A star is born within an especially dense blob tucked within the swirling, whirling folds of a giant, cold, dark and beautiful molecular Cloud. Such clouds float like phantoms through our Milky Way Galaxy in huge numbers, and they serve as the strange cradles of newborn stars. Although it may seem counterintuitive, things have to get very cold in order for a searing-hot, fiery baby star to be born. Young stars are circled by a surrounding disk of gas and dust termed a protoplanetary accretion disk, and these disks contain all of the necessary ingredients from which a system of planets can emerge. Protoplanetary accretion disks form at about the same time the baby star (protostar) does within its obscuring natal cloud. Most of the material contained in the accretion disk gathers at the center, and when this central material becomes massive enough, the process of nuclear fusion lights the newborn star’s stellar fires–for this is how a star is born. In December 2018, astronomers at the University of Leeds (UK) announced that they have captured one of the most detailed views of a young star obtained to date–and their observations revealed something surprising about the unexpected true identity of a companion object orbiting around it.
While carefully observing the young star, the astronomers, led by Dr. John Ilee from the University of Leeds, made the surprising discovery that it is not really one star–but two. The primary object of this duo, dubbed MM ia, is a massive baby star still encircled by its natal protoplanetary accretion disk. The disk was actually the original target of the astronomers’ investigation.
A faint body, dubbed MM 1b, was detected just beyond the disk circling around MM 1a. The University of Leeds astronomers propose that this mysterious object is one of the first examples of a «fragmented» disk to be spotted around a young massive star.
«Stars form within large clouds of gas and dust in interstellar space,» noted Dr. Ilee in a December 14, 2018 Leeds University Press Release. «When these clouds collapse under gravity, they begin to rotate faster, forming a disk around them. In low mass stars like our Sun, it is in these disks that planets can form,» he added. Dr. Ilee is of the School of Physics and Astronomy at Leeds.
«In this case, the star and disk we have observed is so massive that, rather than witnessing a planet forming in the disk, we are seeing another star being born,» he continued to explain.
A Star Is Born
Stars are enormous fluffy balls of searing-hot, glaring, roiling gas. Regardless of their mass, all stars are primarily composed of hydrogen gas. Hydrogen is both the most abundant, as well as the lightest, atomic element in the Universe. In their hidden, hot, roiling hearts, stars transform their abundant supply of hydrogen into progressively heavier and heavier atomic elements. All of the atomic elements that are heavier than helium (the second-lightest atomic element in the Universe) are manufactured in the furnaces of stars–or, alternatively, in the brilliant blaze of a supernova blast that marks the explosive «death» of a massive star. Indeed, the heaviest atomic elements of all–such as gold and uranium–are created in the violent explosion of a brilliant supernova.
The process of stellar nucleosynthesis is the term astronomers use to describe the way that natural abundances of the chemical elements contained within stars experience a strange sea-change. This sea-change is the result of nuclear fusion reactions that occur within stellar cores and their encircling mantles. Stars change as they grow older. These age-related stellar alterations depend on the abundances of atomic elements that stars hold within their seething-hot hearts. Fusion reactions occurring within a star’s core increase the atomic weight of its constituent elements. This causes a reduction of the number of particles. This process may result in the loss of radiation pressure–and the outward push of pressure is necessary to keep the star bouncy against the relentless inward pull of its own merciless gravity. When pressure can no longer battle the devastating pull of gravity, this triggers a contraction that is followed by a skyrocketing of temperature within the dying, doomed star. This means that the glittering multitude of stars inhabiting the Cosmos are all kept fluffy and bouncy as a result of a delicate balance between gravity and pressure. Gravity tries to pull everything in, while radiation pressure tries to push everything out. This precarious balancing act continues from star-birth to star-death–continuing for the entire «lifetime» of the star. A star spends its «lifetime» on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution.
But, all good things must come to an end. When the aging star has finally managed to consume its entire necessary content of hydrogen fuel by way of the process of nuclear-fusion, it must perform its inevitable grand finale to the Universe. At this final, fatal point, gravity wins the ancient battle against its foe, radiation pressure, and the doomed massive star goes supernova.
Planets are also born within the disk that encircles a newborn star. What is left over of the gas and dust, that went into the formation of the central baby star, evenually becomes the protoplanetary accretion disk–from which not only planets, but also moons, asteroids and comets emerge. In the earliest stages, protoplanetary accretion disks are seething hot and extremely massive, and they can hang around their young star for as long as ten million years.
By the time a newborn small star like our Sun has reached what is called the T Tauri stage of its evolution, the disk has grown considerable thinner and much cooler. A T Tauri is a stellar tot–a very young, variable star–that is quite active at the age of a mere 10 million years. These very young, small stars sport large diameters that are several times greater than that of our own «grown up» Sun at present. Stars, like our Sun, can «live» on the hydrogen-burning main-sequence for about 10 billion years, and our Sun is still in mid-life at approximately 4.56 billion years of age.
T Tauris grow smaller as they grow older. Very young Sun-like stars shrink as they «grow up». Indeed, by the time the searing-hot stellar tot has reached the T Tauri stage, less volatile materials have started to condense close to the center of the surrounding accretion disk, forming extremely small and sticky grains of dust. These small and delicate dust particles contain crystalline silicates.
The sticky little dust motes collide with one anoter and merge within the dense environment of the disk. This causes ever larger and larger bodies to form–from grain size, to pebble size, to boulder size, to mountain size, to planet size. These growing bodies eventually become planetesimals–the «building blocks» of planets. Planetesimals can be 1 kilometer across, or even larger, and they are an abundant population within the crowded accretion disk–and some of them can survive long enough to still be present billions of years after a fully-formed system of planets has evolved. In our own Solar Systems, the frozen, dusty comets are the relic primordial building blocks that formed the outer gaseous giant planets: Jupiter, Saturn, Uranus and Neptune. Conversely, the asteroids are what is left of the rocky and metallic planetesimals that formed the inner quartet of solid planets: Mercury, Venus, Earth and Mars.
The Star That Formed Like A Planet
By measuring the amount of radiation produced by the dust, as well as subtle alterations in the frequency of light emitted by the gas, the University of Leeds astronomers were able to calculate the mass of both MM 1a and MM 1b. The research describing the study is published in the December 14, 2018 issue of the Astrophysical Journal Letters, and it reveals the new findings that MM 1a weighs in at 40 times solar-mass, while the smaller orbiting star MM 1b was calculated to weigh less than 50% of the mass of our Sun.
«Many older massive stars are found with nearby companions. But binary stars are often very equal in mass, and so likely formed together as siblings. Finding a young binary system with a mass ratio of 80:1 is very unusual, and suggests an entirely different formation process for both objects,» Dr. Ilee explained in the December 14, 2018 University of Leeds Press Release.
The currently favored explanation for the formation of MM 1b is that it was born in the frigid twilight zone of the outermost region of its star’s accretion disk. These «gravitationally unstable» disks cannot hold themselves up against the merciless pull of their own gravity. For this reason, they collapse into one–or even more–fragments.
Dr. Duncan Forgan, who is a co-author from the Centre for Exoplanet Science at the University of St. Andrews in Scotland, commented in the December 14, 2018 University of Leeds Press Release that «I’ve spent most of my career simulating this process to form giant planets around stars like our Sun. To actually see it forming something as large as a star is really exciting.»
The team of astronomers went on to note that the newly-discovered baby star MM 1b could also be encircled by its very own circumstellar disk, which may have the ability to give birth to a system of planets of its own. However, if MM 1b is to accomplish this stellar parental feat, it will have to be quick.
As Dr. Ilee continued to explain: «Stars as massive as MM 1a only live for around a million years before exploding as powerful supernovae, so while MM 1b may have the potential to form its own planetary system in the future, it won’t be around for long.»
Massive stars «live» fast and «die» young–lasting only millions of years instead of the billions, or trillions, of years that their smaller stellar siblings can survive. Small stars of our Sun’s mass live for about 10 billion years, while the smallest true nuclear-fusing stars in our Galaxy–called red dwarfs–probably have the potential to «live» for trillions of years. Because our Universe is less than 14 billion years old, it is generally thought that no red dwarf has had time enough to «die» since the Big Bang.
The astronomers made their surprising discovery while making use of a one-of-a-kind new instrument that is located high in the Chilean desert–the Atacama Large Millimetre/submillimetre Array (ALMA).
Using the 66 separate radio dishes that compose ALMA, together with a technique called interferometry, the astronomers were able to simulate the power of a single telescope that would be nearly 4 kilometers across. This made it possible for them to image the material encircling the duo of young stars for the first time.
The team of astronomers have been granted additional observing time with ALMA that will enable them to further characterize these fascinating star systems in 2019. The future observations will simulate a telescope that is 16 kilometers across.