Stars do not live forever, and when they perish they can disappear into eternity with either a bang or a whimper, depending on their mass. Furthermore, stars can do some very strange things when they come to the end of that long stellar road–and a distant dying star dubbed V Hydrae has been observed to be behaving very strangely, indeed. In October 2016, astronomers announced that NASA’s Hubble Space Telescope (HST) has spotted some searing-hot blobs of glaring gas, each twice as massive as the planet Mars, being shot out near V Hydrae. The big blobs of super-hot plasma zoom out so fast into the surrounding space that it would take only half-an-hour for them to zip from Earth’s Moon to our planet. This puzzling stellar “cannon fire” has been occurring once every 8.5 years for at least 400 years–and now astronomers have proposed a new explanation for this strange behavior among the stars.
The fiery blobs of super-hot gas present a mystery for astronomers to solve. This is because the rudely evicted material, that is being shot into space like a series of celestial “cannonballs”, cannot have come from V Hydrae. This dying star is a swollen red giant, dwelling 1,200 light-years away from Earth, which has probably ejected at least 50% of its mass into the space between stars during its final death throes. Red giants are the doomed and dying remains of small Sun-like stars that have grown old, after having exhausted their necessary amount of nuclear-fusing-fuel–and have now bloated up to monstrous proportions as they approach the sad end of their shining stellar existence.
Of Red Giant Stars And Celestial “Butterflies”
Red giant stars are enormous, luminous old stars that have evolved from small or intermediate mass stars of approximately 0.3 to 8 times solar-mass. These gigantic, elderly stars are in the late stages of stellar evolution, and are about to meet their inevitable destiny. Yellow-orange to red in color, the outer atmosphere of a red giant is both tenuous and bloated–which is the reason why these old stars possess such huge radii, as well as low surface temperatures of only 5,000 Kelvin–or lower.
Even though red giants are about to go gentle into that good night, they still have some life left in them because they are still capable of fusing гидра онион into helium in a shell surrounding a degenerate core of helium. The closest red giant star to Earth is Gamma Crucis, which is a mere 88 light-years away. However, there is an intriguing orange giant star, dubbed Arcturus, that some astronomers think is a red giant–and it is only 36 light-years away.
Red giants display impressive radii tens to hundreds of times greater than that of our Sun. However, their outer gaseous envelope is much cooler, which is the reason why they show a cool reddish-orange hue. Even though these gigantic old stars are many times more luminous than our Sun, their gaseous envelopes have lower energy density because of their huge size. In fact, these dying stars display luminosities that are approximately one hundred to several hundred times that of our Sun!
Red giants are the remains of small Sun-like stars that were on the hydrogen burning main sequence of the Hertzsprung-Russell Diagram Of Stellar Evolution before they used up their necessary supply of fuel. An infant star, or protostar, is born as the result of the collapse of an especially dense pocket located within the undulating, swirling folds of one of the numerous, billowing, cold, and dark giant molecular clouds that float like lovely phantoms throughout our Milky Way Galaxy. Giant frigid molecular clouds are primarily made up of hydrogen and helium, sprinkled with only a pinch of heavier atomic elements. Hydrogen (the most abundant atomic element in the Universe), and helium are the two lightest of all atomic elements, and both were born in the inflationary Big Bang birth of the Universe almost 14 billion years ago–along with very small amounts of lithium and beryllium. All atomic elements heavier than helium are termed metals in the terminology that astronomers use, and all of the metals were formed in the nuclear-fusing furnaces of our Universe’s myriad of stars–or else in the violent, furiously hot and brilliant supernovae explosions that dramatically herald the raging death throes of massive stars. The heaviest atomic elements of all are born in the fantastic fireworks of glaring supernovae blasts.
The metals are all uniformly dispersed throughout the entire neonatal star. The baby star at last reaches the hydrogen-burning main-sequence when its core grows sufficiently hot enough to commence the process of nuclear-fusion–whereby lighter atomic elements are fused into ever heavier atomic elements. The process of nuclear-fusion begins at the toasty temperature of a few million Kelvin, and at these temperatures the baby star can at last begin to fuse hydrogen in its heart and establish hydrostatic equilibrium. Throughout the entire main-sequence “life” of the star, it gradually continues to fuse the hydrogen in its searing-hot core into helium. The “life” of a star comes to an end when almost all of the hydrogen in its heart has been fused.
Our own Star today was also born within a dense pocket embedded within the billowing, swirling folds of a dark and frigid giant molecular cloud composed of mostly hydrogen gas and a pinch of dust. Today our Sun is a small middle-aged main-sequence Star sometimes called a “yellow dwarf”. There is nothing particularly special about our Sun–it is like billions of others of its kind scattered throughout Space and Time. There are planets, moons, and an assortment of smaller objects circling our Star, which is comfortably situated in the far suburbs of our large, majestic, barred-spiral Milky Way Galaxy, in one of its pin-wheel-like arms.
Today, our Sun is still experiencing an active, productive, and searing-hot middle-age. Alas, like other stars, our Sun is doomed to die. The good news is that our 4.6 billion-year-old Star still has another 5 billion years or so before it comes to the end of the road. Our Sun is still young and bouncy enough to continue to fuse lighter atomic elements into heavier ones (stellar nucleosynthesis). However, when our Sun–and other Sun-like stars–have finally fused their entire necessary supply of hydrogen in their cores, their looks change, and they start to show their age. In the heart of an older Sun-like star lurks a heart of helium that is surrounded by a shell in which hydrogen is still being fused into helium. The shell ultimately swells outward, and the hot helium heart grows larger and larger, as the elderly star ages. The helium heart itself finally shrivels under its own heavy mass, and it grows extremely hot. This triggers a new stage of nuclear-fusion. Now, it is the helium that is fused to form the heavier atomic element, carbon.
About 5 billion years from now, our Sun will contain a searing-hot and relatively small heart that will be pouring out more energy than our still-main-sequence Star is today. The outer gaseous layers of our Sun will have swollen up to monstrous proportions, and it will have undergone a sea-change into a bloated red giant.
The shriveled heart of our dying Star will continue to shrink further. Because our Sun can no longer manufacture energy by way of nuclear-fusion reactions, all further evolution will depend only on the force of gravity. At last, our Star will toss off its outer gaseous layers, but its heart will remain in one piece. Our Star’s material will ultimately collapse into this tiny remnant object that is only approximately the same size as our own small, rocky planet. In this way, our Sun will enter the end-stages of its existence as a form of stellar corpse called a white dwarf. The new white dwarf will be surrounded by a brilliantly dazzling and beautiful expanding shell of varicolored gases called a planetary nebula. These lovely, luminous objects are so beautiful that astronomers frequently refer to them as “the butterflies of the Universe.” However, they are relatively short-lived, and can only dazzle the Cosmos for a few tens of thousands of years–compared to the much lengthier stellar lifetime of several billion years.
A white dwarf is an extremely dense object that radiates away the energy of its stellar collapse, and is made up of a bizarre soup of carbon and oxygen nuclei swimming around in a strange sea of degenerate electrons. Adding more mass to a white dwarf will only force it to shrink even further than it already has–and its central density will grow even larger. The dead star’s radius ultimately shrinks to only a few thousand kilometers–and, at this stage, our Sun and other stars that are similar to it are doomed to grow progressively cooler as time passes.
In the end, our Star will likely undergo a sea-change into an object called a black dwarf. Black dwarf stars are hypothetical bodies because astronomers generally think that none exist in our Universe–as yet! It takes hundreds of billions of years for a white dwarf to cool down to the black dwarf stage–and our Universe is “only” about 13.8 billion years old.
Dazzling Fireballs Are Shot Out From A Dying Star
The V Hydrae system could serve as the archetype to explain a dazzling and diverse variety of beautiful, glowing shapes unveiled by HST that are observed surrounding other dying stars.
“We knew this object had a high-speed outflow from previous data, but this is the first time we are seeing this process in action. We suggest that these gaseous blobs produced during this late phase of a star’s life help make the structures seen in planetary nebulae,” commented Dr. Raghvendra Sahai in an October 6, 2016 Hubblesite Press Release. Dr. Sahai is of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, and lead author of the study.
Over the past twenty years, HST observations have unveiled an enormous diversity and complexity in the structures of planetary nebulae. HST’s high resolution revealed embedded knots of material within the glowing, varicolored clouds of shimmering gas encircling the dying stars. Astronomers have proposed that these captured knots of material are actually jets of material surrounding companion stars that could not be seen in the HST images.
“We want to identify the process that causes these amazing transformations from a puffed-up red giant to a beautiful glowing planetary nebula. These dramatic changes occur over roughly 200 to 1,000 years, which is the blink of an eye in cosmic time,” Dr. Sahai continued to explain.
Dr. Sahai’s and his colleagues used HST’s Imaging Spectrograph (STIS) in order to observe V Hydrae and its surrounding region over a time span of 11 years, first from 2002 to 2004, and then again from 2011 to 2013. Spectroscopy studies the light being emitted by an object, revealing important information concerning its temperature, location, velocity, and motion.
The collected data revealed a steady stream of enormous, glaring, superhot blobs, each with a temperature of over 17,000 degrees Fahrenheit–which makes them twice as hot as the surface of our Sun. The astronomers put together a detailed map of the blobs’ location, enabling them to track the first behemoth clumbs all the way back to 1986. “The observations show the blobs moving over time. The STIS data show blobs that have just been ejected, blobs that have moved a little farther away, and blobs that are even farther away,” Dr. Sahai explained in the October 6, 2016 Hubblesite Press Release. STIS spotted the enormous glaring celestial “cannonballs” as far as 37 billion miles away from V Hydrae–more than eight times farther away than the Kuiper Belt is from our Sun. The Kuiper Belt is a distant, dimly-lit region where a dancing myriad of icy comet nuclei circle our Star beyond the orbit of Neptune–the most distant of the eight major planets from our Sun.
The glaring fireball-like blobs expand and cool as they travel farther away, and as a result they become undetectable in visible light. However, observations obtained at longer sub-millimeter wavelengths in 2004, by the Submillimeter Array in Hawaii, unveiled mysterious knotty, fuzzy structures that are possibly the blobs that were cast out into space four centuries ago.
“This model provides the most plausible explanation because we know that the engines that produce jets are accretion disks. Red giants don’t have accretion disks, but many most likely have companion stars, which presumably have lower masses because they are evolving more slowly. The model we propose can help explain the presence of bipolar planetary nebulae. We think this model has very wide applicability,” Dr. Sahai noted in the JPL Press Release.
The STIS observations were surprising because they revealed that the disk does not fire these enormous glaring “cannonballs” in precisely the same direction every 8.5 years. Amazingly, their direction flip-flops from back-and-forth and from side-to-side. This odd behavior suggests that it may be the result of a wobble in the accretion disk. “This discovery was quite surprising, but it is very pleasing as well because it helped explain some other mysterious things that had been observed about this star by others,” Dr. Sahai continued to explain.
It has also been noted by astronomers that V Hydrae is obscured every 17 years. This observation hints that something is blocking this very strange star’s light. Dr. Sahai and his team propose that as a result of the back-and-forth wobble of the jet direction, the glaring blobs alternate between passing behind and in front of V Hydrae. Therefore, if one of these enormous glaring fireballs shoots out in front of V Hydrae, it blocks the red giant from view.
“The accretion disk engine is very stable because it has been able to launch these structures for hundreds of years without falling apart. In many of these systems, the gravitational attraction can cause the companion to actually spiral into the core of the red giant star. Eventually, though, the orbit of V Hydrae’s companion will continue to decay because it is losing energy in this frictional interaction. However, we do not know the ultimate fate of this companion,” Dr. Sahai explained in the October 6, 2016 Hubblesite Press Release.
The team of astronomers hope to use HST to conduct additional observations of the bizarre V Hydrae system, including the most recent celestial “cannonball” shot out in 2011. The astronomers also plan to use the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to observe blobs ejected over the past few hundred years that are currently too cool to be spotted by HST.