A Superluminous Supernova Surprise

Supernovae are the most brilliant and powerful stellar blasts known, and their fierce traveling light can be observed all the way out to the very edge of the visible Universe. When a doomed massive star has managed to consume its necessary supply of nuclear-fusing fuel–that has kept it bouncy against the relentless crush of its own gravity–it perishes in the violent, raging final tantrum of a supernova explosion. In the aftermath of the massive star’s final blaze of glory, it leaves behind a souvenir to the Universe, telling the tragic story of how there was once a star that is a star no more. The tattle-tale relic that the erstwhile star leaves as its legacy is either a bizarre, dense little “oddball” called a neutron star, or an even weirder stellar ghost known as a black hole of stellar mass. In July 2017, a team of astronomers announced their discovery that an exceptionally bright supernova occurred in a very unusual location–and the discovery of this “heavy metal” supernova challenges current ideas of how and where such ferociously luminous supernovae occur.

For the past decade, astronomers have detected about 50 unusually powerful supernovae out of the thousands already known. These extremely energetic blasts are much brighter than other supernovae caused by the collapse of a doomed and dying massive star. Indeed, they can briefly outshine their entire host galaxy, as they hurl vital newly-forged atomic elements out into space. Known as superluminous supernovae or hypernovae, these extraordinary explosions show a luminosity 10 or more times higher than that of the more common type of supernova.

Superluminous supernovae are responsible for long gamma-ray bursts (GRBs), which can last anywhere from 2 seconds to over a minute. These brilliant bursts were detected for the first time on July 2, 1967 by U.S. military satellites in high orbit, whose purpose it was to spot gamma radiation. The United States, at that time, suspected the USSR of conducting clandestine nuclear tests, even though it had signed the Nuclear Test Ban Treaty of 1963. Also, the U.S. Vela satellites–on the hunt for possible violations of the Test Ban Treaty–were able to spot explosions behind the Moon. Indeed, the U.S. military satellites did detect a signal–but it was unlike that of a nuclear weapon signature, and it could not be correlated to solar flares.

Over the following few decades, the mysterious GRB’s kept the secret of their origin well hidden from the prying eyes of astronomers. Gamma rays need extremely energetic events to produce them, and yet the bewildering GRBs could not be correlated to a supernova blast, solar flares, or any other known activity in space. Their very brief existence made them difficult to trace. However, once their direction could finally be determined, it was found that they were evenly distributed across the sky. For this reason, they could not originate within our Milky Way Galaxy, or even from nearby galaxies. The mysterious bursts had to be coming from distant regions of space.

In February 1997, the Dutch-Italian satellite BeppoSAX successfully traced GRB 970508 to a dim and distant galaxy approximately 6 billion light-years from Earth. When astronomers analyzed the spectroscopic data for both the burst and its host galaxy, they found that a hypernova was the mysterious burst’s place of origin. That same year, hypernovae were studied in greater detail by Princeton University astronomer Dr. Bohdan Paczynski.

The first hypernova to be detected was SN 1998bw. This brilliant stellar blast had a luminosity 100 times higher than a standard Type 1b supernova. The first confirmed superluminous supernova to be connected to a GRB wasn’t detected until 2003, when GRB 030329 lit up the Leo constellation. SN 2003db heralded the explosive death of a star that had been 25 times more massive than our Sun. These fatal stellar fireworks shot material out into space at more than a tenth of the speed of light.

Currently, many astronomers think that dying stars boasting about 40 solar-masses produce superluminous supernovae.

The End Of The Stellar Road

The stars of the Universe produce energy as a result of the process of nuclear fusion. These giant stars possess sufficient mass to fuse atomic elements that have higher masses than small stars like our Sun can fuse. The degeneracy pressure of electrons and the energy manufactured by fusion reactions are sufficient to battle the relentless squeeze of gravity. This pressure prevents the star from collapsing, and in this way maintains stellar equilibrium. The star fuses increasingly higher and higher mass atomic elements, starting with the two lightest elements–hydrogen and helium. The massive star then continues on and on to produce all of the elements listed in the familiar Periodic Table. But, at last, when a core of iron and nickel forms, as a result of nuclear fusion reactions, the star is doomed to go supernova. This is because nuclear fusion of iron and nickel creates no net energy output, and so further fusion comes to an end. As a result, there is no longer energy output that creates an outward pressure to keep the star fluffy against the merciless squeeze of its own powerful gravity. Equilibrium is broken.

When the massive iron-nickel core is greater than the Chandrasekhar limit of 1.4 solar-masses, electron degeneracy alone cannot wage war against the force of gravity. As a result, a cataclysmic supernova explosion occurs within seconds. At this time, the outer core of the dying star reaches an inward velocity of as much as 23% the speed of light–and the inner core’s temperature skyrockets to a horrific 100 billion Kelvin.

A supernova usually blasts the dying massive star to shreds, furiously hurling its multicolored, fiery, and brilliant outer gaseous layers into the space between stars. The most massive stars to dwell in the Universe collapse and blow themselves up–leaving behind a stellar mass black hole. Stars that are massive, but not quite that massive, leave behind an extremely dense relic core, termed a neutron star.

The discovery of numerous superluminous supernovae in the 21st century revealed that not only were they much more luminous–by an order of magnitude–than the more common type of supernovae, but they were unlikely to be powered by the typical radioactive decay that is the culprit behind the observed energy of the more garden-variety supernovae.

Superluminous supernovae blasts use a separate classification scheme to differentiate them from the conventional Type Ia and Type II supernovae. A Type Ia supernova event occurs when a small, dense stellar corpse, of what was once a star of approximately our Sun’s mass, gravitationally sips up material from a binary companion–a hydrogen-burning star that is still on the main sequence of the Hertzsprung-Russell Diagram of Stellar Evolution. A white dwarf is the relic core of a Sun-like star that has finally managed to burn up its necessary supply of hydrogen fuel, and has puffed off its varicolored gaseous outer layers–leaving its core behind. After the white dwarf has stolen sufficient material from its “still-living” companion star–and victim–it “goes critical” and blasts itself to smithereens in a Type Ia supernova event. Type II supernovae, or core-collapse supernovae, occur when a massive star has attained a core of nickel-iron–and can fuse no more. The erstwhile massive star goes supernova. and the explosion itself fuses the heaviest atomic elements of all, such as gold and uranium. The conventional classification of Type Ia and Type II events is used to distinguish between the two types–according to the spectral signature of hydrogen-rich and hydrogen-poor explosions.

Hydrogen-rich superluminous supernovae are classified as Type SLSN-II, with observed radiation sweeping through the changing opacity of an expanding thick hydrogen envelope. Most of the hydrogen-poor explosions are classified as Type SLSN-I, with the visible radiation created from an enormous expanding envelope of material that is being powered by a still-unknown mechanism. A third less common group of SLSNe is also hydrogen-poor–as well as abnormally luminous. However, this type of superluminous supernova is clearly powered by radioactivity from nickel 56.

The increasing numbers of discoveries show that some superluminous supernovae do not fit neatly into these three classes. For this reason, additional sub-classes or unique events have been proposed.

A Superluminous Supernova Surprise

Following the discovery of these superluminous supernovae, a team of astronomers led by Dr. Matt Nicholl from the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, gathered some important clues indicating where some of these mysterious objects originate.

A team of astronomers from Cambridge University’s (UK) Gaia Science Alerts team detected the extraordinarily bright “heavy metal” supernova, dubbed SN 2017egm, on May 23, 2017 using the European Space Agency’s (ESA’s) Gaia satellite. A team led by Dr. Subo Dong of the Kavli Institute for Astronomy and Astrophysics at Peking University in China used the Nordic Optical Telescope to identify it as a superluminous supernova.

The brilliant stellar blast, SN 2017egm, is located in a spiral galaxy that is approximately 420 million light-years from our planet. This makes the supernova about three times closer than any other superluminous supernova observed so far. Dr. Dong realized that the host galaxy itself was surprising. This is because all known superluminous supernovae have been observed in dwarf galaxies that are considerably smaller than large spiral galaxies, like our own Milky Way.

Adding to this new and surprsing discovery, the team of CfA astronomers found that SN 2017egm‘s unusual host galaxy has a high concentration of elements heavier than hydrogen and helium, which are metals in the terminology that astronomers use. For astronomers, any atomic element heavier than helium is classified as a metal, and so the term holds a different meaning for astronomers than it does for chemists. The discovery of SN 2017egm is the first clear evidence for a metal-rich place of birth for a superluminous supernova. The dwarf galaxies that usually host superluminous supernovae are all known to have a low metal content–which up until now had been considered to be an important ingredient for triggering these exceptionally brilliant stellar blasts.

Superluminous supernovae were already the rock stars of the supernova world. We now know that some of them like heavy metal, so to speak, and explode in galaxies like our own Milky Way,” Dr. Nicholl noted in a July 31, 2017 CfA Press Release.

“If one of these went off in our own Galaxy, it would be much brighter than any supernova in recorded human history and would be as bright as the full Moon. However, they’re so rare that we probably have to wait several million years to see one,” commented study co-author, Dr. Edo Berger, in the same Press Release. Dr. Berger is also of the CfA.

The CfA astronomers also uncovered still more clues concerning the true nature of SN 2017egm. Of special importance, their new study supports the theory that a rapidly spinning, highly magnetized neutron star, termed a magnetar, is likely the engine that drives the amazingly brilliant light that is churned out by these dazzling supernovae.

Even though the brightness of SN 2017egm and the properties of the magnetar that powers it, overlap with those displayed by other superluminous supernovae, the quantity of the mass hurled out by SN 2017egm may be lower than what is produced by the average superluminous supernova blast. The difference may show that the massive progenitor star that gave its “life” to produce the brilliant “heavy metal” SN 2017egm relinguished more mass than most superluminous supernova progenitors before blasting itself to pieces. The spin rate of the magnetar may also be slower than average.

The results of this study suggest that the quantity of metals has at most only a small effect on the properties of a superluminous supernova and the engine that drives it. However, the metal-rich variety blow themselves up at only about 10% of the rate of their metal-poor cousins. Similar results have been observed for gamma-ray bursts that have been associated with the explosion of massive stars. This indicates that there may be a close association between these two types of objects.

From July 4th 2017 until September 16th 2017 the “heavy metal” supernova is not observable. This is because it is too close to our Sun. However, after that time span detailed studies should be possible for at least a few more years.

“This should break all records for how long a superluminous supernova can be followed. I’m excited to see what other surprises this object has in store for us,” study co-author Dr. Raffaella Margutti explained in the July 31, 2017 CfA Press Release. Dr. Margutti is of Northwestern University in Evanston, Illinois.

The CfA astronomers observed SN 2017egm on June 18th 2017 with the 60-inch telescope at the Smithsonian Astrophysical Observatory’s Fred Lawrence Whipple Observatory in Arizona.

A research paper by Dr. Nicholl describing these results was accepted for publication in The Astrophysical Journal Letters. In addition to Dr. Berger and Dr. Margutti, the co-authors of the research paper are Dr. Peter Blanchard, Dr. James Guillochon, and Dr. Joel Leja, all of the CfA, and Dr. Ryan Chornock of Ohio University in Athens, Ohio.

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