‘87 Supernova Offers Science Unique Look at Sun’s Origin
About 160,000 years ago, the unstable iron heart of a giant sun suddenly collapsed, triggering a devastating shock wave in less than a second that blew the star to pieces and, for an instant, generated more energy than all the starlight in the universe.
In less than 10 seconds, a torrent of subatomic particles called neutrinos blasted away from the collapsed core, by then probably an ultra-dense neutron star just 30 miles across, carrying news of the stellar catastrophe across the universe at or near the speed of light.
The titanic shock wave created a blinding flare of light that followed close behind. Showers of high-energy gamma rays, reduced to visible light and delayed by collisions with material in the expanding cloud of stellar debris, ultimately followed.
Difficult to Detect
The energy generated in the explosion--temperatures reached more than 10 billion degrees--created heavy metals out of lighter elements. Thus the star’s wreck seeded that region of the universe with the building blocks of future solar systems.
About 160,000 years later, on Feb. 23, 1987, the flood of penetrating neutrinos, which can pass through light-years of solid lead without being stopped, reached Earth’s solar system.
A light-year is the distance light travels in a year at a speed of 186,000 miles per second. By this yardstick, the explosion occurred 160,000 light-years from Earth.
Neutrinos are electrically neutral subatomic particles that have virtually unmeasurable mass, if they have mass at all. They seldom interact with other particles--hence their great penetrating power--and are thus extremely difficult to detect.
Nonetheless, 19 of the elusive particles were detected by sophisticated instruments in the United States and Japan, along with another five detected in Italy that may be associated with the explosion, but no one noticed at the time. And then, three hours later, the light arrived.
On a cold mountaintop in Chile, Canadian astronomer Ian Shelton noticed a star in the Large Magellanic Cloud, a satellite galaxy to Earth’s Milky Way, that was far brighter than any star he could remember in that area.
He quickly realized he had stumbled onto a magnificent discovery. Telegrams were sent to observatories around the world announcing what turned out to be the closest supernova visible from Earth in nearly 400 years.
It was Feb. 24, 1987, and the telegrams marked the event of the year, if not the century, for astronomy.
Unprecedented Detail
Even though about 620 supernovae have been observed throughout recorded history, the vast majority were so far away that it has been extremely difficult to study more than their general behavior. Supernova 1987A is in a class by itself in that regard and, as exciting as its discovery was, the best was yet to come.
Because the supernova is so relatively close as astronomical distances go, scientists have been able to study the rare celestial phenomenon in unprecedented detail using the full array of modern instruments both on the ground and in space.
For the first time, astronomers have been able to identify the original, or “progenitor,” star that exploded--Sanduleak-69 202--which is crucial to understanding what kinds of stars can experience such devastating deaths.
They have detected the neutrino burst that signified the original collapse of the star’s unstable iron core and, as 1987 came to a close, gamma rays were observed for the first time, confirming the creation of heavy metals like iron through a process called “explosive nucleosynthesis.”
Edward Chupp of the University of New Hampshire is the principal investigator of a joint U.S.-West German experiment aboard NASA’s Solar Maximum Mission satellite. The instrument detected gamma rays from the supernova in August.
“The idea is that our sun, when it was formed, was formed out of material debris that was the result of explosions of other stars that were dispersed throughout space,” he said. “So, in a sense, we’re finding out our own origins.”
The discovery was a triumph for modern astronomy.
‘A Long Time to Come’
Astrophysicist Stan Woosley of the University of California at Santa Cruz and M. M. Phillips of the Cerro Tololo Inter-American Observatory in Chile wrote in a paper for the journal Science: “The great beauty of this supernova is that . . . we will be able to observe it at all wavelengths for a long time to come.”
“But the most important and exciting events will come unforetold as supernova 1987A continues to be the answer to an astronomer’s prayer--’Surprise me!’ ”
Supernovae have long fascinated astronomers because they represent the most violent events in the universe. When Sanduleak’s core collapsed, the neutrinos that were emitted in one second carried away 100 times the entire energy output of Earth’s sun over the 5 billion years it has been in existence.
For comparison, Woosley said, all the nuclear weapons in the superpower arsenals could power the sun for only a “few millionths of a second.” Expressed another way, the supernova’s neutrino burst represented more energy than all the starlight in the universe during the first seconds they were emitted.
The death of Sanduleak-69 202 marked a “Type 2” supernova, which astronomers believe to be a common fate for stars at least eight times as massive as Earth’s sun.
A star remains stable by balancing gravity, which constantly pulls inward, against the outward pressure produced by nuclear fusion in its core. When a star’s nuclear fuel is exhausted, gravity triumphs and it collapses.
Temperature Increases
And the force of gravity, as Isaac Newton wrote in 1687, increases as the square of an object’s radius decreases. That is, if a star’s radius shrinks to one-fourth its normal size, the gravity acting on the surface of the star becomes 16 times greater than before.
As a star contracts, the atomic particles that are its substance are forced into a smaller volume. As chemists in the 17th Century discovered, reducing the volume of a gas, for example, increases its temperature.
In a star’s case, the pressure produced by gravitational contraction can raise internal temperatures to such levels that the fusion of heavier elements becomes possible. With renewed nuclear burning, energy becomes available to offset the force of gravity and stability is regained.
A star like Earth’s sun can burn for 10 billion years going through cycles of expansion and contraction until, finally, even the inward pressure of gravity cannot trigger additional burning. At that point stars like the sun typically become shrunken “white dwarfs” and slowly die, radiating their heat away into space.
But for stars much more massive than the sun, advanced burning stages are possible. Sanduleak-69 202 was a “supergiant” about 20 times as massive as the sun and it was destined for a much more violent death.
Such giant stars burn up their nuclear fuel at prodigious rates. Born just 10 million years ago, Sanduleak spent 90% of its life using hydrogen fusion in its core to provide the energy to offset the relentless pull of gravity.
In hydrogen fusion, hydrogen nuclei--protons--are smashed together in a series of reactions that result in the formation of helium nuclei. In the process, matter is converted into energy. Earth’s sun, for example, converts about 140 trillion tons of matter into energy every year.
Eventually, the hydrogen fuel supply in the core is exhausted and at this stage in Sanduleak-69 202’s history, the end was near.
Hydrogen fusion then began in a thick shell surrounding the star’s helium core. The star quickly expanded into a red giant with a radius roughly equal to the distance from the sun to the Earth.
Eventually, helium began to burn in the core, producing carbon and oxygen, and continued to do so for about a million years. When the helium was exhausted, the star contracted, and when temperatures in the core reached 700 million degrees, carbon fusion began and continued for about 1,000 years, producing neon, sodium and magnesium.
After brief stages of neon, oxygen and silicon burning, internal temperature reached about 3.5 billion degrees, which produced isotopes of iron. But iron cannot undergo fusion. Before the temperature can get high enough, the atoms simply disassociate, or “melt.” Suddenly, there was no energy left to offset gravity.
“It is the end of the road for the star,” Woosley and Phillips wrote. “Gravity has not diminished, indeed it has only become stronger with each successive stage of contraction and burning.
“Having no other source of energy to support itself, the core does what it has done ever since the star was born. It contracts and heats up.”
Internal density quickly rose by a factor of 1 million and in less than one second, the inner region of the core collapsed to about 30 miles across from an initial size comparable to that of Earth.
The tremendous collapse smashed electrons into protons and created neutron-rich isotopes. In each such reaction a neutrino was produced, and a torrent of the elusive particles streamed away into space.
But the majority of the neutrinos thrown off by supernova 1987A were created over the next 10 seconds as the core collapsed even farther and swarms of subatomic particles crashed into each other and were destroyed.
So many neutrinos were produced by the supernova that Woosley estimates that a neutrino from the explosion lodged in the bodies of roughly 1 million people on Earth, 160,000 light-years away.
‘Something Was Born’
“The neutrino burst told us one thing for absolute sure, and that is: the iron core of a massive star collapsed. There’s nothing else that could have made that signal. Something was born, and it was either a neutron star or a black hole,” Woosley said in a telephone interview, adding that preliminary evidence supports the neutron star hypothesis.
A neutron star is a tiny, ultra-dense object composed primarily of uncharged neutrons covered by a mantle of iron. Gravity is so intense at the surface, an object would weigh about 100 billion times what it would weigh on Earth.
Sanduleak’s core had collapsed to a point at which it could contract no farther and a rebound phenomenon occurred, creating a catastrophic shock wave.
“So the central half of the core stops like it had run into a brick wall, and the rest of the material, which is raining down at about a fifth the speed of light, runs into that brick wall and bounces,” Woosley said.
The neutrino burst reaches Earth ahead of the visible light of the explosion because the shock wave created by the rebound from the collapse of the core took about two hours to make its way to the surface. When it did, it created the tremendous flare of light that was detected on Earth in February.
It is not yet clear exactly how the shock wave propagates through the in-falling debris, but that it did so is obvious, and the results were catastrophic: The star was blown to pieces and, in the process, heavy elements were created.
“As that shock wave moves out through what were the silicon and oxygen layers of the pre-explosive star, it raises it to very high temperatures,” Woosley said. “All of that material, which is raised to temperatures of more than 5 billion degrees, turns into iron group elements, but not iron like we know on Earth.”
Instead, nickel-56 was created, which has a half-life of about six days. Nickel-56 decayed into cobalt-56, a radioactive material that in turn decayed into stable iron-56--which, at last, is the kind found on Earth.
When each cobalt-56 atom decayed it produced a shower of gamma rays. Most of that radiation was weakened by collisions with material blown away from the core and, by the time it made it into space, it had been reduced to visible light. Although the supernova’s initial flaring was caused by the shock wave, it was sustained by the radiation produced by cobalt decay.
But as the expansion proceeded, the clouds of debris thinned out enough so that gamma rays produced in later cobalt decay reactions were able to make it into space. It was that radiation that was detected by the Solar Max satellite in August and by balloon-borne instruments carried aloft last fall from Australia during a NASA supernova research campaign.
Because the cobalt decay process results in unique gamma rays, they serve as a sort of celestial fingerprint that proves heavy elements were created in the supernova.
“These theories of nucleosynthesis have been developed over the past 30 or 40 years,” said Gerald Fishman, an astrophysicist at NASA’s Marshall Space Flight Center and one of two principal investigators for one of the balloon-borne gamma ray detectors.
“Literally hundreds of astrophysicists have been working on these theories and this is the first direct confirmation.”
But for Woosley, the most exciting element in the supernova saga so far has been the neutrino burst.
“To just think of the energy of that,” he said. “The neutrinos they detected came through the Earth and the power of that neutrino burst exceeded the power of the entire universe at that moment. And it generated that in a region that was only about 30 miles across.”
As the cloud of debris that marks the death of Sanduleak -69 202 continues to expand and thin out.
