Shapes Reveal Supernovae History

These two supernova remnants are part of a new study from NASA’s Chandra X-ray Observatory that shows how the shape of the remnant is connected to the way the progenitor star exploded. Credit: NASA/CXC/UCSC/L. Lopez et al.)

At a very early age, children learn how to classify objects according to their shape. Now, new research suggests studying the shape of the aftermath of supernovas may allow astronomers to do the same. Images of supernova remnants taken by the Chandra X-ray Observatory shows that the symmetry of the debris from exploded stars, or lack thereof, reveals how the star exploded. This is an important discovery because it shows that the remnants retain information about how the star exploded even though hundreds or thousands of years have passed.

“It’s almost like the supernova remnants have a ‘memory’ of the original explosion,” said Laura Lopez of the University of California at Santa Cruz, who led the study. “This is the first time anyone has systematically compared the shape of these remnants in X-rays in this way.”

Astronomers sort supernovas into several categories, or “types”, based on properties observed days after the explosion and which reflect very different physical mechanisms that cause stars to explode. But, since observed remnants of supernovas are leftover from explosions that occurred long ago, other methods are needed to accurately classify the original supernovas.

Lopez and colleagues focused on the relatively young supernova remnants that exhibited strong X-ray emission from silicon ejected by the explosion so as to rule out the effects of interstellar matter surrounding the explosion. Their analysis showed that the X-ray images of the ejecta can be used to identify the way the star exploded. The team studied 17 supernova remnants both in the Milky Way galaxy and a neighboring galaxy, the Large Magellanic Cloud.

Chandra X-ray Image of SNR 0548-70.4  (Credit: NASA/CXC/UCSC/L. Lopez et al.)
Chandra X-ray Image of SNR 0548-70.4 (Credit: NASA/CXC/UCSC/L. Lopez et al.)

For each of these remnants there is independent information about the type of supernova involved, based not on the shape of the remnant but, for example, on the elements observed in it. The researchers found that one type of supernova explosion – the so-called Type Ia – left behind relatively symmetric, circular remnants. This type of supernova is thought to be caused by a thermonuclear explosion of a white dwarf, and is often used by astronomers as “standard candles” for measuring cosmic distances.

On the other hand, the remnants tied to the “core-collapse” supernova explosions were distinctly more asymmetric. This type of supernova occurs when a very massive, young star collapses onto itself and then explodes.

“If we can link supernova remnants with the type of explosion”, said co-author Enrico Ramirez-Ruiz, also of University of California, Santa Cruz, “then we can use that information in theoretical models to really help us nail down the details of how the supernovas went off.”

Models of core-collapse supernovas must include a way to reproduce the asymmetries measured in this work and models of Type Ia supernovas must produce the symmetric, circular remnants that have been observed.

Out of the 17 supernova remnants sampled, ten were classified as the core-collapse variety, while the remaining seven of them were classified as Type Ia. One of these, a remnant known as SNR 0548-70.4, was a bit of an “oddball”. This one was considered a Type Ia based on its chemical abundances, but Lopez finds it has the asymmetry of a core-collapse remnant.

“We do have one mysterious object, but we think that is probably a Type Ia with an unusual orientation to our line of sight,” said Lopez. “But we’ll definitely be looking at that one again.”

While the supernova remnants in the Lopez sample were taken from the Milky Way and its close neighbor, it is possible this technique could be extended to remnants at even greater distances. For example, large, bright supernova remnants in the galaxy M33 could be included in future studies to determine the types of supernova that generated them.

The paper describing these results appeared in the November 20 issue of The Astrophysical Journal Letters.

Source: Chandra

Superbright Supernova First Observed of Antimatter Variety

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The supernova 2007bi wasn’t your typical supernova: it was 10 times brighter than a Type Ia supernova, making it one of the most energetic supernova events ever recorded. Astronomers from the University of California Berkeley have analyzed the explosion, which was recorded by a robotic survey in 2007, and found that it is likely the first confirmed observation ever made of a pair-instability supernova, a type of extremely energetic supernova that has been theorized but never directly confirmed.

The confirmed observation of a pair-instability supernova has been long-awaited – the theory that they exist has been around since the 1960’s – but it appears as if the wait is over. The supernova 2007bi, seen by the Nearby Supernova Factory in April of 2007, is the first observed supernova that fits the bill for the unfathomably huge proportions of pair-instability supernovae explosions. A team of astronomers led by Alex Filippenko of the University of California Berkeley published their analysis in in the December 3rd issue of Nature. The discovery was initially made by the Nearby Supernova Factory, and emission spectra of the event was taken with the Keck Telescope and Very Large Telescope in Chile 

These type of supernovae occur only in stars above 100 solar masses, and are incredibly bright. Energetic gamma rays are created by the intense heat in the core of the star. These gamma rays, in turn, create antimatter pairs of electrons and positrons. Because of this antimatter production, the outward pressure exerted by the nuclear reactions in the core of the star is lessened, and gravity takes over, quickly collapsing the massive core of the star and creating a supernova.

There are theorized to be two kinds: those that explode with just enough force to allow for the mass around the leftover core of the star to recombine, and those that explode completely with not a smidgen left to form a black hole or neutron star. The supernova 2006gy, which had a luminosity 10 times that of a Type Ia supernova, is thought to be of the first variety. Here’s our story on that one, Could Antimatter be Powering Super-Luminous Supernovae? and Eta Carinae may also fit the profile.hese types of pair-instability supernovae will eject the outer shells of the star’s matter, settle down into an equilibrium, and repeat that process until the mass is low enough for a normal supernova to occur.

But 2007bi was much too massive to settle back down and explode multiple times. With a mass of 200 suns, the runaway thermonuclear explosion that happened in its core was energetic enough to effectively vaporize the entire star. Pair-instability supernovae in stars above 130 solar masses leave nothing behind in the way of black holes or neutron stars, but because they are so energetic and luminous, the increasing light from the explosion peaks over a very long time – 70 days in the case of 2007bi.

Though the team detected the supernova almost a week after the peak, they were able to calculate the duration of the light curve. They then studied the remnants of the explosion over the next 555 days as it faded away.

Filippenko said, “The central part of the huge star had fused to oxygen near the end of its life, and was very hot. Then the most energetic photons of light turned into electron-positron pairs, robbing the core of pressure and causing it to collapse. This led to a nuclear runaway explosion that created a large amount of radioactive nickel, whose decay energized the ejected gas and kept the supernova visible for a long time.”

The star was unique in another way: it lies in a nearby dwarf galaxy, which contains little else but the elements hydrogen and helium. Because of this, 2007bi is much like the stars that existed near the beginning of the Universe, before the trillions of supernovae populated the Universe with heavier elements. Looking more closely at dwarf galaxies – the Universe has them in spades, but they are quite dim – may be the key to observing more supernovae of this kind. Being able to study its explosion and aftereffects will give scientists a look into what the earliest massive stars acted like.

Source: Berkeley Lab press release

Supernova

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A supernova is the explosion of a star. In an instant, a star with many times the mass of our own Sun can detonate with the energy of a billion suns. And then, within just a few hours or days, it dims down again. Some explode into a spray of gas and dust, while others become exotic objects like neutron stars or black holes.

Astronomers have classified supernovae into two broad classifications: Type I and Type II. Type I supernovae occur in binary systems, where one star pulls off mass from a second star until it reaches a certain amount of mass. This causes it to explode as a supernova. Type II supernovae are the explosions of massive stars which have reached the end of their lives.

All of the elements heavier than iron were created in supernova explosions. As a massive star runs out of hydrogen fuel, it starts to fuse together heavier and heavier elements. Helium into carbon and oxygen. And then oxygen into heavier elements. It goes up the periodic table this way, fusing heavier elements until it reaches iron. Once a star reaches iron, it’s no longer able to extract energy from the fusion process. The core collapses down into a black hole, and the material around it is fused together into the elements heavier than iron. If you’re wearing any gold jewelry, that was created in a supernova.

In 1054 Chinese astronomers saw a supernova explosion that was so bright it was visible in the middle of the day. The explosion of gas and dust is now visible as the Crab Nebula (that’s the picture at the top of this article). The most recent powerful supernova explosion occurred in 1987, when a star exploded in the Large Magellanic Cloud.

Astronomers use Type I supernovae to judge distances in the Universe. This is because they always explode with approximately the same amount of energy. When a white dwarf star collected approximately 1.4 times the mass of the Sun, it can’t support its mass and collapses. This amount is called the Chandrasekhar Limit. When an astronomer sees a Type I supernova, they know how bright it is, and so they can measure how far away it is.

We’ve written many articles about supernovae for Universe Today. Here’s an article about a slow motion supernova, and here’s an article about a theoretical supernova that was actually found to exist.

If you’d like to see a gallery of supernova photographs, check out this section of the Hubble Space Telescope site, and here’s NASA’s Photo Gallery of Nebulae.

We’ve also recorded several episodes of Astronomy Cast about supernovas. Check out this one, Episode 14: We’re All Made of Supernovae.

References:
http://www.cfa.harvard.edu/supernova//newdata/supernovae.html
http://en.wikipedia.org/wiki/Chandrasekhar_limit

Slow-Motion Supernova

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Supernovae are generally considered as fast and furious events. For the Type II, core collapse supernovae, the core implodes almost instantaneously although it takes some time for the shockwave to escape the star. As it does, the star brightens in what’s known as the “rise time” of the supernova. For most Type II supernovae, this takes about a week.

So what are astronomers to make of supernova 2008iy that had an unprecedented rise time of at least 400 days?

From the time it was discovered, SN 2008iy was an oddball. When its spectra was analyzed, it was placed in the rare IIn subclass. This subclass is reserved for supernovae that feature narrow emission lines. Most supernovae have broad emission lines, if they even have emission lines at all.

To learn more about the history of this unusual case, astronomers at the University of California, Berkeley turned to archival images from the Palomar Quest survey. They searched images of the region to trace back the supernova as far as July of 2007, before which, the star was too faint to appear in images. Thus, the supernova brightening started at least that early and continued until late October of 2008 giving it a rise time at least four times as long as any previously discovered supernova.

The main clue to explain this mystery stemmed from the unusual emission lines. Generally, stars and supernovae are characterized by their absorption spectra which are caused when relatively cool gas stands between a hotter source and our detection. To generate emission lines, there must been a relatively dense medium being excited by the supernova. Furthermore, the fact that the lines were narrow implied that it was fairly motionless.

Together, this pointed to the progenitor undergoing a heightened period of mass loss prior to the detonation. The idea is such that the progenitor had shed large amounts of material. When the supernova occurred, this shell initially obscured the event. But as the ejecta from the supernova overtook the relatively stationary earlier shells, the brighter material slowly seeped out giving rise to the 400 day rise time.

While all stars undergo a period of mass loss in their post main sequence life, such a dense shell would be uncommon. To explain this, the authors turned to a type of star known as a Luminous Blue Variable. These stars are typically near the theoretical limit for the mass of a star (150 times the mass of the sun). Due to their extreme mass, they have strong stellar winds which periodically blow off large amounts of material that could create shells similar to those necessary for SN 2008iy. Unfortunately, this event was so distant that it could not be resolved to search for such a nebula. Even the host galaxy proved difficult to distinguish due to its faintness, although it is believed to be an irregular dwarf galaxy. Eta Carinae is one such luminous blue variable star. If perhaps one day soon it decides to turn into a supernova, it too will unfold in slow-motion.

Eta Carinae- A Naked Eye Enigma

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Eta Carinae is a beast of a star. At more than 100 solar masses and 4 million times the luminosity of our Sun, eta Car balances dangerously on the edge of stellar stability and it’s ultimate fate: complete self-destruction as a supernova. Recently, Hubble Space Telescope observations of the central star in the eta Carinae Nebula have raised an alert on eta Car among the professional community. What they discovered was totally unexpected.

“It used to be, that if you looked at eta Car you saw a nebula and then a faint little core in the middle” said Dr. Kris Davidson, from the University of Minnesota. “Now when you look at it, it’s basically the star with a nebula. The appearance is completely different. The light from the star now accounts for more than half the total output of eta Car. I didn’t expect that to happen until the middle of this century. It’s decades ahead of schedule. We know so little about these very massive objects, that if eta Car becomes a supernova next Thursday we should not be very surprised.”

In 1843, eta Carinae underwent a spectacular eruption, making it the second brightest star in the sky behind Sirius. During this violent episode, eta Car ejected 2 to 3 solar masses of material from the star’s polar regions. This material, traveling at speeds close to 700 km/s, formed two large, bipolar lobes, now known as the Homunculus Nebula. After the great eruption, Eta Car faded, erupted again briefly fifty years later, then settled down, around 8th magnitude. Davidson picks up the story from there.

This light curve depicts the visual apparent brightness of Eta Car from 1822 to date. It contains visual estimates (big circles), photographic (squares), photoelectric (triangles) and CCD (small circles) observations. All of them have been fitted for consistency of the whole data. Red points are recent observations from La Plata (Feinstein 1967; Fernández-Lajús et al., 2009, 2010). Used by permission.
This light curve depicts the visual apparent brightness of Eta Car from 1822 to date. It contains visual estimates (big circles), photographic (squares), photoelectric (triangles) and CCD (small circles) observations. All of them have been fitted for consistency of the whole data. Red points are recent observations from La Plata (Feinstein 1967; Fernández-Lajús et al., 2009, 2010). Used by permission.

“Around 1940, Eta suddenly changed its state. The spectrum changed and the brightness started to increase. Unfortunately, all this happened at a time when almost no one was looking at it. So we don’t know exactly what happened. All we know is that by the 1950’s, the spectrum had high excitation Helium lines in it that it didn’t have before, and the whole object, the star plus the Homunculus, was gradually increasing in brightness. In the past we’ve seen three changes of state. I suspect we are seeing another one happening now.”

During this whole time eta Car has been shedding material via its ferocious stellar winds. This has resulted in an opaque cloud of dust in the immediate vicinity of the star. Normally, this much dust would block our view to the star. So how does Davidson explain this recent, sudden increase in the luminosity of eta Carinae?

“The direct brightening we see is probably the dust being cleared away, but it can’t be merely the expansion of the dust. If it’s clearing away that fast, either something is destroying the dust, or the stellar wind is not producing as much dust as it did before. Personally, I think the stellar wind is decreasing, and the star is returning to the state it was in more than three hundred years ago. In the 1670’s, it was a fourth magnitude, blue, hot star. I think it is returning to that state. Eta Carinae has just taken this long to readjust from its explosion in the 1840’s.”

After 150 years what do we really know about one of the great mysteries of stellar physics? “We don’t understand it, and don’t believe anyone who says they do,” said Davidson.  “The problem is we don’t have a real honest-to-God model, and one of the reasons for that is we don’t have a real honest-to-God explanation of what happened in 1843.”

Can amateur astronomers with modest equipment help untangle the mysteries of eta Carinae? Davidson think so, “The main thing is to make sure everyone in the southern hemisphere knows about it, and anyone with a telescope, CCD or spectrograph should have it pointed at eta Carinae every clear night.”

Found: Theoretical Supernova Actually Exists

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Astronomers have identified a type of supernova that appears to be a type predicted in theory but never actually observed before. Two years ago Lars Bildsten from UC Santa Barbara and his colleagues predicted a new type of supernova in distant galaxies which they dubbed the “.Ia” (point one a) mechanism, involving a helium detonation on a white dwarf, ejecting a small envelope of material. This theoretical explosion would be fainter than most other supernovae and its brightness would rise and fall in only a few weeks. Dovi Poznanski from Berkeley went back and looked at seven-year-old observations and found this unusual kind of supernova. Poznanski and colleagues say supernova 2002bj belongs in its own category, as its spectra suggest that it evolved extremely fast and produced an unusual combination of elements.

Supernovae are usually classified based on tell-tale lines in the spectrum of radiation they emit. The two main types are thought to develop from exploding white dwarfs and collapsing massive stars.

However, Bildsten’s theory said that in rare instances, there is a binary star system where helium flows from one white dwarf onto another and accumulates on the more massive white dwarf.

It is this rare occurrence that leads to unique conditions of the explosive thermonuclear ignition and complete ejection of the accumulated helium ocean. The plethora of unusual radioactive elements made in the rapid fusion leads to a bright light show from the freshly synthesized matter that lasts a few weeks.
The “usual” explosions of white dwarfs are referred to as “Type Ia supernova.” They are brighter than a whole galaxy for more than a month and are quite useful in cosmological studies. The predicted “.Ia” supernovae are only one-tenth as bright for one-tenth the time.

Poznanski and his team say 2002bj fits the bill for this never-seen-before type of supernova.

“This is the fastest evolving supernova we have ever seen,” said Poznanski. “It was three to four times faster than a standard supernova, basically disappearing within 20 days. Its brightness just dropped like a rock.”

Poznanski told Universe Today that he was actually looking at Type II supernovae for another purpose when he hit the spectrum of 2002bj. “My first reaction was great confusion,” he said. “My second reaction, after showing it to other experts was greater confusion. After matching it against every object we know of, and finding nothing the confusion was topped with a lot of excitement. This kept rising until the .Ia idea came up and matched pretty well.”

Then Poznanski and his team re-analyzed their data to make sure, and the rest is history.

This explosion was nothing like a regular Type Ia explosion, said team member Alex Filippenko, because the white dwarf survives the detonation of the helium shell. In fact, it has similarities to both a nova and a supernova. Novas occur when matter – primarily hydrogen – falls onto a star and accumulates in a shell that can flare up as brief thermonuclear explosions. SN 2002bj is a “super” nova, generating about 1,000 times the energy of a standard nova, he said.

“As we have talked about our work over the last years, most astronomers in the audience reminded us that they had never seen such an event,” said Bildsten. “We told them to keep looking! With the sky the limit, the observers are usually ahead of theory, so I am really happy that we were able to make a prediction that allowed for a rapid interpretation of a new phenomena. Even though the supernova was observed in 2002, it took the keen eye of Dovi Poznanski to appreciate its import and relevance.”

Source: Science

What Is The Crab Nebula?

The Crab Nebula, or M1 (the first object in Messier’s famous catalog), is a supernova remnant and pulsar wind nebula. The name – Crab Nebula – is due to the Earl of Rosse, who thought it looked like a crab; it’s not in the constellation Cancer (the Crab), rather Taurus (the Bull).

The supernova which gave rise to the Crab Nebula was seen widely here on Earth in 1054 (and so it’s called SN 1054 by astronomers); it is perhaps the most famous of the historical supernovae. It is certainly one of the brightest (estimated to be –7 at peak), partly because it is so close (only 6,300 light-years away), and partly because it’s not hidden by dust clouds. The expansion of the nebula – as in seen-to-be-getting-bigger, rather than the-gas-is-moving-very-fast – was first confirmed in 1930.

As it was a core collapse supernova (a massive star which ran out of fuel), it left behind a neutron star; by chance, we are in line with its ‘lighthouse beam’, so we see it as a pulsar (all young neutron stars are pulsars, but not all of them have beams which point to us in one part of the cycle). It’s a pretty fast pulsar; the neutron star rotates once every 33 milliseconds. Because it’s so young and so close, the Crab Nebula pulsar was the first to be detected in the visual waveband, and also in x-rays and gamma rays. Being the source of the tremendous output of energy, from both the pulsar wind nebula and the pulsar itself, and as energy is conserved, the pulsar is slowing down, at a rate of 15 microseconds per year.

The inner part of the Crab Nebula, the pulsar wind nebula, contains lots of really hot (‘relativistic’) electrons spiraling around magnetic fields; this creates the eerie blue glow … synchrotron radiation. This makes the Crab Nebula one of the brightest objects in the x-ray and gamma ray region of the electromagnetic spectrum, and as it is a relatively steady source (unlike most high energy objects) it has given its name to a new astronomical unit, the Crab. For example, a new x-ray source may be 2 mCrab (milli-Crab), meaning 0.002 times as strong an x-ray source as the Crab Nebula.

This SEDS page has a lot more information on the Crab Nebula, both historical and contemporary.

Such an intensively studied object, no wonder there are lots of Universe Today stories on it; for example Nearly a Thousand Years After the Death of a Star, Giant Hubble Mosaic of the Crab Nebula, The Peculiar Pulsar in the Crab Nebula, Astronomers Locate High Energy Emissions from the Crab Nebula, and Evidence of Supernovae Found in Ice Core Sample.

Astronomy Cast’s Neutron Stars and Their Exotic Cousins has more on pulsars, and Nebulae more on nebulae.

Sources: Caltech Astronomy, SEDS, Stanford University SLAC

Chandrasekhar Limit

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When a human puts on too much weight, there is an increased risk of heart attack; when a white dwarf star puts on too much weight (i.e. adds mass), there is the mother of all fatal heart attacks, a supernova explosion. The greatest mass a white dwarf star can have before it goes supernova is called the Chandrasekhar limit, after astrophysicist Subrahmanyan Chandrasekhar, who worked it out in the 1930s. Its value is approx 1.4 sols, or 1.4 times the mass of our Sun (the exact value depends somewhat on the white dwarf’s composition how fast it’s spinning, etc).

White dwarfs are the end of the road for most stars; once they have used up all their available hydrogen ‘fuel’, low mass stars shed their outermost shells to form planetary nebulae, leaving a high density core of carbon, oxygen, and nitrogen (that’s a summary, it’s actually a bit more complicated). The star can’t collapse further because of electron degeneracy pressure, a quantum effect that comes from the fact that electrons are fermions (technically, only two fermions can occupy a given energy state, one spin up and one spin down).

So what happens in the core of a massive star, one whose core weighs in at more than 1.4 sols? As long as the star is still ‘burning’ nuclear fuel – helium, then carbon etc, then neon, then … – the core will not collapse because it is very hot (electron degeneracy pressure won’t hold it up ’cause it’s too massive). But once the core gets to iron, no more burning is possible, and the core will collapse, spectacularly, producing a core collapse supernova.

There is a way a white dwarf can go out with a bang rather than a whimper; by getting a little help from a friend. If the white dwarf has a close binary companion, and if that companion is a giant star, some of the hydrogen in its outer shell may end up on the white dwarf’s surface (there are several ways this can happen). The white dwarf thus adds mass, and every so often the thin hydrogen envelope blows up, and we see a nova. One day, though, the extra mass may put it over the limit, the Chandrasekhar limit … the temperature in its center gets high enough that the carbon ‘ignites’, the ‘flame’ spreads throughout the star, and it becomes a special kind of supernova, a Ia supernova.

For more technical details of the Chandrasekhar limit, Richard Fitzpatrick of the University of Texas at Austin has an online Thermodynamics & Statistical Mechanics course, which includes a page on the Chandrasekhar limit.

Supernovae are very important to astronomy, so you won’t be surprised to learn that there are lots of Universe Today stories on the Chandrasekhar limit! Some examples: White Dwarf Theories Get More Proof, White Dwarf “Close” to Exploding as Supernova, and Colliding White Dwarfs Caused a Powerful Supernova.

Astronomy Cast Episode 90 (The Scientific Method) includes a look at how Chandrasekhar worked out the limit that now bears his name, and Where Do Stars Go When They Die? also covers this topic.

References:
Wikipedia
http://www.bluffton.edu/~bergerd/NSC_111/stars.html

Hunt for Supernovae With Galaxy Zoo

How would you like to find a supernova? I can’t think of anyone who wouldn’t be proud to say they have spotted an exploding star. And now, perhaps you can – and without all the work of setting up your telescope and staying up all night (well, that can be fun, too, but…). The great folks who brought you Galaxy Zoo have now partnered with the Palomar Transient Factory to offer the public a chance to hunt and click for supernovae from the comfort of your own computer. And yes, you can still classify galaxies at Galaxy Zoo, but now you can search for for the big guns out in space, too. Sound like fun?

The Palomar Transient Facory uses the famous Palomar Observatory and the Samuel Oschin 1.2 m telescope to look for anything that’s changing in the sky — whether it’s a variable star, an asteroid moving across the sky, the flickering of an active galaxy’s nucleus or a supernova. For now, though, the partnership with Galaxy Zoo will concentrate on finding supernovae, and in particular Type 1A supernovae.

According to Scott Kardel of the Palomar Observatory, “the quantity and quality of the new data that’s been coming in are absolutely mind blowing for astronomers working in this field. On one recent night PTF patrolled a section of the sky about five times the size of the Big Dipper and found eleven new objects.” For the supernova search, it returns to the same galaxies twice a night, every five nights.

That’s where the Zooites from Galaxy Zoo come in: searching through all specially chosen PTF data and looking for supernovae.

“Your task is to search through the candidates found by PTF” said the Galaxy Zoo team. “Waiting for your results are two intrepid Oxford astronomers, Mark and Sarah, who have travelled out to the Roque de los Muchachos Observatory on the Canary Island of La Palma. They have time allocated on the 4.2m William Herschel Telescope to follow up the best of our discoveries.”

Check out Galaxy Zoo’s Supernova page for more info and to sign up to be part of this exciting new Citizen Science project!

For more info on the Palomar Transient Factory, listen to Scott Kardel’s 365 Days of Astronomy podcast.

Variability in Type 1A Supernovae Has Implications for Studying Dark Energy

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The discovery of dark energy, a mysterious force that is accelerating the expansion of the universe, was based on observations of type 1a supernovae, and these stellar explosions have long been used as “standard candles” for measuring the expansion. But not all type 1A supernovae are created equal. A new study reveals sources of variability in these supernovae, and to accurately probe the nature of dark energy and determine if it is constant or variable over time, scientists will have to find a way to measure cosmic distances with much greater precision than they have in the past.

“As we begin the next generation of cosmology experiments, we will want to use type 1a supernovae as very sensitive measures of distance,” said lead author Daniel Kasen, of a study published in Nature this week. “We know they are not all the same brightness, and we have ways of correcting for that, but we need to know if there are systematic differences that would bias the distance measurements. So this study explored what causes those differences in brightness.”

Kasen and his coauthors–Fritz Röpke of the Max Planck Institute for Astrophysics in Garching, Germany, and Stan Woosley, professor of astronomy and astrophysics at UC Santa Cruz–used supercomputers to run dozens of simulations of type 1a supernovae. The results indicate that much of the diversity observed in these supernovae is due to the chaotic nature of the processes involved and the resulting asymmetry of the explosions.

For the most part, this variability would not produce systematic errors in measurement studies as long as researchers use large numbers of observations and apply the standard corrections, Kasen said. The study did find a small but potentially worrisome effect that could result from systematic differences in the chemical compositions of stars at different times in the history of the universe. But researchers can use the computer models to further characterize this effect and develop corrections for it.

A type 1a supernova occurs when a white dwarf star acquires additional mass by siphoning matter away from a companion star. When it reaches a critical mass–1.4 times the mass of the Sun, packed into an object the size of the Earth–the heat and pressure in the center of the star spark a runaway nuclear fusion reaction, and the white dwarf explodes. Since the initial conditions are about the same in all cases, these supernovae tend to have the same luminosity, and their “light curves” (how the luminosity changes over time) are predictable.

Some are intrinsically brighter than others, but these flare and fade more slowly, and this correlation between the brightness and the width of the light curve allows astronomers to apply a correction to standardize their observations. So astronomers can measure the light curve of a type 1a supernova, calculate its intrinsic brightness, and then determine how far away it is, since the apparent brightness diminishes with distance (just as a candle appears dimmer at a distance than it does up close).

The computer models used to simulate these supernovae in the new study are based on current theoretical understanding of how and where the ignition process begins inside the white dwarf and where it makes the transition from slow-burning combustion to explosive detonation.

The simulations showed that the asymmetry of the explosions is a key factor determining the brightness of type 1a supernovae. “The reason these supernovae are not all the same brightness is closely tied to this breaking of spherical symmetry,” Kasen said.

The dominant source of variability is the synthesis of new elements during the explosions, which is sensitive to differences in the geometry of the first sparks that ignite a thermonuclear runaway in the simmering core of the white dwarf. Nickel-56 is especially important, because the radioactive decay of this unstable isotope creates the afterglow that astronomers are able to observe for months or even years after the explosion.

“The decay of nickel-56 is what powers the light curve. The explosion is over in a matter of seconds, so what we see is the result of how the nickel heats the debris and how the debris radiates light,” Kasen said.

Kasen developed the computer code to simulate this radiative transfer process, using output from the simulated explosions to produce visualizations that can be compared directly to astronomical observations of supernovae.

The good news is that the variability seen in the computer models agrees with observations of type 1a supernovae. “Most importantly, the width and peak luminosity of the light curve are correlated in a way that agrees with what observers have found. So the models are consistent with the observations on which the discovery of dark energy was based,” Woosley said.

Another source of variability is that these asymmetric explosions look different when viewed at different angles. This can account for differences in brightness of as much as 20 percent, Kasen said, but the effect is random and creates scatter in the measurements that can be statistically reduced by observing large numbers of supernovae.

The potential for systematic bias comes primarily from variation in the initial chemical composition of the white dwarf star. Heavier elements are synthesized during supernova explosions, and debris from those explosions is incorporated into new stars. As a result, stars formed recently are likely to contain more heavy elements (higher “metallicity,” in astronomers’ terminology) than stars formed in the distant past.

“That’s the kind of thing we expect to evolve over time, so if you look at distant stars corresponding to much earlier times in the history of the universe, they would tend to have lower metallicity,” Kasen said. “When we calculated the effect of this in our models, we found that the resulting errors in distance measurements would be on the order of 2 percent or less.”

Further studies using computer simulations will enable researchers to characterize the effects of such variations in more detail and limit their impact on future dark-energy experiments, which might require a level of precision that would make errors of 2 percent unacceptable.

Source: EurekAlert