Like the legendary Phoenix Bird rising from the ashes of its own funeral pyre to soar again through the sky, a pulsar rises from the wreckage of its massive progenitor star–that has recently perished in the fiery blast of a supernova. A pulsar is a newborn neutron star; a dense, rapidly rotating city-sized relic of an erstwhile massive star that has collapsed under the stupendous weight of its own crushing gravity–to the fatal point that its constituent protons and electrons have merged together to form neutrons. Indeed, the fiery explosions of doomed stars as supernovae are sometimes so bright that they out-dazzle–for one brief shining moment–their entire home galaxy. In September 2018, a team of astronomers announced that they are the first to have witnessed the birth of a pulsar emerging from the funeral pyre of its dead parent-star. This came at the very same time that the Selection Committee of the Breakthrough Prize in Fundamental Physics recognized the British astrophysicist Dr. Jocelyn Bell Burnell for her discovery of pulsars–a detection first announced in February 1968.
This Special Breakthrough Prize was given to Dr. Bell Burnell “for fundamental contributions to the discovery of pulsars, and a lifetime of inspiring leadership in the scientific community.” Her discovery of pulsars half a century ago proved to be one of the biggest surprises in the history of astronomy. This discovery elevated neutron stars right out of the realm of science fiction to reach the status of scientific reality in a very dramatic way. Among a large number of later important ramifications, it resulted in several strong tests of Albert Einstein’s General Theory of Relativity (1915), and also led to a new understanding of the origin of heavy elements in the Universe. Called metals by astronomers, heavy atomic elements are all those that are heavier than helium.
The supernovae that give birth to pulsars can take months or even years to fade away. Sometimes, the gaseous leftovers of the fierce stellar explosion itself crash into hydrogen-rich gas and–for a short time–regain their former brilliance. However, the question that needs to be answered is this: could they remain luminous without this sort of interference, resulting in their bright encore performance?
In an effort to answer this nagging question, Dr. Dan Milisavljevic, an assistant professor of physics and astronomy at Purdue University in West Lafayette, Indiana, announced that he had witnessed such an event six years after a supernova–dubbed SN 2012au–had blasted its progenitor star to smithereens.
“We haven’t seen an explosion of this type, at such a late timescale, remain visible unless it had some kind of interaction with hydrogen gas left behind by the star prior to explosion. But there’s no spectral spike of hydrogen in the data–something else was energizing this thing,” Dr. Milisavljevic explained in a September 12, 2018 Purdue University Press Release.
If a newborn pulsar sports a magnetic field and rotates rapidly enough, it is able to speed-up nearby charged particles and evolve into what astronomers term a pulsar wind nebula. This is probably what happened to SN 2012au, according to the new study published in The Astrophysical Journal Letters.
“We know that supernova explosions produce these types of rapidly rotating neutron stars, but we never saw direct evidence of it at this unique time frame. This is a key moment when the pulsar wind nebula is bright enough to act like a lighbulb illuminating the explosions outer ejecta,” Dr. Milisavlievic continued to explain in the Purdue University Press Release.
Lighthouses In The Sky
Pulsars shoot out a regular beam of electromagnetic radiation, and weigh-in at approximately double our Sun’s mass, as they spin wildly about 7 times each second! The beams emanating from brilliant pulsars are so extremely regular that they are frequently likened to lighthouse beams on Earth, and this beam of radiation is detectable when it sweeps our way. The radiation streaming out from a pulsar can only be seen when the light is targeted in the direction of our planet–and it is also responsible for the pulsed appearance of the emission. Neutron stars are extremely dense, and they have brief, regular rotational periods. This creates a very precise interval between the pulses that range approximately from milliseconds to seconds for any individual pulsar. Astronomers discover most pulsars through their radio emissions.
Neutron stars can wander around space either as solitary “oddballs” or as members of a binary system in close contact with another still “living” main-sequence (hydrogen-burning) star–or even in the company of another stellar-corpse just like itself. Neutron stars have also been observed nesting within brilliant, beautiful, and multicolored supernova remnants. Some neutron stars can even be orbited by a system of doomed planets that are utterly and completely inhospitable spheres that suffer a constant shower of deadly radiation screaming out from their murderous stellar parent. Indeed, the first bundle of exoplanets, discovered in 1992, were the tragic planetary offspring of a deadly parent-pulsar. Pulsars switch off and on brilliantly, hurling their regular beams of light through the space between stars. Certain pulsars even rival atomic clocks in their accuracy at keeping time.
The first observation of a pulsar was made on November 28, 1967, by Dr. Bell Burnell and Dr. Antony Hewish. The newly-spotted pulses were separated by 1.35 second intervals that originated from precisely the same location in space, and kept to sidereal time. Sidereal time is determined from the movement of Earth (or a planet) relative to the distant stars (rather than in respect to our Sun).
In their efforts to explain these exotic pulses, Dr. Bell Burnell and Dr. Hewish came to the realization that the extremely brief period of the pulses ruled out most known astrophysical sources of radiation, such as stars. Indeed, because the pulses followed sidereal time, they could not be explained by radio frequency interference originating from intelligent aliens living elsewhere in the Cosmos. When more observations were conducted, using a different telescope, they confirmed the existence of this truly odd and mysterious emission, and also ruled out any sort of instrumental effects. The two astronomers nicknamed their discovery LGM-1, for “little green men”. It was not until a second similarly pulsating source was discovered in a different region of the sky that the playful “LGM” theory was completely ruled out. The word “pulsar” itself is a contraction of “pulsating star”, that first appeared in print in 1968.
All stars are immense spheres composed of fiery, roiling searing-hot gas. These enormous glaring stellar objects are mostly composed of hydrogen gas that has been pulled into a sphere very tightly as the result of the relentless squeeze of the star’s own gravity. This is the reason why a star’s core becomes so hot and dense. Stars are so extremely hot because their raging stellar fires have been lit as a result of nuclear fusion, which causes the atoms of lighter elements (such as hydrogen and helium) to fuse together to form increasingly heavier and heavier atomic elements. The production of heavier atomic elements from lighter ones, occurring deep within the searing-hot heart of a star, is termed stellar nucleosynthesis. The process of stellar nucleosynthesis begins with the fusion of hydrogen, which is both the lightest and most abundant atomic element in the Cosmos. The process ends with iron and nickel, that are fused only by the most massive stars. This is because smaller stars like our Sun are not hot enough to manufacture atomic elements heavier than carbon. The heaviest atomic elements–such as uranium and gold–are created in the supernovae explosions that end the “lives” of massive stars. Smaller stars go gentle into that good night and puff off their beautiful multicolored outer gaseous layers into the space between stars. These lovely objects, called planetary nebulae, are so beautiful that astronomers call them the “butterflies of the Universe”. Literally all of the atomic elements heavier than helium–the metals–were made in the hot hearts of the Universe’s myriad stars.
The process of nuclear fusion churns out a monumental amount of energy. This is the reason why stars shine. This energy is also responsible for creating a star’s radiation pressure. This pressure creates a necessary and delicate balance that battles against the relentless squeeze of a star’s gravity. Gravity tries to pull all of a stars material in, while pressure tries to push everything out. This eternal battle keeps a star bouncy against its inevitable collapse that will come when it runs out of its necessary supply of nuclear-fusing fuel. At that tragic point, gravity wins the battle and the star collapses. The progenitor star has reached the end of that long stellar road, and if it is sufficiently massive, it goes supernova. This powerful, relentless, merciless gravitational pulling speeds up the nuclear fusion reactions in the doomed star. Where once a star existed, a star exists no more.
Before they meet their inevitable demise, massive stars succeed in fusing a core of iron in their searing-hot hearts. Iron cannot be used for fuel, and at this point the progenitor star-that-was makes its sparkling farewell performance to the Cosmos–sometimes leaving behind a wildly spinning pulsar.
Before the new study, astronomers already knew that SN 2012au was an unusual beast inhabiting the celestial zoo. The weird relic was extraordinary and odd in a number of ways. Even though the supernova blast wasn’t brilliant enough to be termed a “superluminous supernova”, it was bright enough to be quite energetic and last for a long time. It finally dimmed in a similarly slow light curve.
Dr. Milisavljevic predicts that if astronomers continue to observe the sites of extremely bright supernovae, they might see similar sea-changes.
“If there truly is a pulsar or magnetar wind nebula at the center of the exploded star, it could push from the inside out and even accelerate the gas. If we return to some of these events a few years later and take careful measurements, we might observe the oxygen-rich gas racing away from the explosion even faster,” Dr. Milisavljevic commented in the September 12, 2018 Purdue University Press Release.
Superluminous supernovae are transient celestial objects of great interest in the astronomical community. This is because they are potential sources of gravitational waves and black holes, and many astronomers also theorize that they might be related to other forms of celestial blasts, such as gamma-ray bursts and fast radio bursts. Astronomers are trying to understand the fundamental physics that is the basis for them, but they are hard to observe. This is because they are relatively rare and are situated very far from Earth.
The next generation of telescopes, which astronomers call Extremely Large Telescopes, will have the technological ability to observe these mysterious events in greater detail.
This new study aligns with one of Purdue University’s Giant Leaps, space, which is a part of Purdue’s Sesquicentennial 150 Years of Giant Leaps.
Dr. Milisavljevic continued to note that “This is a fundamental process in the Universe. We wouldn’t be here unless this was happening. Many of the elements essential to life come from supernova explosions–calcium in our bones, oxygen we breathe, iron in our blood–I think it’s crucial for us, as citizens of the Universe to understand this process.”
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 some of the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.