A Bizarre Binary Star System

Cataclysmic variables are binary star systems that are usually composed of a type of superdense stellar corpse called a white dwarf and a normal star like our own Sun. They are small systems–and the entire cataclysmic variable is usually only about the same size as our own Earth-Moon system, with an orbital period of a very brief 1 to 10 hours. In July 2015, an international team of astronomers, along with the valuable assistance of amateur astronomers, announced that the Gaia satellite has discovered a unique cataclysmic variable where one star is devouring the other–but neither of the two sister stars has any hydrogen! Hydrogen is the most abundant element in the Universe, and most stars are primarily composed of hydrogen, so this very bizarre system can provide an important tool for understanding how binary stars might blast themselves to pieces in mighty supernovae explosions at the end of their normal, main-sequence (hydrogen-burning) lives! The system can be used as an important laboratory for the scientific study of ultra-bright supernova explosions, which provide a vital tool for measuring the expansion of the Universe.The system, dubbed Gaia14aae, is located about 730 light years away from Earth in the Draco constellation. It was discovered by the European Space Agency’s (ESA’s) Gaia satellite in August 2014 when it suddenly and dramatically became five times brighter over the course of only one day! Gaia was designed for astronometry, and it aims to create a 3D space catalog of approximately 1 billion astronomical objects in our Milky Way Galaxy–mostly very bright stars. It was launched on December 19, 2013.Gaia14aae is a unique binary stellar system that is the first of its kind to be discovered by astronomers. The system includes one sister star that completely eclipses the other. In this system, the white dwarf is gulping down gas from its companion star–and victim–effectively “cannibalizing” it.Astronomers led by the University of Cambridge in the UK analyzed the information collected from Gaia and determined that the sudden, brilliant, and dramatic outburst resulted from the fact that the white dwarf is eating its larger companion star. A white dwarf is so dense that a teaspoon of its material would weigh as much as a whale.Additional observations of this strange system were conducted by the Center for Backyard Astrophysics (CBA), which is a collaboration between amateur and professional astronomers. The astronomers discovered that the weird system is a rare eclipsing binary, where one star floats directly in front of the other, totally blocking out its fiery stellar light when observed from Earth. The stellar duo are tightly orbiting each other, resulting in the occurrence of a total eclipse every 50 minutes.There are probably more than a million of these CVs in our Galaxy, but only those close to our Star–several hundred–have been studied in X-rays so far. This is because CVs are dim in X-rays.White Dwarf SupernovaeOur Milky Way Galaxy’s 200 to 400 billion stars were born as a result of the gravitational collapse of an especially dense blob embedded in one of the very numerous frigid, dark, and enormous molecular clouds that float around throughout our Galaxy.Cold molecular clouds are primarily composed of gas, with a smaller amount of dust, and they can be found everywhere in our Milky Way. The dark, billowing, and frigid clouds serve as strange nurseries for baby stars (protostars), and these undulating, ghostly stellar cradles tend to mix themselves up together and combine. However, stars that share a kindred chemistry commonly reveal themselves within the same clouds at about the same time.Our Solar System formed from jumbled fragments left over from the dead, nuclear-fusing cores of previous generations of stars. In the secretive folds of a dark, vast molecular cloud, a dense fragment ultimately collapsed under the pull of its own gravity to give rise to the new protostar. In the hidden billowing depths of such mysterious, dark clouds, dense pockets form, where fragile threads of material gradually clump together and merge–growing in size for hundreds of thousands of years. Then squeezed together tightly by the crush of gravity, the hydrogen atoms within this dense pocket suddenly fuse, lighting a fabulous stellar fire that will flame for as long as the protostar lives–for that is how a star is born.Our Sun is a middle-aged, main-sequence (hydrogen-burning), relatively small Star. As stars go, it is not particularly special. There are planets and an assortment of other objects, both large and small, orbiting our Sun, which is located in the distant suburbs of our Galaxy in one of its starlit spiral arms.In another 5 billion years, or so, our Sun will die. Stars do not live forever, and a star of our Sun’s relatively small mass lives for about 10 billion years. Our 4.56 billion year old Sun, and stars like our Sun that are still in main-sequence middle-age, have retained enough of their youthful bounce and roiling heat to go on actively burning hydrogen in their cores by way of nuclear fusion–which serves to form heavier atomic elements out of lighter ones in a process termed stellar nucleosynthesis. When our Sun, and stars like our Sun, have finally consumed their necessary supply of hydrogen fuel in their hot nuclear-fusing hearts, their appearance starts to change dramatically. A terrible beauty is born, and the doomed star is now old. In the seething hot core of an elderly Sun-like star, there is a core of helium, surrounded by a shell in which hydrogen is still being fused into helium. Helium is the second-lightest atomic element in the Universe, after the lightest and most abundant element, hydrogen. The shell starts to swell outward, and the heart of the elderly star grows bigger as the star ages. The helium core itself then begins to shrink under its own weight, and it grows very, very hot until, finally, it becomes sufficiently hot at the core for a new era of nuclear fusion to begin. Now, at this new stage, it is the helium that is being fused to create the still heavier atomic element, carbon. Five billion years from now, our Star will harbor a small and searing-hot heart that will be emitting more energy than our middle-aged Star is at present. The outer gaseous layers of our roiling Sun will have swollen up to hideous proportions, and it will no longer be a beautiful, small, brilliant little Star. It will have evolved into a fiery-red, swollen, hot and enormous sphere of gas that is termed a red giant. Our Sun, in its swollen red giant phase will grow large enough to consume Mercury in its stellar flames, before it goes on to swallow Venus–and afterwards, possibly, our own scorched planet. The temperature at the surface of this immense swollen red sphere of hot gas will be considerably cooler than that of our Sun’s surface today. This explains its (comparatively) “cool” red hue. Nevertheless, our gigantic red, flaming, swollen elderly Sun will still be hot enough to alter the frigid denizens of the distant Kuiper Belt, such as the ice dwarf Pluto and its moons and other frozen kin, into tropical paradises–at least for a while. The core of our dying, old Sun will continue to shrink, and because it is no longer able to manufacture radiation by way of the process of nuclear fusion, all further evolution will be the result of gravity. Our Sun will finally hurl off its outer layers. The heart of our Star, however, will stay in one piece, and all of our Sun’s material will ultimately collapse into a tiny, relic stellar-corpse–the superdense white dwarf, that is only about the same size as Earth. The new white dwarf will be encircled by a lovely shell of expanding multicolored gases termed a planetary nebula. These beautiful, shimmering objects are sometimes referred to as the “butterflies of the Cosmos” because of their great beauty.A white dwarf radiates away the energy of its collapse, and is normally composed of carbon and oxygen nuclei swimming around in a pool of degenerate electrons. The equation of state for degenerate matter is “soft”. This basically means that any contribution of more mass to the object will result in an even smaller white dwarf. Adding ever more and more mass to the white dwarf only results in further shrinkage, and its central density will become even greater. The dead star’s radius ultimately shrinks to a mere few thousand kilometers. Therefore, a white dwarf star, like our future Sun will become, is destined to grow cooler and cooler over time.Small stars like our Sun die much more quietly than their more massive stellar kin–if they are solitary stars, like our Sun. However, if a white dwarf dwells in close contact with another star in a binary system, explosive things can occur. Massive stars blow themselves up at the end of their stellar lives in the fireworks of a supernova blast. Similarly, when a cannibalistic white dwarf has managed to reach the Chandrasekhar limit of 1.4 solar masses–after lunching on too much of its sister star–it may acquire sufficient mass to blow itself to smithereens in a supernova blast, just like the big guys. This violent and explosive event is termed a Type Ia supernova. As the small star approaches this limit, pressure mounts up and the internal temperature skyrockets enough for carbon fusion to take place. Most white dwarfs are composed primarily of carbon, and when this fusion occurs, all of the carbon experiences fusion instantly. The result is a Type Ia supernova.A Bizarre Binary Star System”It’s rare to see a binary system so well-aligned. Because of this, we can measure this system with great precision in order to figure out what these systems are made of and how they evolved. It’s a fascinating system–there’s a lot to be learned from it,” explained Dr. Heather Campbell in a July 2015 Cambridge University Press Release. Dr. Campbell, who led the follow-up campaign for Gaia14aae, is of Cambridge’s Institute of Astronomy in the UK.Using spectroscopy from the William Herschel Telescope in the Canary Islands, Dr. Campbell and her team discovered that Gaia14aae harbors large quantities of helium, but no hydrogen–which is very odd because hydrogen is the most common atomic element in the Universe. Because of this lack of hydrogen, the astronomers were able to classify Gaia14aae as an extremely rare type of system termed an AM Canum Venaticorum (AM CVn), a special type of cataclysmic variable system where both sister stars have lost all of their hydrogen. This is the first known AM CVn system where one star completely eclipses the other.”It’s really cool that the first time that one of these systems was discovered to have one star completely eclipsing the other, that it was amateur astronomers who made the discovery and alerted us. This really highlights the vital contribution that amateur astronomers make to cutting edge scientific research,” Dr. Campbell noted in the Cambridge University Press Release.AM CVn systems are composed of a hot, small white dwarf, which is feeding on its larger, still-”living” companion. The gravitational effects from the superdense, searing-hot white dwarf are so extremely powerful that it has forced its sister star to balloon up to enormous proportions and travel towards it.The still-”living” companion star is approximately 125 times the volume of our own Sun, and it is considerably larger than the petite, but sinister, white dwarf–which is only about the same size as Earth! The difference in size of these two very unusual stellar sisters has been compared to a hot air balloon and a marble. However, the swollen companion star is light in weight and it weighs in at a mere one percent of the white dwarf’s mass.Astronomers consider AM CVn systems valuable because they could shed light on one of the greatest and most nagging mysteries in modern astrophysics: what causes Ia supernova explosions? That is the question. Type Ia supernovae occur in binary systems, and they are important because their extreme brilliance makes them an important tool to measure the expansion rate of the Cosmos.In the case of Gaia14aae , it is unknown whether the stellar duo composing the system will collide and trigger a supernova blast, or whether the white dwarf will manage to eat its unlucky companion first.”Every now and then, these sorts of binary systems may explode as supernovae, so studying Gaia14aae helps us understand the brightest explosions in the Universe,” explained Dr. Morgan Fraser to the press. Dr. Fraser is of Cambridge’s Institute of Astronomy.”This is an exquisite system: a very rare type of binary system in which the component stars complete orbits faster than the minute hand on a clock, oriented so that one eclipses the other. We will be able to measure their sizes and masses to a higher accuracy than any similar system; it whets the appetite for the many new discoveries I expect from the Gaia satellite,” Dr. Tom Marsh commented in the July 2015 Cambridge University Press Release. Dr. Marsh is of the University of Warwick in the UK.Dr. Simon Hodgkin, who is leading the hunt for more transients in Gaia data, told the press in July 2015 that “This is an awesome first catch for Gaia, but we want it to be the first of many. Gaia has already found hundreds of transients in its first few months of operation, and we know there are many more out there for us to find.”