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.