Stars are shimmering, roiling, glaring, and gigantic spheres of seething-hot gas–they are luminous, brilliant balls of plasma held together by the powerful grip of their own mighty gravity. When a star is born, it is surrounded by a whirling disk made up of gas and motes of swirling dust, termed a protoplanetary accretion disk, and these circling, gas-laden rings contain the necessary ingredients from which a family of planets–and other objects–can form. Indeed, the protoplanetary accretion disks surrounding baby stars contain enormous amounts of very nutritious gas and dust that serve the important function of feeding growing newborn protoplanets. But which stars make the best stellar-parents for infant planets? In September 2017, a new study was released that used data derived from NASA’s Chandra X-ray Observatory and the European Space Agency’s (ESA’s) XMM-Newton, that showed that X-rays emitted by a planet’s parent star may provide important clues to just how hospitable a particular stellar system could be. A team of astronomers peered at 24 stars similar to our own Sun, each at least one billion years old, and how their X-ray brightness changed as time went by.
Stellar X-rays mirror a star’s magnetic activity. Because of this, X-ray observations can reveal to astronomers important information about the high-energy environment around a star. In the new study, the X-ray data derived from Chandra and XMM-Newton revealed that stars similar to our Sun, and their even less massive kin, calm down from the turbulence of their wild, flaming youth surprisingly fast–thus becoming the truly stellar parents of baby planets at a relatively young age. 바카라사이트
Our own Solar System, as well as other planetary systems, circling stars beyond our own Sun, form when a very dense, but relatively small blob that is embedded within the undulating, billowing folds of an enormous cold, dark molecular cloud experiences gravitational collapse as a result of its own hefty weight. Ghostly, frigid molecular clouds are beautiful objects that haunt our Milky Way Galaxy in huge numbers–and these clouds serve as the strange cradles of sparkling baby stars. Molecular clouds are composed primarily of gas, but they also harbor smaller quantities of dust. Most of the collapsing gaseous and dusty blob collects at the center, and eventually ignites with a fierce fire as a result of the process of nuclear fusion–thus forming a new star (protostar). The remainder of the gas and dust, that did not go into the formation of the protostar, ultimately evolves into the protoplanetary accretion disk from which planets, moons, asteroids, and comets eventually emerge. In their earliest stages of development, protoplanetary accretion disks are both extremely massive and searing-hot–and they can hang around the young star for as long as ten million years.
By the time a glaring, roiling, searing-hot stellar baby has reached what is called the T Tauri phase of its development, the hot, massive surrounding disk has grown considerably cooler and thinner. A T Tauri star is a mere tot by star standards–a very young, variable Sun-like star that is extremely active at the tender age of only ten million years. These stellar toddlers sport impressively large diameters that are several times larger than that of our Sun today. However, T Tauri stars are still in the process of shrinking. This is because young Sun-like stars, unlike human children, shrink as they grow up. By the time the fiery young star has reached this stage of its development, less volatile materials have started to condense close to the center of the encircling disk, creating very sticky, fine particles of dust. The dust of the disk does not resemble the dust that we frequently sweep away on Earth. Instead, this cosmic dust resembles clouds of billowing smoke. The very fine and fragile dust motes also carry crystalline silicates.
Because the accretion disk environment is crowded, the very tiny, sticky motes of dust bump into one another frequently, and merge as a result. Ultimately, larger and larger objects grow–from pebble size, to boulder size, to mountain size, to asteroid size–and finally, to planet-size. These growing objects evolve into planetesimals, which are primordial planetary building blocks. The asteroids and comets that populate our own Solar System are lingering planetesimals. The asteroids resemble the solid, rocky building blocks that constructed the quartet of inner planets: Mercury, Venus, Earth, and Mars. In contrast, the icy, frozen comets are the relic building blocks of the four giant, gaseous outer planets: Jupiter, Saturn, Uranus, and Neptune. The asteroids and comets of our Sun’s family show that lingering primordial planetesimals can still be hanging around their parent-star billions of years after a mature planetary system has developed.