Interstellar Dust From Our Solar System’s Birth
By Judith E Braffman-Miller
Our Sun formed from the jumbled leftovers of the nuclear-fusing furnaces of earlier generations of stars–and like other stars, it was born within a dense, frigid blob cradled within one of the giant, dark, interstellar molecular clouds that float through our Milky Way Galaxy in huge numbers. This especially dense blob eventually collapsed under its own gravitational pull to create our brilliant baby Star. Most of the gas and dust that swirls within beautiful, ghostly molecular clouds originates from the stellar furnaces of earlier generations of doomed stars, that either blasted themselves to shreds in a supernova explosion, or (if they were smaller stars), more gently puffed their outer gaseous layers into interstellar space. From this lingering material, left as a legacy by a multitude of long-dead stars, new stars were born from the wreckage of previous stellar generations. In June 2018, a team of scientists announced their new discovery that certain interplanetary dust particles are primordial leftovers from the initial birth of our Solar System.
The team of scientists, led by University of Hawaii at Manoa (UH Manoa) School of Ocean and Earth Science and Technology (SOEST) researcher Dr. Hope Ishii, was funded by NASA’s Cosmochemistry, Emerging Worlds and Laboratory Analysis of Returned Samples Programs and was enabled, in part, by the Advanced Electron Microscopy Center at the University of Hawaii. Portions of the research were also performed at national user facilities at the Molecular Foundry and the Advanced Light Source at Lawrence Berkeley National Laboratory, supported by the U.S. Department of Energy.
The first solids out of which our Solar System emerged were composed mostly of amorphous silicate, carbon and ices. This primordial dust was almost entirely destroyed and altered by processes that eventually resulted in the formation of planets. Surviving samples of pre-solar dust are probably preserved in comets. Comets are small, cold objects that inhabit our Solar System’s outer limits: the Kuiper Belt, Scattered Disk, and Oort Cloud. Here, in our Solar System’s deep freeze, the icy and dusty dancing comet nuclei preserve, in their frozen hearts, the mysterious ancient dust of our baby Solar System. Comets formed in the outer fringes of the original solar nebula.
Tucked within a relatively obscure class of interplanetary dust particles (IDPs), believed to originate from comets, are very small glassy grains dubbed GEMS, or glass embedded with metal and sulfides that are typically only tens to hundreds of nanometers in diameter. This is less than 1/100th the thickness of a strand of human hair.
Although we often think of vast regions of interstellar space as being empty, this is not the case. Much of the space between stars is brimming with atomic and molecular gas–primarily hydrogen and helium–and extremely tiny tidbits of solid particles or dust. This dust is composed mainly of silicon, oxygen, and carbon. In certain regions the gas and dust density is very low.
In the secretive depths of enormous, dark molecular clouds–that contain this gas and dust–extremely fragile threads of material slowly merge, clump, and grow for hundreds of thousands of years. Then, mercilessly squeezed by the relentless crush of gravity, the hydrogen atoms within these clumps dramatically and suddenly fuse. This initial episode of nuclear fusion lights a baby star’s fire that will last for as long as the new star “lives”.
All stars, regardless of their mass, are gigantic spheres of primarily hydrogen gas. The Big Bang birth of the Universe, about 13.8 billion years ago, produced only the lightest atomic elements–hydrogen, helium, and trace quantities of lithium (Big Bang Nucleosynthesis). All of the atomic elements heavier than helium–called metals by astronomers–are produced in the nuclear-fusing cores of the Universe’s stars (Stellar Nucleosynthesis) or, in the case of the heaviest atomic elements of all (such as gold and uranium), in the supernova explosion that heralds the death of a massive star.
Stars “live” on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution as a result of the process of nuclear fusion–that is, by producing increasingly heavier and heavier atomic elements out of lighter ones. The fusion process begins with hydrogen and, in the case of massive stars, continues until the star has a core of iron. Iron cannot be used as fuel, and so that’s the end of the massive star. Smaller stars, like our Sun, are not able to continue fusion all the way up to the point that they possess a core of iron. However, they do fuse lighter atomic elements from their supply of hydrogen fuel, such as carbon and oxygen. This is why solitary small stars, like our Sun, don’t “go supernova”.
Nuclear fusion creates radiation pressure that tries to push everything outward and away from the star, while gravity does the opposite and tries to pull everything in and towards the star. The eternal battle between radiation pressure and gravity keeps a main-sequence star bouncy–until it has managed to burn its entire necessary supply of nuclear-fusing fuel, which marks the end of the long stellar road for the doomed star. At that unfortunate point, gravity wins the war against its arch-enemy, radiation pressure, and the star is ready to make its final farewell performance to the Cosmos. If the star is massive, it will blow itself to smithereens in a brilliant supernova blast, that will send its newly forged supply of freshly fused metals screaming out into interstellar space. For a short time, this explosion can be so bright that it outshines its entire host galaxy. Massive stars, that have exploded in the furious tantrum of a fiery supernova, leave either a neutron star or stellar mass black hole behind as testimony of their former existence.
Small stars–like our Sun–go much more “gentle into that good night”, and puff their metals out into interstellar space, as they leave their relic core behind in the form of a dense stellar corpse termed a white dwarf. The new white dwarf is born surrounded by the multicolored shimmering, glimmering shroud of what was once its small progenitor star’s outer gaseous layers. Indeed, these glowing candy-colored stellar shrouds are so beautiful that they are frequently referred to as the “butterflies of the Universe.” This will be our Sun’s fate.
Today our Sun is a small, middle-aged star. Stars of our Sun’s mass live for approximately 10 billion years on the hydrogen-burning main-sequence. Since our Sun is only 4.56 billion years old, it will not have its grand finale for about another 5 billion years. As stars go, our Sun is rather ordinary. There are eight major planets and a rich assortment of other objects orbiting our Sun, which is located in the far suburbs of our majestic spiral Milky Way. If we trace the history of atoms on our planet today back about 8 billion years, we would likely find them spread all over our Galaxy. Some of these formerly widely dispersed atoms exist in a single strand of human genetic material (DNA)–even though, in more ancient times, they were born within alien stars inhabiting our young Milky Way.
Our solitary Sun was born with company, just like billions of other stars that do their mesmerizing stellar dance within our Galaxy. Our own Star was likely born a member of a dense open cluster along with thousands of other glittering stellar siblings. However, our Sun’s stellar sisters have gone missing, wandering off to more remote regions of our Milky Way–and there well may be as many as 3,500 of these long-lost solar kin.
All stars, our own included, are born surrounded by a whirling disk composed of gas and dust called a protoplanetary accretion disk. These whirling, nurturing gaseous rings, that linger around newborn stars, contain the necessary ingredients from which a family of planets can emerge. Astronomers have observed many protoplanetary accretion disks circling distant young stars, and these disks form at about the same time that the new star (protostar) is born within its veiling natal cloud.
Most of the material of the collapsing, dense blob that is cradled within the giant, dark molecular cloud, gathers at the center, and eventually evolves into a protostar. The leftover gas and dust becomes the surrounding accretion disk, from which planets, moons, and smaller objects eventually accrete. These disks are both extremely hot and massive, and they can linger around the young star for as long as 10 million years.
By the time a fiery baby star, that is similar to our own Sun, reaches what is called the T Tauri stage of development, the disk has become both cooler and thinner. A T Tauri star is a young variable star, that will eventually become a small star that is similar to our Sun. T Tauris are very active at the tender age of about 10 million years, and these stellar toddlers sport large diameters that are several times greater than that of our Sun–but they will shrink. Unlike human children, T Tauris shrink as they grow older. By the time the stellar tot has reached the T Tauri stage, less volatile materials have started to condense close to the center of the encircling accretion disk, creating very fine and sticky dust motes. The delicate dust particles contain crystalline silicates.
The sticky dust motes collide with one another in the crowded disk environment, and “glue” themselves to one another–forming ever larger, and larger, and larger objects–from pebble size, to boulder size, to mountain size to moon size, to planet size. These growing bodies evolve into planetesimals–the primordial building blocks of planets. Planetesimals constitute an abundant population within the disk, and some of them can linger around their star for billions of years. In our own Solar System, the asteroids and comets are what is left of this ancient population of planetesimals. The asteroids, that are mostly found in the Main Asteroid Belt between Mars and Jupiter, are akin to the rocky and metallic planetesimals that constructed the four solid, inner planets: Mercury, Venus, Earth, and Mars. In a similar way, comets are the relics of the icy, dirty planetesimals that formed the quartet of outer Solar System gaseous behemoths: Jupiter, Saturn, Uranus, and Neptune.
Interstellar Dust Tells Its Ancient Tale
Dr. Ishii and her colleagues used transmission electron microscopy to make maps of the element distributions and found that the glassy grains (GEMS) are composed of even smaller subgrains that merged together in a different environment–probably before the formation of their parent-comet nucleus.These glassy grains are also encapsulated by carbon of a different type than the carbon that creates a matrix gluing together GEMS and other components of cometary dust.
The forms of carbon that coat the subgrains, and create the matrix within these particles, tends to fall apart even when only slightly warmed up. This means that the GEMS could not have been born in the searing-hot inner solar nebula close to the intense fiery heat of our newborn Sun. Therefore, they must have formed in a frigid, radiation-rich environment. This type of environment would have likely existed in the outer solar nebula or within the swirling folds of the natal pre-solar molecular cloud.
“Our observations suggest that these exotic grains represent surviving pre-solar interstellar dust that formed the very building blocks of planets and stars. If we have at our fingertips the starting materials of planet formation from 4.6 billion years ago, that is thrilling and makes possible a deeper understanding of the processes that formed and have since altered them,” Dr. Ishii explained in a June 12, 2018 University of Hawaii Press Release.
In the future, Dr. Ishii and her team plan to go on the hunt for additional comet dust particles, especially those that were well-protected during their dive down through the Earth’s atmosphere. The team wants to increase scientific understanding of the distribution of carbon hiding within GEMS, as well as the size distribution of GEMS subgrains.
“This is an example of research that seeks to satisfy the human urge to understand our world’s origins and serves the people of Hawaii by boosting our reputation for excellence in space science and as a training ground for our students to be engaged in exciting science,” Dr. Ishii continued to comment.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others some of the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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