Stellar Thief Is the Surviving Companion to a Supernova

Seventeen years ago, astronomers witnessed supernova 2001ig go off 40 million light-years away in the galaxy NGC 7424, in the southern constellation Grus, the Crane. Shortly after, scientists photographed the supernova with the European Southern Observatory’s Very Large Telescope (VLT) in 2002. Two years later, they followed up with the Gemini South Observatory, which hinted at the presence of a surviving binary companion. As the supernova’s glow faded, scientists focused Hubble on that location in 2016. They pinpointed and photographed the surviving companion, which was possible only due to Hubble’s exquisite resolution and ultraviolet sensitivity. Hubble observations of SN 2001ig provide the best evidence yet that some supernovas originate in double-star systems. Credits: NASA, ESA, S. Ryder (Australian Astronomical Observatory), and O. Fox (STScI). Hi-res image

Seventeen years ago, astronomers witnessed a supernova go off 40 million light-years away in the galaxy called NGC 7424, located in the southern constellation Grus, the Crane. Now, in the fading afterglow of that explosion, NASA's Hubble Space Telescope has captured the first image of a surviving companion to a supernova. This picture is the most compelling evidence that some supernovas originate in double-star systems.

“We know that the majority of massive stars are in binary pairs,” said Stuart Ryder from the Australian Astronomical Observatory (AAO) in Sydney, Australia, and lead author of the study. “Many of these binary pairs will interact and transfer gas from one star to the other when their orbits bring them close together.”

The companion to the supernova’s progenitor star was no innocent bystander to the explosion. It siphoned off almost all of the hydrogen from the doomed star’s stellar envelope, the region that transports energy from the star’s core to its atmosphere. Millions of years before the primary star went supernova, the companion’s thievery created an instability in the primary star, causing it to episodically blow off a cocoon and shells of hydrogen gas before the catastrophe.

The supernova, called SN 2001ig, is categorized as a Type IIb stripped-envelope supernova. This type of supernova is unusual because most, but not all, of the hydrogen is gone prior to the explosion. This type of exploding star was first identified in 1987 by team member Alex Filippenko of the University of California, Berkeley.

How stripped-envelope supernovas lose that outer envelope is not entirely clear. They were originally thought to come from single stars with very fast winds that pushed off the outer envelopes. The problem was that when astronomers started looking for the primary stars from which supernovas were spawned, they couldn’t find them for many stripped-envelope supernovas.

“That was especially bizarre, because astronomers expected that they would be the most massive and the brightest progenitor stars,” explained team member Ori Fox of the Space Telescope Science Institute in Baltimore. “Also, the sheer number of stripped-envelope supernovas is greater than predicted.” That fact led scientists to theorize that many of the primary stars were in lower-mass binary systems, and they set out to prove it.

Looking for a binary companion after a supernova explosion is no easy task. First, it has to be at a relatively close distance to Earth for Hubble to see such a faint star. SN 2001ig and its companion are about at that limit. Within that distance range, not many supernovas go off. Even more importantly, astronomers have to know the exact position through very precise measurements.

In 2002, shortly after SN 2001ig exploded, scientists pinpointed the precise location of the supernova with the European Southern Observatory’s Very Large Telescope (VLT) in Cerro Paranal, Chile. In 2004, they then followed up with the Gemini South Observatory in Cerro Pachón, Chile. This observation first hinted at the presence of a surviving binary companion.

Knowing the exact coordinates, Ryder and his team were able to focus Hubble on that location 12 years later, as the supernova’s glow faded. With Hubble’s exquisite resolution and ultraviolet capability, they were able to find and photograph the surviving companion—something only Hubble could do.

Prior to the supernova explosion, the orbit of the two stars around each other took about a year.

When the primary star exploded, it had far less impact on the surviving companion than might be thought. Imagine an avocado pit—representing the dense core of the companion star—embedded in a gelatin dessert—representing the star’s gaseous envelope. As a shock wave passes through, the gelatin might temporarily stretch and wobble, but the avocado pit would remain intact.

In 2014, Fox and his team used Hubble to detect the companion of another Type IIb supernova, SN 1993J. However, they captured a spectrum, not an image. The case of SN 2001ig is the first time a surviving companion has been photographed. “We were finally able to catch the stellar thief, confirming our suspicions that one had to be there,” said Filippenko.

Perhaps as many as half of all stripped-envelope supernovas have companions—the other half lose their outer envelopes via stellar winds. Ryder and his team have the ultimate goal of precisely determining how many supernovas with stripped envelopes have companions.

Their next endeavor is to look at completely stripped-envelope supernovas, as opposed to SN 2001ig and SN 1993J, which were only about 90 percent stripped. These completely stripped-envelope supernovas don’t have much shock interaction with gas in the surrounding stellar environment, since their outer envelopes were lost long before the explosion. Without shock interaction, they fade much faster. This means that the team will only have to wait two or three years to look for surviving companions.

In the future, they also hope to use the James Webb Space Telescope to continue their search.

The paper on this team’s current work was published on March 28, 2018, in the Astrophysical Journal.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

For NASA's Hubble webpage, visit: www.nasa.gov/hubble

For more images and information, visit: http://hubblesite.org/news_release/news/2018-20

For the science paper, visit: https://media.stsci.edu/preview/file/science_paper/file_attachment/321/Ryder_published_ApJ_paper.pdf


Ann Jenkins / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4488 / 410-338-4514 
jenkins@stsci.edu / villard@stsci.edu

Ori Fox
Space Telescope Science Institute, Baltimore, Maryland
410-338-6768 
ofox@stsci.edu

Stuart Ryder
Australian Astronomical Observatory, Sydney, Australia
011-61-2-93724843
011-61-419-970834 (cell) 
sdr@aao.gov.au

Alex Filippenko
University of California, Berkeley, California 
afilippenko@berkeley.edu

Source: NASA/Supernova

Supernova 1993J in Spiral Galaxy M81

This is an artist's impression of supernova 1993J, an exploding star in the galaxy M81 whose light reached us 21 years ago. The supernova originated in a double-star system where one member was a massive star that exploded after siphoning most of its hydrogen envelope to its companion star. After two decades, astronomers have at last identified the blue helium-burning companion star, seen at the center of the expanding nebula of debris from the supernova. The Hubble Space Telescope identified the ultraviolet glow of the surviving companion embedded in the fading glow of the supernova. Credit: NASA, ESA, and G. Bacon (STScI). Release Images

Supernova 1993J in Spiral Galaxy M81

This Hubble Space Telescope photo composite shows the location of supernova 1993J inside the majestic spiral galaxy M81. Though astronomers saw the star explode as a supernova 21 years ago, the glow of that explosion is still present, as seen in the inset image. The supernova has faded to the point where astronomers are confident that they have picked up the ultraviolet glow of a very hot companion star. This is the first time astronomers have been able to put constraints on the properties of the companion star in this unusual class of supernova called Type IIb. Hubble observations in ultraviolet light confirm the theory that the explosion originated in a double-star system where one star fueled the mass-loss from the aging primary star.

Photo Credit: NASA, ESA, A. Zezas (CfA), and A. Filippenko (UC Berkeley)

Acknowledgment: Hubble Heritage Team (STScI/AURA)

Science Credit: NASA, ESA, and O. Fox (University of California, Berkeley), A. Bostroem (STScI), S. Van Dyk (Caltech), A. Filippenko (University of California, Berkeley), C. Fransson (Stockholm University), T. Matheson (NOAO), S. Cenko (University of California, Berkeley, and NASA/GSFC), P. Chandra (National Center for Radio Astrophysics/Pune University, India), V. Dwarkadas (University of Chicago), W. Li and A. Parker (University of California, Berkeley), and N. Smith (Steward Observatory)

Scenario for Type IIb SN 1993J

This illustration shows the key steps in the evolution of a Type IIb supernova.
Panel 1: Two very hot stars orbit about each other in a binary system.

Panel 2: The slightly more massive member of the pair evolves into a bloated red giant and spills the hydrogen in its outer envelope onto the companion star.

Panel 3: The more massive star explodes as a supernova.

Panel 4: The companion star survives the explosion. Because it has locked up most of the hydrogen in the system, it is a larger and hotter star than when it was born. The fireball of the supernova fades. Credit: NASA, ESA, and A. Feild (STScI)

Astronomers using NASA's Hubble Space Telescope have discovered a companion star to a rare type of supernova. This observation confirms the theory that the explosion originated in a double-star system where one star fueled the mass-loss from the aging primary star.

This detection is the first time astronomers have been able to put constraints on the properties of the companion star in an unusual class of supernova called Type IIb. They were able to estimate the surviving star's luminosity and mass, which provide insight into the conditions that preceded the explosion.

"A binary system is likely required to lose the majority of the primary star's hydrogen envelope prior to the explosion. The problem is that, to date, direct observations of the predicted binary companion star have been difficult to obtain since it is so faint relative to the supernova itself," said lead researcher Ori Fox of the University of California (UC) at Berkeley.

Astronomers estimate that a supernova goes off once every second somewhere in the universe. Yet they don't fully understand how stars explode. Finding a "smoking gun" companion star provides important new clues to the variety of supernovae in the universe. "This is like a crime scene, and we finally identified the robber," quipped team member Alex Filippenko, professor of astronomy at UC Berkeley. "The companion star stole a bunch of hydrogen before the primary star exploded."

The explosion happened in the galaxy M81, which is about 11 million light-years away from Earth in the direction of the constellation Ursa Major (the Great Bear). Light from the supernova was first detected in 1993, and the object was designated SN 1993J. It was the nearest known example of this type of supernova, called a Type IIb, due to the specific characteristics of the explosion. For the past two decades astronomers have been searching for the suspected companion, thought to be lost in the glare of the residual glow from the explosion.

Observations made in 2004 at the W.M. Keck Observatory on Mauna Kea, Hawaii, showed circumstantial evidence for spectral absorption features that would come from a suspected companion. But the field of view is so crowded that astronomers could not be certain if the spectral absorption lines were from a companion object or from other stars along the line of sight to SN 1993J. "Until now, nobody was ever able to directly detect the glow of the star, called continuum emission," Fox said.

The companion star is so hot that the so-called continuum glow is largely in ultraviolet (UV) light, which can only be detected above Earth's absorbing atmosphere. "We were able to get that UV spectrum with Hubble. This conclusively shows that we have an excess of continuum emission in the UV, even after the light from other stars has been subtracted," said team member Azalee Bostroem of the Space Telescope Science Institute (STScI), in Baltimore, Maryland.

When a massive star reaches the end of its lifetime, it burns though all of its material and its iron core collapses. The rebounding outer material is seen as a supernova. But there are many different types of supernovae in the universe. Some supernovae are thought to have exploded from a single-star system. Other supernovae are thought to arise in a binary system consisting of a normal star with a white dwarf companion, or even two white dwarfs. The peculiar class of supernova called Type IIb combines the features of a supernova explosion in a binary system with what is seen when single massive stars explode.

SN 1993J, and all Type IIb supernovae, are unusual because they do not have a large amount of hydrogen present in the explosion. The key question has been: how did SN 1993J lose its hydrogen? In the model for a Type IIb supernova, the primary star loses most of its outer hydrogen envelope to the companion star prior to exploding, and the companion continues to burn as a super-hot helium star.

"When I first identified SN 1993J as a Type IIb supernova, I hoped that we would someday be able to detect its suspected companion star," said Filippenko. "The new Hubble data suggest that we have finally done so, confirming the leading model for Type IIb supernovae."

The team combined ground-based data for the optical light and images from two Hubble instruments to collect ultraviolet light. They then constructed a multi-wavelength spectrum that matched what was predicted for the glow of a companion star.

Fox, Filippenko, and Bostroem say that further research will include refining the constraints on this star and definitively showing that the star is present.

The results were published in the July 20 Astrophysical Journal.

For images and more information about Hubble, visit:  http://hubblesite.org/news/2014/38 - http://www.nasa.gov/hubble

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the telescope. STScI conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.

CONTACT

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

Ori Fox
University of California, Berkeley, California
ofox@astro.berkeley.edu

Alex Filippenko
University of California, Berkeley, California
afilippenko@berkeley.edu

Source: Hubble Site

Confirmed: Stellar Behemoth Self-Destructs in a Type IIb Supernova

A star in a distant galaxy explodes as a supernova: while observing a galaxy known as UGC 9379 (left; image from the Sloan Digital Sky Survey; SDSS) located about 360 million light years away from Earth, the team discovered a new source of bright blue light (right, marked with an arrow; image from the 60-inch robotic telescope at Palomar Observatory). This very hot, young supernova marked the explosive death of a massive star in that distant galaxy.

A detailed study of the spectrum (the distribution of colors composing the light from the supernova) using a technique called “flash spectroscopy” revealed the signature of a wind blown by the aging star just prior to its terminal explosion, and allowed scientists to determine what elements were abundant on the surface of the dying star as it was about to explode as a supernova, providing important information about how massive stars evolve just prior to their death, and the origin of crucial elements such as carbon, nitrogen and oxygen.

The Palomar 48 inch telescope

Photo by: Iair Arcavi, Weizmann Instiute of Science

Berkeley Lab Researchers Help Catch a Wolf-Rayet Hours After it Goes Supernova

Our Sun may seem pretty impressive: 330,000 times as massive as Earth, it accounts for 99.86 percent of the Solar System’s total mass; it generates about 400 trillion trillion watts of power; and it has a surface temperature of about 10,000 degrees Celsius. Yet for a star, it’s a lightweight. 

The real cosmic behemoths are Wolf-Rayet stars, which are more than 20 times as massive as the Sun and at least five times as hot. Because these stars are relatively rare and often obscured, scientists don’t know much about how they form, live and die. But this is changing, thanks to an innovative sky survey called the intermediate Palomar Transient Factory (iPTF), which uses resources at the National Energy Research Scientific Computing Center (NERSC) and Energy Sciences Network (ESnet), both located at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), to expose fleeting cosmic events such as supernovae.

For the first time ever, scientists have direct confirmation that a Wolf-Rayet star—sitting 360 million light years away in the Bootes constellation—died in a violent explosion known as a Type IIb supernova. Using the iPTF pipeline, researchers at Israel’s Weizmann Institute of Science led by Avishay Gal-Yam caught supernova SN 2013cu within hours of its explosion. They then triggered ground- and space-based telescopes to observe the event approximately 5.7 hours and 15 hours after it self-destructed. These observations are providing valuable insights into the life and death of the progenitor Wolf-Rayet.

“Newly developed observational capabilities now enable us to study exploding stars in ways we could only dream of before. We are moving towards real-time studies of supernovae,” says Gal-Yam, an astrophysicist in the Weizmann Institute’s Department of Particle Physics and Astrophysics. He is also the lead author of a recently published Nature paper on this finding.

“This is the smoking gun. For the first time, we can directly point to an observation and say that this type of Wolf-Rayet star leads to this kind of Type IIb supernova,” says Peter Nugent, who heads Berkeley Lab’s Computational Cosmology Center (C3) and leads the Berkeley contingent of the iPTF collaboration.

“When I identified the first example of a Type IIb supernova in 1987, I dreamed that someday we would have direct evidence of what kind of star exploded. It’s refreshing that we can now say that Wolf-Rayet stars are responsible, at least in some cases,” says Alex Filippenko, Professor of Astronomy at UC Berkeley. Both Filippenko and Nugent are also co-authors on the Nature paper.

Elusive Signatures Illuminated in a Flash of Light

Some supermassive stars become Wolf-Rayets in the final stages of their lives. Scientists find these stars interesting because they enrich galaxies with the heavy chemical elements that eventually become the building blocks for planets and life.

“We are gradually determining which kinds of stars explode, and why, and what kinds of elements they produce,” says Filippenko. “These elements are crucial to the existence of life. In a very real sense, we are figuring out our own stellar origins.”

All stars—no matter what size—spend their lives fusing hydrogen atoms to create helium. The more massive a star, the more gravity it wields, which accelerates fusion in the star’s core, generating energy to counteract gravitational collapse. When hydrogen is depleted, a supermassive star continues to fuse even heavier elements like carbon, oxygen, neon, sodium, magnesium and so on, until its core turns to iron. At this point, atoms (even subatomic particles) are packed in so closely that fusion no longer releases energy into the star. It is now solely supported by electron degeneracy pressure—the quantum mechanical law that prohibits two electrons from occupying the same quantum state.

When the core is massive enough, even electron degeneracy won’t support the star and it collapses. Protons and electrons in the core merge, releasing a tremendous amount of energy and neutrinos. This, in turn, powers a shockwave that tears through the star ejecting its remains violently into space as it goes supernova.

The Wolf-Rayet phase occurs before the supernova. As nuclear fusion slows, the heavy elements forged in the star’s core rise to the surface setting off powerful winds. These winds shed a tremendous amount of material into space and obscure the star from prying telescopes on Earth.

“When a Wolf-Rayet star goes supernova, the explosion typically overtakes the stellar wind and all information about the progenitor star is gone,” says Nugent. “We got lucky with SN 2013cu—we caught the supernova before it overtook the wind. Shortly after the star exploded, it let out an ultraviolet flash from the shock wave that heated and lit up the wind. The conditions that we observed in this moment were very similar to what was there before the supernova.”

Before the supernova debris overtook the wind, the iPTF team managed to capture its chemical light signatures (or spectra) with the ground-based Keck telescope in Hawaii and saw the telltale signs of a Wolf-Rayet star.  When the iPTF team performed follow-up observations 15 hours later with NASA’s Swift satellite, the supernova was still quite hot and strongly emitting in the ultraviolet. In the following days, iPTF collaborators rallied telescopes around the globe to watch the supernova crash into material that had been previously ejected from the star. As the days went by, the researchers were able to classify SN 2013cu as a Type IIb supernova because of the weak hydrogen signatures and strong helium features in the spectra that appeared after the supernova cooled.

“With a series of observations, including data I took with the Keck-I telescope 6.5 days after the explosion, we could see that the supernova’s expanding debris quickly overtook the flash-ionized wind that had revealed the Wolf-Rayet features. So, catching the supernova sufficiently early is hard—you’ve got to be on the ball, as our team was,” says Filippenko.

“This discovery was totally shocking, it opens up a whole new research area for us,” says Nugent. “With our largest telescopes you might have a chance of getting a spectrum of a Wolf-Rayet star in the nearest galaxies to our Milky Way, perhaps 4 million light years away. SN 2013cu is 360 million light years away—further by almost factor of 100.”

And because the researchers caught the supernova early—when the ultraviolet flash lit up the progenitor’s stellar wind—they were able to take several spectra.  “Ideally, we’d like to do this again and again and develop some interesting statistics, not just for supernovae with Wolf-Rayet progenitors but other types as well,” says Nugent.

Pipeline Upgrade Leads to Unexpected Discoveries

Since February 2014, the iPTF survey has been scanning the sky nightly with a robotic telescope mounted on the 48-inch Samuel Oschin Telescope at Palomar Observatory in Southern California. As soon as observations are taken, the data travel more than 400 miles to NERSC in Oakland via the National Science Foundation’s High Performance Wireless Research and Education Network and the Department of Energy’s ESnet. At NERSC, the Real-Time Transient Detection Pipeline sifts through the data, identifies events to follow up on and sends an alert to iPTF scientists around the globe.

The survey was built on the legacy of the Palomar Transient Factory (PTF), designed in 2008 to systematically chart the transient sky by using the same camera at Palomar Observatory. Last year Nugent and colleagues at Caltech and UC Berkeley made significant modifications to the transient detection pipeline for the iPTF project. Working with NERSC staff, Nugent upgraded the pipeline’s computing and storage hardware.  The iPTF team also made improvements to the machine learning algorithms at the heart of the detection pipeline and incorporated the Sloan Digital Star Survey III star and galaxy catalogs so the pipeline could immediately reject known variable stars.

They even added an asteroid rejection feature to the automated workflow, which calculates the orbit of every known asteroid at the beginning of the night, determines where the asteroids are in an individual image, and then rejects them.

“All of our modifications significantly sped up our real-time transient detection; we now send high quality supernova alerts to astronomers all around the globe in less than 40 minutes after taking an image at Palomar,” says Nugent. “In the case of SN 2013cu, that made all the difference.”

*** 

The automated real-time detection pipeline was created under the DOE Office of Science’s Scientific Discovery through Advanced Computing (SciDAC) program and through additional support from NASA. 

NERSC provided the storage and systems infrastructure. NERSC and ESnet are also supported by the DOE Office of Science.

Led by Shri Kulkarni of Caltech, iPTF has discovered more than 2000 supernovae during its four and a half years of observations, including many rare and exotic types of cosmic outbursts. The iPTF survey is a scientific collaboration among Caltech, Los Alamos National Laboratory, the University of Wisconsin, Milwaukee, the Oskar Klein Center, the Weizmann Institute of Science, the TANGO Program of the University System of Taiwan, and the Kavli Institute for the Physics and Mathematics of the Universe.

This research was supported by the I-CORE Program “The Quantum Universe” of the Planning and Budgeting Committee and The Israel Science Foundation; grants from the ISF, BSF, GIF, Minerva, the FP7/ERC, and a Kimmel Investigator award; support from the Hubble and Carnegie-Princeton Fellowships; support from the Arye Dissentshik career development chair and a grant from the Israeli MOST; support from the NSF; support from an NSF Postdoctoral Fellowship; support from the TABASGO Foundation, the Christopher R. Redlich Fund, and NSF grant AST-1211916. Some of the data were obtained at the W. M. Keck Observatory, which was made possible by the generous financial support of the W. M. Keck Foundation.

Source: Lawrence Berkeley National Laboratory

Scientists Hold Séance for Supernova

Credit: NASA/JPL-Caltech/ Y. Kim (Univ. of Arizona/ Univ. of Chicago)

Astronomers have unearthed secrets from the grave of a star that blasted apart in a supernova explosion long ago. By decoding ghostly echoes of light traveling away from the remains of a supernova called Cassiopeia A, the scientists have pieced together what the star looked like in life, and ultimately how it met its demise.

The discovery, made using primarily NASA's Spitzer Space Telescope and Japan's Subaru telescope on Mauna Kea in Hawaii, represents the first time astronomers have been able to resurrect the life history of a supernova remnant in our own galaxy.

"Cassiopeia A lies in our cosmic backyard and offers the sharpest view of what is left hundreds of years after a supernova explosion," said Oliver Krause of the Max Planck Institute for Astronomy in Germany, lead author of a paper about the discovery appearing in this week's Science. "The echoes of light we found around Cassiopeia A provide us with a time machine to go back and see its past."

Cassiopeia A is one of the most explored objects in our sky and the subject of more than 1,000 scientific papers. It is the burnt-out corpse of a massive star that ended its life in a fiery supernova about 11,300 years ago. In fact, until recently, it was the youngest supernova remnant in our Milky Way galaxy (the new record holder, G1.9+0.3, was recently discovered using NASA's Chandra X-ray Observatory and other ground-based telescopes). Because Cassiopeia A is 11,000 light-years from Earth, the light from its explosion would have reached Earth, sweeping right past it, about 300 years ago.

Astronomers had thought this supernova light was never to be seen again, until 2005, when Krause and his colleagues discovered hints of it still bouncing around clouds surrounding the remnant (http://www.spitzer.caltech.edu/Media/releases/ssc2005-14/index.shtml).

Using Spitzer's infrared eyes, they found so-called infrared echoes, which occur when a flash of light from the supernova blasts through clouds, heating them up and causing them to glow in infrared. As the light rolls outward, the infrared echoes continue to flare up and travel away from the star.

In the new study, the astronomers used Cassiopeia A's infrared echoes to hone in on faint visible-light echoes with Subaru and other ground-based telescopes. Visible-light echoes, known simply as light echoes, occur when visible light from the supernova scatters off dust. Unlike infrared echoes, they are direct signals from the graves of exploded stars, bearing all the information about the nature of the original blast.

Next, the astronomers had to act quickly because these echoes can fade within weeks. They used Subaru's spectrometer instrument to break the light apart and reveal signatures of atoms present when Cassiopeia A exploded. The resulting spectrum of light revealed hydrogen and helium -- telltale signs that Cassiopeia A was once a huge red supergiant star whose core collapsed in a rare supernova referred to as Type IIb. Previously, scientists did not know the supernova class to which Cassiopeia A belonged.

"This is an exciting result," said Alex Filippenko of the University of California, Berkeley, a supernova expert not affiliated with the study. "Cassiopeia A has been studied extensively with many telescopes over a wide range of wavelengths. It is gratifying that we finally know what kind of star exploded so long ago."

The findings also offer insight into another mystery shrouding Cassiopeia A. When Cassiopeia A's original star erupted, the event should have been widely witnessed on Earth as a bright star lighting up the sky. The most likely possible sighting is by the Astronomer Royal John Flamsteed in 1680, but he made just one observation of a dim star. The fact that almost no one saw the event is a classic problem in supernova lore.

Now that astronomers have learned how Cassiopeia A was forged, they think they might know why its death went unnoticed. "Type IIb supernovas fade quickly," said co-author George Rieke of the University of Arizona in Tucson. "This, plus a few cloudy nights, might explain the historical enigma around Cassiopeia A."

Recently, astronomers using Chandra, ESA's XMM-Newton Observatory and the Gemini Observatory in Chile, were able to use light echoes to identify the origins of a supernova outside our galaxy. That study, together with the new one, demonstrates the power of light echoes for conjuring up the "ghosts" of long-dead stars.

Other co-authors include Stephan Birkmann and Miwa Goto of the Max Planck Institute for Astronomy; Tomonori Usuda and Takashi Hattori of the National Astronomical Observatory of Japan in Hawaii; and Karl Misselt of the University of Arizona. 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 California Institute of Technology, also in Pasadena. For more information about Subaru, operated by the National Astronomical Observatory of Japan, visit http://subarutelescope.org.

Whitney Clavin 818-354-4673

Jet Propulsion Laboratory, Pasadena, Calif.