CLASH : the Cluster Lensing And Supernova search with Hubble

In 2010 the Hubble Space Telescope launched three bold new initiatives that came to be called the Multi-Cycle Treasury programs. One was the CANDELS program, the parent of this blog.  Another was the Panchromatic Hubble Andromeda Treasury program (PHAT), a deep and detailed study of the nearby galaxy M31, led by Julianne Dalcanton of the University of Washington. The third program was called CLASH: the Cluster Lensing And Supernova survey with Hubble (tortured acronyms were a prerequisite for approval of the HST time). The CLASH team (not to be confused with The Clash) is led by Marc Postman from the Space Telescope Science Institute, and includes about 50 astronomers at some 25 institutions around the world.  This survey is in many ways a close sister to the CANDELS program, and indeed there is significant overlap across the two groups, especially in the supernova search component, which has been a joint CLASH+CANDELS effort.

  

Galaxy cluster MACS J1206.2-0847 (or MACS 1206 for short) as viewed
through Hubble in the CLASH program. Credit: NASA, ESA,
M. Postman (STScI), and the CLASH Team

The CLASH program takes a deep look at 25 massive galaxy clusters. These are collections of galaxies (a few hundred in each), hot gas (heated to above 10 million degrees), and dark matter (more on that mysterious stuff below). The clusters in the CLASH sample sit at redshifts between about 0.2 and 0.9, so we are seeing them at a fairly recent epoch in terms of cosmic history (the universe was already more than 6 billion years old when the light we see left these clusters). Several of these clusters have been studied in great detail, but the CLASH program has opened up a new window to look in at one of the great mysteries of the universe: the nature of dark matter.

When Hubble looks at a galaxy cluster in the CLASH survey, it captures the ultraviolet, optical, and infrared light emitted by billions upon billions of stars in the many galaxies that live within the cluster. Astronomers have long known, however, that these stars make up only a small fraction of the total contents of these clusters.  Far more important is the hot gas in the Intra-Cluster Medium (ICM). This superheated gas (mostly Hydrogen and Helium) has been stripped away from the galaxies by tidal gravitational forces and the effects of ram pressure. The gas is so hot that it emits x-ray radiation, which can be observed using x-ray observatories like Chandra and XMM-Newton. The mass of gas in a typical galaxy cluster is almost 10 times greater than the total mass of all the stars in all the member galaxies. However, even after counting up all of the stars and gas, we still have only captured about 10% of the total mass of the galaxy cluster.  The other 90% is (presumably) in the form of dark matter.
 

"Dark matter" is the name we assign to all the mass in the universe that does not emit any light. There are a number of theories as to what this dark matter could be, and the most promising idea right now seems to be that it is some form of elementary particle that does not interact with other matter -- except through the force of gravity. In galaxy clusters, we have two primary lines of evidence that reveal the presence of a large concentration of dark matter. First, the motions of the galaxies in the cluster show that there must be a large central mass pulling the galaxies in and through the cluster (more mass than we can account for in stars and gas). Second, we see the effect of the dark matter on background galaxies through gravitational lensing.
 

Einstein's theory of relativity tells us that the force of gravity is in fact a warping of spacetime. This distortion of the fabric of our universe affects all forms of matter -- as we see in the motions of planets, stars and galaxies -- and it also affects light itself. In the CLASH clusters, the warping is sufficiently strong to bend the pathway of light rays passing through the cluster. This results in a lensing effect, as light rays are distorted and redirected such that they focus on our location here in the Milky Way. We see the extraordinary evidence for this lensing in the form of absurdly stretched galaxies, long arcs, and impossibly bright background sources that have been distorted and magnified by the cluster's gravitational lens.
 

The principal aim of the CLASH program is to use these lensing artifacts to construct detailed models of the matter content of each of the 25 clusters. The cluster models are built by piecing together these distorted background sources to make a map of the dark matter that Hubble cannot see. Adding in evidence from the star light and the x-ray gas emission provides a complete picture of all the content in the cluster. With all of this information, the CLASH team has been able to improve our understanding of how these clusters are formed, and even to put new constraints on the nature of the dark matter fluid that dominates the cluster.

The tiny red blob (just a fraction of the size of our Milky Way) is among
the most distant galaxies ever observed. The object is observed just
420 million years after the big bang, and is only visible to the Hubble
Space Telescope due to the magnification from the massive galaxy
cluster MACS0647,  which lies in between us and the distant
proto-galaxy. Credit: NASA, ESA, M. Postman and D. Coe (STScI),
and the CLASH Team 

The gravitational lenses in the CLASH clusters also provide a powerful tool for looking back into the very early universe. The CLASH team has twice discovered objects behind the gravitational clusters that are among the most distant galaxies ever seen, first in April 2012, and then again in November 2012. These very distant background sources would normally be far too faint for even Hubble to see, but the intervening cluster acts like a natural telescope, focusing the light from the far-off galaxies so that Hubble can just barely detect them.

The science work of the CLASH team is still in progress, and we expect many more exciting discoveries are yet to come. Hubble is not done with deep galaxy cluster surveys, either, as the new Frontier Fields initiative has already begun to follow in the footsteps of CLASH. 

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. 

Meet Steve Rodney

From time to time we'll bring you a biographical post introducing one of the astronomers writing for the CANDELS blog.  This week, we introduce Steve Rodney.  You can find his earlier posts here.

View of Hanauma Bay on Oahu.

Currently I'm a postdoctoral researcher and a Hubble fellow at the Johns Hopkins University (JHU) in Baltimore, Maryland. Before landing here on the shores of the Chesapeake, I was a graduate student at the University of Hawaii Institute for Astronomy. Several of my classmates from Hawaii are also part of the CANDELS team: Dale Kocevski, Liz McGrath, and Jeyhan Kartaltepe were all Hawaii grad students while I was there. We're now scattered across the country, but team meetings give us the occasional opportunity to reminisce about shave ice, moonlight surfing, and Andy's sandwich shop. 

Me and the kids in Baltimore.

JHU is a great place to be, and I am very lucky to be working here with some fantastic people… but unfortunately living in Baltimore is a source of some significant environmental stress for me. Not because of the drugs and crime of Baltimore (excellent dramatizations notwithstanding, Baltimore is actually a great city to live in). No, I'm referring to the deep emotional trauma that comes from growing up a fan of the hapless Cleveland Browns, and now watching my 4-year old daughter wearing her Ravens jersey to pre-school. (If you're not an NFL fan, then think of the Ravens vs Browns as Manchester United vs Liverpool, but imagine Liverpool is hopelessly inept over many decades. If you're not a soccer fan either, then... well, nevermind). I can only hope that my daughter will eventually outgrow this phase of moral depravity.

One of the conversations that will come up for most any astronomer from time to time is the discussion of "Why astronomy?" Sometimes this is about personal choices: "Why did you choose to become an astronomer instead of a chemist or a doctor?" For myself, astronomy has always been a study that I was drawn to because of the stories. As a kid, I read the legends of Greek mythology, and was fascinated to find them echoed on the sky in the constellations. I loved to trace the stories of ancient heroes, gods and monsters that were painted across the heavens. Later, the study of physics opened up for me the fascinating stories within the stars, from the birth of a star incubated in a dusty envelope of gas, to the fragile beauty of stellar death throes. Studying astronomy takes these wonderful images and unfolds them to reveal the complex puzzles and deep mysteries of the universe. For me personally, this was the hook that drew me in. 

Cherry blossoms and the Jefferson
Memorial in Washington, D.C.

Sometimes the "why astronomy" question is framed more broadly: "OK, maybe astronomy is great for you, but what practical use does it serve in our society?" A glance at recent Nobel Prizes can illustrate this very legitimate question. Astronomers won a Nobel prize in 2011 for discovering Dark Energy. Exotic and fascinating, but also intangible and wholly disconnected from everyday human lives. In contrast, the recently announced 2012 Nobel prize in chemistry recognizes advances in the understanding of G-protein-coupled receptors (GCPRs). This work is fundamentally connected to drugs used in the treatment of a wide range of ailments, from common allergies and high blood pressure to breast cancer and schizophrenia. One might look at these two fields and legitimately question whether we as a society should be investing so much (both in dollars and in human capital) in the study of distant stars, when there are real problems with real people that can be addressed with other avenues of scientific exploration. 

I think this is an important question, and a conversation that we astronomers and fans of astronomy should have more often and more publicly. Especially in a time of tight budgets for research funding and significant skepticism about the value of science in general, we all should have a coherent argument for why basic research is important. For me, the answer is that the pursuit of understanding is a fundamental quality of humanity. What separates us from other species on this planet is our ability to consider the universe, to seek a deeper understanding of how it works and what is our place within the grand cosmos. In a way, astronomy is a bit like poetry, art and music. It is a discipline that provides its own reward by enriching our lives. We astronomers should never forget that it is a special privilege to be able to devote ourselves to this task, and that we have a responsibility to share what we learn with the world around us. 

Sunrise over Maui, viewed from Makapu'u point on Oahu.

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.