Our solitary Sun blasts our daytime sky with its shimmering fires, sending both welcoming warmth and light to Earth. Our Sun, and its familiar family of planets, moons, and smaller objects, emerged about 4.56 billion years ago from jumbled relics left over from the now-dead, nuclear-fusing furnaces of ancient stars that have long since vanished–their light was switched off forever when they ran out of their necessary supply of fuel to keep them searing-hot and shining. Our Sun, as well as other stars, are born within the swirling, whirling depths of one of the many beautiful, dark, and cold molecular clouds that roam through our Milky Way Galaxy in huge numbers. When a dense pocket tucked within the undulating whorls of one of these eerie clouds collapses, under the merciless pull of its own relentless gravity, a new star is born. Yet, despite the many new discoveries scientists have made about our mysterious Cosmos, many uncertainties still remain about the birth of our own Solar System. In December 2017, researchers from the University of Chicago announced that they have laid out a new comprehensive theory for how our Solar System could have formed in the wind-blown bubbles around a giant, long-dead star.
The prevailing theory describing how our Solar System came into being proposes that it formed billions of years ago near a supernova–a powerful, brilliant stellar explosion that heralds the “death” of a massive star. According to this model, the heavy atomic elements in our Solar System, as well as on our own planet, can only be explained as the outcome of a supernova conflagration. All atomic elements, heavier than helium, that are called metals in the terminology of astronomers, were manufactured in the nuclear-fusing cores of the many stars inhabiting the Universe. The Big Bang birth of the Cosmos–thought to have occurred about 14 billion years ago–produced only the very lightest of atomic elements: hydrogen, helium, and trace quantities of lithium. The stars produced the rest, and then hurled them throughout the Universe, when they perished explosively in the fatal fires of a supernova blast. The supernova blast itself is responsible for the heaviest atomic elements of all, such as gold and uranium.
The “parent supernova” of our Solar System would have probably left behind a souvenir of its erstwhile existence in the form of either a neutron star or stellar-mass black hole. However, there is no way for astronomers to determine this. That is because our Solar System’s supernova “parent”, that blasted a progenitor star to smithereens between 4.5 to 5 billion years ago [the age of our Solar System] perished a long, long time ago, and there is no way for astronomers to determine what our Galaxy was like after the passage of such a vast amount of time. In addition, there is also no way to know what important events have occurred within our Galactic neighborhood between the present time and 5 billion years ago. During those 5 billion years, our Sun has roamed around our Milky Way Galaxy about 20 to 30 times. Also, the “parent supernova” relic could have experienced a number of significant events. Stars usually wander around their host galaxy–but they also travel relative to one another, and it is quite a challenge to determine the exact movements the proposed “parent” supernova–and its lingering remnants–experienced 5 billion years ago. After all this time, the “parent” supernova and its relic might have even left our Milky Way–and zipped into intergalactic space.
Many astronomers hypothesize that our infant Sun was either unceremoniously evicted from its birth cluster, or it simply drifted away of its own accord, as it traveled to more remote regions of our Milky Way Galaxy. Indeed, there may well have been as many as 3,500 of these nomadic stellar siblings, according to recent supercomputer simulations. Evidence of our Sun’s birth cluster may be preserved in the anomalous chemical abundances and structure of our Solar System’s distant, frigid Kuiper belt. The Kuiper belt is situated in the outer limits of our Sun’s family, where a vast multitude of small–as well as not so small–icy objects circle our Star beyond the orbit of the deep-blue-banded, ice-giant planet Neptune–the outermost of the eight major planets from our Sun. Some of the frozen inhabitants of the Kuiper belt are dynamically “hot”. This means that they were shaken up and scattered by the gravity of at least one of our Sun’s neighboring cluster siblings. These sister stars zipped closely past one another very long ago–soon after their birth. Like other open clusters, however, our Sun’s birth cluster fell apart as time passed. Our Star’s lost sparkling sisters have by now traveled far away, and many of them are probably lost to us forever.
Our Sun, like its shining sisters, was born within a particularly dense blob embedded within a molecular cloud. This blob eventually collapsed under the pull of its own gravity to give birth to a new star. In the secret depths of these vast, dark clouds, composed of gas and dust, fragile and delicate tendrils of material gradually merge and clump together–growing and growing for hundreds of thousands of years. Finally, squeezed together by the crush of gravity, hydrogen atoms within this clump suddenly and dramatically fuse. This lights the baby star’s stellar fire that will burn for as long as the new star lives, for that is how a star is born.
All of the billions of stars in our Milky Way Galaxy were born this way–from the collapse of a dense blob tucked within a cold molecular cloud composed of gas and dust. These star-birthing clouds carry within them the newly-forged heavy metals manufactured by older generations of stars–now “dead”. These giant clouds tend to mix themselves up together, thus combining their various contents. However, stars of kindred chemistry usually show up within the same clouds at about the same time.
The new model proposed by the University of Chicago scientists differs from the “parent supernova” scenario. According to the new model, the story does not start with a supernova blast–but begins instead with a type of giant star called a Wolf-Rayet star. These stars are more than 40 to 50 times the size of our Sun.
Wolf-Rayet stars are a rare heterogeneous set of stars that display odd spectra that show prominent broad emission lines of highly ionized helium and nitrogen or carbon. The spectra suggest that there is a very high surface enhancement of heavy metals, depletion of hydrogen, and powerful stellar winds. Their surface temperatures range from 30,000 K to approximately 200,000 K, which make Wolf-Rayets hotter than most other stars.
Classic or Population I Wolf-Rayets are evolved, massive stars, that have completely lost their outer hydrogen and are in the process of fusing helium or heavier elements in their cores. All Wolf-Rayet stars are extremely luminous spheres as a result of their high temperatures. Indeed, these massive stars are so luminous that they sport a bolometric luminosity that is thousands of times that of our Sun–Population I Wolf-Rayets can be over a million times more luminous than our Star.
A trio of stars Gamma Velorum and Theta Muscae, as well as the most massive of all stars known to astronomers, R136a1, which is located in 30 Doradus, are all Wolf-Rayet stars.
Because they are the hottest known stars, Wolf-Rayets manufacture tons of atomic elements which they hurl off their surfaces in a powerful stellar wind. As the Wolf-Rayet sheds its mass, the ferocious stellar wind shoots through the material that had been surrounding it, thus creating a bubble structure with a dense shell.
“The shell of such a bubble is a good place to produce stars,” because dust and gas become imprisoned inside where they can then condense into fiery newborn stars, commented study co-author Dr. Nicolas Dauphas in a December 22, 2017 University of Chicago Press Release. Dr. Dauphas is a professor in the University of Chicago’s Department of Geophysical Sciences. The study’s authors estimate that 1 percent to 16 percent of all stars like our Sun could be born in just such strange stellar cradles.
How Our Solar System Formed: The Answer Is Blowin’ In The Wind
The new theory, proposed by the University of Chicago scientists, differs from the supernova model in order to explain the existence of two bewildering isotopes that occurred in strange proportions in our primordial Solar System–when compared to the rest of our Milky Way Galaxy. Meteorites lingering from the early Solar System reveal to scientists that there was a large amount of aluminum-26. Furthermore, studies increasingly indicate that our Solar System had less of the isotope iron-60 than the rest of our Galaxy.
This, of course, causes some confusion. This is because supernovae produce both isotopes. “It begs the question of why one was injected into the Solar System and the other was not,” explained study co-author Dr. Vikram Dwarkadas in the December 22, 2017 University of Chicago Press Release. Dr. Dwarkadas is a research associate professor in Astronomy and Astrophysics at the University of Chicago.
This is what inspired the scientists to consider Wolf-Rayet stars, which hurl out large amounts of aluminum-26, but no iron-60.
“The idea is that aluminum-26 flung from the Wolf-Rayet star is carried outwards on grains of dust formed around the star. These grains have enough momentum to punch through one side of the shell, where they are mostly destroyed–trapping the aluminum inside the shell,” Dr. Dwarkadas explained in the University of Chicago Press Release. Ultimately, part of the shell collapses inward as a result of the pull of gravity–thus forming our Solar System.
As for the fate of the parental Wolf-Rayet star that sheltered us, and is responsible for our existence–well, its own “life” was over very long ago. This massive star probably perished in a supernova explosion, or directly collapsed to a black hole. A direct collapse to a black hole would churn out very little iron-60; if it was a supernova, the iron-60 produced in the horrific stellar explosion may not have torn through the bubble walls–or was distributed unevenly.
Other authors on the paper included University of Chicago undergraduate student Peter Boyajian and Michael Bojazi and Brad Meyer of Clemson University.
The article, titled Triggered star formation inside the shell of a Wolf-Rayet bubble as the origin of the solar system,” is published in the December 22, 2017 edition of The Astrophysical Journal.