You may be surprised by how much we have learned about life on Earth by observing some of the most distant objects we find in the universe: supernovae. These are the brief, but brilliant, explosions that end the life of certain types of stars and can outshine the collected light from the hundreds of billions of living stars in their home galaxy. Incredibly, these explosions have governed the history of life on Earth and challenged and informed the most basic tenets of our understanding of the universe for millennia. Revolutions in astronomical technology are providing us with a whole new understanding of supernovae and the things that they influence — which, it turns out, is nearly everything in the universe!
The Elements of Supernovae
We couldn’t live without supernovae. The initial result of the Big Bang was a universe composed almost entirely of hydrogen and helium, the two lightest chemical elements . However, life relies on a complex system of chemical reactions based on heavier elements, such as carbon and oxygen. Heavier elements can be formed from lighter ones by nucleosynthetic chain reactions, where the nuclei of atoms fuse together to form heavier elements. But these reactions don’t just happen anywhere. The nuclei of atoms are filled with positively charged particles called protons, which naturally repel each other. Atoms will only have enough energy to overcome this repulsion and undergo nuclear fusion if they experience extremely high temperature and pressure.
The dense, hot core of a star is therefore an ideal environment for nucleosynthetic reactions to take place. Indeed, stars live off the energy produced by the fusion of lighter elements into heavier ones, but those fusion products usually stay trapped in the star’s core for billions of years. However, they can be released quickly if the star explodes as a supernova. In addition, there is another set of elements that are produced only during a supernova by nucleosynthetic processes unique to those extreme conditions.
Core collapse supernovae shaped our world
“Core collapse” supernovae, the death throes of stars larger than about eight times the mass of our sun, are the primary source of the elements necessary for life on Earth. This is the most common type of supernovae, and they start exploding soon after the first stars in a new galaxy form. This is because very massive stars generally live far shorter lives than stars like our sun, which will live about ten billion years. Consequently, core collapse supernovae flooded the galaxy with the building blocks for Earth long before our solar system formed. However, having too many supernovae is not good for life. A planet sitting in a region of very active star formation, where many massive stars are forming and exploding, may be bombarded by too much high-energy radiation for life to survive. This balance of factors helps determine where and when life can arise in a galaxy.
The history of a galaxy can be viewed as a gas feedback cycle, in which supernovae play a prominent role. In this cycle, new stars and planetary systems are constantly forming from the debris of dead ones. Because supernovae enrich the gas by injecting nucleosynthesis products, they change the properties of the next generation of stars. Although the massive stars that produce core collapse supernovae are relatively rare, these stars are especially important because they violently eject their debris at many miles per second. The winds from these explosions can accelerate star formation by forcing gas together. They may also disrupt or delay star formation by dispelling pockets of gas that would otherwise collapse into stars.
Supernovae explosions have dramatic effects on the formation of dwarf galaxies as well as on larger galaxies like ours. Watch how supernovae eruptions constantly reshape the gas distribution in this simulated dwarf.
Dark Energy From Bright Explosions
Observations of supernovae have played a remarkable role in our understanding of how the universe changes over time (a field known as cosmology). Until the advent of modern astronomical techniques in the 20th century, only a handful of supernovae were bright enough to be observed. The first recorded supernova arguably dates back to a sighting in China in 185 CE , and later supernova sightings had profound effects on the development of Western culture. To a society convinced by Aristotle that the celestial sphere was immaculate, eternal, and unchanging, the appearance of a “new” (nova) star in the heavens was deeply shocking. In fact, Tycho Brahe’s observations of a supernova in 1572 are often given significant credit for discrediting the Aristotelian world-view and setting the stage for the Scientific Revolution .
|The remnant of the supernova Tycho Brahe observed in 1572 taken by the Chandra X-Ray Space Telescope. The remnant is a spherical gas shell >1000 times larger in radius than our solar system, superheated to tens of millions of degrees, and expanding at thousands of miles per second!|
In the 1920s, the American astronomer Edwin Hubble made the next giant leap in cosmology. He observed stellar pulsations: the dramatic changes in the size and temperature of certain stars over time . Among pulsating stars, some are called standard candles because we can use their pattern of pulsation to predict the true brightness of the star. Comparing this estimate of the star’s intrinsic brightness to the brightness we observe tells us our distance from the star. Using the Doppler effect, Hubble also determined the speed at which the stars were moving toward or away from us. A star moving away from us looks redder than it truly is, just as the pitch of a police siren deepens as it passes due to the wave nature of both light and sound. Using the speed and distance estimates of pulsating stars, Hubble demonstrated that there are other galaxies beyond the Milky Way, thereby ending decades of debate. Moreover, he found something completely unexpected: the farther away a galaxy is from us, the faster it moves away! He concluded that the universe itself is expanding.
An equally momentous cosmological discovery came by observing Type Ia supernovae. These supernovae can also be used as standard candles because we can predict their brightness from the rate at which they fade away. Figuring out why these explosions are so predictable and what type of stars produce them is an exciting field of research. Most signs point to white dwarfs, which are the tiny cores left over by small stars like our sun when they die. White dwarfs could sit quietly forever, unless something causes them to become so massive they collapse on themselves. The leading theories are that they have siphoned off material from a companion star or collided with another white dwarf .
In 1998, two groups of astronomers studying type Ia supernovae discovered that the universe is not only expanding, but that something is causing the rate of expansion to accelerate. Because supernovae are so startlingly bright, they can be used to measure the speed and distance of much more distant objects than Hubble’s pulsating stars. You might expect the expansion of the universe to slow down as gravity pulls galaxies together. The fact that it is accelerating suggests an unexpected force is pushing them apart. This force has been named dark energy.
A revolution in supernova detection
Although supernovae are famous for their brightness, they are, ironically, difficult to detect. Astronomers have found 274 remnants of supernova in our own galaxy, but most new explosions are hidden by the shroud of dust that sits in the disk of the Milky Way. In fact, no new supernova has been observed within our own galaxy since the invention of the telescope .
In 1987, a supernova (SN 1987a) appeared that could be seen with the naked eye because it occurred in a galaxy very close to our own . This unprecedented opportunity to observe a core collapse supernova close up and watch as its debris interacts with the surrounding environment taught us that the geometry of these explosions can be much more complicated than a simple expanding sphere. However, astronomers have yet to solve many of 1987a’s mysteries. Core collapse supernovae usually leave behind a neutron star or black hole, but we have yet to detect one at the site of 1987a.
Astronomers looking for supernovae are faced with millions of known potential host galaxies, so deciding which ones to check can be challenging. The number of galaxies we can observe is limited by a tradeoff between the fraction of the sky we look at (the telescope’s field of view) and how closely we can look (depth). Moreover, because supernovae fade away within weeks after exploding, we need to frequently check each galaxy in order to catch a supernova in the act. The brightest galaxies are frequently observed. But if we had the resources to check fainter galaxies, we would not only find many more supernovae, but also learn about the supernovae produced by more diverse galactic environments.
How do we determine what types of stars produce supernovae? In rare cases, we can actually see the star that has disappeared. (A) “Before” shot of the explosion site of supernova 2008bk from 2001. (B) Same region of the sky during the 2008 explosion. (C) “After” shot in 2010. The cross-hairs mark the site of the supernova progenitor, a typical red supergiant star about 8.5 times the mass of the sun, which disappeared after the explosion. Figure from Mattila et al., 2010.
Technological advancements have provided us with remarkable supernova detection capabilities. About twice as many supernovae were discovered in the past decade than ever before, and the pace of discovery is accelerating. Astronomers have made terrific use of existing instruments as well as purpose-built telescopes to monitor vast swathes of the sky. For example, the Pan-STARRS consortium (which includes Harvard University) has built four 1.8-meter telescopes in Hawaii, each with a 1400-megapixel camera . The telescopes have such incredibly wide fields-of-view that they can canvas the entire Hawaiian sky in about a week. Thus, Pan-STARRS can discover supernovae in countless galaxies of all types. The consortium estimates that Pan-STARRS will discover about 5000 new type Ia supernovae, roughly twice the number known today.
Ongoing searches like Pan-STARRS and its successors promise to revolutionize supernovae research by rapidly expanding the dataset astronomers have to study. Finding more Type Ia supernovae will improve our understanding of how these spectacular explosions are produced and reveal more about the nature of dark energy. More core collapse supernova observations will lead to more accurate models of how the chemical elements are produced and dispersed in galaxies. Dying stars have volumes to teach us about the history of Earth and the universe â€“ we just need to watch and learn.
-  An from the UC Berkeley Cosmology Group.
-  Stephenson, F.R. and Green, D, Historical Supernovae and their Remnants. New York: Oxford University Press, 2002.
-  Marschall, L. The Supernova Story. New York: Plenum Press, 1988.
-  A brief Edwin Hubble biography from NASA.
-  Overbye, Dennis. “From the Clash of White Dwarfs, the Birth of a Supernova.” The New York Times 22 February 2010.
-  NASA press release on the 20th anniversary of supernova 1987a.
-  The Pan-STARRS supernova project.