The Quiet Explosion

Object intermediate between normal supernovae

and gamma-ray bursts found

A European-led team of astronomers are providing hints that a recent supernova may not be as normal as initially thought. Instead, the star that exploded is now understood to have collapsed into a black hole, producing a weak jet, typical of much more violent events, the so-called gamma-ray bursts. The object, SN 2008D, is thus probably among the weakest explosions that produce very fast moving jets. This discovery represents a crucial milestone in the understanding of the most violent phenomena observed in the Universe.

ESO PR Photo 23a/08
A Galaxy and two Supernovae

These striking results, partly based on observations with ESO's Very Large Telescope, will appear tomorrow in Science Express, the online version of Science.

Stars that were at birth more massive than about 8 times the mass of our Sun end their relatively short life in a cosmic, cataclysmic firework lighting up the Universe. The outcome is the formation of the densest objects that exist, neutron stars and black holes. When exploding, some of the most massive stars emit a short cry of agony, in the form of a burst of very energetic light, X- or gamma-rays.

In the early afternoon (in Europe) of 9 January 2008, the NASA/STFC/ASI Swift telescope discovered serendipitously a 5-minute long burst of X-rays coming from within the spiral galaxy NGC 2770, located 90 million light-years away towards the Lynx constellation. The Swift satellite was studying a supernova that had exploded the previous year in the same galaxy, but the burst of X-rays came from another location, and was soon shown to arise from a different supernova, named SN 2008D.

Researchers at the Italian National Institute for Astrophysics (INAF), the Max-Planck Institute for Astrophysics (MPA), ESO, and at various other institutions have observed the supernova at great length. The team is led by Paolo Mazzali of INAF's Padova Observatory and MPA.

"What made this event very interesting," says Mazzali, "is that the X-ray signal was very weak and 'soft' [1], very different from a gamma-ray burst and more in line with what is expected from a normal supernova."

So, after the supernova was discovered, the team rapidly observed it from the Asiago Observatory in Northern Italy and established that it was a Type Ic supernova.

"These are supernovae produced by stars that have lost their hydrogen and helium-rich outermost layers before exploding, and are the only type of supernovae which are associated with (long) gamma-ray bursts," explains Mazzali. "The object thus became even more interesting!"

Earlier this year, an independent team of astronomers reported in the journal Nature that SN 2008D is a rather normal supernova. The fact that X-rays were detected was, they said, because for the first time, astronomers were lucky enough to catch the star in the act of exploding.

Mazzali and his team think otherwise. "Our observations and modeling show this to be a rather unusual event, to be better understood in terms of an object lying at the boundary between normal supernovae and gamma-ray bursts."

The team set up an observational campaign to monitor the evolution of the supernova using both ESO and national telescopes, collecting a large quantity of data. The early behaviour of the supernova indicated that it was a highly energetic event, although not quite as powerful as a gamma-ray burst. After a few days, however, the spectra of the supernova began to change. In particular Helium lines appeared, showing that the progenitor star was not stripped as deeply as supernovae associated with gamma-ray bursts.

Over the years, Mazzali and his group have developed theoretical models to analyse the properties of supernovae. When applied to SN2008D, their models indicated that the progenitor star was at birth as massive as 30 times the Sun, but had lost so much mass that at the time of the explosion the star had a mass of only 8-10 solar masses. The likely result of the collapse of such a massive star is a black hole.

"Since the masses and energies involved are smaller than in every known gamma-ray burst related supernova, we think that the collapse of the star gave rise to a weak jet, and that the presence of the Helium layer made it even more difficult for the jet to remain collimated, so that when it emerged from the stellar surface the signal was weak," says Massimo Della Valle, co-author.

"The scenario we propose implies that gamma-ray burst-like inner engine activity exists in all supernovae that form a black hole," adds co-author Stefano Valenti.

"As our X-ray and gamma-ray instruments become more advanced, we are slowly uncovering the very diverse properties of stellar explosions," explains Guido Chincarini, co-author and the Principal Investigator of the Italian research on gamma-ray bursts. "The bright gamma-ray bursts were the easiest to discover, and now we are seeing variations on a theme that link these special events to more normal ones."

These are however very important discoveries, as they continue to paint a picture of how massive star end their lives, producing dense objects, and injecting new chemical elements back into the gas from which new stars will be formed.
More Information

The metamorphosis of Supernova SN 2008D/XRF 080109: a link between Supernovae and GRBs/Hypernovae, by Paolo Mazzali et al., Science Express, 24 July 2008.

[1] Astronomers classify X-rays as soft when the relative amount of high-energy X-rays is smaller than that of lower-energy ones.

The team is composed of Paolo A. Mazzali (INAF-Padova, Italy and Max-Planck Institute for Astrophysics/MPA, Germany), Stefano Valenti, Deborah Hunter, Kate Maguire, Andrea Pastorello, Stephen Smartt, and Carrie Trundle (Queen's University, Belfast, Northern Ireland, UK), Massimo Della Valle (INAF-Napoli, Italy, and ESO), Guido Chincarini, Raffaella Margutti, Francesco Pasotti (University of Milano-Bicocca, Italy), Daniel N. Sauer, Nancy Elias-Rosa (MPA), Stefano Benetti, Filomena Bufano, Enrico Cappellaro, Paola Marziani, and Hripsime Navasardyan (INAF-Padova), Elena Pian(INAF-Trieste), Tsvi Piran, Re'em Sari (Hebrew University, Jerusalem, Israel), Valerio D'Elia and L. Angelo Antonelli (INAF-Roma), Sergio Campana, Stefano Covino, Paolo D'Avanzo, Dino Fugazza, and Gianpiero Tagliaferri (INAF-Milano), Fabrizio Fiore (INAF-Roma), Roberto Gilmozzi and Felix Mirabel (ESO), Elisabetta Maiorano, Nicola Masetti, Eliana Palazzi (Istituto Nazionale di Astrofisica, Bologna, Italy), Ken'ichi Nomoto, Masaomi Tanaka, Nozomu Tominaga (University of Tokyo, Japan), Nino Panagia (ESA-STSCI), L.J. Pellizza (Instituto de Astronomia y Fisica del Espacio, Buenos Aires, Argentina), and Massimo Turatto (INAF-Catania).


Contacts: Paolo Mazzali INAF-Osservatorio Astronomico di Padova (Italy) and Max-Planck Insitute for Astrophysics Garching, Germany Phone: +49 89 30000-2221 E-mail: mazzali (at) MPA-Garching.MPG.de Massimo Della Valle INAF-Capodimonte Astronomical Observatory (Italy) and ESO Garching, Germany Phone: +49 89 320 6851 Mobile: +39 3394320350 E-mail: mdellava (at) eso.org Guido Chincarini INAF-Osservatorio Astronomico di Brera, Italy Mobile: +39 340 280 3612 Phone: +39 39 999 1157 Email: guido.chincarini (at) brera.inaf.it

Barred Spiral Galaxies Are Latecomers to the Universe

Credit: NASA, ESA, K. Sheth (Spitzer Science Center, California Institute of Technology, Pasadena, Calif.), and P. Capak and N. Scoville (California Institute of Technology)

A frequent sign of the maturity of a spiral galaxy is the formation of a ribbon of stars and gas that slices across the nucleus, like the slash across a "no smoking" sign.

In a landmark study of more than 2,000 spiral galaxies from the largest galaxy census conducted by NASA's Hubble Space Telescope, astronomers found that so-called barred spiral galaxies were far less plentiful 7 billion years ago than they are today, in the local universe.

The study's results confirm the idea that bars are a sign of galaxies reaching full maturity as the "formative years" end. The observations are part of the Cosmic Evolution Survey (COSMOS).

This new detailed look at the history of bar formation, made with Hubble's Advanced Camera for Surveys, provides clues to understanding when and how spiral galaxies formed and evolved over time.

A team led by Kartik Sheth of the Spitzer Science Center at the California Institute of Technology in Pasadena discovered that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with nearly 70 percent of their modern counterparts.

Bars have been forming steadily over the last 7 billion years, more than tripling in number. "The recently forming bars are not uniformly distributed across galaxy masses, however, and this is a key finding from our investigation," Sheth explained. "They are forming mostly in the small, low-mass galaxies, whereas among the most massive galaxies, the fraction of bars was the same in the past as it is today."

The findings, Sheth continued, have important ramifications for galaxy evolution. "We know that evolution is generally faster for more massive galaxies: They form their stars early and fast and then fade into red disks. Low-mass galaxies are known to form stars at a slower pace, but now we see that they also made their bars slowly over time," he said.

COSMOS covers an area of sky nine times larger than the full Moon, surveying 10 times more spiral galaxies than previous observations. In support of the Hubble galaxy images, the team derived distances to the galaxies in the COSMOS field using data from Hubble and an assortment of ground-based telescopes.

Bars form when stellar orbits in a spiral galaxy become unstable and deviate from a circular path. "The tiny elongations in the stars' orbits grow and they get locked into place, making a bar," explained team member Bruce Elmegreen of IBM's research Division in Yorktown Heights, N.Y. "The bar becomes even stronger as it locks more and more of these elongated orbits into place. Eventually a high fraction of the stars in the galaxy's inner region join the bar."

Added team member Lia Athanassoula of the Laboratoire d'Astrophysique de Marseille in France: "The new observations suggest that the instability is faster in more massive galaxies, perhaps because their inner disks are denser and their gravity is stronger."

Bars are perhaps one of the most important catalysts for changing a galaxy. They force a large amount of gas towards the galactic center, fueling new star formation, building central bulges of stars, and feeding massive black holes.

"The formation of a bar may be the final important act in the evolution of a spiral galaxy," Sheth said. "Galaxies are thought to build themselves up through mergers with other galaxies. After settling down, the only other dramatic way for galaxies to evolve is through the action of bars."

Our Milky Way Galaxy, another massive barred spiral, has a central bar that probably formed somewhat early, like the bars in other large galaxies in the Hubble survey. "Understanding how bars formed in the most distant galaxies will eventually shed light on how it occurred here, in our own backyard," Sheth said.

Other members of the study include Debra Elmegreen (Vassar College); Nick Scoville (COSMOS principal investigator); Peter Capak, Richard Ellis, Mara Salvato, and Lori Spalsbury (California Institute of Technology); Roberto Abraham (University of Toronto); Bahram Mobasher (University of California, Riverside); Eva Schinnerer (Max Planck Institute for Astronomy, Heidelberg); Linda Strubbe and Andrew West (University of California, Berkeley); Mike Rich (University of California, Los Angeles); and Marcella Carollo (ETH Zurich).

CONTACT
Donna Weaver/Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

Kartik Sheth
Spitzer Science Center, California Institute of Technology, Calif.
626-395-1821
astrokartik@gmail.com

IC 4406 - A Seemingly Square Nebula

Credit: C. R. O'Dell (Vanderbilt U.) et al., Hubble Heritage Team, NASA

How can a round star make a square nebula? This conundrum comes to light when studying planetary nebulae like IC 4406. Evidence indicates that IC 4406 is likely a hollow cylinder, with its square appearance the result of our vantage point in viewing the cylinder from the side. Were IC 4406 viewed from the top, it would likely look similar to the Ring Nebula. This representative-color picture is a composite made by combining images taken by the Hubble Space Telescope in 2001 and 2002. Hot gas flows out the ends of the cylinder, while filaments of dark dust and molecular gas lace the bounding walls. The star primarily responsible for this interstellar sculpture can be found in the planetary nebula's center. In a few million years, the only thing left visible in IC 4406 will be a fading white dwarf star.

Astronomy Picture of the Day

Central IC 1805

Credit & Copyright: Keith Quattrocchi

Cosmic clouds seem to form fantastic shapes in the central regions of emission nebula IC 1805. Of course, the clouds are sculpted by stellar winds and radiation from massive hot stars in the nebula's newborn star cluster (aka Melotte 15). About 1.5 million years young, the cluster stars appear on the right in this colorful skyscape, along with dark dust clouds silhouetted against glowing atomic gas. A composite of narrow and broad band telescopic images, the view spans about 15 light-years and includes emission from hydrogen in green, sulfur in red, and oxygen in blue hues. Wider field images reveal that IC 1805's simpler, overall outline suggests its popular name - The Heart Nebula. IC 1805 is located about 7,500 light years away toward the constellation Cassiopeia.

Astronomy Picture of the Day

Bridge Across Space: "Keenan's System" by Martin Winder and Dietmar Hager

NGC 5216 - Keenan's System Credit: by Winder/Hager

Take a very close look at this image of NGC 5216 and companion galaxy NGC 5218 and you'll see a bridge of galactic material that joins these two isolated galaxies. Located in the constellation of Ursa Major (RA 12 30 30 Dec +62 59), this tidally connected pair known as Keenan's System has been well-studied but you'll find they have rarely been imaged.

First discovered by Friedrich Wilhelm Herschel in 1790 and later studied as Intergalactic Nebulae in 1926 by Edwin Hubble, it wasn't until 1935 until PC Keenan noted this double galaxy mystery seemed to be connected by "luminous debris" - a connection that spans 22,000 light years. Keenan noted the peculiar structure in his paper but it would be 1958 before the bridge of material was "rediscovered" by observers at Lick and Palomar observatories in "The Interaction of Galaxies and the Nature of Their Arms, Spanning Filaments and Tails".

By 1966, peculiar type spiral NGC 5216 and the globular galaxy NGC 5218 were included as Arp 104 into Halton Arp's Catalog of Peculiar Galaxies and the 17.3 million light year distant pair were beginning to capture the attention they deserved. Studies were conducted of active galactic nuclei among interacting galaxies and galaxies with extreme tidal distortions and it wasn't long before science realized these two galaxies had collided - stripping stars, gas and dust from each other which appear about them like skewed halos. Once interaction has occurred, the bridge between them fills with "stars in new and perturbed orbits".

In infrared studies done by Bushouse (et al), even more fascinating details have been revealed as we learn that galaxy-to-galaxy collisions can produce higher infrared emissions. "Only the most strongly interacting systems in the sample show extreme values of infrared excess, suggesting that deep, interpenetrating collisions are necessary to drive infrared emission to extreme levels. Comparisons with optical indicators of star formation show that infrared excess and color temperatures correlate with the level of star-formation activity in the interacting galaxies. All interacting galaxies in our sample that exhibit an infrared excess and have higher than normal color temperatures also have optical indicators of high levels of star formation. It is not necessary to invoke processes other than star formation to account for the enhanced infrared luminosity in this sample of interacting galaxies."

What's happening between the pair is causing starburst activity, perhaps from the sharing of gases. According to Casaola (et al); "From the data it appears that interacting galaxies have a higher gas content than normal ones. Galaxies classified as ellipticals have both a dust and gas content one order of magnitude higher than normal. Spirals have in most part a normal dust and HI content but an higher molecular gas mass. The X-ray luminosity also appears higher than that of normal galaxies of same morphological type, both including or excluding AGNs. We considered the alternative possibilities that the molecular gas excess may derive from the existence of tidal torques which produce gas infall from the surrounding regions… it appears that interacting galaxies possess a higher molecular mass than normal galaxies but with a similar star formation efficiency."

Plate 3: Credit: Zwicky - Palomar Observatory courtesy of Caltech

However, the single most interesting point is the remarkable filament which connects NGC 5216 and companion galaxy NGC 5218 - a "concentrated string-like formation connecting the two systems and the fingerlike extension, or countertide, protruding from the globular cluster NGC 518 and starting on the same tangent as the interconnecting filament." It was this very string of material which has been a very recent study of Beverly Smith (et al) in the Spitzer infrared, Galaxy Evolution Explorer UV, Sloan Digitized Sky Survey and Southeastern Association for Research in Astronomy. Their studies helped to reveal these "beads on a string": a series of star-formation complexes. According to their findings; "Our model suggests that bridge material falling into the potential of the companion overshoots the companion. The gas then piles up at apogalacticon before falling back onto the companion, and star formation occurs in the pile-up."

The light data for this awesome image was gathered by AORAIA member Martin Winder and processed by Dr. Dietmar Hager. This particular image took nearly 10 hours of exposure time and untold hours of processing to turn it into the beautiful, study-grade photo you see here. We thank Mr. Winder and Dr. Hager for sharing this exclusive photo with us!

Universe Today

Team uses gravitational lenses to weigh 70 galaxies

This Hubble Space Telescope image shows the Einstein ring of one of the SLACS gravitational lenses, with the lensed background galaxy enhanced in blue.

Credit: A. Bolton (UH/IfA) for SLACS and NASA/ESA

The findings show that the fraction of dark matter relative to a star increases with the galaxies mass.
Provided by the University of Hawaii

An international team of astronomers, including Adam S. Bolton of the University of Hawaii's Institute for Astronomy, has recently announced a finding that helps to settle a long-standing debate over the relationship between mass (the amount of matter) and luminosity (brightness) in galaxies.

The team achieved this result by compiling the largest-ever single collection of gravitational lens galaxies (70 in all). A gravitational lens is a phenomenon similar to a terrestrial mirage, but it occurs on a scale of many thousands of light-years. When two galaxies happen to be precisely aligned with one another in the sky, the gravitational field of the nearer galaxy distorts the image of the more distant galaxy into multiple arc-shaped images or even into a complete ring, known as an Einstein ring. These Einstein ring images can be up to 30 times brighter than the image of the distant galaxy would be in the absence of the lensing effect.

The discovery represents the culmination of the Sloan Lens ACS (or SLACS) Survey. The gravitational lenses were originally identified using data from the Sloan Digital Sky Survey, a major project that has used a dedicated 2.5-meter telescope in New Mexico to measure precise distances to nearly one million distant galaxies and quasars throughout one quarter of the sky. To observe and measure the details of the Einstein ring images, the SLACS astronomers then took advantage of the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope, which delivers pictures of unparalleled sharpness.

"The SLACS collection of lenses is especially powerful for science," says Bolton, lead author of two papers describing these latest results, which will be published in the Astrophysical Journal in August and September. "For each lens, we measured the apparent sizes of the Einstein rings on the sky using the Hubble images, and we measured the distances to the two galaxies of the aligned pair using Sloan data. By combining these measurements, we were able to deduce the mass of the nearer galaxy."

In other lens surveys of this scale, distances to the lens and background galaxies—and hence the lens galaxy masses—have not been measured precisely.

By considering these galaxy masses along with measurements of their sizes, brightness, and stellar velocities, the SLACS astronomers were able to infer the presence of dark matter in addition to the visible stars within the galaxies. Dark matter is the mysterious, invisible material that is the majority of matter in the universe. And with such a large number of lens galaxies across a range of masses, they found that the fraction of dark matter relative to stars increases systematically when going from galaxies of average mass to galaxies of high mass.

The existence of gravitational lenses was first predicted by Albert Einstein in the 1930s, but the first example was not discovered until the late 1970s. In the 30 years since then, many more lenses have been discovered, but their scientific potential has been limited by the disparate assortment of known examples. The SLACS Survey has significantly changed this situation by discovering a single large and uniformly selected sample of strong lens galaxies. The SLACS collection promises to form the basis of many further scientific studies.

Astronomy.com

Ancient Galactic Magnetic Fields Stronger than Expected

Spiral galaxy M 51 with magnetic field data.

Credit: MPIfR Bonn

Mining the far reaches of the universe for clues about its past, a team of scientists including Philipp Kronberg of Los Alamos National Laboratory has proposed that magnetic fields of ancient galaxies like ours were just as strong as those existing today, prompting a rethinking of how our galaxy and others may have formed.

With powerful telescopes and sophisticated measurements, the team probed back in time to see the ancient universe as it existed some 8 to 9 billion years ago. Their research was published in the July 17 edition of Nature.

Until now, a prevailing view in the astrophysical community has been that galactic magnetic fields gradually increased over cosmic time up to their present strengths and that in the nascent universe, magnetic fields were initially very weak. Astrophysicists explain this gradual growth of magnetism over time with the large-scale "galactic dynamo" model.

The letter in the current issue of Nature extends a parallel, larger study by Kronberg et al. of early magnetic fields from the March 2008 edition of The Astrophysical Journal. That study, whose contributors also included LANL colleagues David Higdon and Margaret Short, relied mostly on Faraday rotation measures (RM) taken at radio wavelengths, beyond what is visible to the human eye.

By measuring how far the radio waves were pulled toward the red end of the spectrum-known as "redshift"-Kronberg and his colleagues homed in on the location of magnetic fields in the distant universe.

What allowed the team to take a more detailed look at the ancient universe in this Nature letter was the addition of high-resolution optical spectra by Martin Bernet, Francesco Miniati, and Simon Lilly at the ETH Zürich (the Swiss Federal Institute of Technology) from the European Southern Observatory's 8-meter telescope, located in Chile's Atacama Desert. Their measurements at optical wavelengths of more than 70 quasars were combined with the RM data Kronberg has been collecting for more than 25 years - data based on accurate radio RM measurements from several of the world's most powerful radio telescopes, including the Very Large Array near Soccoro, New Mexico, and the 100-meter dish in Effelsberg, Germany.

"It was thought that, looking back in the past, earlier galaxies would not have generated much magnetic field," Kronberg said. "The results of this study show that the magnetic fields within Milky Way-like galaxies have been every bit as strong over the last two-thirds of the Universe's age as they are now-and possibly even stronger then."

Serving as a looking glass into the past, the powerful telescope at the European Southern Observatory, adding to the radio RM data, allowed the scientists to make observations of high magnetic fields between 8 billion and 9 billion years ago for 70 intervening galaxies whose faint optical absorption spectra revealed them as "normal" galaxies. That means that several billion years before the existence of our own sun, and within only a few billion years of the Big Bang, ancient galaxies were exerting the tug of these strong magnetic fields.

This research suggests that the magnetic fields in galaxies did not arise due to a slow, large-scale dynamo effect, which would have taken 5 billion to 10 billion years to reach their current measured levels. "There must be some other explanation for a much quicker and earlier amplification of galactic magnetic fields," Kronberg said. "From the time when the first stars and galaxies formed, their magnetic fields have probably have been amplified by very fast dynamos. One good possibility is that it happened in the explosive outflows that were driven by supernovae, and possibly even black holes in the very earliest generations of galaxies."

This realization brings a new focus on the broader question of how galaxies form. Instead of the commonly held view that magnetic fields have little relevance to the genesis of new galaxies, it now appears that they are indeed important players. If so, strong magnetic fields a long time ago are one of the essential ingredients that explain the very existence of our galaxy and others like it.

Los Alamos National Laboratory is a multidisciplinary research institution engaged in strategic science on behalf of national security. The Laboratory is operated by a team composed of Bechtel National, the University of California, BWX Technologies, and Washington Group International for the Department of Energy's National Nuclear Security Administration.

Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health and global security concerns.

Contact: John C. Cannon, jcannon@lanl.gov

"No Organics" Zone Circles Pinwheel

Image credit: NASA/JPL-Caltech/STScI 

The Pinwheel galaxy, otherwise known as Messier 101, sports bright reddish edges in this new infrared image from NASA's Spitzer Space Telescope. Research from Spitzer has revealed that this outer red zone lacks organic molecules present in the rest of the galaxy. The red and blue spots outside of the spiral galaxy are either foreground stars or more distant galaxies.

The organics, called polycyclic aromatic hydrocarbons, are dusty, carbon-containing molecules that help in the formation of stars. On Earth, they are found anywhere combustion reactions take place, such as barbeque pits and exhaust pipes. Scientists also believe this space dust has the potential to be converted into the stuff of life. 

Spitzer found that the polycyclic aromatic hydrocarbons decrease in concentration toward the outer portion of the Pinwheel galaxy, then quickly drop off and are no longer detected at its very outer rim. According to astronomers, there's a threshold at the rim where the organic material is being destroyed by harsh radiation from stars. Radiation is more damaging at the far reaches of a galaxy because the stars there have less heavy metals, and metals dampen the radiation. 

The findings help researchers understand how stars can form in these harsh environments, where polycyclic aromatic hydrocarbons are lacking. Under normal circumstances, the polycyclic aromatic hydrocarbons help cool down star-forming clouds, allowing them to collapse into stars. In regions like the rim of the Pinwheel -- as well as the very early universe -- stars form without the organic dust. Astronomers don't know precisely how this works, so the rim of the Pinwheel provides them with a laboratory for examining the process relatively close up. 

In this image, infrared light with a wavelength of 3.6 microns is colored blue; 8-micron light is green; and 24-micron light is red. All three of Spitzer's instruments were used in the study: the infrared array camera, the multiband imaging photometer and the infrared spectrograph. 

XMM-Newton discovers the star that everyone missed

Credits: Contours: ESA/ XMM-Newton/ EPIC (adapted from R. Saxton et al.), Background: NASA/ ESA/ Hubble/ DSS

XMM-Newton has discovered an exploding star in the Milky Way. Usually that would be important in itself, but this time there is a special twist. Calculations show that the explosion must have been clearly visible to the unaided eye but was missed by the legions of star watchers around the planet.

On 9 October 2007, ESA’s orbiting X-ray observatory XMM-Newton was turning from one target to another. As it did so, it passed across a bright source of X-rays that no one was expecting. The source was not listed in any previous X-ray catalogue, yet XMM-Newton was receiving some 50 X-rays every second from this mysterious object.

The nova V598 Puppis, accidentally discovered in the XMM-Newton slew survey. An optical image is shown on the left, and the X-ray discovery is shown on the right.

Credits: ESA/ XMM-Newton/ EPIC (adapted from A. Read et al.)

The only celestial object the XMM-Newton team could find at this location was a faint star, known only by its catalogue number USNO-A2.0 0450-03360039. Acting quickly, Andy Read of the University of Leicester and Richard Saxton of ESA’s European Space Astronomy Centre (ESAC), Spain, arranged for an astronomical telegram to be circulated across the Internet, informing other astronomers of the newly-discovered X-ray source.

Astronomers using the 6.5-m Magellan-Clay telescope at Las Campanas Observatory in Chile, found that USNO-A2.0 0450-03360039 had dramatically brightened by more than 600 times. Analysing the light from the source meant that they could classify the object as a nova.

Novae occur when a compact star, called a white dwarf, feeds off the gas of a nearby companion star. When sufficient gas builds up on the white dwarf, a nuclear reaction begins releasing large quantities of energy, prompting the white dwarf to shoot up in brightness.


This map of our galaxy shows all the objects that were detected in the XMM-Newton slew survey, one of which was the nova V598 Puppis. The plot is colour-coded such that sources of a lower energy are red and those with a higher energy are blue. Also, the brighter the source, the larger it appears on the map. The plot is in galactic coordinates (the centre of the plot corresponds to the centre of the Milky Way).

Credits: ESA/ XMM-Newton/ EPIC (adapted from R. Saxton et al.)

But there was a puzzle. The incandescent explosion does not immediately release X-rays; the expanding cloud of debris created in the detonation temporarily masks them. As this clears, the X-rays shine through. So, for XMM-Newton to see this nova, the explosion must have taken place many days before. Yet, no one had reported seeing it.

Usually, dedicated amateur and professional astronomers find novae by regularly sweeping the night sky for stars that suddenly brighten. This one, it seemed, had slipped the net. Saxton contacted the robotic survey project ASAS and asked them to check their data. They found the nova. It had taken place on 5 June 2007 and had been clearly visible, even to the unaided eye.

“Anyone who went outside that night and looked towards the constellation of Puppis would have seen it,” says Saxton.

The nova is now officially designated V598 Puppis and is one of the brightest for almost a decade, doubling the irony that it was not spotted during its brilliant peak. As news of it spread, the global effort to track its fading light became intense. “Suddenly there was all this data being collected about the star. For variable star work like this, the contribution of the amateur community can be at least as important as that from the professionals,” says Read.

Thanks to XMM-Newton, this story has a happy ending, but it does make astronomers wonder whether there are other discoveries going unnoticed too.

Notes for editors:

This nova was discovered in the XMM-Newton slew survey; a systematic processing of data taken while the satellite moves between objects. To date, the survey has covered 30% of the sky and produced a catalogue of 7700 X-ray sources that has been released to the public.

XMM-Newton slew survey discovery of the nova XMMSL1 J070542.7-381442 by Read et al. is published in Astronomy and Astrophysics.

The first XMM-Newton slew survey catalogue: XMMSL1 by Saxton et al. is published in Astronomy and Astrophysics.

The XMM-Newton science teams are based in several European and US institutes, grouped into three instrument teams and the XMM-Newton Survey Science Centre (SSC). Science operations are managed at ESA’s European Space Astronomy Centre (ESAC), at Villanueva de la Cañada near Madrid, Spain. Spacecraft operations are managed at ESA’s European Space Operations Centre (ESOC) in Darmstadt, Germany.

Three Red Spots Mix it Up on Jupiter

Credit: NASA, ESA, A. Simon-Miller (Goddard Space Flight Center),
N. Chanover (New Mexico State University),
and G. Orton (Jet Propulsion Laboratory)

This sequence of Hubble Space Telescope images offers an unprecedented view of a planetary game of Pac-Man among three red spots clustered together in Jupitear's atmosphere.

The time series shows the passage of the "Red Spot Jr." in a band of clouds below (south) of the Great Red Spot (GRS). "Red Spot Jr." first appeared on Jupiter in early 2006 when a previously white storm turned red.

This is the second time, since turning red, it has skirted past its big brother apparently unscathed.

But this is not the fate of "baby red spot," which is in the same latitudinal band as the GRS. This new red spot first appeared earlier this year.

The baby red spot gets ever closer to the GRS in this picture sequence until it is caught up in the anticyclonic spin of the GRS.

In the final image the baby spot is deformed and pale in color and has been spun to the right (east) of the GRS.

These three natural-color Jupiter images were made from data acquired on May 15, June 28, and July 8, 2008, by the Wide Field Planetary Camera 2 (WFPC2).

Each one covers 58 degrees of Jovian latitude and 70 degrees of longitude (centered on 5 degrees South latitude and 110, 121, and 121 degrees West longitude, respectively).

Brightest Star in the Galaxy has New Competition

Image Credit: NASA/JPL-Caltech

A contender for the title of brightest star in our Milky Way galaxy has been unearthed in the dusty metropolis of the galaxy's center.

Nicknamed the "Peony nebula star," the bright stellar bulb was revealed by NASA's Spitzer Space Telescope and other ground-based telescopes. It blazes with the light of an estimated 3.2 million suns.

The reigning "brightest star" champion is Eta Carina, with a whopping solar wattage of 4.7 million suns. But according to astronomers, it's hard to pin down an exact brightness, or luminosity, for these scorching stars, so they could potentially shine with a similar amount of light.

"The Peony nebula star is a fascinating creature. It appears to be the second-brightest star that we now know of in the galaxy, and it's located deep into the galaxy's center," said Lidia Oskinova of Potsdam University in Germany. "There are probably other stars just as bright if not brighter in our galaxy that remain hidden from view." Oskinova is principal investigator for the research and second author of a paper appearing in a future issue of the journal Astronomy and Astrophysics.

Scientists already knew about the Peony nebula star, but because of its sheltered location in the dusty central hub of our galaxy, its extreme luminosity was not revealed until now. Spitzer's dust-piercing infrared eyes can see straight into the heart of our galaxy, into regions impenetrable by visible light. Likewise, infrared data from the European Southern Observatory's New Technology Telescope in Chile were integral in calculating the Peony nebula star's luminosity.

"Infrared astronomy opens extraordinary views into the environment of the central region of our galaxy," said Oskinova.

The brightest stars in the universe are also the biggest. Astronomers estimate the Peony nebula star kicked off its life with a hefty mass of roughly 150 to 200 times that of our sun. Stars this massive are rare and puzzle astronomers because they push the limits required for stars to form. Theory predicts that if a star starts out too massive, it can't hold itself together and must break into a double or multiple stars instead.

Not only is the Peony nebula star hefty, it also has a wide girth. It is a type of giant blue star called a Wolf-Rayet star, with a diameter roughly 100 times that of our sun. That means this star, if placed where our sun is, would extend out to about the orbit of Mercury.

With so much mass, the star barely keeps itself together. It sheds an enormous amount of stellar matter in the form of strong winds over its relatively short lifetime of a few million years. This matter is pushed so hard by strong radiation from the star that the winds speed up to about 1.6 million kilometers per hour (one million miles per hour) in only a few hours.

Ultimately, the Peony nebula star will blow up in a fantastic explosion of cosmic proportions called a supernova. In fact, Oskinova and her colleagues say that the star is ripe for exploding soon, which in astronomical terms mean anytime from now to millions of years from now.

"When this star blows up, it will evaporate any planets orbiting stars in the vicinity," said Oskinova. "Farther out from the star, the explosion could actually trigger the birth of new stars."

In addition to the star itself, the astronomers noted a cloud of dust and gas, called a nebula, surrounding the star. The team nicknamed this cloud the Peony nebula because it resembles the ornate flower.

"The nebula was probably created from the spray of dust leaking off the massive Peony nebula star," said Andreas Barniske of Potsdam University, lead author of the study.

Wolf-Rainer Hamann, also of Potsdam University, is another co-author of the paper and the principal investigator of a Spitzer program enabling this research.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA. Spitzer's infrared spectrograph, which was used to determine the luminosity of the Peony nebula star, was built by Cornell University, Ithaca, N.Y. Its development was led by Jim Houck of Cornell. For more information about Spitzer, visit http://www.spitzer.caltech.edu/spitzer and http://www.nasa.gov/spitzer.

Whitney Clavin 818-354-4673/818-648-9734

Jet Propulsion Laboratory, Pasadena, Calif.

Rare 'Star-Making Machine' Found In Distant Universe

Distant Starburst Galaxy

Credit:NASA/JPL-Caltech/P. Capak (Spitzer Science Center) Telescopes: Hubble, Spitzer, Chandra, Galex, Keck, CFHT, Subaru, UKIRT, JCMT, VLA, and the IRAM 30m.

Astronomers have uncovered an extreme stellar machine -- a galaxy in the very remote universe pumping out stars at a surprising rate of up to 4,000 per year. In comparison, our own Milky Way galaxy turns out an average of just 10 stars per year.

The discovery, made possible by several telescopes including NASA's Spitzer Space Telescope, goes against the most common theory of galaxy formation. According to the theory, called the Hierarchical Model, galaxies slowly bulk up their stars over time by absorbing tiny pieces of galaxies -- and not in one big burst as observed in the newfound "Baby Boom" galaxy.

"This galaxy is undergoing a major baby boom, producing most of its stars all at once," said Peter Capak of NASA's Spitzer Science Center at the California Institute of Technology, Pasadena. "If our human population was produced in a similar boom, then almost all of the people alive today would be the same age." Capak is lead author of a new report detailing the discovery in the July 10th issue of Astrophysical Journal Letters.

The Baby Boom galaxy, which belongs to a class of galaxies called starbursts, is the new record holder for the brightest starburst galaxy in the very distant universe, with brightness being a measure of its extreme star-formation rate. It was discovered and characterized using a suite of telescopes operating at different wavelengths. NASA's Hubble Space Telescope and Japan's Subaru Telescope, atop Mauna Kea in Hawaii, first spotted the galaxy in visible-light images, where it appeared as an inconspicuous smudge due to is great distance.

It wasn't until Spitzer and the James Clerk Maxwell Telescope, also on Mauna Kea in Hawaii, observed the galaxy at infrared and submillimeter wavelengths, respectively, that the galaxy stood out as the brightest of the bunch. This is because it has a huge number of youthful stars. When stars are born, they shine with a lot of ultraviolet light and produce a lot of dust. The dust absorbs the ultraviolet light but, like a car sitting in the sun, it warms up and re-emits light at infrared and submillimeter wavelengths, making the galaxy unusually bright to Spitzer and the James Clerk Maxwell Telescope.

To learn more about this galaxy's unique youthful glow, Capak and his team followed up with a number of telescopes. They used optical measurements from Keck to determine the exact distance to the galaxy -- a whopping12.3 billion light-years. That's looking back to a time when the universe was 1.3 billion years old (the universe is approximately 13.7 billion years old today).

"If the universe was a human reaching retirement age, it would have been about 6 years old at the time we are seeing this galaxy," said Capak.

The astronomers made measurements at radio wavelengths with the National Science Foundation's Very Large Array in New Mexico. Together with Spitzer and James Clerk Maxwell data, these observations allowed the astronomers to calculate a star-forming rate of about 1,000 to 4,000 stars per year. At that rate, the galaxy needs only 50 million years, not very long on cosmic timescales, to grow into a galaxy equivalent to the most massive ones we see today.

While galaxies in our nearby universe can produce stars at similarly high rates, the farthest one known before now was about 11.7 billion light-years away, or a time when the universe was 1.9 billion years old.

"Before now, we had only seen galaxies form stars like this in the teenaged universe, but this galaxy is forming when the universe was only a child," said Capak. "The question now is whether the majority of the very most massive galaxies form very early in the universe like the Baby Boom galaxy, or whether this is an exceptional case. Answering this question will help us determine to what degree the Hierarchical Model of galaxy formation still holds true."

"The incredible star-formation activity we have observed suggests that we may be witnessing, for the first time, the formation of one of the most massive elliptical galaxies in the universe," said co-author Nick Scoville of Caltech, the principal investigator of the Cosmic Evolution Survey, also known as Cosmos. The Cosmos program is an extensive survey of a large patch of distant galaxies across the full spectrum of light.

"The immediate identification of this galaxy with its extraordinary properties would not have been possible without the full range of observations in this survey," said Scoville.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA. For more information about Spitzer, visit http://www.spitzer.caltech.edu/spitzer and http://www.nasa.gov/spitzer .

Whitney Clavin 818-354-4673/818-648-9734
Jet Propulsion Laboratory, Pasadena, Calif.

What's My Age? Mystery Star Cluster Has 3 Different Birthdays

Credit: NASA, ESA, and L. Bedin (STScI)

Imagine having three clocks in your house, each chiming at a different time. Astronomers have found the equivalent of three out-of-sync "clocks" in the ancient open star cluster NGC 6791. The dilemma may fundamentally challenge the way astronomers estimate cluster ages, researchers said.

Using NASA's Hubble Space Telescope to study the dimmest stars in the cluster, astronomers uncovered three different age groups. Two of the populations are burned-out stars called white dwarfs. One group of these low-wattage stellar remnants appears to be 6 billion years old, another appears to be 4 billion years old. The ages are out of sync with those of the cluster's normal stars, which are 8 billion years old.

"The age discrepancy is a problem because stars in an open cluster should be the same age. They form at the same time within a large cloud of interstellar dust and gas. So we were really puzzled about what was going on," explained astronomer Luigi Bedin, who works at the Space Telescope Science Institute in Baltimore, Md.

Ivan King of the University of Washington and leader of the Hubble study said: "This finding means that there is something about white dwarf evolution that we don't understand."

After extensive analysis, members of the research team realized how the two groups of white dwarfs can look different and yet have the same age. It is possible that the younger- looking group consists of the same type of stars, but the stars are paired off in binary-star systems, where two stars orbit each other. Because of the cluster's great distance, astronomers see the paired stars as a brighter single star.

"It is their brightness that makes them look younger," said team member Maurizio Salaris of Liverpool John Moores University in the United Kingdom.

Binary systems are also a significant fraction of the normal stellar population in NGC 6791, and are also observed in many other clusters. This would be the first time they have been found in a white-dwarf population.

"Our demonstration that binaries are the cause of the anomaly is an elegant resolution of a seemingly inexplicable enigma," said team member Giampaolo Piotto the University of Padova in Italy.

Bedin and his colleagues are relieved that they now have only two ages to reconcile: an 8- billion-year age of the normal stellar population and a 6-billion-year age for the white dwarfs. All that is needed is a process that slows down white-dwarf evolution, the researchers said.

Hubble's Advanced Camera for Surveys analyzed the cooling rate of the entire population of white dwarfs in NGC 6791, from brightest to dimmest. Most star clusters are too far away and the white dwarfs are too faint to be seen by ground-based telescopes, but Hubble's powerful vision sees many of them.

White dwarfs are the smoldering embers of Sun-like stars that no longer generate nuclear energy and have burned out. Their hot remaining cores radiate heat for billions of years as they slowly fade into darkness. Astronomers have used white dwarfs as a reliable measure of the ages of star clusters, because they are the relics of the first cluster stars that exhausted their nuclear fuel.

White dwarfs have long been considered dependable because they cool down at a predictable rate-the older the dwarf, the cooler it is, making it a seemingly perfect clock that has been ticking for almost as long as the cluster has existed.

NGC 6791 is one of the oldest and largest open clusters known, about 10 times larger than most open clusters and containing roughly 10,000 stars. The cluster is located in the constellation Lyra.

The first results appeared in the May 10 issue of The Astrophysical Journal, and the clarification about binaries was in the May 20 issue of The Astrophysical Journal Letters.

Other members of the research team are Santi Cassisi of the Collurania Astronomical Observatory in Italy, and Jay Anderson, of the Space Telescope Science Institute.

CONTACT:

Donna Weaver/Ray Villard
Space Telescope Science Institute, Baltimore, Md. 410-338-4493/4514; dweaver@stsci.edu/villard@stsci.edu

Luigi Bedin
Space Telescope Science Institute, Baltimore, Md.
410-338-5061;
bedin@stsci.edu

Ivan King University of Washington, Seattle, Wash.
206-685-9010;
king@astro.washington.edu

Einstein Was Right, Astrophysicists Say

The double pulsar PSR J0737-3039A/B consists of a binary system made up of two pulsars in a 2.4-hour orbit. Each pulsar emits radio waves along its magnetic poles that illuminate Earth-based radio-telescopes like rotating lighthouse beacons as they spin; one every 23 milliseconds and the other every 2.8 seconds. The fortunate almost-perfect alignment of our line of sight with the orbital plane of the system gives rise to an eclipse of the 23-ms pulsar, once per orbit, as it moves behind its 2.8-s pulsar companion. The eclipse is created by the magnetosphere of the 2.8-s pulsar, a region in which a dense cloud of plasma is trapped by the magnetic field of the pulsar. These eclipses allow us to infer the orientation the 2.8-s pulsar since changes in the geometry would affect the way that light emitted by the other pulsar is transmitted to us during the eclipse. According to classical Newtonian physics, the spin axis about which a star rotates should remain fixed with respect to the background stars as it orbits another star. Einstein's general relativity predicts, however, that the spin axis should slowly precess, like the gentle wobble of a tilted spinning top.

Credit: Daniel Cantin, DarwinDimensions. McGill University

Researchers at McGill University's Department of Physics -- along with colleagues from several countries -- have confirmed a long-held prediction of Albert Einstein's theory of general relativity, via observations of a binary-pulsar star system.

Their results will be published July 3 in the journal Science.

Pulsars are small, ultradense stellar objects left behind after massive stars die and explode as supernovae. They typically have a mass greater than that of our Sun, but compressed to the size of a city like Montreal. They spin at staggering speeds, generate huge gravity fields and emit powerful beams of radio waves along their magnetic poles.

These illuminate Earth-based radio-telescopes like rotating lighthouse beacons as the pulsar spins. More than 1,700 pulsars have been discovered in our galaxy, but PSR J0737-3039A/B, discovered in 2003, is the only known double-pulsar system; that is, two pulsars locked into close orbit around one another. The two pulsars are so close to each other, in fact, that the entire binary could fit within our Sun. PSR J0737-3039A/B lies about 1,700 light years from Earth.

This new test of Einstein's theory was led by McGill astrophysics PhD candidate René Breton and Dr. Victoria Kaspi, leader of the McGill University Pulsar Group.

"A binary pulsar creates ideal conditions for testing general relativity's predictions because the larger and the closer the masses are to one another, the more important relativistic effects are," Breton explained.

"Binary pulsars are the best place to test general relativity in a strong gravitational field," agreed Kaspi, McGill's Lorne Trottier Chair in Astrophysics and Cosmology and Canada Research Chair in Observational Astrophysics. ""Einstein's theory predicted that, in such a field, an object's spin axis should slowly change direction as the pulsar orbits around its companion. Imagine a spinning top when its slightly non-vertical: the spin axis slowly changes direction, an elegant motion called 'precession.'"

The researchers discovered that one of the two pulsars is indeed precessing -- just as Einstein's 1915 theory predicts. If Einstein had been wrong, the pulsar wouldn't be precessing, or would precess in some other way.

Pulsars are too small and too distant to to allow us to directly observe their orientation, the researchers explained. However, they soon realized they could make such measurements using the eclipses visible when one of the twin pulsars passes in front of its companion. When this occurs, the magnetosphere of the first pulsar partly absorbs the radio "light" being emitted from the other, which allows the researchers to determine its spatial orientation. After four years of observations, they determined that its spin axis precesses just as Einstein predicted.

Even though spin precession has been observed in Earth's solar system, differences between general relativity and alternative theories of gravity might only shake out in extremely powerful gravity fields such as those near pulsars, Breton said.

"However, so far, Einstein's theory has passed all the tests that have been conducted, including ours. We can say that if anyone wants to propose an alternative theory of gravity in the future, it must agree with the results that we have obtained here."

Breton, Kaspi and colleagues in Canada, the United Kingdom, the U.S., France and Italy studied the twin-pulsar using the 100-metre Robert C. Byrd Green Bank Radio Telescope at the National Radio Astronomy Observatory in Green Bank, WV.

"I think that if Einstein were alive today, he would have been absolutely delighted with these results," said Dr. Michael Kramer, Associate Director of the Jodrell Bank Centre for Astrophysics at Manchester University. "Not only because it confirms his theory, but also because of the novel way the confirmation came about."

Adapted from materials provided by McGill University.
ScienceDaily

First Measurements Of The Solar Wind Termination Shock By Voyager 2 Spacecraft

The Voyager 2 spacecraft, which has been traveling outward from the Sun for 31 years, has made the first direct observations of the solar wind termination shock (Credit: NASA)

Two University of Iowa space physicists report that the Voyager 2 spacecraft, which has been traveling outward from the Sun for 31 years, has made the first direct observations of the solar wind termination shock, according to a paper published in the July 3 issue of the journal Nature.

At the termination shock the solar wind, which continuously expands outward from the sun at over a million miles per hour, is abruptly slowed to a subsonic speed by the interstellar gas. Don Gurnett, professor of physics in the College of Liberal Arts and Sciences and principal investigator for the plasma wave instrument on Voyager 2, and Bill Kurth, UI research scientist and Voyager co-investigator, said that the shock crossing was marked by an intense burst of plasma wave turbulence detected by the UI instrument, as well as by various effects detected by other instruments on the spacecraft.

At the time of the shock crossing, August 31, 2007, Voyager 2 was at a distance of 83.7 astronomical units (AU), roughly twice the distance between the Sun and Pluto. At this great distance, it took 11.2 hours for the radio signal from the spacecraft to reach Earth.

Shock waves in the thin, ionized gas -- called plasma -- that exists in space are similar in some respects to the shock waves produced by an airplane in supersonic flight. Shock waves in space are believed to play an important role in the acceleration of cosmic rays, which are very energetic atomic particles that continually bombard Earth. The most energetic cosmic rays, which are potentially hazardous to astronauts, are believed to be produced in intense shock waves caused by supernova explosions -- immense stellar explosions that occur in massive stars toward the end of their lives.

The termination shock is believed to be responsible for the origin of less energetic cosmic rays called "anomalous cosmic rays." The recent observations at the termination shock are expected to help physicists understand how cosmic rays are produced by the turbulent fields that exist in such shocks. Gurnett said, "There is no way for us to make direct measure of a super nova shock, so the Voyager 2 measurements at the termination shock provide us the best opportunity in the foreseeable future to understand how cosmic rays are produced by supernova cosmic shocks."

Kurth noted that while some aspects of the termination shock matched scientists' expectations, a number of the observations made by Voyager were surprising and will cause a number of theories to be revised.

Gurnett noted that Voyager 2, launched in 1977, is moving at a speed of 38,000 miles an hour. Even at this considerable speed, the spacecraft will still take 30,000 years to reach a distance equal to that of the nearest star.

The sounds of Voyager's encounter with shock waves at various planets and other sounds of space can be heard by visiting the space audio Web site at: http://www-pw.physics.uiowa.edu/space-audio/.

The University of Iowa research was supported by NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif., a division of Caltech. JPL manages the Voyager mission for NASA's Office of Space Science, Washington, D.C.

Adapted from materials provided by University of Iowa.
ScienceDaily

Hubble Sees Stars and a Stripe in Celestial Fireworks

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

ABOUT THIS IMAGE:

A delicate ribbon of gas floats eerily in our galaxy. A contrail from an alien spaceship? A jet from a black-hole? Actually this image, taken by NASA's Hubble Space Telescope, is a very thin section of a supernova remnant caused by a stellar explosion that occurred more than 1,000 years ago.

On or around May 1, 1006 A.D., observers from Africa to Europe to the Far East witnessed and recorded the arrival of light from what is now called SN 1006, a tremendous supernova explosion caused by the final death throes of a white dwarf star nearly 7,000 light-years away. The supernova was probably the brightest star ever seen by humans, and surpassed Venus as the brightest object in the night time sky, only to be surpassed by the moon. It was visible even during the day for weeks, and remained visible to the naked eye for at least two and a half years before fading away.

It wasn't until the mid-1960s that radio astronomers first detected a nearly circular ring of material at the recorded position of the supernova. The ring was almost 30 arcminutes across, the same angular diameter as the full moon. The size of the remnant implied that the blast wave from the supernova had expanded at nearly 20 million miles per hour over the nearly 1,000 years since the explosion occurred.

In 1976, the first detection of exceedingly faint optical emission of the supernova remnant was reported, but only for a filament located on the northwest edge of the radio ring. A tiny portion of this filament is revealed in detail by the Hubble observation. The twisting ribbon of light seen by Hubble corresponds to locations where the expanding blast wave from the supernova is now sweeping into very tenuous surrounding gas.

The hydrogen gas heated by this fast shock wave emits radiation in visible light. Hence, the optical emission provides astronomers with a detailed "snapshot" of the actual position and geometry of the shock front at any given time. Bright edges within the ribbon correspond to places where the shock wave is seen exactly edge on to our line of sight.

Today we know that SN 1006 has a diameter of nearly 60 light-years, and it is still expanding at roughly 6 million miles per hour. Even at this tremendous speed, however, it takes observations typically separated by years to see significant outward motion of the shock wave against the grid of background stars. In the Hubble image as displayed, the supernova would have occurred far off the lower right corner of the image, and the motion would be toward the upper left.

SN 1006 resides within our Milky Way Galaxy. Located more than 14 degrees off the plane of the galaxy's disk, there is relatively little confusion with other foreground and background objects in the field when trying to study this object. In the Hubble image, many background galaxies (orange extended objects) far off in the distant universe can be seen dotting the image. Most of the white dots are foreground or background stars in our Milky Way galaxy.

This image is a composite of hydrogen-light observations taken with Hubble's Advanced Camera for Surveys in February 2006 and Wide Field Planetary Camera 2 observations in blue, yellow-green, and near-infrared light taken in April 2008. The supernova remnant, visible only in the hydrogen-light filter was assigned a red hue in the Heritage color image.

Photo Credit: NASA, ESA, and Z. Levay (STScI)

ABOUT THIS IMAGE:

This image is a composite of visible (or optical), radio, and X-ray data of the full shell of the supernova remnant from SN 1006. The radio data show much of the extent that the X-ray image shows. In contrast, only a small linear filament in the northwest corner of the shell is visible in the optical data. The object has an angular size of roughly 30 arcminutes (0.5 degree, or about the size of the full moon), and a physical size of 60 light-years (18 parsecs) based on its distance of nearly 7,000 light-years. The small green box along the bright filament at the top of the image corresponds to the dimensions of the Hubble release image.

The optical data was obtained at the University of Michigan's 0.9-meter Curtis Schmidt telescope at the National Science Foundation's Cerro Tololo Inter-American Observatory (CTIO) near La Serena, Chile. H-alpha, continuum-subtracted data were provided by F. Winkler (Middlebury COllege) et al. The X-ray data were acquired from the Chandra X-ray Observatory's AXAF CCD Imaging Spectrometer (ACIS) at 0.5-3keV, and were provided by J. Hughes (Rutgers University) et al. The radio data, supplied by K. Dyer (NRAO, Socorro) et al., were a composite from the National Radio Astronomy Observatory's Very Large Array (NRAO/VLA) in Socorro, New Mexico, along with the Green Bank Telescope (GBT) in Green Bank, West Virginia. Data of the supernova remnant were blended on a visible-light stellar background created using the Digitized Sky Survey's Anglo-Australian Observatory (AAO2) blue and red plates.

Illustration Credit: NASA, ESA, and L. Frattare (STScI)

ABOUT THIS IMAGE:

Comparison of visible hydrogen emission in the NW filament of SN 1006 in data taken at the CTIO Curtis-Schmidt H-alpha, continuum-subtracted (Winkler, et al.) in 1998 (shown in green), and the Hubble ACS data (Raymond et. al) in 2006 (shown in red). The stellar background is from WFPC2 broadband B, V, and I data from 2008 (Hubble Heritage Team).

For additional information, contact:

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu

William Blair
Johns Hopkins University, Baltimore, Md.
410-516-8447
wpb@pha.jhu.edu