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X-Rays Reveal Which Stars Make The Best Stellar Parents
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.
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.
Star Light, Star Bright
For the active, youthful years of its life, a star shines brilliantly as a result of the process of thermonuclear fusion of hydrogen to helium in its core. This reaction causes the star to release energy that travels through the star’s interior and then radiates out into interstellar space. Almost all of the naturally occurring atomic elements that are heavier than helium are formed by way of this process–termed stellar nucleosynthesis–during the star’s “lifetime”. However, some of the heaviest atomic elements of all are forged in the dying furnaces of massive stars when they go supernova. The heaviest of atomic elements–such as gold and uranium–form as a result of the final, fatal supernova explosion of a doomed massive star. As a star approaches the end of that long stellar road, it can also contain degenerate matter.
Astronomers determine the age, mass, and metallicity of a star by studying its motion through space, its spectrum, and its luminosity. In astronomy, all atomic elements heavier than helium are termed metals, and so the term does not carry the same meaning for astronomers that it does for chemists. The metallicity of a star refers to the percentage of metals it contains as opposed to hydrogen–the lightest and most abundant of atomic elements. However, all stars, regardless of their metal content, are mostly composed of hydrogen. While hydrogen, helium, and trace quantities of lithium and beryllium were formed in the Big Bang birth of the Universe about 13.8 billion years ago, all of the metals were created in the nuclear fusing furnaces of stars–or in the supernova blasts that marked their tragic end.
The total mass of a star is what determines how it will evolve and finally perish. Other attributes of a star, including temperature and diameter, evolve as time goes by–while a star’s environment influences its movement and rotation. A plot of the temperature of numerous stars as opposed to their luminosities are recorded in a plot known as the Hertzspring-Russell Diagram of Stellar Evolution (H-R diagram). Plotting a particular star on that diagram enables astronomers to determine the age and evolutionary state of that star.
After a star has been born as the result of the gravitational collapse of a dense, gaseous blob within its natal molecular cloud, and its core has become sufficiently dense, the star’s supply of hydrogen is steadily converted into helium by way of the process of nuclear fusion. Helium is the second lightest atomic element after hydrogen, and this fusion reaction releases energy. The remainder of the stellar interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star’s internal pressure is what prevents it from collapsing further as a result of the relentless pull of its own powerful gravitational squeeze. Stars with masses greater than 0.4 times that of our Sun will expand to become a red giant when it has exhausted its necessary supply of hydrogen fuel in its core. In some cases, a star will fuse heavier atomic elements–the metals–at its core or in a shell surrounding its core. As the star continues to expand, it hurls a percentage of its mass, enriched with those newly-forged metals, out into the space between stars. The newly created metals then wander through interstellar space, where they ultimately may be incorporated into a giant molecular cloud–only to be recycled later in the production of new and sparkling baby stars. Meanwhile, the star’s core morphs into a stellar corpse–a white dwarf, neutron star, or–if extremely massive–a black hole of stellar mass.
Binary and multiple stellar systems consist of two or more sibling stars that are bound to each other gravitationally, and generally travel around each other in stable orbits. When a duo of such sibling stars sport a relatively close orbit, their gravitational dance can produce a dramatic impact on their evolution. Most stars are observed to be members of binary systems, and the characteristics of those binaries result from the conditions in which the duo formed. A gas cloud must lose its angular momentum in order to collapse and form a baby star. The fragmentation of the cloud into multiple stars spreads some of that angular momentum.
Stars spend about 90% of their stellar lives fusing hydrogen into helium in the high-temperature, high-pressure reactions near their nuclear fusing cores. Such stars are said to be on the main-sequence of the H-R Diagram. The time that a star spends on the main-sequence depends on how much fuel it has and the rate at which it fuses it. For example, our 4.56 billion year old Sun has a “life” expectancy of 10 billion years–and it is still considered to be an active middle-aged star, although it has calmed down considerably since its flaming youth billions of years ago. Stars that are much more massive than our Sun burn their fuel very, very quickly–by star standards–and don’t “live” on the hydrogen-burning main sequence very long. Massive stars live fast, and die young–living for merely millions, as opposed to billions, of years. Lucky low-mass stars, on the other hand, take their time burning their supply of fuel, and “live” on the main-sequence for a very long time. Stars that are less massive than 0.25 solar-mass, called red dwarfs, are able to fuse nearly all of their mass while stars of about our Sun’s somewhat more hefty mass can only fuse approximately 10% of their mass. The combination of their relatively abundant usuable fuel supply and their lazy fuel-consumption enables stars of low-mass to “live” on the main-sequence for one trillion years. Red dwarfs become hotter, and hotter, and hotter as they accumulate more and more helium. When they ultimately consume their necessary supply of hydrogen, they shrivel up and undergo a sea-change into a white dwarf–and grow cooler, and cooler, and cooler. However, since the “life expectancy” of such small stars is much greater than the current age of our 13.8 billion year old Universe, there are no stars of such a small mass that have had time enough to die. Red dwarfs are also the most abundant type of star in our Milky Way Galaxy.
During their hydrogen-burning “lives” massive stars, weighing in at more than 9 times our Sun’s mass, first expand to morph into a blue supergiant and, after that, a red supergiant. Stars that are especially massive may evolve into a Wolf-Rayet star, which show spectra dominated by emission lines of elements heavier than hydrogen. These heavier atomic elements have reached the stellar surface as a result of intense mass loss and strong convection.
When the helium of a massive star has been used up, its core shrivels and the temperature and pressure rise enough to cause the doomed star to begin to fuse carbon. Successive stages of nuclear fusion produce neon, oxygen, and silicon. The process continues until the star winds up with a nickel-iron core–and goes supernova.
X-rays Reveal Which Stars Make The Best Stellar Parents
X-rays can provide valuable information about whether a star system will be hospitable to life emerging on its planets. This is because stellar X-rays mirror magnetic activity, which can churn out energetic radiation and eruptions that may impact a stellar parent’s planets. Scientists used Chandra and XMM-Newton to observe 24 stars like our Sun that were at least one billion years old. The most recent observations suggest that older Sun-like stars calm down relatively quickly. This encourages life to emerge and evolve on planets that exist around them.
In order to gain a new understanding about how rapidly a star’s magnetic activity level changes as time goes by, astronomers need accurate ages for a variety of different stars. This is not an easy task, but recent precise age estimates have now become available thanks to studies of the way a star pulsates using NASA’s Kepler Space Telescope and ESA’s CoRoT missions. These recent age estimates were used for most of the 24 stars being observed in this study.
Astronomers know that most stars are very magnetically active when they are young. This is because young stars are rotating rapidly. As the rotating young star begins to lose energy over time, the star starts to spin more slowly and the magnetic activity level, along with the associated X-ray emission, plummets.
Although it has not been determined why older stars settle down relatively quickly, astronomers have some ideas that they are currently exploring. One theory suggests that the decrease in spin rate of older stars occurs more rapidly than it does for the more youthful stars. A second suggestion is that the X-ray brightness drops more rapidly with time for older, more sluggishly rotating stars than it does for younger stars.
A paper describing these new results has been accepted for publication in the Monthly Notices of the Royal Astronomical Society (UK). The other co-authors of the paper are Dr. Victor Silva Aguirre from Aarhus University in Denmark and Dr. Scot Wolk from Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts.
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