Hungry Baby Stars Are Nourished In Their Stellar Nursery

Fiery baby stars are born within dark and frigid stellar nurseries that are tenderly tucked within the amorphous, swirling, whirling folds of beautiful, giant molecular clouds. These strange and numerous clouds are widely dispersed throughout our Milky Way Galaxy. Primarily composed of hydrogen, along with lesser amounts of dust, these phantom-like star-birthing clouds contain the mass of thousands, or even millions, of Suns–and they hide dense blobs that are embedded like black pearls in their mysterious, undulating folds, that eventually condense to give birth to new, brilliant, voracious baby stars. In February 2016, an international team of astronomers, led by researchers at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) in Taiwan, announced that they have used a new infrared imaging technique to unveil the first dramatic, hidden, and secret moments of star and planet birth. These seem to occur when ambient material somersaults down to very active neonatal stars, which then feed hungrily on it, even as they remain hidden within their nurturing natal nursery clouds.

The team of scientists used the High Contrast Instrument for the Subaru Next-Generation Adaptive Optics (HICIAO) camera on the Subaru 8-meter Telescope in Hawaii to observe a batch of beautiful baby stars. The results of their research sheds new light on our scientific understanding of how stars and planets form.

A molecular cloud, which is a type of interstellar cloud, is sometimes called a stellar nursery–if star-birth is occurring within it. The density and size of these clouds enable the formation of molecules–mostly molecular hydrogen (H2). This contrasts with other areas of the space between stars that contain mostly ionized gas.

Within our Milky Way Galaxy, molecular clouds account for less than one percent of the volume of the interstellar medium (ISM), and yet they also represent the densest portion of the medium, comprising approximately 50% of the total gas mass interior to our Sun’s Galactic orbit. Most of the molecular gas is located within a ring that is between 11,000 and 24,000 light-years from the central heart of our Galaxy.

As time goes by, gravity in the densest parts of these dark, phantom-like clouds pulls in the surrounding gas and dust, resulting in what is termed accretion. It is generally assumed that this process is continuous and smooth. However, this steady downpour of gas and dust explains only a small percentage of the final mass of each sparkling baby star that is born within the folds of these strange clouds. Astronomers are still hard at work trying to gain an understanding of when and how the remaining material is collected during the process of star and planet birth.

Most of the infalling nourishing gas and dust tumbles into the center to become the baby star, or protostar. The extremely hot core of the protostar is situated in the very heart of the collapsing blob of dense material. Three-dimensional supercomputer simulations show that the swirling blobs of collapsing dust and gas fragment into two or three separate portions. This explains why most of the stars in our own Milky Way dwell in binary and multiple star systems, rather than being solitary–like our lonely Sun. As the blob collapses under the relentless pull of its own gravity, the seething hot core takes shape and begins to pull in gas and dust. But not all of this material becomes part of the new star. The remaining material that circles around the baby star may become planets, moons, comets, and asteroids–or it may merely hang around the newborn star as dust.

Within the dark and secretive folds of molecular clouds, baby stars light up the ambient darkness with their furious fires, as they burst into sparkling stellar existence within their starry nurseries. Deep within the billows of these giant, dark clouds, fragile threads of star-making material twist themselves around each other, and ultimately merge, continuing to grow larger and larger for hundreds of thousands of years. The merciless hug of gravity finally becomes so crushing that the hydrogen atoms within these blobs suddenly and dramatically fuse. This ignites the baby star’s fabulous fires, and the brilliant flames will continue to rage brightly for as long as the baby star “lives.”

Nuclear fusion is the process that lights up a star, and it creates ever heavier and heavier atomic elements out of lighter ones (stellar nucleosynthesis). Searing-hot, roiling, sparkling protostars keep themselves “alive” by balancing two warring forces: gravity and radiation pressure. All stars that are on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of stellar evolution maintain a necessary balance between these two opposing forces. While the relentless tugging of of crushing gravity pulls in the ambient gas, radiation pressure keeps the star bouncy and fluffy by pushing everything out and away from the star. The critical balance that exists between gravity and radiation pressure keeps the star brilliantly “alive”, and on the hydrogen-burning main-sequence. But stars, like people, inevitably must grow old and ultimately perish–and when an elderly star has finally managed to burn its entire necessary supply of hydrogen fuel, its core collapses, and the star is ready to make its final farewell performance to the Universe. Stars that are relatively small, like our Sun, perish peacefully and beautifully by tossing their outer layers of shimmering, glimmering varicolored gases into the space between stars. The tragic remnant core of a small star eventually evolves into a type of stellar ghost termed a white dwarf. However, stars that are larger and more massive do not die gently. Instead, massive stars meet their tragic fate with a terrible and awe-inspiring fury. Massive stars, when they have reached the end of the line, blow themselves up into shreds in the raging tantrum of a Type II (core-collapse) supernova.

Giant, billowing, cold and dark molecular clouds are the precursors of what are called HII regions. HII regions are brilliant and dazzling objects as they toss their magnificent light out into the interstellar medium. The immense molecular clouds can remain in a stable state for a very long time, but collisions between clouds, magnetic interactions, and supernova blasts can trigger collapse. When this collapse occurs, sparkling baby stars are born. An HII region usually looks irregular and clumpy, and could easily serve as the cradle for literally thousands of protostars over the passage of several million years. Some of these sparkling protostars can cause the HII region to shine with light, and also sculpt its shape. HII regions, in fact, have been observed to sport a variety of shapes, because the gas and stars embedded within them are irregularly scattered.

Once the baby stars reach the stellar toddler stage of their “lives”, they become the source of fierce and powerful winds that are made up of escaping particles that soar their screaming way in the opposite direction away from these brilliant, ferocious young stars. The rushing winds both shape and blast away the surrounding gases.

A few baby stars have been observed to be associated with a dramatic and violent “feeding” frenzy from within their stellar nursery. When these sparkling protostars voraciously feed on the surrounding material, their visible light flares up very suddenly–by a factor of about a hundred. These dramatic, rapid “outbursts” of visible light are termed FU Orionis outbursts because they were first observed toward the star FU Orionis.

Only a small number of stars are found to be associated with such flare-ups–only a dozen out of thousands of stars observed. However, astronomers speculate that all baby stars may undergo such flare-ups as part of their growth into stellar adulthood. Perhaps the reason why FU Ori outbursts have been observed for only a few protostars, is simply because they are relatively peaceful and quiet most of the time.

What are the detailed physical mechanisms of these outbursts? That is the question! The answer lies in the region surrounding the baby star. Astronomers realize that the optical flare-ups are associated with the disk of material circling the new star–the accretion disk. The baby star grows dramatically brighter when the disk gets heated up to searing-hot temperatures that are about the same as those of volcanic lava flows on Earth.

Hungry Baby Stars Are Nourished In Their Stellar Nursery

An international team of astronomers, led by Dr. Hauyu Baobab Liu and Dr. Hiro Takami of the ASIAA, used a new imaging technique made available at the Subaru Telescope in order to answer this quesion. The new technique–imaging polarimetry with coronagraphy–presents great advantages for imaging the mysterious environments of the accretion disks. In particular, its high angular resolution and sensitivity provide astronomers with the ability to “see” the light emanating from the accretion disk more readily. How does this new technique work?

Circumstellar material is a mixture of gas and dust, but the quantity of dust is considerably smaller than the quantity of gas in the natal cloud–and so the dust has only a very minor effect on the way the material moves. However, dust particles scatter (reflect) light flowing out from the brilliant protostar, lighting up all of the ambient material. The HICIAO camera that is mounted on the Subaru 8.2-meter telescope, which is one of the largest near-infrared (NIR) and optical telescopes on our planet, is very well-suited to observe this faint interstellar light. It successfully enabled the scientists to observe a quartet of stars in the process of undergoing FU Ori outbursts.

The astronomers’ targeted stars are situated about 1,500-3,500 light-years from our Solar System. The images that were obtained showing these outbursting stellar infants were both fascinating and surprising, because they were like nothing previously observed surrounding young stars. A trio of the four stars show weird looking tails. One reveals an “arm”, which is a feature formed by the movement of the material around the star: Another displays odd spiky features, which may result from an optical outburst blowing away circumstellar gas and dust. None of the stars observed match the steady growth scenario. Instead, the sparkling quartet reveal a rather chaotic messy environment–very much like the very messy way a human baby eats her food.

In order to understand the structures seen surrounding these four baby stars, theorists on the team carefully studied one of several mechanisms proposed to explain FU Ori outbursts. The observations indicate that gravity in circumstellar gas and dust clouds carve complicated structures that resemble the way cream looks when it is stirred into a cup of black coffee. These weirdly shaped collections of material tumble down onto the star at irregular intervals. The scientists then conducted additional supercomputer simulations for the way light would be scattered from the outburst. Although more simulations are necessary to match the simulations to the observed images, these images do indeed reveal that this is a promising explanation.

Studying the distant structures may also show how some planetary systems form. Astronomers already know that some exoplanets–planets that orbit a star other than our Sun–are observed to be very far away from their central parent-stars. Some of the exoplanets have even been observed to orbit a thousand times the distance between Earth and Sun, which is about 93,000,000 miles. This means that the orbits of these exoplanets carry them much farther from their star than Neptune is from our Sun. Neptune is the outermost of the eight major planets from our Star, and its orbit is about 30 times the Earth-Sun separation. These distances are also considerably larger than orbits explained by standard theories of planet formation. Supercomputer simulations of complicated circumstellar structures like the ones observed in the HICIAO views also predict that some of the dense clumps in the material may grow to become gas giant planets. This would naturally explain the presence of the exoplanets observed to have such large orbits.

But, despite these very exciting new results, there is still a great amount of work left to be done to enable scientists to understand the mysterious mechanisms of star and planet birth. More detailed comparisons between observation and theory are necessary. Additional observations, especially with the Atacama Large Millimeter/Submillimeter Array, will take the watchful eyes of observers much more deeply into circumstellar gas and dust clouds. The array will permit observations to be made of the surrounding dust and gas with unprecedented angular resolution and sensitivity. Astronomers are also making plans to build telescopes significantly larger than Subaru in the decades to come. The new telescopes will include the Thirty Meter Telescope (TMT) and the European Extremely Large Telescope. These should permit detailed new studies of the mysterious, fascinating regions very close to newborn baby stars!

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