The Massive Star That Died, Not With A Bang, But With A Whimper
By Judith E Braffman-Miller
A black hole of stellar mass is born when a massive star experiences a gravitational collapse, and perishes in the brilliantly beautiful, and furiously fatal, fiery core-collapse of a supernova explosion. At least that’s the way it usually happens, but some especially massive stars travel to the beat of a different drum, and refuse to go gentle into that good night–dying not with a bang, but with a whimper. N6946-BH1 is just such an erstwhile massive star, that weighed-in at 25 times the mass of our Sun during its life on the hydrogen burning main-sequence–and, therefore, should have blasted itself to pieces in an extremely bright supernova conflagration. Instead, this “failed supernova” simply fizzled out–leaving behind only a whimpering black hole, telling the tragic story of how once there was a star, that is a star no more. In May 2017, astronomers reported that they had witnessed the quiet and peaceful demise of this massive, dying star, that had mysteriously vanished out of their sight. As many as 30 percent of such exceptionally massive stars, it seems, may quietly collapse into black holes–leaving behind no brilliant supernova to disclose their terrible, tragic end.
It required the combined power of the Large Binocular Telescope (LBT), and NASA’s Hubble (HST) and Spitzer Space Telescopes to go on the hunt for the tattered wreckage of what was once a star–only to find that this particular star had disappeared without a trace.
“Massive fails” like this one, in a galaxy not far away, could account for why astronomers seldom observe supernovae from the most massive stellar inhabitants of the Universe, explained Dr. Christopher Kochanek in a May 25, 2017 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Kochanek is a professor of astronomy at The Ohio State University in Columbus, and the Ohio Eminent Scholar in Observational Cosmology. The JPL is in Pasadena, California.
Dr. Kochanek leads a team of astronomers who published their most recent results in the April 1, 2017 Monthly Notices of the Royal Astronomical Society (UK) under the title: The Search for failed supernovae with Large Binocular Telescope: confirmation of a disappearing star.
“The typical view is that a star can form a black hole only after it goes supernova. If a star can fall short of a supernova and still make a black hole, that would help to explain why we don’t see supernovae from the most massive stars,” Dr. Kochanek continued to comment.
Playfully referred to as the “Fireworks Galaxy”, NGC 6946 is among the galaxies that the team of astronomers were observing. It is a spiral galaxy, 22 million light-years away, and it was given its nickname because dazzling, fiery supernovae often explode there. Indeed, SN 2017eaw, discovered on May 14, 2017, is showing itself off right now, and is currently sparkling brilliantly at its maximum, fiery brightness.
Beginning in 2009, the star, dubbed N6946-BH1, began to softly brighten with only a weak glow. However, by 2015, this strange, dim star had gone out like a blown candle, and had vanished. The mysterious disappearing star was nowhere to be seen.
Going Out With A Bang
Core-collapse –or Type II–supernovae mark the final, fatal act of a massive star that has finished burning its necessary supply of fuel as a result of the process of nuclear-fusion. In order for an elderly, doomed star to suffer through this form of rapid, catastrophic collapse–followed by a devastating supernova explosion–it must have at least eight times, and no more than 40 to 50 times, the mass of our own small Sun. A supernova explosion can shine so brightly that it briefly outshines its entire host galaxy.
Supernovae are the most powerful of all known stellar explosions, and they can be observed all the way out to the very edge of the visible Universe. When a massive star perishes in the final, furious tantrum of a fatal supernova, it leaves behind in its own wreckage, either an extremely dense, relatively small “oddball” termed a neutron star, or a stellar mass black hole.
Stars manufacture energy by way of the process of nuclear fusion. Unlike our relatively small Sun, heavier stars contain sufficient mass to fuse atomic elements that are heavier than hydrogen and helium–the two lightest of all atomic elements. Stars perform this act of atomic metamorphosis at increasing temperatures and pressures. The degeneracy pressure of electrons and the energy manufactured by fusion reactions are sufficient to wage war against the force of gravity and prevent the star from collapsing–thus maintaining stellar equilibrium. The star fuses increasingly heavier and heavier atomic elements, starting with hydrogen and helium, and then progressing on and on through all of the atomic elements up to iron and nickel. In the end, when a core of iron and nickel is created by the elderly, massive star, it has reached the end of that long stellar road. Nuclear fusion of iron and nickel can produce no net energy output, and therefore no further fusion can occur–leaving the nickel-iron core inert. Alas, because there is no longer energy output creating outward pressure, equilibrium is broken, and the star is ready for its grand finale.
When the heavy mass of the iron-nickel core is greater than what is termed the Chandrasekhar limit of 1.4 solar masses, electron degeneracy alone cannot wage war against the force of gravity, thus maintaining stellar equilibrium. A horrific, fiery, brilliant supernova explosion occurs within seconds, in which the outer core attains an inward velocity of as much as 23% of the speed of light, while the temperature of the inner core soars to as much as 100 billion Kelvin.
Supernovae usually destroy the doomed massive star, blasting it to shreds, and violently hurling its brilliant rainbow of multicolored gaseous layers out into the space between stars. The most massive stars in the Universe collapse and blow themselves up–leaving behind a stellar mass black hole. That is what usually happens–but not always.
Stellar-mass black holes have masses ranging from about 5 to several tens of solar masses. A black hole, of any mass, can possess only three fundamental properties: electric charge, mass, and angular momentum (spin). The spin of a black hole is the result of the conservation of angular momentum of the star that produced it.
According to Albert Einstein’s Theory of General Relativity (1915), a black hole of any mass could exist in nature. The smaller the mass, the higher the density of matter must be in order to create a black hole. There is no known process that can form a black hole with a mass less than a few times solar-mass.
Our Milky Way Galaxy hosts several candidate stellar mass black holes which are situated closer to our Earth than the supermassive black hole that lurks in the Galactic center. Our Galaxy’s resident supermassive black hole weighs millions of times more than our Sun. Most of the other black hole candidates are members of X-ray binary systems. These binary systems contain one vampire-like compact object that sips up the material from its unfortunate binary companion by way of an accretion disk. The probable black holes that exist in these binary systems can range from three to over a dozen times the mass of our Sun.
The Massive Star That Died, Not With A Bang, But With A Whimper
After the LBT survey for failed supernovae turned up the mysterious star, N6946-BH1, astronomers used the HST and Spitzer space telescopes to see if it was really still there–merely hiding, after having dimmed. The astronomers also used the infrared Spitzer space telescope to hunt for any infrared radiation streaming out from the location where the vanished star could be in hiding, thus eluding their prying eyes. This infrared radiation would have provided an important clue, revealing that the star was still present, but was merely hiding behind the obscuring veil of a dust cloud.
However, all the tests came out negative. The star was not there. By a careful process of elimination, the astronomers finally concluded that the star must have become a black hole.
It is still too early in the project to know for certain how frequently massive stars undergo this type of supernova failure. However, Dr. Scott Adams was able to make a preliminary estimate. “NGC6946-BH1 is the only likely failed supernova that we found in the first seven years of our survey. During this period, six normal supernovae have occurred within the galaxies we’ve been monitoring, suggesting that 10 to 30 percent of massive stars die as failed supernovae,” Dr. Adams noted in the May 25, 2017 JPL Press Release. Dr. Adams, a former doctoral student at Ohio State University, is now an astrophysicist at the California Institute of Technology (Caltech) in Pasadena.
“This is just the fraction that would explain the very problem that motivated us to start the survey, that is, that there are fewer observed supernovae than should be occurring if all massive stars die this way,” he added.
For Dr. Krzysztof Stanek, a study co-author, the really fascinating part of their discovery is the implications that it holds for the origins of supermassive black holes. Supermassive black holes, weighing in at millions to billions of times more than our Sun, dwell with secret and sinister intent in the centers of every large galaxy in the observable Universe–including our own. These supermassive black holes are precisely the kind that the Laser Interferometer Gravitational-Wave Observatory (LIGO) observed when it detected gravitational waves. Gravitational waves are ripples in the fabric of Spacetime, predicted by Albert Einstein in his Theory of General Relativity.
However, Dr. Stanek noted in the May 25, 2017 JPL Press Release that it doesn’t necessarily make sense that a massive star could go supernova and still have enough mass left over to give birth to a massive black hole on the scale of those that LIGO detected. This is because a supernova blows off much of the dying massive star’s outer layers. Dr. Stanek is a professor of astronomy at Ohio State.
“I suspect it’s much easier to make a very massive black hole if there is no supernova,” he concluded.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.