type ia supernovae

Wispy remains of a supernova explosion hide a possible ‘survivor.’ Of all the varieties of exploding stars, the ones called Type Ia are perhaps the most intriguing. Their predictable brightness lets astronomers measure the expansion of the universe, which led to the discovery of dark energy. Yet the cause of these supernovae remains a mystery. Do they happen when two white dwarf stars collide? Or does a single white dwarf gorge on gases stolen from a companion star until bursting? If the second theory is true, the normal star should survive. Astronomers used the Hubble Space Telescope to search the gauzy remains of a Type Ia supernova in a neighboring galaxy called the Large Magellanic Cloud. They found a sun-like star that showed signs of being associated with the supernova. Further investigations will be needed to learn if this star is truly the culprit behind a white dwarf’s fiery demise.

 This supernova remnant is located 160,000 light-years from Earth. The actual supernova remnant is the irregular shaped dust cloud, at the upper center of the image. The gas in the lower half of the image and the dense concentration of stars in the lower left are the outskirts of a star cluster.

Image credit: NASA, ESA and H.-Y. Chu (Academia Sinica, Taipei)


Ask Ethan: What Surprises Might NASA’s Future Space Telescopes Discover?

“One of the primary science goals for WFIRST is to survey the sky out to very large distances to look for new type Ia supernovae. These are the same events that led to the discovery of dark energy, but instead of tens or hundreds, it will collect many thousands, and out to very large distances. And what it will allow us to measure is not just the rate of expansion of the Universe, but how it’s changed over time, to about ten times better precision than we can currently measure. If dark energy is different from a cosmological constant by even 1%, we’ll find it. And if it’s even 1% more negative than a cosmological constant’s negative pressure, our Universe will end in a Big Rip.”

We know what NASA’s James Webb and WFIRST are designed for, and we know what we expect to find. James Webb will be the largest space telescope ever, focused mostly on infrared observations probing exoplanets, star-forming nebulae, galaxy evolution and the first stars and galaxies in the Universe. WFIRST will be just like Hubble, except with better instruments and 100 times the field-of-view. But the best discoveries from Hubble were things like dark energy: things we didn’t expect to find! What might some of the surprises be – without hypothesizing radical new physics – that these two observatories might uncover? They range from signatures of exoplanetary life to being able to possibly falsify dark matter, and they’re all incredible.

Find out seven of the most tantalizing possibilities today, on this week’s astonishing Ask Ethan!

Of all the varieties of exploding stars, the ones called Type Ia are perhaps the most intriguing. Their predictable brightness lets astronomers measure the expansion of the universe, which led to the discovery of dark energy. Yet the cause of these supernovae remains a mystery. Do they happen when two white dwarf stars collide? Or does a single white dwarf gorge on gases stolen from a companion star until bursting?

If the second theory is true, the normal star should survive. Astronomers used NASA’s Hubble Space Telescope to search the gauzy remains of a Type Ia supernova in a neighboring galaxy called the Large Magellanic Cloud. They found a sun-like star that showed signs of being associated with the supernova. Further investigations will be needed to learn if this star is truly the culprit behind a white dwarf’s fiery demise.

This image, taken with NASA’s Hubble Space Telescope, shows the supernova remnant SNR 0509-68.7, also known as N103B. It is located 160,000 light-years from Earth in a neighboring galaxy called the Large Magellanic Cloud. N103B resulted from a Type Ia supernova, whose cause remains a mystery. One possibility would leave behind a stellar survivor, and astronomers have identified a possible candidate.

The actual supernova remnant is the irregular shaped dust cloud, at the upper center of the image. The gas in the lower half of the image and the dense concentration of stars in the lower left are the outskirts of the star cluster NGC 1850.

The Hubble image combines visible and near-infrared light taken by the Wide Field Camera 3 in June 2014.

Image credit:andnbsp;NASA, ESA and H.-Y. Chu (Academia Sinica, Taipei)
Text: Space Telescope Science Institute
Media contact: Rob Gutro, NASA’s Goddard Space Flight Center, Greenbelt, Md.

Hubble Space Telescope

Time And Space

Light rays from a supernova bent by the curvature of space-time around a galaxy

An international research team led by Ariel Goobar at Stockholm University has detected for the first time multiple images from a gravitationally lensed Type Ia supernova. The new observations suggest promising new avenues for the study of the accelerated expansion of the Universe, gravity and distribution of dark matter in the universe.

Type Ia supernovae, nature’s own “standard candles,” have been used for many years by astronomers to measure cosmological distances. These studies led to the discovery of the accelerated expansion of the Universe, a sensational discovery that won the 2011 Nobel prize in Physics. Professor Ariel Goobar at the Department of Physics at Stockholm University was a member of the team led by one of the Nobel laureates, Saul Perlmutter.

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Could a new type of supernova eliminate dark energy?

“Imagine you had a box of candles that you thought were all identical to one another: you could light them up, put them all at different distances, and immediately, just from measuring the brightness you saw, know how far away they are. That’s the idea behind a standard candle in astronomy, and why type Ia supernovae are so powerful.

But now, imagine that these candle flames aren’t all the same brightness! Suddenly, some are a little brighter and some are a little dimmer; you have two classes of candles, and while you might have more of the brighter ones close by, you might have more of the dimmer ones far away. That’s what we think we’ve just discovered with supernovae: there are actually two separate classes of them, where one’s a little brighter in the blue/UV, and one’s a little brighter in the red/IR, and the light curves they follow are slightly different. This might mean that, at high redshifts (large distances), the supernovae themselves are actually intrinsically fainter, and not that they’re farther away.”

Back in the 1990s, scientists were quite surprised to find that when they measured the brightness and redshifts of distant supernovae, they appeared fainter than one would expect, leading us to conclude that the Universe was expanding at an accelerating rate to push them farther away. But a 2015 study put forth a possibility that many scientists dreaded: that perhaps these distant supernovae were intrinsically different from the ones we had observed nearby. Would that potentially eliminate the need for dark energy altogether? Or would it simply change ever-so-slightly the amount and properties of dark energy we required to explain modern cosmology? A full analysis shows that dark energy is here to stay, regardless of the supernova data.



With the help of an automated supernova-hunting pipeline and a galaxy sitting 2 billion light-years away from Earth that’s acting as a “magnifying glass,’’ astronomers have captured multiple images of a Type Ia supernova – the brilliant explosion of a star – appearing in four different locations on the sky. So far this is the only Type Ia discovered that has exhibited this effect.

This phenomenon called ‘gravitational lensing’ is an effect of Einstein’s theory of relativity – mass bends light. This means that the gravitational field of a massive object – like a galaxy – can bend light rays that pass nearby and refocus them somewhere else, causing background objects to appear brighter and sometimes in multiple locations. Astrophysicists believe that if they can find more of these magnified Type Ia’s, they may be able to measure the rate of the universe’s expansion to unprecedented accuracy and shed some light on the distribution of matter in the cosmos.

Fortunately, by taking a closer look at the properties of this rare event, two Lawrence Berkeley National Laboratory (Berkeley Lab) researchers have come up with a method – a pipeline – for identifying more of these so-called “strongly lensed Type Ia supernovae” in existing and future wide-field surveys. A paper describing their approach was recently published in the Astrophysical Journal Letters. Meanwhile, a paper detailing the discovery and observations of the 4 billion year old Type Ia supernova, iPTF16geu, will be published in Science on April 21.

“It is extremely difficult to find a gravitationally lensed supernova, let alone a lensed Type Ia. Statistically, we suspect that there may be approximately one of these in every 50,000 supernovae that we identify,” says Peter Nugent, an astrophysicist in Berkeley Lab’s Computational Research Division (CRD) and an author on both papers. “But since the discovery of iPTF16geu, we now have some thoughts on how to improve our pipeline to identify more of these events.”

Cosmic Surprise Sheds New Light on Cosmology

For many years, the transient nature of supernovae made them extremely difficult to detect. Thirty years ago, the discovery rate was about two per month. But thanks to the Intermediate Palomar Transient Factory (iPTF), a new survey with an innovative pipeline, these events are being detected daily, some within hours of when their initial explosions appear.

The process of identifying transient events, like supernovae, begins every night at the Palomar Observatory in Southern California, where a wide-field camera mounted on the robotic Samuel Oschin Telescope scans the sky. As soon as observations are taken, the data travel more than 400 miles to the Department of Energy’s (DOE’s) National Energy Research Scientific Computing Center (NERSC), which is located at Berkeley Lab. At NERSC, machine learning algorithms running on the facility’s supercomputers sift through the data in real-time and identify transients for researchers to follow up on.

On September 5, 2016, the pipeline identified iPTF16geu as a supernova candidate. At first glance, the event didn’t look particularly out of the ordinary. Nugent notes that many astronomers thought it was just a typical Type Ia supernova sitting about 1 billion light-years away from Earth.

Like most supernovae that are discovered relatively early on, this event got brighter with time. Shortly after it reached peak brightness (19th magnitude) Stockholm University Professor in Experimental Particle Astrophysics Ariel Goobar decided to take a spectrum – or detailed light study – of the object. The results confirmed that the object was indeed a Type Ia supernova, but they also showed that, surprisingly, it was located 4 billion light-years away. A second spectrum taken with the OSIRIS instrument on the Keck telescope on Maunakea, Hawaii, showed without a doubt that the supernova was 4 billion light-years away, and also revealed its host galaxy and another galaxy located about 2 billion light-years away that was acting as a gravitational lens, which amplified the brightness of the supernova and caused it to appear in four different places on the sky.

“I’ve been looking for a lensed supernova for about 15 years. I looked in every possible survey, I’ve tried a variety of techniques to do this and essentially gave up, so this result came as a huge surprise,” says Goobar, who is lead author of the Science paper. “One of the reasons I’m interested in studying gravitational lensing is that it allows you to measure the structure of matter – both visible and dark matter – at scales that are very hard to get.”

According to Goobar, the survey at Palomar was set up to look at objects in the nearby universe, about 1 billion light-years away. But finding a distant Type Ia supernova in this survey allowed researchers to follow up with even more powerful telescopes that resolved small-scale structures in the supernova host galaxy, as well as the lens galaxy that is magnifying it.

“There are billions of galaxies in the observable universe and it takes a tremendous effort to look in a very small patch of the sky to find these kind of events. It would be impossible to find an event like this without a magnified supernova directing you where to look,” says Goobar. “We got very lucky with this discovery because we can see the small scale structures in these galaxies, but we won’t know how lucky we are until we find more of these events and confirm that what we are seeing isn’t an anomaly.”

Another benefit of finding more of these events is that they can be used as tools to precisely measure the expansion rate of the universe. One of the keys to this is gravitational lensing. When a strong gravitational lens produces multiple images of a background object, each image’s light travels a slightly different path around the lens on its way to Earth. The paths have different lengths, so light from each image takes a different amount of time to arrive at Earth.

“If you measure the arrival times of the different images, that turns out to be a good way to measure the expansion rate of the universe,” says Goobar. “When people measure the expansion rate of the universe now locally using supernovae or Cepheid stars they get a different number from those looking at early universe observations and the cosmic microwave background. There is tension out there and it would be neat if we could contribute to resolving that quest.”

New Methods Sniff Out Lensed Supernovae

According to Danny Goldstein, a UC Berkeley astronomy graduate student and an author of the Astrophysical Journal letter, there have only been a few gravitationally lensed supernovae of any type ever discovered, including iPTF16geu, and they’ve all been discovered by chance.

“By figuring out how to systematically find strongly lensed Type Ia supernovae like iPTF16geu, we hope to pave the way for large-scale lensed supernova searches, which will unlock the potential of these objects as tools for precision cosmology,” says Goldstein, who worked with Nugent to devise a method for finding them in existing and upcoming wide-field surveys.

The key idea of their technique is to use the fact that Type Ia supernovae are “standard candles” – objects with the same intrinsic brightness – to identify ones that are magnified by lensing. They suggest starting with supernovae that appear to go off in red galaxies that have stopped forming stars. These galaxies only host Type Ia supernovae and make up the bulk of gravitational lenses. If a supernova candidate that appears to be hosted in such a galaxy is brighter than the “standard” brightness of a Type Ia supernova, Goldstein and Nugent argue that there is a strong chance the supernova does not actually reside in the galaxy, but is instead a background supernova lensed by the apparent host.

“One of the innovations of this method is that we don’t have to detect multiple images to infer that a supernova is lensed,” says Goldstein. “This is a huge advantage that should enable us to find more of these events than previously thought possible.”

Using this method, Nugent and Goldstein predict that the upcoming Large Synoptic Survey Telescope should be able to detect about 500 strongly lensed Type Ia supernovae over the course of 10 years – about 10 times more than previous estimates. Meanwhile, the Zwicky Transient Facility, which begins taking data in August 2017 at Palomar, should find approximately 10 of these events in a three-year search. Ongoing studies show that each lensed Type Ia supernova image has the potential to make a four percent, or better, measurement of the expansion rate of the universe. If realized, this could add a very powerful tool to probe and measure the cosmological parameters.

“We are just now getting to the point where our transient surveys are big enough, our pipelines are efficient enough, and our external data sets are rich enough that we can weave through the data and get at these rare events,” adds Goldstein. “It’s an exciting time to be working in this field.”

TOP IMAGE….This composite image shows the gravitationally lensed type Ia supernova iPTF16geu, as seen with different telescopes. The background image shows a wide-field view of the night sky as seen with the Palomar Observatory located on Palomar Mountain, California. Far Left Image: Captured by the Sloan Digital Sky Survey, this optical light observation shows the lens galaxy and its surrounding environment in the sky. Center Left Image: Captured by the Hubble Space Telescope, this is a 20x zoom infrared image of the lens galaxy. Center Right Image: Captured by the Hubble Space Telescope, this 5x optical light zoom reveals the four gravitationally lensed images of iPTF16geu. Far Right Image: Captured by the Keck Telescope, this infrared observation features the four gravitationally lensed images of iPTF16geu and the gravitational “arc” of its host galaxy.
(Image Credit: Joel Johansson, Stockholm University)

LOWER IMAGE….The supernova iPTF16geu exploded at a distance corresponding to a time 4.3 billion years ago. It could only be detected because a foreground galaxy lensed the light of the explosion, making it 50 times brighter for observers on Earth. It also caused the supernova to appear in four distinct places on the sky, surrounding the lensing galaxy in the foreground. Credit: ESA/Hubble, NASA


The Milky Way’s most recent supernova was hidden… until now!

“Close to the galactic center, the supernova remnant G1.9+0.3 was first discovered in the radio thanks to the Very Large Array (VLA), with its origin unknown. The fact that it was so small on the sky, despite being at a distance of 25,000 light years, brought up the possibility that this was a very young supernova: perhaps the youngest of all supernovae in the Milky Way. Follow-up observations took place in the 2000s with the Two-Micron All-Sky Survey in the infrared and with the Chandra X-ray observatory, where a wonderful surprise came to light: this supernova remnant was expanding at an incredible pace!

In 1604, Kepler’s supernova went off, the last Milky Way supernova visible to naked-eye skywatchers here on Earth. Yet since the development of radio and X-ray astronomy, other, more recent supernova remnants in our galaxy have been found. They’ve only been invisible to the naked eye because of the galactic gas and dust that blocks their visible light. In 1984/5, the VLA discovered the most recent known remnant near the galactic center, and follow-up observations showed a rapid expansion. The most recent data not only dates this remnant to be only 110 years old, but it teaches us that it’s a Type Ia supernova that formed from the merger of two white dwarfs. The standard model — of one white dwarf accruing matter from a binary companion — may not only be a minority of Type Ia events, perhaps it doesn’t occur at all.

The universe is expanding at an accelerating rate, or is it?

Five years ago, the Nobel Prize in Physics was awarded to three astronomers for their discovery, in the late 1990s, that the universe is expanding at an accelerating pace.

Their conclusions were based on analysis of Type Ia supernovae – the spectacular thermonuclear explosion of dying stars – picked up by the Hubble space telescope and large ground-based telescopes. It led to the widespread acceptance of the idea that the universe is dominated by a mysterious substance named ‘dark energy’ that drives this accelerating expansion.

Now, a team of scientists led by Professor Subir Sarkar of Oxford University’s Department of Physics has cast doubt on this standard cosmological concept. Making use of a vastly increased data set – a catalogue of 740 Type Ia supernovae, more than ten times the original sample size – the researchers have found that the evidence for acceleration may be flimsier than previously thought, with the data being consistent with a constant rate of expansion.

The study is published in the Nature journal Scientific Reports.

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Supernova 1994D and the Unexpected Universe : Long ago, far away, a star exploded. Supernova 1994D, visible as the bright spot on the lower left, occurred in the outskirts of disk galaxy NGC 4526. Supernova 1994D was not of interest for how different it was, but rather for how similar it was to other supernovae. In fact, the light emitted during the weeks after its explosion caused it to be given the familiar designation of a Type Ia supernova. If all Type 1a supernovae have the same intrinsic brightness, then the dimmer a supernova appears, the farther away it must be. By calibrating a precise brightness-distance relation, astronomers are able to estimate not only the expansion rate of the universe . The large number and great distances to supernovae measured over the past few years, when combined with other observations, are interpreted as indicating that we live in a previously unexpected universe. via NASA


Hubble Detects Link Between Weak Supernovae and Zombie Stars

A team of astronomers, with the help of the Hubble Space Telescope, have detected a possible “zombie star” left behind as a result of a weak supernova.

Supernovae are violent explosions, signaling the end of a star’s life, and the dying star (usually a white dwarf) is obliterated. In this instance, astronomers believe they have found the zombified remains of the supernova’s progenitor – a surviving portion of a white dwarf star. Since white dwarfs typically do not survive supernovae explosions, any potential remnant is referred to a “zombie star”. In cases like these, the white dwarfs are not fully disrupted, only battered and bruised so-to-speak. 

Astronomers carefully examined previous Hubble images, taken years before the supernova, and were able to identify a white dwarf and its blue companion. Eventually, the companion star shed enough material onto the white dwarf to make the system unstable, igniting a nuclear explosion. The resulting blast was classified as a Type Iax supernova and is less common and dimmer than Type Ia supernovae. Overall, astronomers have identified over 30 of these mini-supernovae (Type Iax), that could potentially leave behind “zombie stars”.

These particular stellar explosions are important in our understanding of Type Ia supernovae. For decades, astronomers have searched for star systems that produce Type Ia explosions. These cosmic explosions are important, as they serve as cosmic mile markers, and help us measure the Universe’s expansion and vast cosmic distances. 

Currently, astronomers have not conclusively identified the Type Ia progenitor. Since there are many similarities between Type Ia’s and Type Iax’s, it’s important we understand this type of white dwarf explosion. For every one Type Iax explosion, there are five Type Ia’s and Type Iax’s only release between one and fifty percent the amount of energy as a Type Ia explosion. 

Approximately 110 million light-years from Earth, in the galaxy NGC 1309, lies the weak supernova, SN 2012Z. Hubble’s Advanced Camera for Surveys first captured images of this would-be explosion prior to its outburst. Then in January 2012, the Lick Observatory Supernova Search caught the stellar explosion on camera. These two surveys provided astronomers with before-and-after images to compare. 

The Hubble images were subsequently sharpened, and astronomers were pleasantly surprised to see the progenitor system. They expected any system would be too faint to observe, as was the case in previous searches for Type Ia progenitors. Analysis and comparisons with computer simulations, researchers concluded that they were looking at a star that shed its outer hydrogen envelope, revealing a helium core. 

Astronomers plan to employ Hubble again next year to observe the area, after the supernova’s light has faded, hoping to see any potential “zombie star” and helium companion in order to confirm their hypothesis. They also hope these finding will lead to improved models of white dwarf explosions, as well as a better understanding of the relationship between Type Ia, Type Iax explosions, and their host systems.

Another example of a Type Iax explosion has been located in the galaxy UGC 12682. Hubble images of the supernova 2008ha were recorded in January 2013, four years after the explosion occurred. In the images, astronomers could see an object but so far have been able to tell if it is the zombie star, or its companion. 

This second finding shows us how diverse these explosions can be. SN 2012Z is one of the most powerful examples, while SN 2008ha is one of the weakest. Such diversity could be directly related to how each star explodes. Since the white dwarf is not completely obliterated, some explosions could eject a small amount of material, while some could eject a lot more. 

Image & Source Credit: NASA/Hubble 

Supernova SN 2014J explodes

New data from NASA’s Chandra X-ray Observatory has provided stringent constraints on the environment around one of the closest supernovas discovered in decades. The Chandra results provide insight into possible cause of the explosion, as described in our press release. On January 21, 2014, astronomers witnessed a supernova soon after it exploded in the Messier 82, or M82, galaxy. Telescopes across the globe and in space turned their attention to study this newly exploded star, including Chandra.  Astronomers determined that this supernova, dubbed SN 2014J, belongs to a class of explosions called “Type Ia” supernovas. These supernovas are used as cosmic distance-markers and played a key role in the discovery of the Universe’s accelerated expansion, which has been attributed to the effects of dark energy.  Scientists think that all Type Ia supernovas involve the detonation of a white dwarf. One important question is whether the fuse on the explosion is lit when the white dwarf pulls too much material from a companion star like the Sun, or when two white dwarf stars merge. This image contains Chandra data, where low, medium, and high-energy X-rays are red, green, and blue respectively. The boxes in the bottom of the image show close-up views of the region around the supernova in data taken prior to the explosion (left), as well as data gathered on February 3, 2014, after the supernova went off (right).  The lack  of the detection of X-rays detected by Chandra is an important clue for astronomers looking for the exact mechanism of how this star exploded. The non-detection of X-rays reveals that the region around the site of the supernova explosion is relatively devoid of material. This finding is a critical clue to the origin of the explosion. Astronomers expect that if a white dwarf exploded because it had been steadily collecting matter from a companion star prior to exploding, the mass transfer process would not be 100% efficient, and the white dwarf would be immersed in a cloud of gas. If a significant amount of material were surrounding the doomed star, the blast wave generated by the supernova would have struck it by the time of the Chandra observation, producing a bright X-ray source. Since they do not detect any X-rays, the researchers determined that the region around SN 2014J is exceptionally clean. A viable candidate for the cause of SN 2014J must explain the relatively gas-free environment around the star prior to the explosion.  One possibility is the merger of two white dwarf stars, in which case there might have been little mass transfer and pollution of the environment before the explosion. Another is that several smaller eruptions on the surface of the white dwarf cleared the region prior to the supernova.  Further observations a few hundred days after the explosion could shed light on the amount of gas in a larger volume, and help decide between these and other scenarios.

Image credit: NASA/CXC/SAO/R.Margutti et al

The universe is expanding even faster than expected

Astronomers using NASA’s Hubble Space Telescope have discovered that the universe is expanding 5 percent to 9 percent faster than expected.

“This surprising finding may be an important clue to understanding those mysterious parts of the universe that make up 95 percent of everything and don’t emit light, such as dark energy, dark matter, and dark radiation,” said study leader and Nobel Laureate Adam Riess of the Space Telescope Science Institute and The Johns Hopkins University, both in Baltimore, Maryland.

The results will appear in an upcoming issue of The Astrophysical Journal.

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Cosmic Detective Work

Supernovae are thought to be massive explosions signaling the end of a star’s life; however, stellar demise is not always the cause of these explosions. Supernovae explosions are not created equal and are divided into categories - the most common of which is a Type Ia. This class of supernova involves the detonation of small, dense, already dead stars called white dwarfs. 

New observations from NASA’s Spitzer Space Telescope have shed light on a new rare type of Type Ia supernova, involving a zombie star of sorts. The dead star (white dwarf) feeds off a neighboring aging star just like a zombie, until it has “eaten” so much material, it triggers an explosion. This new data shows just how exceptional these events are. Astronomers at NASA’s Goddard Space Flight Center in Greenbelt, Maryland get to play detective and search through the stellar remains for any clue that will explain exactly what causes these powerful explosions. 

Supernovae provide cosmic fuel, spewing out all the elements necessary to the Universe - including heavy metals like the iron found in our blood. Typically Type Ia explosions are very consistent and astronomers use these explosions as distance markers and use them to study how the universe is expanding. 

Over the past ten years, researchers have been mounting evidence that Type Ia explosions occur when two white dwarfs collide - with only one exception - Kepler’s supernova. Named for the famous astronomer who witnessed it back in 1604, Kepler’s supernova is thought to be caused by one white dwarf and its elderly red giant companion. Scientists know the companion was a red giant based on the white dwarf remnant sitting among the gas pool and dust shed by the aging star.

These new observations show that Spitzer has now observed a second explosion just like Kepler’s supernova, dubbed N103B. Located approximately 160,000 light-years away in our galactic neighbor, the Large Magellanic Cloud, lies Kepler’s older cousin - N103B. However, unlike Kelper’s supernova, there are no recorded historical sightings of the explosion. 

Both N103B and Kepler’s supernova are thought to have formed from an aging red giant orbiting and already “dead” white dwarf. Typically older stars molt (meaning they shed their outer layers) and this material falls onto the companion white dwarf. Once the white dwarf has accumulated enough mass, it will become unstable and ultimately explode. 

Just a decade ago, scientists thought the white dwarf red giant scenario was the common cause of Type Ia explosions, scientists now suggest that a pair of white dwarf stars are the more likely (and common) culprit. The new Spitzer data shows how complex these explosions are and how there are many different triggers. However, the exact reason as to why these “dead” stars explode is still to be determined. 


Image & Source Credit: NASA/JPL-CalTech

No X-rays from SN 2014J

Last January, telescopes in observatories around planet Earth were eagerly used to watch the rise of SN 2014J, a bright supernova in nearby galaxy M82. Still, the most important observations may have been from orbit where the Chandra X-ray Observatory saw nothing. Identified as a Type Ia supernova, the explosion of SN2014J was thought to be triggered by the buildup of mass on a white dwarf star steadily accreting material from a companion star. That model predicts X-rays would be generated when the supernova blastwave struck the material left surrounding the white dwarf. But no X-rays were seen from the supernova. The mostly blank close-ups centered on the supernova’s position are shown in the before and after inset panels of Chandra’s false color X-ray image of the M82 galaxy. The stunning lack of X-rays from SN 2014J will require astronomers to explore other models to explain what triggers these cosmic explosions.

Image credit: NASA / CXC / SAO / R. Margutti et al.

Why Type Ia Supernovae Continue to Burn Bright

Three years after its explosion, a type Ia supernova continues to shine more brightly than expected, new research finds. The observations, made with the Hubble Space Telescope and published today in The Astrophysical Journal, suggest that powerful explosions like this one produce a heavy form of cobalt that gives the heat from nuclear decay an energy boost.

The work could help researchers pinpoint the parents of type Ia supernovae and reveal the mechanics behind these events. These particular types of stellar explosions are frequently used to measure distances to faraway galaxies, and have grown more important to the field in recent decades, after they were used to demonstrate that expansion of the universe is accelerating. But researchers still have many questions about the phenomenon.

“We still do not know exactly what type of star system explodes as a type Ia supernova or how the explosion takes place,“ said lead author Or Graur, a research associate in the American Museum of Natural History’s Department of Astrophysics and a postdoctoral researcher at New York University. "A lot of research has gone into these two questions, but the answers are still elusive.”

Current research suggests that type Ia supernovae begin in binary star systems, where two stars orbit one another, and where at least one star is a white dwarf. The explosion is the result of a thermonuclear chain reaction, which produces vast quantities of heavy elements. The light that researchers see when a type Ia supernova explodes comes from the radioactive decay of these elements, notably when an isotope of nickel (56Ni) decays into an isotope of cobalt (56Co) and then into a stable isotope of iron (56Fe).

Read the full story on the Museum blog. 


The universe is expanding at an accelerating rate – or is it?

Five years ago, the Nobel Prize in Physics was awarded to three astronomers for their discovery, in the late 1990s, that the universe is expanding at an accelerating pace.

Their conclusions were based on analysis of Type Ia supernovae - the spectacular thermonuclear explosion of dying stars - picked up by the Hubble space telescope and large ground-based telescopes. It led to the widespread acceptance of the idea that the universe is dominated by a mysterious substance named ‘dark energy’ that drives this accelerating expansion.

Now, a team of scientists led by Professor Subir Sarkar of Oxford University’s Department of Physics has cast doubt on this standard cosmological concept. Making use of a vastly increased data set - a catalogue of 740 Type Ia supernovae, more than ten times the original sample size - the researchers have found that the evidence for acceleration may be flimsier than previously thought, with the data being consistent with a constant rate of expansion.

The study is published in the Nature journal Scientific Reports.
Professor Sarkar, who also holds a position at the Niels Bohr Institute in Copenhagen, said: 'The discovery of the accelerating expansion of the universe won the Nobel Prize, the Gruber Cosmology Prize, and the Breakthrough Prize in Fundamental Physics. It led to the widespread acceptance of the idea that the universe is dominated by “dark energy” that behaves like a cosmological constant - this is now the “standard model” of cosmology.

'However, there now exists a much bigger database of supernovae on which to perform rigorous and detailed statistical analyses. We analysed the latest catalogue of 740 Type Ia supernovae - over ten times bigger than the original samples on which the discovery claim was based - and found that the evidence for accelerated expansion is, at most, what physicists call “3 sigma”. This is far short of the “5 sigma” standard required to claim a discovery of fundamental significance.

'An analogous example in this context would be the recent suggestion for a new particle weighing 750 GeV based on data from the Large Hadron Collider at CERN. It initially had even higher significance - 3.9 and 3.4 sigma in December last year - and stimulated over 500 theoretical papers. However, it was announced in August that new data shows that the significance has dropped to less than 1 sigma. It was just a statistical fluctuation, and there is no such particle.’

There is other data available that appears to support the idea of an accelerating universe, such as information on the cosmic microwave background - the faint afterglow of the Big Bang - from the Planck satellite. However, Professor Sarkar said: 'All of these tests are indirect, carried out in the framework of an assumed model, and the cosmic microwave background is not directly affected by dark energy. Actually, there is indeed a subtle effect, the late-integrated Sachs-Wolfe effect, but this has not been convincingly detected.

'So it is quite possible that we are being misled and that the apparent manifestation of dark energy is a consequence of analysing the data in an oversimplified theoretical model - one that was in fact constructed in the 1930s, long before there was any real data. A more sophisticated theoretical framework accounting for the observation that the universe is not exactly homogeneous and that its matter content may not behave as an ideal gas - two key assumptions of standard cosmology - may well be able to account for all observations without requiring dark energy. Indeed, vacuum energy is something of which we have absolutely no understanding in fundamental theory.’

Professor Sarkar added: 'Naturally, a lot of work will be necessary to convince the physics community of this, but our work serves to demonstrate that a key pillar of the standard cosmological model is rather shaky. Hopefully this will motivate better analyses of cosmological data, as well as inspiring theorists to investigate more nuanced cosmological models.

Significant progress will be made when the European Extremely Large Telescope makes observations with an ultrasensitive “laser comb” to directly measure over a ten to 15-year period whether the expansion rate is indeed accelerating.’

Oldest recorded supernova

This image combines data from four space telescopes to create a multi-wavelength view of all that remains of RCW 86, the oldest documented example of a supernova. Chinese astronomers witnessed the event in 185 A.D., documenting a mysterious “guest star” that remained in the sky for eight months. X-ray images from NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton Observatory were combined to form the blue and green colors in the image. The X-rays show the interstellar gas that has been heated to millions of degrees by the passage of the shock wave from the supernova.

Infrared data from NASA’s Spitzer Space Telescope and WISE, Wide-Field Infrared Survey Explorer, shown in yellow and red, reveal dust radiating at a temperature of several hundred degrees below zero, warm by comparison to normal dust in our Milky Way galaxy.

By studying the X-ray and infrared data, astronomers were able to determine that the cause of the explosion was a Type Ia supernova, in which an otherwise-stable white dwarf, or dead star, was pushed beyond the brink of stability when a companion star dumped material onto it. Furthermore, scientists used the data to solve another mystery surrounding the remnant - how it got to be so large in such a short amount of time. By blowing away wind prior to exploding, the white dwarf was able to clear out a huge “cavity,” a region of very low-density surrounding the system. The explosion into this cavity was able to expand much faster than it otherwise would have.

This is the first time that this type of cavity has been seen around a white dwarf system prior to explosion. Scientists say the results may have significant implications for theories of white-dwarf binary systems and Type Ia supernovae.

RCW 86 is approximately 8,000 light-years away. At about 85 light-years in diameter, it occupies a region of the sky in the southern constellation of Circinus that is slightly larger than the full moon. This image was compiled in October 2011.

Image credit: X-ray: NASA/CXC/SAO & ESA; Infared: NASA/JPL-Caltech/B. Williams (NCSU)

Rare white dwarf systems do a doubletake

For those of us who remain forever fascinated by astronomy, nothing could spark our imaginations more than a cosmic curiosity. In this case, the unusual object is a star cataloged as AM Canum Venaticorum (AM CVn) located in the constellation of Canes Venatici. What makes this dual star system of interest? Try the fact that the pair revolve completely around each other in a brief 18 minutes. What’s more, they are the stuff of which Einstein dreamed… creators of ripples in space-time known as gravitational waves.

Like other astronomical anomalies, AM CVn became the forerunner of a new class of stellar objects. It is a white dwarf, a sun-like star which has exhausted its fuel and collapsed to around the size of Earth. Yet it also has a white dwarf companion – a very compact orb which is delivering matter to its neighbor. AM Canum Venaticorum is not alone, however. There are similar systems where the stellar pairs complete their rotations in about an hour and even as rapidly as five minutes! Can you imagine the crackling amount of energy a system like this produces?!

Even though we have been aware of systems like AM CVn for almost five decades, no one is quite sure how they originate. Now, through the use of X-ray and optical observations, astronomers are taking a look at newly evolved double stars systems which one day might become a dueling duo dwarf. Heading their list are two binary systems, J0751 and J1741. These candidates were observed in the X-ray part of the electromagnetic spectrum by NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton telescope. In addition, observations at optical wavelengths were made using the McDonald Observatory’s 2.1-meter telescope in Texas, and the Mt. John Observatory 1.0-meter telescope in New Zealand.

What’s happening here? As the pair of white dwarf stars whip around each other, they are releasing gravitational waves which constrict the orbit. In time, the heavier, diminutive dwarf will begin stripping material from its lighter, larger companion. This material consumption will continue for perhaps a 100 million years, or until the collected matter reaches a critical mass and releases a thermonuclear explosion.

Another scenario is the thermonuclear explosion could annihilate the larger white dwarf completely in what astronomers call a Type Ia supernova. An event like this is well-known and gives a measurement in standard candles for cosmic distance. However, chances are better the explosion will happen on the surface of the star – an event known as .Ia supernovae. While .Ia supernovae events have been recorded in other galaxies, J0751 and J1741 are the first binary stars which have the potential to erupt in .Ia supernovae.

“The optical observations were critical in identifying the two white dwarfs in these systems and ascertaining their masses. The X-ray observations were needed to rule out the possibility that J0751 and J1741 contained neutron stars.” says the Chandra team. “A neutron star – which would disqualify it from being a possible parent to an AM CVn system – would give off strong X-ray emission due to its magnetic field and rapid rotation. Neither Chandra nor XMM-Newton detected any X-rays from these systems.”

Are AM CVn systems riding the gravitational wave? While astronomers haven’t been able to detect them yet, these new observations are highly important because equipment to verify their presences is currently being developed. It won’t be long until we can see the wave and have a whole new way of looking at the Universe!

The Supernova Next Door

Exciting news for astronomers today! A fresh, new supernova has been detected in the M82 galaxy (if by “fresh” you mean 12 million years old). M82 lies in Ursa Major, and this particular galaxy contains a dense, active birthing garden for new stars. The image above (via Wikipedia) shows M82 as it appeared in December 2013 and again on january 21, 2014.

This supernova, currently christened with the mouthful-of-a-name “PSN J09554214+6940260” is the closest supernova detected in over 25 years (but it’s still far enough away that we have nothing to worry about). It is classified as a Type Ia supernova, a class that astronomers still don’t completely understand. 

Currently, it’s still dim enough that you’d need a telescope to see, but it may brighten enough in the next couple weeks that binoculars will do the trick (but really, who owns binoculars?) … an exciting reminder that the universe is a constantly evolving place, both here and 12 million light years away, inside of a space bear.

Phil Plait has the full sciencey rundown at Bad Astronomy!