Discovering A Famous Supernova’s Secret Past

Supernova 1987A (SN 1987A) heralded the explosive and brilliant death of a massive star, whose powerful, fiery, wandering light reached Earth on February 23, 1987. The stellar blast occurred about 168,000 light-years from our planet, and it was bright enough to be observed with the naked eye as it ferociously exploded in the outer limits of the Tarantula Nebula in the Large Magellanic Cloud (LMC). The LMC is a small, amorphous dwarf galaxy that is a satellite of our own enormous and ancient barred-spiral Galaxy–the Milky Way. SN 1987A provided the very first opportunity for modern astronomers to observe the mysterious evolution of a supernova in detail, and the resulting valuable investigations have shed new light into how core-collapse Type II supernovae occur and develop through the passage of time. In August 2016, astronomers announced that they had been able to “see” millions of years into the secret and well-hidden past of SN 1987A, and had successfully investigated the tattered stellar ruins of this famous blast from the past that had made a brilliant spectacle of itself 29 years earlier.

The research was led by a doctoral student investigating the lingering remains of the famous star-that-was, and the new findings about supernova remnant 1987A will greatly help astrophysicists understand how similar core-collapse supernovae are triggered and evolve in general. Just like archaeologists excavating the ruins of ancient civilizations on Earth, and then interpreting what these ancient remains reveal to them about those vanished civilizations, the ancient stellar ruins of SN 1987A can open a new window into the vanished massive progenitor star’s secret past.

Peering into the hidden heart of SN 1987A, the team of astronomers were able to observe its past, millions of years before its famous final blaze of glory, using a telescope in the remote outback region of Australia at a site that was not polluted by FM radio interference. The research group, led by a doctoral student at the University of Sydney in Australia, includes an international team of astronomers exploring the region at the lowest-ever radio frequencies. These investigations have helped to provide the scientists with a better understanding of supernovae.

The astronomers explained that SN 1987A, the first supernova to be detected in 1987, had a brightness that peaked in May 1987, with an apparent magnitude of about 3. The apparent magnitude gradually dimmed over the next several months.

The new research depicts the doomed star’s life, long before its explosive stellar farewell performance, when it was still a bright, active, and youthful hydrogen-burning star on the main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution. The new study then went on to show how this massive progenitor main-sequence star evolved to become both the brightest and closest supernova seen from Earth–collapsing in an explosive grand finale about three decades ago.

A Brilliant Stellar Grand Finale

Supernovae blasts are the most powerful stellar explosions known. Indeed, they are so extremely powerful and brilliant that they can be observed all the way out to the very edge of the visible Universe. When a massive star, such as the SN 1987A progenitor, at last reaches the unfortunate end of the road, it perishes in the rage of a core-collapse Type II supernova. However, the erstwhile star usually leaves behind a relic of its former “life” in the form of a weird, extremely dense, relatively small stellar ghost termed a neutron star–or, alternatively, in the case of the most massive stars of all, an even more bizarre inhabitant of our strange Cosmos termed a black hole of stellar mass.

The myriad of magnificent, sparkling, and fiery stars that frolic around our Universe shine brightly because they churn out energy as the product of nuclear fusion. Unlike our own comparatively small Star, the Sun. much more massive stars contain sufficient mass to fuse elements that have an atomic mass greather than hydrogen and helium–the two lightest of all atomic elements–at ever-increasing temperatures and pressures. The degeneracy pressure of electrons and the energy churned out by these fusion reactions are sufficient to wage war against the force of gravity–thus preventing the still-“living” star from collapsing. This process enables the massive main-squence star to maintain stellar equilibrium. A main-sequence star fuses increasingly heavier and heavier atomic elements out of lighter ones, starting the process with hydrogen and helium, and then continuing on to manufacture all of the atomic elements listed in the familiar Periodic Table. This process, termed stellar nucleosynthesis, continues until a core of nickel or iron forms. The nuclear fusion of nickel or iron produces no energy output. This basically means that no further fusion is possible–leaving the nickel-iron stellar core inert. Because there is no longer any energy output that creates an outward pressure, stellar equilibrium is broken. The star is doomed.

When the extremely heavy mass of the nickel-iron core is greater than what is termed the Chandrasekhar limit of about 1.4 times the mass of our Sun, electron degeneracy alone cannot hold its own against the force of gravity and maintain stellar equilibrium. The result is the catastrophic and spectacular death of the massive star in a supernova explosion that occurs within mere seconds. Indeed, during this final blaze of stellar fury, the outer core of the dying star reaches an inward velocity of as much as 23% of the speed of light–and the temperature of the inner core soars to as much as 100 billion Kelvin.

Supernovae blasts usually tear the massive dying star to shreds, violently hurling its brilliant and fiery rainbow of multicolored outer gaseous layers into interstellar space. The most massive stars to frolic in the Cosmos collapse and violently blow themselves into oblivion–leaving behind a mysterious, weird stellar mass black hole. Massive stars–that are not that massive–leave behind a wildly spinning, city-sized neutron star. A teaspoon full of neutron star stuff can weigh as much as a herd of dinosaurs.

There are several differing classes of core-collapse supernova explosions. The classes differ as a result of light curve–a graph of time versus luminosity–in the aftermath of the blast. Type II-L supernovae show a linear drop of their light curve in the immediate aftermath of the explosion, whereas Type II-P display a period of considerably less rapid decline (a plateau) in their light curve that is followed by a normal decay. Type Ib and Type Ic supernovae are core-collapse supernovae that involve a massive star that has previously hurled off its outer envelope of hydrogen and (for Type Ic) helium, as well. As a result, Type Ib and Type Ic blasts appear to be devoid of these two light elements.

SN 1987A was discovered by Dr. Ian Shelton and Dr. Oscar Duhalde at the Las Campanas Observatory in Chile on February 24, 1987. Within the same 24 hour period, it was independently discovered by the late Dr. Albert Jones in New Zealand. From March 4-12, 1987, the supernova was observed from space by Astron, which was the largest ultraviolet space telescope at the time.

SN 1987A‘s progenitor star was a blue supergiant located about 168,000 light-years away. Blue supergiants are extremely massive and gigantic stars that are both brilliantly bright and searing-hot, with surface temperatures of between 2,000-50,000 degrees Celsius. The brightest star in the constellation of Orion, dubbed Rigel, is the most famous blue supergiant.

As well as being the closest supernova to be observed in hundreds of years, SN 1987A was also a very important event for astronomers because it was the first time that neutrinos were detected shooting out from an astronomical source other than our own Sun. Neutrinos are subatomic particles that are produced as a result of the decay of radioactive elements and have no electric charge. These almost massless subatomic particles are theoretically predicted to be born in great numbers during core-collapse supernovae explosions. Therefore, their discovery during the SN 1987A blast, strengthed some of the most basic and widely held theories about the mysterious mechanisms going on within supernovae.

About two to three hours before the visible light emanating from SN 1987A reached Earth, a flood of neutrinos was detected at three separate neutrino observatories. The reason for this observation is generally thought to be that neutrino emission occurs simultaneously with the stellar core collapse–and this occurs before there is the emission of visible light. The transmission of visible light is a slower process that can occur only after the shock wave has reached the doomed star’s surface.

Even though the actual neutrino count was only 24, it was considered to be important because it was a significant rise from the previously observed background level. This was the very first time that neutrinos, shot out from a supernova, had been directly observed–and this heralded the beginning of neutrino astronomy. The observations proved to be consistent with theoretical supernova scenarios proposing that 99% of the energy produced as the result of the stellar core collapse is radiated away in the form of neutrinos. The observations are also consistent with the scenarios’ prediction of a total neutrino count of 10 to the 58th power with a total energy of 10 to the 46th power joules.

Revealing SN 1987’s Secret Past

Astronomers knew a great deal about the immediate past of SN 1987A from studying the cosmic wreckage of the dying star’s collapse in the LMC back in 1987. However, it was the discovery of the very faintest of hisses through low-frequency radio astronomy that provided the most recent insights.

Previously, only the final fatal fraction of the dead star’s multi-million-year-long-life, about 0.1% or 20,000 years, had been observable. This latest study, released in August 2016, helped astrophysicists probe into the supernova’s mysterious past life millions of years further back in time than was previously possible. The study was led by Joseph Callingham, a doctoral student at the University of Sydney in Australia and the ARC Centre of Excellence fo All-Sky Astrophysics. This work was done under the supervision of former Young Australian of the Year Dr. Bryan Gaensler, who is now at the University of Toronto in Canada.

Operating the Murchison Widefield Array located in the West Australian desert, the team of radio astronomers managed to “see” all the way back in time to when the dying star was still in its long-lasting red supergiant phase. Red supergiants are the largest stars in the Universe. Callingham explained, in an August 2, 2016 University of Sydney Press Release, that earlier studies had focused on the material that was hurled out into space when the star was in its final blue supergiant phase.

“Just like excavating and studying ancient ruins that teach us about the life of a past civilization, my colleagues and I have used low-frequency radio observations as a window into the star’s life,” Callingham explained in the August 2, 2016 University of Sydney Press Release.

The team of astronomers discovered that the red supergiant lost its matter at a slower rate and produced slower winds than had been assumed earlier. The slow winds took their time pushing into its ambient environment.

“Our new data improves our knowledge of the composition of space in the region of supernova 1987A; we can now go back to our simulations and tweak them, to better reconstruct the physics of supernova explosions,” Callingham continued to explain.

Dr. Gaensler added in the same August 2, 2016 University of Sydney Press Release that the key to obtaining these new insights was due to the quiet environment in which the radio telescope they used is situated.

“Nobody knew what was happening at low radio frequencies, because the signals from our own earthbound FM radio drown out the faint signals from space. Now, by studying the strength of the radio signal, astronomers for the first time can calculate how dense the surrounding gas is, and thus understand the environment of the star before it died,” Dr. Gaensler added.

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