A New Type Ia Supernova in M82

Just a couple of days ago, a dim, but quickly brightening, supernova was discovered in M82, the beautiful "cigar galaxy." At "only" 12 million light years away, this is the nearest supernova to Earth since 1987 and the nearest Type Ia supernova since 1972. With the enormous changes in our imaging technology since then (including the launch and subsequent improvements to the Hubble Space Telescope), this is a fantastic opportunity for precision measurements of one of the brightest and most mysterious explosions in the universe.

The new supernova in M82, discovered by students at the University College London
Observatory.  Photo by Adam Block/Mount Lemmon SkyCenter/University of Arizona

Discovering more about the nature of Type Ia supernovae has been one of the primary goals of the CANDELS project. These supernovae begin as stars like our sun, which have shed their outer layers at the end of their lives and become white dwarfs. White dwarfs are the extremely dense cores of a burned-out star, and although they're only the size of our earth, they have the mass of our entire sun.  The detonation happens when a nearby star adds even more mass onto this dwarf -- when the weight becomes too much, nuclear fusion ignites it and a supernova occurs.

In CANDELS, we study the most distant Type Ia supernovae that we can find, the farthest of which stands at over 10 billion light years away. Our supernovae tell us about the early expansion of the universe (and its Dark Energy), the chemical evolution of the universe, and how quickly supernovae form and explode around 8-10 billion years ago -- at the peak of star formation in the universe.

This nearby galaxy offers a completely different, and rarer, perspective. In 1972, when the last Type Ia supernova this close to Earth exploded, it was still a year before anyone proposed the idea that these supernovae were formed in binary star systems. It was 12 years before someone realized that both stars could be white dwarfs, and 18 years before supernovae could be studied from space with the Hubble Space Telescope. It was over 25 years before such supernovae were used to discover that Dark Energy was accelerating the expansion of our universe.

Motivated by the knowledge and technology gained since the last close Type Ia supernova went off, scientists will be asking an entirely different set of questions this time around. First, we'll be looking for a giant companion star that could have fed mass onto the white dwarf. If a companion star is visible, this would be the first direct evidence that a system with one white dwarf can lead to a supernova; if a companion star is not found, the theory that two white dwarfs can make a Type Ia supernova will gain credibility.

Artist's conception of the single-degenerate (one white dwarf)
theory of Type Ia supernova explosions, wherein
a white dwarf accretes mass from its companion
star.  (original) © ESA and Justyn Maund (Queens Univ. Belfast)

Artist's conception of the double-degenerate theory
of Type Ia supernova explosions, in which two white dwarfs merge
together as they emit gravitational waves. (original) © NASA,
Tod Strohmayer (GSFC),  and Dana Berry (Chandra X-ray Observatory)

Second, scientists will be studying the geometry of the supernova from the fraction of polarized light emitted. Polarization, the orientation of a light ray's electric field, is entirely random when it originates from a spherically symmetric star. However, if one side becomes longer than the other, the light's polarization will have a preferential direction that can be measured on Earth. As the outer layers of the M82 supernova expand, they will become transparent and expose the inner material. Over the next month, scientists will be able to measure the shape of different layers and examine the three-dimensional explosion. With this structural information, we'll learn more about how supernova detonation occurs; specifically, how nuclear fusion begins and spreads through the layers of the white dwarf.

The location of M82 on the night sky from Sky and Telescope.
A more detailed chart is available here

Lastly, Type Ia supernovae are nearly uniform in brightness, serving as excellent distance indicators for most of the visible universe. CANDELS supernova principal investigator Adam Riess -- among others -- will be measuring the distance and doppler shift velocity (the reddening of its light) of this supernova to determine how fast the local universe is expanding -- and infer the amount of the mysterious Dark Energy that surrounds us.

This supernova is particularly rare in that it offers opportunities not only to scientists, but for anyone with access to a dark night sky. It will brighten for approximately a week and a half, and at its peak it will be visible near Ursa Major (the Big Dipper) to anyone with a set of binoculars. Although it's impossible to predict when the next close supernova will be, I'm looking forward to seeing an exploding star with my own eyes - it may be 40 years before there's another opportunity.

Rapid Response Astronomy

Nothing in astronomy ever changes.  

That is to say, nearly every thing we study in astronomy is effectively unchanging. The stars and galaxies we look at through our telescopes are just about as constant and eternal as you can get. Even a massive star with a "fast" life cycle takes millions of years to exhibit any visible change, far longer than the time available to human observers. So for most astronomical observations we can really take our time; there's never any hurry to catch a galaxy or a cluster of stars before it disappears.

For an observatory like the Hubble Space Telescope, this means that most of the observations being done are fully designed and scheduled well in advance. The typical process for observing with HST is spread over many months. First, astronomers prepare a proposal describing the science they want to do, and how they'll use HST to do it.  These are submitted each year around the first week of March. Then in May a panel of volunteer astronomers is gathered in Baltimore at the Space Telescope Science Institute (STScI) to review all the proposals and select the ones that will be awarded time on HST. The successful proposers then go through another round of preparation, where they pin down the details of exactly how the observations will be done. The observations can happen anytime over the next year or so, and special large programs like CANDELS get spread out over multiple years. 

The Hubble Space Telescope.
 Image Credit:  NASA, Z. Levay

The specifics of when HST actually collects the data are hammered out by a dedicated team of STScI research support staff. These program coordinators and calendar builders have the job of piecing together the puzzle of many hundreds of different HST observations. Each observation has a unique set of constraints to consider: When is the target visible? What does the rotation of the telescope need to be? Are there bright stars near the target that HST can use to lock its position? Each week the calendar builders balance these competing requirements and put together a very detailed schedule for exactly what HST will do two weeks in the future. Efficiently packing and organizing those observations is a big task, and one with real significance. Astronomers and telescope operators know that every observation with HST is a precious resource, and represents a substantial investment in this science. The total cost of HST divided by its lifetime works out to about $15 per second, or $54,000 per hour. All the careful advance planning is really critical for maximizing the science return from that investment. 

For CANDELS, however, we can't plan out all of our observations many months in advance. One of our primary science goals is to detect and analyze distant supernovae: stars reaching the end of their life-cycle with a violent explosion. The explosion itself occurs without warning in a fraction of a second, and we can observe the after-glow for weeks and months afterward. There is no way to predict when and where these explosions may appear, but when we do spot one, we often need to quickly mobilize HST for follow-up observations, while the supernova is still bright enough to see. For this type of object, the HST operators allow a special mode for submitting observation plans, called the Target Of Opportunity (ToO) mode.

Here's how it works:

When we discover a new supernova of interest, like the record-setting SN Wilson, we sift through all the available data and decide that we want to get a quick follow-up observation, maybe as soon as next week. We quickly contact our program coordinators at STScI and tell them that we're going to trigger a ToO observation. Then we plan out the observations and submit them for review. To make room for our new ToO supernova, the calendar builders then pull out some of the pre-planned observations from other programs (they'll get put back in sometime later in the year). 

The bright star in the lower left is SN 1994D
in the galaxy NGC 4526.
Image Credit: High-z SN Search Team, HST, NASA

With experience and good organization, the detailed observing plan for a new SN can be arranged in a few hours - but sometimes we only have a few hours to spare. For most ground-based observatories, normal ToO observations can be slotted in on the same night that a supernova is discovered. For HST, however, it is much more complex and risky to make sudden changes, so each week the HST schedule gets locked in place on Wednesday morning. We need to give the program coordinators and calendar builders at least 4-5 hours to process a new observation, so that means that we have a weekly deadline of 12 noon each Tuesday for any new ToO interruptions.   

This brings us to the peculiar situation that if we happen to discover a new supernova on a Wednesday, then we have almost a week to leisurely examine the data and decide whether it warrants a ToO trigger. If that same supernova is found on a Monday, though, we are scrambling to get all our decisions made and plans in place before the Tuesday noon deadline. This can lead to some long hours on Monday nights when CANDELS observations are coming down from HST - but its exciting and rare to get any kind of astronomy in rapid action. We supernova hunters really appreciate what a unique privilege it is to get to push around an orbiting space telescope at the last minute when our science requires it. 

CANDELS Finds the Most Distant Type Ia Supernova Yet Observed

Image credit: NASA

A couple weeks ago, the supernova science team from CANDELS was pleased to announce the discovery of the most distant Type Ia supernova ever seen at more than 10 billion light years away, a time when the universe was only about 3.5 billion years old. There wasn't ever much doubt that supernovae existed more than 10 billion years ago -- but it's still an exciting moment for us when we're able to photograph one with the Hubble Space Telescope. The image on the right shows the supernova's location in one of the five fields that CANDELS is searching. Almost every single dot in the upper panel is a distant galaxy, filled with billions of stars. Our team searches through these images every couple of months, hoping that one of these stars has exploded.

But ironically, the most exciting thing about this supernova for many of us is that we haven't seen more. In terms of gaining an understanding of how these objects are created, its really the lack of supernovae in the early universe that is the most telling observation. The Hubble has the power to observe one of these objects from more than 11 billion light years away, and we've been staring at the sky in search of them for nearly 3 years. So far, we've found only a single confirmed Type Ia supernova from 10-11 billion years ago -- a billion years of cosmic history.

In astronomy, our uncertainty about the universe is somewhat striking. 70% of the Universe is the mysterious "dark energy", completely unknown until about 15 years ago. Another 25% is dark matter -- also a mystery, since we only observe its gravitational effects. This leaves 5% of the universe that is composed of matter more familiar to us. In the context of this uncertainty, the supernova progenitor mystery might not seem so fundamental. And yet, every second, about 5-10 supernovae are going off somewhere in the observable universe with 10 billion times the brightness of our sun. These supernovae produce about half of the iron in the universe, some of the raw material for creating planets like the Earth. And the ways that these explosions happen -- both the stellar evolutionary steps that lead to the explosion and the physics of the detonation itself -- remain mysterious.

The composition of the universe.  Only 5% of the universe is normal matter, and only .03% of the universe consists of the heavier elements we're most familiar with on Earth.
Image source: http://www.lsst.org/lsst/public/dark_energy

This dearth of supernovae must be telling us something fundamental about the nature of these objects -- so what is it? Finding the earliest supernovae sheds light on this puzzle in a couple of different ways because fundamentally, the early universe was a very different place than the universe we live in today. There were fewer heavy elements, stars were younger, and galaxies were producing stars at a faster rate than they are today. The way in which supernovae form in this environment tells us something about their nature.

First of all, if supernovae are formed from younger stars, do they explode in the same way? Some theorists think that the answer is no -- it's predicted that only more massive stars have time to become supernovae within the first 3-4 billion years of the Universe, and nuclear fusion in more massive stars will result in a different blend of heavy elements in the core.  When the star's remnant (called a white dwarf) gains matter from a binary companion and explodes, its brightness is powered by the radioactive decay of elements fused in the explosion; a different chemical composition results in a different brightness. We're looking for such a change in brightness -- it can tell us a bit more about how supernovae are created.

Secondly, if a burst of star formation occurs in a galaxy, how long will it be before the Type Ia supernovae start exploding? How quickly stars evolve and form supernovae can tell us a lot. If most Type Ia supernovae occur when two white dwarfs form in a binary system and then slowly merge together, the time it takes them to explode will be based on the distribution of initial separations that binary stars are born with. If, on the other hand, they tend to occur when a normal or giant star is slowly pouring mass onto a white dwarf at a given speed, one might expect something closer to a single characteristic time from formation to explosion. By knowing when most of the stars formed in the universe, and observing how quickly supernovae are exploding at different ages of the universe, we can determine which model for supernovae is correct. This knowledge may also make it easier to understand the physics of a white dwarf's detonation.

All of this work can be tied back to the fact that Type Ia supernovae are not just interesting in and of themselves; they are cosmological tools with a characteristic brightness that can be used to set a distance scale for the Universe. Understanding better how supernovae explode can be used to answer questions like: why are supernovae dimmer in smaller galaxies? What process creates unpredictable and unusual supernovae like Type Iax? How can we better calibrate these tools to learn more about dark energy?

These are some of the questions that our team hopes to answer in the coming years. In the meantime, we're searching the night sky for exploding stars -- and each time we find one, it tells us a bit more about the universe we live in.

Supernovae Part II – Supernova Environments

In a previous post, we introduced readers to the exciting field of supernovae. Today we are going to delve a little deeper and discuss the environments where supernovae explode. These environments are interesting because they can provide information about the nature of the stellar systems that produce supernovae.  This can help us understand how and why some of the brightest events in the universe are produced.  

Type Ia supernovae are known as “standardizable candles”. Without going too deep into the science, a "standard candle" is an object whose observed brightness only depends on how far away it is. If you hold a flashlight 5 feet from your eye, it looks much brighter to you than if you hold it 100 feet away. By knowing the difference in brightness, you can estimate the distance to the flashlight. Now picture a flashlight with dying batteries. If you only see the flashlight far away, and have no idea how bright it would be up close, you can't estimate the distance. But if you somehow know how much power is left in the batteries, you can use that information to correct your distance estimate. In a nutshell, this is a "standardizable candle". The distance to a "standardizable candle" is not only based on it's observed brightness, but also on intrinsic properties of the object. For supernovae, what this means is that by measuring certain properties of the explosion through our observations, we can estimate how bright it should be if it were nearby, and therefore we can calculate how far away it is. 

In one of the most astounding tenets of astronomy, looking at objects that are farther away is analogous to looking back in time, because the speed of light is finite. It takes a long time for the light from these supernovae and their host galaxies to reach us. Finding supernovae that are farther away therefore allows us to trace the history of the expansion of the universe. The 2011 Nobel Prize in Physics was awarded for the role of type Ia supernovae in discovering dark energy, the mysterious force that is driving the acceleration of the expansion of the universe. Several CANDELS team members played key roles in the discovery, and team member Adam Riess was one of the prize recipients.

Recent research suggests that the brightness (and therefore our estimated distance) of a type Ia supernova depends in some way on the properties of it’s host galaxy. This suggests that the explosion mechanism for type Ia supernovae depends in some way on the surrounding environment. In the CANDELS Supernova Survey, we are searching for supernovae from a time when the universe was only 3 or 4 billion years old (we have measured the age of the universe to be around 14 billion years old). Galaxies from the early universe do not look like our own Milky Way galaxy; they are smaller, bluer (because young, hot stars are blue), and less polluted with heavier elements such as iron which are produced in the interior of stars and in supernovae. Studying the environments of these very far away supernovae is therefore important for tracing the expansion history, but also in our basic understanding of the systems that produce type Ia supernovae.

In the rest of this post, we will show some of the diverse galaxies that supernovae have been discovered in by the CANDELS supernova team.

CANDELS images of supernova host galaxies: images a), b), c), e) and f) are 0.004 degrees on each side. Image d) is 0.008 degrees on each side. For reference each image is approximately the size of a US quarter dollar viewed at 1/4 of a mile away.

a) This galaxy is one of the most nearby supernova hosts that we’ve discovered in CANDELS. The light that reached the Hubble Space Telescope to produce this image was emitted around 2 billion years ago. It is a fairly typical “edge-on” galaxy; we are viewing the galaxy right along the plane of the disk. If you look closely, this galaxy has a faintly visible dust lane; this material blocks the light emitted behind it and is therefore slightly darker.

b) This is a similar looking galaxy, but slightly farther away. The light in this image was emitted about 4 billion years ago.

c) Likely a similar shape galaxy to the previous two galaxies, however this one is rotated 90 degrees so that we’re viewing it from a very different angle. This galaxy is also quite a bit farther away; the light you are looking at was emitted 7 billion years ago! The blue clumps are likely bright, blue, massive, young stars, indicating significant recent star-formation. The supernova that went off in this galaxy may be a core-collapse supernova (core-collapse supernovae are much more likely to be found in blue regions like the ones in this image) instead of a type Ia supernova. A core-collapse supernova is the death of a massive star; when the star runs out of nuclear fuel, the outer regions of the star collapse onto a very dense, compact core, and the rebound of this material causes the explosion that we observe. The CANDELS supernova team is still working on the classification of all of our discovered supernovae.

d) Don’t be distracted by the galaxies to the left; the compact object in the center is the host galaxy of a supernova that exploded 6 billion years ago. This galaxy is observationally very different from the first 3 hosts. It is likely much less massive, and though it is difficult to draw conclusions about current star formation from just this image, it is not very blue, implying that it contains alot of fairly old stars.

e) Again not to be distracted by surrounding objects, the faint blue galaxy in the center of this image hosted a supernova 7 billion years ago. The size and shape of this galaxy is similar to d), but this galaxy is much more blue. It is highly likely that this galaxy has had more recent star formation.

f) These three galaxies all appear to be at the same distance from Earth, and so we believe they may be interacting, merging together at some point to form a larger galaxy. We are seeing them as they were about 8 billion years ago. A supernova that exploded in one of these galaxies is very likely a type Ia, and the environment appears different from some of the nearby type Ia supernovae. 

The CANDELS supernova team is hard at work classifying and analyzing the supernovae that we are discovering. You can see the diverse nature of galaxies that host supernovae; while star-forming (blue) galaxies are more likely to host a core-collapse supernova than old, red galaxies, this does not mean that type Ia supernovae cannot explode in blue galaxies. There are many questions that remain to be answered regarding the role of the host galaxy environment on type Ia supernova explosions. Understanding the environment of each supernova will be an important tool in constraining the nature of type Ia supernova progenitors. As the supernova environment may evolve as we discover them farther and farther away, understanding the role that the host galaxy plays in our distance estimate is a goal of the CANDELS supernova team.

Supernova Hunting

Somewhere in the observable universe, a star is exploding right now. Actually, something like 30 stars are exploding right this second, adding up to 2.5 million supernovae each day. That may sound like a ridiculously high number of exploding stars (If the universe is popping off supernovae so fast, then how do we have any stars left!?). Lets see if we can unpack it a bit. 

An average galaxy like our own produces roughly one supernova per century (I'll explain where this number comes from below). There are roughly 100 billion galaxies near enough to be observed by the Hubble Space Telescope (HST). If each of those observable galaxies gives us one supernova each century, then we expect about 100 billion supernovae every hundred years. One century is equal to about 3.15 billion seconds (that's about π x 10⁹ seconds per year, as a handy way to remember it). So we divide those 100 billion supernovae over 3 billion seconds, and get roughly 30 supernovae per second.

August, 2010 (pre-Supernova) 

With so many supernovae blinking on every night, it is actually not too hard to find one of these objects. The three-step process is simple:

1. take a picture of the sky 

2. wait a few days or weeks, and take another picture

3. look for any new "stars" that weren't in the first picture

October, 2010 (see anything new?)

Subtracting off the August image
reveals the newly arrived SN Primo.

Stars and galaxies don't appear or disappear on the timescale of weeks (or years or centuries...) so there are very few astronomical objects that can appear so suddenly in between two pairs of images like that. Fast moving objects (like asteroids and comets) might move into your frame, but these are easy to sort out: take a third picture and you'll see that they keep moving. Anything that blinks on, stays in place, and then shows a steady rise and fall in brightness is most probably a supernova. The figure below shows two infrared images from HST.  The first was taken in early August, 2010, and the second was taken two months later, in October, 2010. The third image shows what happens when we subtract off the September picture: all the galaxies and stars are unchanged, so they get subtracted cleanly away, and we're left with just one new star. This particular supernova was the first one discovered in the CANDELS survey. Nicknamed "SN Primo,"  it is currently the most distant supernova of its kind. SN Primo and other stellar explosions we find with CANDELS will eventually be used to measure distances in the universe, helping us to understand the nature of the mysterious dark energy that is driving the accelerated expansion of space.

Supernova hunting is not limited to the professional astronomers with access to multi-million dollar observatories. Unlike many areas of physics, dedicated amateurs can and do make significant contributions to astronomy - especially in this sub-field of supernova science. The renowned Australian amateur Robert Evans has discovered over 40 supernovae himself, primarily using his own visual memory of the sky.  Lets take a moment to consider that, because this is really quite extraordinary: Rev. Evans was able to discover dozens of supernovae without using any of the careful image subtraction that astronomers rely on. He simply scanned the sky each night with his telescope, and looked for the single new pinpoint of light around a familiar galaxy that signals the death of another star and the start of a new supernova. We professional astronomers didn't get to be as efficient as Evans until the advent of robotic telescopes in the mid 90's.

Young amateurs are in on the supernova hunt, too. The unique object SN 2008ha was discovered by 14-year old Caroline Moore in upstate New York. In recent years this object has become a prototype for a whole new class of supernovae, which are still puzzling astronomers today. Alas, Caroline's record as the youngest person to find a supernova didn't last too long:  two years later SN 2010lt was discovered by Kathryn Gray, a 10 year old girl from Fredericton, New Brunswick in Canada.  

SN 2008ha was discovered by a 14-year-old amateur, and
astronomers now believe it to be the prototype of a new 
class of supernovae.  This picture was taken with the 2.2m
Telescope of the Calar Alto Observatory in southern Spain.
Image credit: Stefan Taubenberger, MPA

So there are 30 new supernovae every second, and we've got world-class telescopes and dedicated backyard astronomers on the hunt... but unfortunately we still don't actually see most of those supernovae. Some fraction are screened by dust, or hidden behind millions of other stars in the bright cores of their host galaxies. But most of the easily observable supernovae are missed simply because we aren't looking for them. To catch them all, we'd need a few million telescopes like HST observing every corner of the sky every day around the clock. We'll never have that, but there are some exciting new telescopes on the ground that can observe the sky much more efficiently than HST - although they don't go as deep or as distant. Amid the alphabet soup of astronomical acronyms, there's Pan-STARRS, PTF, and LSST, just to name a few. Eventually these wide-field surveys will really clean up in the local universe, detecting basically all the nearby supernova explosions.

This brings us back to the question of how do we know just how many supernovae are exploding each second. One critical piece of information is the rate of supernova explosions in an average galaxy. I stated at the top that this rate is about one supernova per century in a galaxy like our own Milky Way. We could measure that number by observing our own galaxy over a century and counting up the number of supernova explosions. That is painfully slow, and rather imprecise, but we can do effectively the same thing by watching a hundred galaxies for one year. But why stop there? It's far better to watch thousands or tens of thousands of galaxies over several years. Then we count up a large number of supernova detections, divide by the number of galaxies and the number of years and come up with the observed rate of one supernova per galaxy per century.   

This is precisely what we are doing with the CANDELS supernova survey - but with an important twist. The other wide-field surveys I mentioned above (like Pan-STARRS and PTF) are observing many thousands of galaxies each night, but they are limited to (relatively) nearby galaxies that are bright enough to observe in short exposures from the ground. The unique difference in the CANDELS survey is that we use very deep infrared imaging from HST. This allows us to look to higher redshifts (farther back in time) and catch supernova explosions within very distant galaxies in the early universe. Right now, our HST survey is the only program able to measure the supernova rate at a time when the universe was only about 3 billion years old. We can compare that observed rate from the early universe with the observed rate in the present-day universe to learn something about how the supernova population has changed. Do these early universe supernovae look the same as local supernovae? Are they exploding at the same rate as they do locally? These are the first questions that we're beginning to address with the CANDELS supernova program, and we hope the answers will help us understand more about these extraordinary events. 

Supernovae

For most stars, death is not so much an event as it is a process. A typical star will swell and contract as its nuclear fuels are gradually depleted, eventually shedding its outer layers into a gently expanding shell. The stellar core will be left behind as a pale remnant that fades into a dark, cool white dwarf. It makes a lovely display but doesn’t have much impact beyond its immediate surroundings (the death of a star is not good for any orbiting planets, but other nearby stars would hardly notice). A prominent minority of stars, however, will end their evolution in spectacular fashion with an explosion that can be observed from across the cosmos. This is a supernova: a powerful stellar explosion that can briefly outshine all the light from all the other stars in an entire galaxy.

Hubble Space Telescope image of Supernova 1994D in 
galaxy NGC 4526.  The supernova - visible in the lower 
left of the image - appeared in the outskirts of this dusty spiral 
galaxy, outshining millions of stars in the galaxy core. (original)
©NASA/ESA, The Hubble Key Project Team,
and the High-z Supernova Search Team

For decades, studying these spectacular events has led us to extraordinary advances in our understanding of the universe. Most recently, supernovae have made headlines with the award of the 2011 Nobel Prize in Physics "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae" (a.k.a. dark energy). Our own colleague and the head of the CANDELS supernova team, Adam Riess, shares that prize with astronomers Saul Perlmutter and Brian Schmidt.

In today's post I'll first sketch out some recipes for how to brew a supernova. Then I'll scratch the surface of modern supernova science, describing how supernovae play three important roles in the astronomer's toolbox: as laboratories, factories and light houses. In future posts I'll come back to say a bit more about how CANDELS supernova discoveries are helping us better understand supernovae, dark energy and the universe.

How to Brew a Supernova

Astronomers divide supernova explosions into two broad categories. The first set includes several flavors of supernovae resulting from the death of giant stars, called core collapse supernovae. The second set I will call white dwarf supernovae, referred to by astronomers as Type Ia ("Type one-A") supernovae. These are extremely useful for cosmology - more on that below. If you are setting out to make a supernova explosion, these two categories require two very different approaches.

Making a core collapse supernova is relatively simple. All you need is a very massive star.  At least 8 times the mass of the sun, and the more mass you pile on the more interesting it gets. Let's suppose you want to see some real fireworks, and go with 20 times the mass of the sun. This star of yours will age very rapidly (in astronomical terms), requiring only about 10 million years from cradle to grave (for contrast, our own sun is now ~5 billion years old, and will go on essentially unchanged for about another 5 billion more).  At birth, this big baby of yours operates much like our own sun (on steroids), with a powerful nuclear fusion engine in its core, burning up hydrogen atoms and turning them into helium. After 8 million years your star runs out of hydrogen and has to start burning helium instead, producing an "ash" of oxygen and carbon. That will keep it going for another million years, until it runs out of helium and has to start burning carbon. The carbon stage lasts only about a thousand years, before your star turns in rapid succession to neon, then oxygen and then silicon. Finally, after burning silicon into iron (for only about two weeks) your star hits the end of the road:  it cannot produce energy by fusing iron together, so the central engine of nuclear fusion fails. The core cools down, the outer layers start to collapse, and the whole star falls in on itself. The core itself has already been compacted into a dense, nearly incompressible sphere, so when the loose gas from above the core falls onto that hard surface... it bounces. That bounce sets off the supernova explosion, tearing off the outer layers and lighting them up with a glow that we can see from billions of light years away. (There's a lot of interesting and controversial physics I'm glossing over here. For example, we don't know precisely how the energy of collapse gets transformed into the energy of explosion. )

Artist's conception of a possible pre-supernova binary star
system: a white dwarf cannibalizing its giant stellar companion.
(original) © ESA and Justyn Maund (Queens Univ. Belfast)

Another artistic impression: two white dwarf stars spiraling
in toward a collision, emitting gravitational waves as the
orbit decays.  (original) © NASA, Tod Strohmayer (GSFC),
and Dana Berry (Chandra X-ray Observatory)

Now, the other option for your home supernova construction kit is a Type Ia, or white dwarf supernova.  Here the recipe is not so clear. We know that you need a binary star system, with two stars locked in a close orbit.  One of these stars must be a very dense white dwarf star: as massive as the sun, but much cooler, and as small as the Earth. Somehow this white dwarf star has to steal a lot of mass from its companion. This could happen by slow accretion: over millions of years the white dwarf slowly cannibalizes its neighbor, swallowing gas from the outer layers and engorging itself. Or it could happen with an orbital death-spiral: the companion star is another white dwarf and the two are locked in a decaying orbit, dancing closer and closer as they lose orbital energy through gravitational waves until eventually they coalesce and merge. We don't yet have any clear evidence which of these two scenarios is correct (perhaps they both occur).   Regardless, the end result is a white dwarf that has acquired more mass than it can handle. It is already too dense for further collapse, so instead it heats up rapidly, reaching a temperature where it can suddenly ignite thermonuclear fusion of carbon atoms. This compact star can't handle the sudden rush of new energy, so it sets off a thermonuclear explosion that ignites the whole star like an atomic bomb.

Now that we know (more or less) how to make a supernova, what can we do with them?   Supernovae play three important roles in the astronomer's toolkit:

1. The Stellar Lab

An entomologist who wants to know how an insect breathes can go catch some insects, open them up and examine their parts. An astronomer who wants to know how the interior of a star works does not have the luxury of slicing it open to peer inside. Instead, we have to make do with the laboratories that the universe has provided for us. Supernovae make exceptional stellar labs, as they very obligingly open themselves up, spewing out a wealth of information about their interiors that becomes accessible to us. We study the changing light of the explosion and the expanding shell of ejected material, measuring the speed, shape, color and content. Comparing these observations to computer models can tell us about the star's pre-explosion structure and its life cycle. Each supernova gives us a truly unique lab for learning about the physics of nuclear fusion, explosions, and energy transport.

2. The Atomic Factory

All stars have at their core a nuclear furnace, steadily burning hydrogen into helium and eventually making some heavier elements such as carbon, nitrogen and oxygen. Those heavier elements are extremely useful to have around if you ever want to construct a planet, especially one with things (carbon) that breath air (oxygen and nitrogen) and drink water (hydrogen and oxygen). The vast majority of stars, however, are extremely stingy about releasing their elements. The heavy elements are all created deep in the stellar core, and in a typical star like our sun that core remains intact as the star slowly dies. After spending billions of years constructing those precious carbon atoms, they all end up trapped inside a cold fading core for the rest of the life of the universe.

Supernovae, however, have much more powerful nuclear furnaces - especially during the explosion. They are able make many more interesting elements, going well beyond carbon and oxygen to produce everything else in the periodic table: gold, silver, nickel, plutonium, etc. What's more, the supernova explosion sends those elements out into empty space, polluting the cosmos with a spray of atoms. Eventually those little bits of supernova stuff will cool and settle down, and some of it will coalesce into new stars and form planets with small curious creatures who read and write blogs. This is basically the only mechanism that our universe has for generating and distributing the heavy elements that form the building blocks of planets and life. As Carl Sagan was fond of saying: "we are all star stuff."

3. The Cosmic Light House

Theoretical physicists have crafted some wonderful and exotic models of the universe, and it is nice to test those from time to time. One of the best methods for testing cosmological models is to measure distances to far-away objects and map out the geometry of the observable universe. To do this, one can use a tool that we call a "standard candle": some class of objects that all have the same intrinsic brightness. If you see a faint star in the sky, you can't know at a glance if it is nearby and naturally dim (like a firefly), or if it is actually quite bright, but appears faint because it is very far away (like a distant light house). For astronomers, when we observe a standard candle that appears faint, we can immediately determine its distance because we know already that it's a light house, not a firefly.

It happens that white dwarf supernovae (Type Ia) are excellent standard candles. They all have very similar intrinsic brightness, and they also happen to be extremely bright, so we can find them at great distances. This characteristic is what enabled the 2011 Nobel laureates and their collaborators to discover dark energy in 1998. They measured the brightnesses of distant supernovae and found them to be fainter than expected, unless they introduced this peculiar accelerating expansion, driven by an unknown and unseen force. In a future post I'll come back to explain how the CANDELS supernova team is now pushing these supernova discoveries out to record distances, finding these cosmic light houses at distances of more than 9 billion light years.