When stars go bang!

Astronomers estimate that every second, somewhere in the observable universe, a star undergoes a supernova explosion. These stars are the source of chemical elements heavier than boron — like the calcium in our bones, the iron in our blood, and the sodium and potassium that orchestrate nerve impulses.

So, while it’s true that we are “star stuff”, we’re most deeply connected to those suns that end their days in explosions and scatter newly forged elements across the cosmos. Supernovae leave behind the most extreme objects we know of. Neutron stars are no larger than Manhattan Island and contain more matter than the Sun. Stranger still are black holes, where gravity is so strong that not even light escapes.

And at least some supernovae are linked to gamma-ray bursts (GRBs), the most powerful explosions in the universe. Scientists have detected GRBs from as early as about 13 billion years ago, when the universe was 4 percent of its current age. They don’t see supernovae as far in the past — the record is now about 12 billion years ago — but the nearest events offer important opportunities.

Thanks to archival images and data, astronomers have been able to study 15 doomed stars before they blew up. Nearby supernovae also provide the best chance for catching the explosions long before they reach peak brightness in order to better understand how they unfold. “There is now a real push to locate and study supernovae as close to the start of the explosion as possible, ideally within the first 24 hours”, says Stephen Smartt, an astronomer at Queen’s University Belfast in Northern Ireland.

  While scientists understand reasonably well the most common types of supernovae, new wide-field surveys are finding extreme examples that explode in unusual environments. One of those projects, the Palomar Transient Factory (PTF), has discoveredmore than 1,800 supernovae since 2009. Shri Kulkarni, the lead investigator on PTF, sums up the situation this way: “We have not been imaginative enough in understanding how stars die”.

The big picture

Nova derives from the Latin for “new star”, a term that entered the scientific lexicon in 1573 when the great Danish astronomer Tycho Brahe reported on the previous year’s brilliant example.

A typical nova rapidly brightens by thousands of times and then, over a period of weeks, fades back into obscurity. Astronomers now know that novae occur in binary systems where a normal star transfers matter onto a white dwarf — the dense Earth-sized remnant of a star like the Sun.

Hydrogen gas piles onto the white dwarf and heats up until the layer eventually ignites in a runaway thermonuclear reaction that blows off the accreted gas, creating the nova outburst.

The white dwarf itself remains intact and may undergo this process repeatedly. One of the best examples is RS Ophiuchi, a system where the white dwarf “goes nova” about every 20 years. For it to “go supernova” would require the white dwarf’s complete disruption, a scenario that until recently was not thought possible in this type of system.

  In 1934, long before the physical details of the nova process were clear, astronomers Fritz Zwicky and Walter Baade began referring to the most exceptional novae as “super-novae”, and they made a convincing case that such events represent the collapse of an ordinary star into a neutron star. At the time, observational proof that neutron stars even existed was 30 years away, but Zwicky and Baade’s basic picture still describes the most populated supernova (SN) class. SN 1987A, the most recent “superstar” visible to the naked eye, dramatically confirmed this theory. Before-and-after images demonstrate that a star did disappear.

Scientists also detected a burst of neutrinos particles that don’t easily interact with matter, travel at nearly the speed of light, and carry away most of the energy of a stellar collapse — at Earth before the visible explosion. Depending on mass, composition, and other factors, such a core-collapse supernova may leave behind a neutron star or a stellar-mass black hole. Spreading starlight into a spectrum of its component colors, rainbow-like, allows astronomers to see the absorption or emission of energy produced by elements in the star’s gases, clues that reveal the stellar atmosphere’s composition and motion.

In 1941, Rudolph Minkowski found that the visible spectra of supernovae come in two distinct flavors at peak brightness: Type I supernovae show little evidence of hydrogen, while in type II blasts it is plentiful. Until recently, the general picture was that a type Ia supernova arises from the detonation and total destruction of a white dwarf in a matter-transferring binary system with a normal star.

In this scenario, the dwarf accumulates the gas rather than blowing it off in repeated nova explosions. All other supernovae — types lb and Ic, which are associated with GRBs, and all type II — involve stars born with more than about eight times the Sun’s mass. Such stars ultimately become unstable, collapse, and explode. The variations between different supernova types are what separate them.

  At 2:13 A.M. EDT on March 19, 2008, alert observers fortunate enough to be looking toward the constellation Boates at a dark site might have glimpsed gamma-ray burst (GRB) 080319B, one of the most luminous bursts in X-ray and gamma-ray energies and the brightest yet seen in visible light. The gamma-ray spike triggered NASA’s Swift satellite, which alerted astronomers around the globe.

Two robotic wide-field optical cameras — TORTORA and Pi of the Sky, both located at observatories in Chile — automatically slewed to observe the burst’s position. Both recorded the emergence of a dim star that, over a period of about 50 seconds, rose to a maximum brightness of magnitude 5.3 on the astronomical scale (or about three times brighter than the visual threshold) and then quickly faded to invisibility.

Incredibly, the light from this dying star had been traveling to Earth for 7.5 billion years. This was the birth cry of a black hole born when the universe was less than half its present age, long before the Sun was born. Astronomers say the event was so bright because its parti-cle jet happened to point almost directly toward Earth. They expect similar dead-on alignments with GRB jets to occur about once a decade, on average.