A white dwarf star is the ghastly, ghostly relic of a star similar to our Sun that has perished after having consumed its entire necessary supply of fuel in its nuclear-fusing heart. Dense, and sometimes deadly, this particular form of strange stellar relic emerges from the ashes of a funeral pyre belonging to a relatively small star, and it frequently threatens the survival of a still-living companion star that is unlucky enough to be trapped in a binary system with it. In July 2016, a team of astronomers using the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT), along with other telescopes, both on Earth and in space, announced their discovery of a new type of exotic and bizarre binary star. Far, far away, in a system named AR Scorpii, the astronomers found that a rapidly spinning white dwarf star powers a mysterious ray of electrons up to almost the speed of light. Alas, these extremely high energy particles release strong bursts of radiation that crash into its companion red dwarf star, and cause the entire binary system to pulse dramatically every 1.97 minutes with radiation ranging from ultraviolet to radio. The new research describing this strange discovery is published in the July 28, 2016 issue of the journal Nature.
The story behind this weird discovery begins in May 2015, when a group of amateur astronomers from Germany, Belgium, and the UK spotted a star system that was displaying weird behavior unlike anything they had ever seen before. Additional follow-up observations led by the University of Warwick in the UK, using a multitude of telescopes both Earth-bound and Space-borne, have now revealed the true nature of this previously bewitching, bothersome, and bewildering system.
The binary stellar system AR Scorpii, or AR Sco for short, dwells in the constellation Scorpius, which is 380 light-years from Earth. It is composed of a rapidly spinning white dwarf, that is about the same size as our planet, but contains 200,000 times more mass, and a very unfortunate little cool red dwarf companion star that is approximately one-third the mass of our Sun. The ghostly white dwarf and the still-living red dwarf orbit one another every 3.6 hours in a bizarre cosmic waltz that is as regular as clockwork.
Little red dwarf stars are the runts of the true stellar litter. Relatively cool and petite, and still on the hydrogen-burning main sequence of the Hertzsprung-Russell Diagram of Stellar Evolution, they range in mass from a mere 0.075 solar-masses to approximately 0.50 solar-masses, and they possess a surface temperature of less than about 4,000 degrees Kelvin–which makes them relatively cold when compared to other, larger stars.
However, what red dwarfs lack in mass, they make up for in numbers. In fact, red dwarfs are by far the most abundant type of star in our Milky Way Galaxy, at least in our Sun’s general neighborhood. However, because of their low luminosity, individual red dwarfs cannot be easily seen from Earth, and not even one is visible to the naked eye. Proxima Centauri–which is the closest star to our Sun–is a red dwarf, as are twenty of the next thirty of the nearest stars to our own. Some estimates propose that red dwarfs compose three-quarters of all of the stellar inhabitants of our Galaxy.
Red dwarfs that are less than 0.35 solar-masses are fully convective according to stellar models. This means that the helium manufactured by the thermonuclear fusion of hydrogen is constantly being remixed throughout these tiny stars, thus avoiding the buildup of helium in their hot nuclear-fusing cores and prolonging the period of fusion. Convection occurs as a result of the opacity of the stellar interior, which has a very high density compared to the temperature. As a result, energy transfer by radiation is decreased, and convection becomes the primary form of energy transport to the surface of these little stars. Red dwarfs weighing in above 0.35 solar-masses will contain a region around their cores where convection does not occur.
Therefore, little light-weight red dwarfs lead peaceful, lazy, slow “lives”, and potentially can live to a ripe old age, maintaining a constant luminosity and spectral type for trillions of years–until their supply of fuel is finally depleted. Because our Universe is “only” about 13.8 billion years old, no red dwarfs exist at advanced stages of stellar evolution. The less massive the star, the longer its “life.” Unlike massive stars that live fast, and pay for this by dying young, little red dwarfs wisely take their time, and die very old–very, very old! In fact, it has been calculated that a 0.16 solar-mass red dwarf would stay on the hydrogen-burning main sequence for 2.5 trillion years. This would then be followed by five billion years that the evolving star would spend as a blue dwarf, during which the doomed star would possess one third of our Sun’s luminosity, and have a surface temperature of 6,500 to 8,500 degrees Kelvin.
Because of their puny mass, red dwarfs have relatively low pressures, a low rate of nuclear fusion, and a comparatively low temperature. The energy that is churned out is the product of nuclear fusion of hydrogen into helium. These little stars, therefore, do not emit much light. Even the largest red dwarfs possess only about 10% of our Sun’s luminosity.
All of the red dwarfs that have been observed by astronomers contain metals. In the terminology that astronomers use a metal is any atomic element heavier than helium. The Big Bang model postulates that the first generation of stars could only have been composed of hydrogen, helium, and trace quantities of lithium, and hence have a low metallicity. All of the atomic elements that are heavier than helium–the metals–were created in the nuclear-fusing furnaces of the stars that progressively fused heavier atomic elements out of lighter ones (stellar nucleosynthesis). Because the first stars to dance in our Universe had no predecessors to fuse elements heavier than helium, these most ancient of stars could only be manufactured from the very light elements formed in the Big Bang (Big Bang nucleosynthesis).
Because of their very long life spans, any tiny red dwarfs that were among the first generation of primordial stars, should still exist today. But low-metallicity red dwarfs are rare. There are several theories about why metal-poor red dwarfs are rare objects, but the currently favored explanation is that, in the absence of heavy metals, only large and massive stars can form. These massive stars burn out quickly–by star standards–and blast themselves to smithereens in supernovae conflagrations, hurling the newly forged heavy metals out into space where they can then be incorporated into younger generations of stars–allowing higher metallicity stars, including red dwarfs, to be born. The heaviest atomic elements, such as gold and uranium, are formed in the supernova blast itself. Alternative theories explaining the scarcity of metal-poor red dwarfs are considered to be less probable explanations for this mystery because they seem to be in conflict with current stellar-evolution models.
Small, solitary stars that are similar to our Sun–but larger than red dwarfs–do not perish in explosive and deadly supernovae blasts. Instead they undergo a metamorphosis into a bloated red giant star, before they undergo a sea-change into that strange and ghostly stellar relic that astronomers call a white dwarf. A neonatal white dwarf is an extremely dense “oddball” that radiates away the energy of its progenitor star’s collapse, and is usually made up of a soup of oxygen and carbon nuclei swimming around in a bizarre sea of degenerate electrons. White dwarfs usually sit at the heart of a beautiful planetary nebula, composed of shimmering multicolored gases, that are really the ejected outer gaseous layers of the doomed star that it once was.
The first white dwarf discovered by astronomers was a denizen of a triple stellar system named 40 Eridani, which plays host to a relatively bright main sequence star called 40 Eridani A, which in turn is circled at a distance by a closer binary stellar system composed of a white dwarf dubbed 40 Eridani B and a main sequence red dwarf called 40 Eridani C. The binary composed of 40 Eridani B and C was discovered by the German-born British astronomer William Herschel on January 31, 1783.
In a rather strange twist, the binary stellar system AR Sco is exhibiting some unusual, macabre, and disturbingly brutal behavior. AR Sco’s white dwarf constituent is spinning wildly and is highly magnetic. This speedily whirling white dwarf is responsible for accelerating electrons up to almost the speed of light. As these very high energy particles fly screaming through space, they emit radiation in the form of a lighthouse-like beam which lashes across the unfortunate face of the companion red dwarf. This powerful beam of electrons causes the entire system to brighten and then fade dramatically every 1.97 minutes. These very strong pulses include radiation at radio frequencies, which has never been spotted before emanating from a white dwarf system.
“AR Scorpii was discovered over 40 years ago, but its true nature was not suspected until we started observing it in 2015. We realized we were seeing something extraordinary within minutes of starting the observations,” commented lead researcher Dr. Tom Marsh in a July 27, 2016 ESO Press Release. Dr. Marsh is of the University of Warwick’s Astrophysics Group.
The observed properties exhibited by AR Sco are unique. They are also mysterious, bewitching, and intriguing. The radiation across a broad range of electromagnetic frequencies suggests emission from electrons accelerated in magnetic fields, which can be explained by the rapid spin of AR Sco’s wildly whirling white dwarf. However, the source of the electrons themselves remains a major mystery–it has not been determined whether it is associated with the white dwarf itself, or its cooler, tormented, and unfortunate companion red dwarf.
AR Sco was first observed in the early 1970s and regular fluctuations in brightness every 3.8 hours led it to be mistakenly classified as a solitary variable star. The true and bizarre source of AR Sco’s varying luminosity was revealed as a result of the combined efforts of both amateur and professional astronomers. Similar pulsing behavior has been seen before, but from neutron stars. Neutron stars are some of the densest objects in the Universe–they are the ghostly relics of progenitor stars that, during their “lifetime,”were much more massive than our Sun. These massive stars perished in the brilliant and fiery blast of a core-collapse Type II supernova explosion. White dwarfs are very dense, but they are not as dense as neutron stars. This type of pulsating behavior had never before been observed coming from a white dwarf star.
Dr. Boris Gansicke, co-author of the new study, also at the University of Warwick, explains: “We’ve known pulsing neutron stars for nearly fifty years, and some theories predicted white dwarfs could show similar behavior. It’s very exciting that we have discovered such a system, and it has been a fantastic example of amateur astronomers and academics working together.”