Small stars can hang around for billions of years and slowly fade away, but large stars have a spectacular way to wave goodbye. When a large star collapses, it can start a new round of nuclear fusion that powers an explosion called a supernova.
These starbursts produce a gargantuan flash of light and energy that may briefly outshine the star’s home galaxy. In its aftermath, a supernova may leave a super-dense neutron star or a black hole, surrounded by an expanding nebula of dust and gas.
Larger stars tend to produce larger supernovas. One of the largest on record was detected in 2007, when a star roughly 200 times as massive as our sun went kaput.
A supernova is triggered by an imbalance between the inward force of gravity and the outward force exerted by the star’s energy production. A star can simply run out of hydrogen and helium fuel, and collapse as nuclear fusion slows and gravity overcomes the fading outward pressure. Stars with 10 to 140 times the mass of our sun may have iron cores that can collapse, forming a supernova and leaving a neutron star in its wake.
Second biggest bang?
A more exotic process can create stupendous explosions in humongo stars with more than 140 solar masses.
This week, Nature published a study by Avishay Gal-Yam, of the Weizmann Institute of Science, in Rehovot, Israel, of supernova 2007bi, a mammoth explosion located 1.5 billion light years away. The explosion, one of the largest ever seen, was observed by multiple large telescopes and released 10 53 ergs of energy. According to our calculations, that’s as much energy as our sun would release in almost 1 trillion years!
The march of superlatives continues: spectral analysis shows that the explosion coughed up 50 to 100 solar masses of heavy elements, such as carbon, oxygen, sodium, magnesium, calcium and iron. The debris included 3 to 10 solar masses of nickel.
Immediately after the explosion, this huge pile junk was departing scene at 12,000 kilometers per second.
That’s rapid transit, in our book. And it’s another example of the fact that the heavy elements in the universe – essentially every element more massive than hydrogen and helium – was formed inside stars.
As Carl Sagan once said, “We are all star stuff.”
The explosion was the first convincing example of a theoretical phenomenon called pair instability, says Gal-Yam, who points out that many measurements of the actual supernova matched predictions made by theorists. In the insanely hot (1 billion degrees Kelvin) core of this heavy star, energetic light particles, called photons, are converted into pairs of electrons and positrons, the electron’s anti-matter counterpart.
A loss of equilibrium
The electron-positron pair quickly annihilates itself, making more photons, which make more electron-positron pairs. But if the pair is present only for an instant, and it releases energy as it self-annihilates, how does this cause the star to collapse? It’s all a question of balance, says Gal-Yam. “At any time, you have transformed some photons into electron-positron pairs, which is enough to take away some fraction of the energy. The star was at equilibrium before this process, the pressure of the energy and gravity were balanced.”
But converting the energy in photons into the mass of electrons and positrons disturbs the equilibrium. “Each particle may not live a long time, but there are always some particles taking away some energy, so it goes out of equilibrium, and the core begins to contract,” Gal-Yam says. “When this gets severely out of equilibrium, the inertia of the infalling material, moving under gravity, will cause the oxygen core to start to fuse into heavier elements. Runaway fusion begins, and you get the big explosion.”
We’ve spent our supply of superlatives, but these jumbo explosions could be a source of gamma ray bursts, the brief showers of gamma rays that must originate in hideously strong astrophysical sources.
A done deal?
“My gut reaction is that this probably is a pair-instability supernova, and that’s pretty exciting,” says Richard Townsend, professor of astronomy at the University of Wisconsin-Madison, but he says the argument is not yet “watertight.” A previous claim for a pair-instability supernova has faded, he says, because it appears that some of its energy was made by the ejected material that blasted into stuff that the star had previously coughed up, called circumstellar material. “If ejected material plows into circumstellar material, radiant energy can be produced to augment the radiated energy of the actual explosion, making it appear more energetic.”
Townsend, who studies the evolution of stars, says “The authors’ basic claim is that because there is no evidence for interaction with circumstellar material, the only alternative is a pair-instability supernova, yet the paper contains no direct evidence” for pair instability. “It’s more that this is the only option left standing once they have ruled out other choices.”
The existence of super-massive stars, much larger than those seen today, would help explain events during the first billion years of the universe, Townsend adds, and finding such a star in the “modern” universe could be key to understanding how the ancient universe developed into the one we see today.
Speaking selfishly, the pair-instability supernova could be a major source of 90 of the 92 elements that are found in nature, which astro-types call “metal.” Otherwise, the universe would consist largely of a boring mix of hydrogen and helium. No cars, no telescopes, no paychecks. Not even pay-per-view TV.
An unstable pair
It’s too soon to say how much of the metals originated in pair-instability supernovas. “We have measured what the explosion made,” says Gal-Yam, “and theory predicts how much will be made, but we don’t know how common these explosions are. If they are very rare, then there’s not much of a contribution. If they are less rare, they might be important.”
A series of new telescopes that can quickly identify fast-changing light sources should provide a more examples of these titanic supernovas, and help determine their role in seeding the universe with the elements in our planet and our bodies.
David J. Tenenbaum