, Mignogna will share her prize with her children, as well as her brother and mother. “My mom lives in central Pennsylvania, where there are no lights from the city,” she said.

Malerbo presented Mignogna with the telescope, a one-year subscription to Astronomy magazine, and other gifts from Astronomy August 28 at the Carnegie Science Center in Pittsburgh.

K/T impactor source identified

Researchers suggest that the impactor believed to have wiped out the dinosaurs and other life forms on Earth 65 million years ago can been traced back to a breakup event in the main asteroid belt.

Don Davis

September 5, 2007
The impactor believed to have wiped out the dinosaurs and other life forms on Earth some 65 million years ago has been traced back to a breakup event in the main asteroid belt.

A joint U.S.-Czech team from Southwest Research Institute (SwRI) and Charles University in Prague suggests that the parent object of asteroid (298) Baptistina disrupted when it was hit by another large asteroid, creating numerous large fragments that would later create the Chicxulub crater on the Yucatan Peninsula as well as the prominent Tycho crater found on the Moon.

The team of researchers, including Dr. William Bottke (SwRI), Dr. David Vokrouhlicky (Charles University, Prague) and Dr. David Nesvorny (SwRI), combined observations with several different numerical simulations to investigate the Baptistina disruption event and its aftermath. A particular focus of their work was how Baptistina fragments affected the Earth and Moon.

At approximately 170 kilometers in diameter and having characteristics similar to carbonaceous chondrite meteorites, the Baptistina parent body resided in the innermost region of the asteroid belt when it was hit by another asteroid estimated to be 60 kilometers in diameter. This catastrophic impact produced what is now known as the Baptistina asteroid family, a cluster of asteroid fragments with similar orbits. According to the team’s modeling work, this family originally included approximately 300 bodies larger than 10 kilometers and 140,000 bodies larger than 1 kilometer.

Once created, the newly formed fragments’ orbits began to slowly evolve due to thermal forces produced when they absorbed sunlight and re-radiated the energy away as heat. According to Bottke, “By carefully modeling these effects and the distance traveled by different-sized fragments from the location of the original collision, we determined that the Baptistina breakup took place 160 million years ago, give or take 20 million years.”

The gradual spreading of the family caused many fragments to drift into a nearby “dynamical superhighway” where they could escape the main asteroid belt and be delivered to orbits that cross Earth’s path. The team’s computations suggest that about 20 percent of the surviving multi-kilometer- sized fragments in the Baptistina family were lost in this fashion, with about 2 percent of those objects going on to strike the Earth, a pronounced increase in the number of large asteroids striking Earth.

Support for these conclusions comes from the impact history of the Earth and Moon, both of which show evidence of a two-fold increase in the formation rate of large craters over the last 100 to 150 million years. As described by Nesvorny, “The Baptistina bombardment produced a prolonged surge in the impact flux that peaked roughly 100 million years ago. This matches up pretty well with what is known about the impact record.”

Bottke adds, “We are in the tail end of this shower now. Our simulations suggest that about 20 percent of the present-day, near-Earth asteroid population can be traced back to the Baptistina family.”

The team then investigated the origins of the 180 kilometer diameter Chicxulub crater, which has been strongly linked to the extinction of the dinosaurs 65 million years ago. Studies of sediment samples and a meteorite from this time period indicate that the Chicxulub impactor had a carbonaceous chondrite composition much like the well-known primitive meteorite Murchison. This composition is enough to rule out many potential impactors but not those from the Baptistina family. Using this information in their simulations, the team found a 90 percent probability that the object that formed the Chicxulub crater was a refugee from the Baptistina family.

These simulations also showed there was a 70 percent probability that the lunar crater Tycho, an 85 kilometer crater that formed 108 million years ago, was also produced by a large Baptistina fragment. Tycho is notable for its large size, young age and its prominent rays that extend as far as 1,500 kilometers across the Moon. Vokrouhlicky says, “The probability is smaller than in the case of the Chicxulub crater because nothing is yet known about the nature of the Tycho impactor.”

This study demonstrates that the collisional and dynamical evolution the main asteroid belt may have significant implications for understanding the geological and biological history of Earth.

As Bottke says, “It is likely that more breakup events in the asteroid belt are connected in some fashion to events on the Earth, Moon and other planets. The hunt is on!”

The article, “An asteroid breakup 160 Myr ago as the probable source of the K/T impactor,” was published in the September 6 issue of Nature.