Astronomy magazine podcast: Voyager marks 30 years in space

Voyager 1
With the potential to last until 2020, Voyager 1 has far exceeded its mission expectations.
JPL / NASA
August 30, 2007
NASA’s two venerable Voyager spacecraft are celebrating three decades of flight as they head toward interstellar space. Their ongoing odysseys mark an unprecedented and historic accomplishment.

Voyager 2 launched August 20, 1977, and Voyager 1 launched September 5, 1977. They continue to return information from distances more than three times farther away than Pluto.

In this week’s show, Voyager project manager Ed Massey talks about the spacecraft.

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

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New stars found in galaxy

IC 10
The central starburst region of the IC 10 irregular dwarf galaxy.
UCB/NASA/Keck Observatory
August 30, 2007
Edwin Hubble once called IC 10 “one of the most curious objects in the sky,” and new observations of the extremely faint, lightweight dwarf galaxy are giving scientists new clues about how populations of stars are born.

Though the properties of stars is one of the most well-studied topics in astronomy, scientists still don’t fully understand all the mechanisms involved in star formation and evolution, particularly in galaxies with low levels of oxygen, nitrogen and other heavy elements. But scientists studying the IC 10 galaxy may soon understand how stars might have looked like in the distant past, when the universe was in a younger, more pristine form.

“A few years ago these types of studies would have been impossible from the ground,” said Taft Armandroff, director of the W. M. Keck Observatory, whose own research includes the study of dwarf galaxies. “We can now study individual stars of galaxies several million light years from Earth to understand how star formation events may have affected the evolution of the Milky Way galaxy. This galaxy can teach us what the most common types of galaxies in the universe might be like.”

New images of IC 10 reveal a small region of space teeming with nearly a thousand stars. The image, obtained with NASA’s Hubble Space Telescope and the W. M. Keck Observatory in Hawaii, shows evidence of a vigorous star formation event that took place within the last 10 million years.

William Vacca at the NASA Ames Research Center led the study and says IC 10 may answer many unresolved questions about stellar evolution. “IC 10 is a remarkable galaxy,” he said. “It is the only one we’ve seen that falls outside an established pattern of having a certain number of massive nitrogen-type stars for each carbon-type star. This imbalance has caused us to wonder if our past conclusions about massive stars have been correct. Do we need to revise the models of stellar evolution?”

Astronomers have known that IC 10 has more giant, rare stars called “Wolf-Rayet stars” than all other nearby dwarf galaxies combined. Wolf-Rayet stars are extremely hot blue stars losing enormous amounts of mass to the interstellar medium. In addition, the proportion of Wolf-Rayet stars in IC 10 seems to be wildly out of balance. For the number of stars containing carbon, astronomers expected to see a certain number containing nitrogen.

But so far, very few nitrogen stars have been found. Could IC 10 be hiding a population of stars?

Using a combination of Hubble and Keck telescope images, Vacca’s team found many previously undiscovered stars in the IC 10 galaxy. Each new star can now be measured to determine its chemical composition. If the newly found stars contain nitrogen, then part of the “missing nitrogen” puzzle might be solved.

“The combination of HST images in the optical and Keck Laser Guide Star images in the infrared has been a major breakthrough in our understanding of dense stellar regions,” said Dr. Graham. “IC 10 has so many stars in such a tiny region of space that ground-based studies have been confused. But the combination of HST and Keck has been revolutionary in our understanding of this object, and for any object with a dense region of stars.”

The new images of IC 10 are centered on a bright star first thought to possibly be the most luminous Wolf-Rayet star in IC 10. Follow up studies then found the star to be comprised of at least three or more components.

Now, new data from Keck show the bright star ([MAC92] 24) is actually six or more stars, perhaps even a cluster of stars.

“This is the first time this sort of study has been done using adaptive optics,” said co-author Chris Sheehy of the University of Chicago. “It gives us the ability to make these kinds of measurements accurately from the ground and there’s no shortage of targets in need of a fresh look. The potential is exciting.”

The new data has also enabled scientists to measure the precise distance to IC 10, a figure that has eluded scientists since the object’s discovery more than 100 years ago. Dr. Vacca and his collaborators calculated the distance to IC 10 to be about 2.4 million light years from Earth, or 750 kiloparsecs. This is in good agreement with some previous estimates.

IC 10 was first discovered by Lewis Swift in 1889 at the Warner Observatory in Rochester, New York. The “Index Catalogue” (IC) is a catalogue of galaxies, nebulae and star clusters that supplements the more modern New General Catalogue (NGC). First published in 1895, the catalogue first described IC 10 as a “faint star involved in extremely faint and very large nebula.” It wasn’t until 1935 that IC 10 was first proposed as an extragalactic object and Edwin Hubble later proposed IC 10 might be a member of the Local Group. It took another 30 years before these suspicions could be confirmed using radial velocity and distance measurements.

Astronomers now know IC 10 is similar in many ways to the Large Magellenic Cloud of the Southern Hemisphere. But unlike the Large Magellenic Cloud, IC 10 orbits Andromeda, not the Milky Way. The study of IC 10 is giving astronomers a picture of what the Milky Way might have looked like billions of years ago before the galaxy’s interstellar medium was enriched with elements such as oxygen and nitrogen.

The paper, “Imaging of the Stellar Population of IC 10 with Laser Guide Star Adaptive Optics and the Hubble Space Telescope,” was published in the June 10 issue of Astrophysical Journal. The research was made possible with grants provided by the National Science Foundation and NASA.

Neutron stars warp space-time

disk of hot gas whipping around a neutron star
An artist depicts a disk of hot gas whipping around a neutron star. The gas in the inner part of the disk whirls around the neutron star at about 40 percent the speed of light, so fast that it experiences effects predicted by Einstein’s theories of relativity. Superheated iron atoms in this region emit X-rays at a characteristic wavelength, but the spectral feature is highly distorted by the relativistic effects.
NASA/Dana Berry
August 28, 2007
Using European and Japanese/NASA X-ray satellites, astronomers have seen Einstein’s predicted distortion of space-time around three neutron stars, and in doing so they have pioneered a groundbreaking technique for determining the properties of these ultradense objects.

Neutron stars contain the densest observable matter in the universe.

They cram more than a sun’s worth of material into a city-sized sphere, meaning a few cups of neutron-star stuff would outweigh Mount Everest. Astronomers use these collapsed stars as natural laboratories to study how tightly matter can be crammed under the most extreme pressures that nature can offer.

“This is fundamental physics,” says Sudip Bhattacharyya of NASA’s Goddard Space Flight Center in Greenbelt, Md. and the University of Maryland, College Park. “There could be exotic kinds of particles or states of matter, such as quark matter, in the centers of neutron stars, but it’s impossible to create them in the lab. The only way to find out is to understand neutron stars.”

To address this mystery, scientists must accurately and precisely measure the diameters and masses of neutron stars. In two concurrent studies, one with the European Space Agency’s XMM-Newton X-ray Observatory and the other with the Japanese/NASA Suzaku X-ray observatory, astronomers have taken a big step forward.

Using XMM-Newton, Bhattacharyya and his NASA Goddard colleague Tod Strohmayer observed a binary system known as Serpens X-1, which contains a neutron star and a stellar companion. They studied a spectral line from hot iron atoms that are whirling around in a disk just beyond the neutron star’s surface at 40 percent the speed of light.

Previous X-ray observatories detected iron lines around neutron stars, but they lacked the sensitivity to measure the shapes of the lines in detail. Thanks to XMM-Newton’s large mirrors, Bhattacharyya and Strohmayer found that the iron line is broadened asymmetrically by the gas’s extreme velocity, which smears and distorts the line because of the Doppler Effect and beaming effects predicted by Einstein’s special theory of relativity. The warping of space-time by the neutron star’s powerful gravity, an effect of Einstein’s general theory of relativity, shifts the neutron star’s iron line to longer wavelengths.

“We’ve seen these asymmetric lines from many black holes, but this is the first confirmation that neutron stars can produce them as well. It shows that the way neutron stars accrete matter is not very different from that of black holes, and it gives us a new tool to probe Einstein’s theory,” says Strohmayer.

A group led by Edward Cackett and Jon Miller of the University of Michigan, which includes Bhattacharyya and Strohmayer, used Suzaku’s superb spectral capabilities to survey three neutron-star binaries:
Serpens X-1, GX 349+2, and 4U 1820-30. This team observed a nearly identical iron line in Serpens X-1, confirming the XMM-Newton result.
It detected similarly skewed iron lines in the other two systems as well.

“We’re seeing the gas whipping around just outside the neutron star’s surface,” says Cackett. “And since the inner part of the disk obviously can’t orbit any closer than the neutron star’s surface, these measurements give us a maximum size of the neutron star’s diameter.
The neutron stars can be no larger than 18 to 20.5 miles across, results that agree with other types of measurements.”

“Now that we’ve seen this relativistic iron line around three neutron stars, we have established a new technique,” adds Miller. “It’s very difficult to measure the mass and diameter of a neutron star, so we need several techniques to work together to achieve that goal.”

Knowing a neutron star’s size and mass allows physicists to describe the “stiffness,” or “equation of state,” of matter packed inside these incredibly dense objects. Besides using these iron lines to test Einstein’s general theory of relativity, astronomers can probe conditions in the inner part of a neutron star’s accretion disk.

The XMM-Newton paper appeared in the August 1 Astrophysical Journal Letters. The Suzaku paper has been submitted for publication in the same journal.

Asteroids redden as they age

Astronomers who study asteroids note differences between one side of a space rock and another. Research conducted by Clark Chapman of the Southwest Research Institute (SWRI) in Boulder, Colorado, and others suggest exposure to space over millions of years “weathers” an asteroid’s surface in a process called space weathering.

Hubble sees Uranus’ rings on edge

Hubble Captures Full View of Uranus's Rings
Uranus’ edge-on rings appear as spikes above and below the planet. The rings cannot be seen running fully across the face of the planet because the bright glare of the planet has been blocked out in the photo (a small amount of residual glare appears as a fan-shaped image artifact, along with an edge between the exposure for the inner and outer rings).
NASA, ESA, and M. Showalter (SETI Institute)
August 27, 2007
This series of images from NASA’s Hubble Space Telescope shows how the ring system around the distant planet Uranus appears at ever more oblique (shallower) tilts as viewed from Earth — culminating in the rings being seen edge-on in three observing opportunities in 2007. The best of these events appears in the far right image taken with Hubble’s Wide Field Planetary Camera 2 on August 14, 2007.

The edge-on rings appear as two spikes above and below the planet. The rings cannot be seen running fully across the face of the planet because the bright glare of the planet has been blocked out in the Hubble photo (a small amount of residual glare appears as a fan-shaped image artifact). A much shorter color exposure of the planet has been photo-composited to show its size and position relative to the ring plane.

Earthbound astronomers only see the rings’ edge every 42 years as the planet follows a leisurely 84-year orbit about the Sun. However, the last time the rings were tilted edge-on to Earth astronomers didn’t even know they existed.

With further analysis of the Hubble data, astronomer Mark Showalter of the SETI Institute in Mountain View, Calif., hopes to detect some of the small moons that may shepherd the debris into distinct rings.

Until Voyager 2 flew by Uranus in January 1986, the rings were only known from the way they temporarily blocked the light of stars passing behind the planet. Hubble provided some of the first images of the ring system as viewed from Earth’s distance of approximately 2 billion miles. The advent of adaptive optics gave ground-based observers using large telescopes comparatively sharp views.

The rings were discovered in 1977, so this is the first time for a Uranus ring crossing to be observed from Earth. Earth’s orbit around the Sun permits three opportunities to view the rings edge-on: Uranus made its first ring crossing as seen from Earth on May 3; it made its second crossing on August 16; and will cross for the third time on February 20, 2008. Though the last ring crossing relative to Earth will be hidden behind the Sun, most of Earth’s premier telescopes, including Keck, Hubble, the European Southern Observatory’s Very Large Telescope and the Hale Telescope on Mt. Palomar, plan to focus on the planet again in the days following December 7, 2007. On December 7 the rings will be perfectly edge-on to the Sun.

Showalter is a member of a team led by Imke de Pater of the University of California, Berkeley, who reported that the rings of micron-sized dust have changed significantly since the Voyager 2 spacecraft photographed the Uranus system 21 years ago. Observations were also gleaned from near-infrared adaptive optics observations with the Keck II telescope on May 28, 2007, and reported in an article appearing on August 23 in Science Express, the online edition of Science.

Did Supernova 1987A leave behind a pulsar?

The physics of a core-collapse supernova is pretty straightforward. Once a star weighing more than about 8 solar-masses has cycled through its available nuclear fuel, its core ends up as an inert ball of iron. The core’s mass continually increases as silicon in the overlying layer continues to fuse and dump more iron onto the core. Once the pressure exerted by electrons can no longer support the mounting weight, the core collapses.

Gaping hole in universe

The Early Cosmic Web
Cosmologists made this computer model of the universe at an age of about 2 billion years. In the simulated universe gravity causes the primordial matter to arrange itself in thin filaments, much like a spider’s web. The color coding indicates the density of the gas, yellow for highest, red for medium, and blue for the lowest density. In the high density (yellow) regions the gas will undergo collapse and ignite bursts of star formation.
Max-Planck-Institute for Astrophysics: Tom Theuns
This artist’s illustration shows the effect of intervening matter in the cosmos on the cosmic microwave background (CMB). On the right, the CMB is released shortly after the Big Bang, with tiny ripples in temperature due to fluctuations in the early universe. As this radiation traverses the universe, filled with a web of galaxies, clusters, superclusters and voids, it experiences slight perturbations. In the direction of the giant newly-discovered void, the WMAP satellite sees a cold spot, while the VLA sees fewer radio galaxies.
Bill Saxton/NRAO/AUI/NSF/NASA
August 24, 2007
U niversity of Minnesota astronomers found a hole in the universe nearly a billion light-years across, empty of both normal matter such as stars, galaxies and gas, as well as mysterious “dark matter.” While earlier studies have shown holes, or voids, in the large-scale structure of the universe, this new discovery dwarfs them all.

“Not only has no one ever found a void this big, but we never even expected to find one this size,” said Lawrence Rudnick, an astronomy professor at the University of Minnesota. Rudnick, along with grad student Shea Brown and associate professor Liliya Williams, reported their findings this week.

“What we’ve found is not normal, based on either observational studies or on computer simulations of the large-scale evolution of the universe,” Williams said.

The astronomers studied data from the NRAO VLA Sky Survey (NVSS), a project that imaged the entire sky visible to the Very Large Array (VLA) radio telescope, part of the National Science Foundation’s National Radio Astronomy Observatory (NRAO). Data showed a remarkable drop in the number of galaxies in a region of the constellation Eridanus, southwest of Orion.

“We already knew there was something different about this spot in the sky,” Rudnick said. The region had been dubbed the “WMAP Cold Spot,” because it stood out in a map of the Cosmic Microwave Background (CMB) radiation. The CMB is the earliest “baby picture” available of the universe. Irregularities in the CMB show structures that existed only a few hundred thousand years after the Big Bang.

The WMAP satellite measured temperature differences in the CMB that are only millionths of a degree. The cold region in Eridanus was discovered in 2004.

“Although our surprising results need independent confirmation, the slightly lower temperature of the CMB in this region appears to be caused by a huge hole devoid of nearly all matter roughly 6-10 billion light-years from Earth,” Rudnick said.

How does a lack of matter cause a lower temperature in the Big Bang’s remnant radiation as seen from Earth?

The answer lies in dark energy, which became a dominant force in the universe very recently, when the universe was already three-quarters of the size it is today. Dark energy works opposite gravity and is speeding up the expansion of the universe. Thanks to dark energy, CMB photons that pass through a large void just before arriving at Earth have less energy than those that pass through an area with a normal distribution of matter.

In a simple expansion of the universe, without dark energy, photons approaching a large mass — such as a supercluster of galaxies — pick up energy from its gravity. As they pull away, the gravity saps their energy, and they wind up with the same energy as when they started.

But photons passing through matter-rich space when dark energy became dominant don’t fall back to their original energy level. Dark energy counteracts the influence of gravity and so the large masses don’t sap as much energy from the photons as they pull away. Thus, these photons arrive at Earth with a slightly higher energy, or temperature, than they would in a dark energy-free universe.

Conversely, photons passing through a large void experience a loss of energy. The acceleration of the universe’s expansion, and thus dark energy, were discovered less than a decade ago. The physical properties of dark energy are unknown, though it is by far the most abundant form of energy in the universe today. Learning its nature is one of the most fundamental current problems in astrophysics.

Wrinkles found in Moon’s basins

Full Moon mosaic
Data from SMART-1 revealed connections between formation of lunar basins and volcanic activity on the Moon hundreds of millions of years after the Moon’s formation.
Robert Reeves
August 23, 2007
T he combination of high-resolution data from SMART-1’s AMIE micro-camera and data from the U.S. Clementine mission is helping scientists determine the tectonics of the Moon’s giant basins and the history of volcanic flooding of mid-sized craters, inside and around the lunar basins.

“Thanks to low-elevation solar illumination on these high resolution images,” says SMART-1 Project Scientist Bernard Foing. “It is now possible to study fine, small scale geological features that went undetected earlier.”

The study provides new information on the thermal and tectonic history of the Moon and the processes following the formation of the large basins. There are approximately 50 recognizable lunar basins more than 300 kilometers in diameter believed to be created by the impact of asteroids or comets during the Lunar Late Heavy Bombardment period (between 350 and 750 million years after the formation of the Moon). Some of these basins were then filled by lava originating from volcanic activity.

Combining information from SMART-1 and Clementine makes it possible to assess the link between fine geological structures, identified for the first time with AMIE’s high resolution, and the area’s chemical composition.

SMART-1 AMIE images were used to compose this mosaic of the edge of Mare Humorum. Basalt deposits are at right, and graben, or elongated, trench-like, erosional fault structures, are around the basin. The images were taken from a distance of 1070 kilometers, centred at 46° west and 27° south. The overall image field measures 200 by 130 km.
ESA/SMART-1/SPACE-X
The Humorum basin is a circular, compact, and moderately thick basin that was created by a simple impact event, showing a thin crust and mass concentration within a small area (from Clementine topography and gravity data).

The Procellarum basin, or Oceanus Procellarum, is a large, extended, complex basin that is moderately thick and shows no mass concentration. It may have been formed by faulting associated with the formation of the adjacent Imbrium crater (3.84 thousand million years ago), rather than by a gargantuan impact.

The Humorum basin shows concentric graben, or elongated, trench-like erosional features around the edge of the basin. These are formed as the crust is deformed due to the presence of a mascon, or mass concentration.

“Lunar crust is like a fragile skin, wrinkled due to local mascons or its thermal history”, Foing says. “We searched for these skin-imprints but some may be masked underneath the latest basalt layers”

For the first time, strike-slip faults have been observed with SMART-1 in the Humorum basin. These are faults where the rupture is vertical and one side slides past the other. An example is the San Andreas fault along the western United States, however there is no multi-plate tectonic activity on the Moon.

Procellarum is an extended basin, where magma has arisen from under the surface, between 4 and 2 thousand million years ago, as the crust is thin enough. SMART-1 images do not show geological faults, or surfaces where the rock ruptures due to differential movement, in the Procellarum basin.

It however shows wrinkle ridges that are not distributed radially around the basin. Due to their location, they do not seem associated with mascon tectonics, but mostly are results of thermal and mechanical deformation resulting from volcanic activity — basalt extruded by the lava causes compression in the area. The Procellarum basin contains the youngest basalt found on the Moon so far, up to 2 thousand million years old.

Different pulses of volcanic activity in lunar history created units of lava on the surface. The flooding of mid-sized craters with lava due to volcanic activity in the region is reflected in the mineralogical map. Differences in the mineralogical composition provide a tool to study the geological history of the region. Flooded and unflooded craters are found in the region, reflecting the evolution of volcanic activity with time.

“This analysis shows the potential of the AMIE camera,” says Jean-Luc Josset, Principal Investigator for the AMIE camera. “We are still analyzing other datasets that make use of the varying illumination conditions during the operation of SMART-1 over one and half years.”