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.
The Vela supernova remnant illustrates the beauty of a massive star’s death. The stellar explosion occurred some 11,000 years ago; since then, the supernova’s shock waves have collided with and compressed interstellar gas, causing it to glow
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.
A type la supernova explosion occurs once a white dwarf has collected enough mass from a companion to reach about 1.4 times the Sun’s mass. Astronomers think the donor star can be a red giant (left), a normal star (middle), or another white dwarf
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.

NASA’s Swift satellite imaged the extremely luminous afterglow of gamma-ray burst 080319B with its X-ray telescope (left) and its optical/ ultraviolet instrument.
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.
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