Our Solar System emerged about 4.56 billion years ago from the mixed remnants still lingering from the long-dead, nuclear-fusing searing-hot cores of earlier generations of ancient stars. Our Sun was born the same way as other stars of its generation–from a dense frigid blob tenderly tucked within the billowing, undulating folds of one of the many giant, dark, and beautiful molecular clouds that haunt our Milky Way Galaxy like lovely ghosts floating around in the space between stars. These enormous dark clouds, composed of gas and dust, are the strange cradles of baby stars. Even though it may seem counterintuitive, things have to get very cold in order for a fiery, hot newborn star to be born. Stars keep their secrets well, hiding their many mysteries from those who seek to understand them and their secretive nature. In July 2017, a team of astronomers, using new numerical supercomputer simulations and observations announced that scientists may now be able to explain why our Sun’s magnetic field reverses every eleven years–and this important discovery explains how the duration of the magnetic cycle of a star depends on its rotation, helping to shed new light on the turbulent space weather around our Sun and kindred stars.
The magnetic field of our Sun, and other stars like it, is generated by the motion of conductive plasma within the star. This motion is created as a result of convection, which is a form of energy transport that involves the physical movement of material. A localized magnetic field exerts a powerful force on the stellar plasma, that effectively increases the pressure without a comparable gain in density. Because of this, the magnetized region rises relative to what is left of the plasma–at least until it reaches the star’s photosphere. This causes starspots to form on the star’s surface, as well as creating the related phenomenon of coronal loops.
A star’s magnetic field can be measured by using what is called the Zeeman effect. The atoms within a star’s atmosphere will usually absorb certain frequencies of energy in the electromagnetic spectrum. As a result, this produces characteristic lines in the stellar spectrum. However, when the atoms are within a magnetic field, these lines split into multiple, closely spaced lines. The energy also becomes polarized with an orientation that is dependent on the orientation of the magnetic field. Therefore, the direction and strength of any given star’s magnetic field can be calculated by examination of the Zeeman effect lines.
Stellar spectropolarimeters are used to measure the magnetic field of a star. This instrument is composed of a spectrograph that is used in combination with a polarimeter. The first instrument to be dedicated to the examination of stellar magnetic fields was NARVAL, which was mounted on the Bernard Lyot Telescope at Pic du Midi de Bigorre in the French Pyrenees mountains.
Various other measurements were made by scientists using magnetometer measurements over the past century-and-a-half. The existence of carbon 14 in tree rings, and Beryllium 10 in ice cores, revealed that there has been substantial magnetic variability of our Sun on decadal, centennial and millennial time scales.
The Secret Lives Of Stars
Our Sun is a lonely star–a sparkling sphere of fire in Earth’s daytime sky. However, it probably was not always this solitary, because our Star is likely to have been born as a glittering member of a dense open stellar cluster hosting literally thousands of other brilliant sibling stars. Many astronomers propose that our neonatal Star was either thrown out of its birth cluster, as the result of unfortunate gravitational interactions with other stars, or it simply floated away from its stellar siblings about 4.5 billion years ago. The missing solar siblings have long since floated away to distant regions of our Milky Way Galaxy–and there well may be as many as 3,500 of these vanished sisters of our Star inhabiting faraway corners of interstellar space.
Our Galaxy’s myriad of fiery stars, including our Sun, were born the same way–as a result of the gravitational collapse of a dense pocket embedded within the secretive swirls of a giant molecular cloud. These dark clouds contain the relic gas and dust scattered throughout our Milky Way by older generations of ancient stars that perished long ago. These star-birthing clouds tend to mix themselves up together, but stars that display a kindred chemistry usually reveal themselves inhabiting the same clouds at about the same time.
There are three generations of stars in the observable Universe. Stars belonging to stellar Population III are the oldest stars. These very ancient stars were born from pristine hydrogen and helium, produced in the Big Bang birth of the Universe itself, almost 14 billion years ago. For this reason, it is thought that Population III stars probably formed differently from the two populations of younger stars. This is because the younger stars are not composed of pristine gases, but instead are “polluted” by heavier atomic elements manufactured by older stars. Indeed, Population III stars are depleted of what astronomers call metals, which are all of the atomic elements heavier than helium. Therefore, the term metal for astronomers has a different meaning than it does for chemists. The metals were manufactured in the nuclear-fusing furnaces of the stars–or, alternatively, in the supernovae conflagrations that heralded the demise of the most massive stellar inhabitants of the Cosmos. The heaviest metals, such as gold and uranium, were formed as a result of these violent and brilliant stellar death throes.
Our Sun is a sparkling member of stellar Population I–the youngest of the three generations of stars, and it carries within it the heavy metals fused in the furnaces of the two older generations of stars.
Population II stars, the stellar “sandwich” generation, are younger than Population III stars, but older than Population I stars like our Sun. Population II stars contain very small quantities of metals, but because they are not metal free, there has to have been a population of stars that came before them to create those metals–hence, there has to have been a Population III.
However, the reality is somewhat more complicated. This is because even Population I stars are composed primarily of hydrogen gas–just like the two earlier stellar generations. Population I stars contain more metals than the two older generations of stars, but they are still mostly composed of hydrogen gas. All of the stars, belonging to all three stellar generations, are primarily composed of hydrogen.
Today our Sun is a middle-aged, hydrogen burning star that is still on the main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution. By star-standards, our Sun is ordinary. There are planets, moons, and an assortment of smaller objects in orbit around our Star, which dwells in the far suburbs of a typical starlit, barred-spiral Galaxy–our Milky Way. If we trace the history of atoms on our Earth today back to about 7 billion years, we would likely find them scattered throughout our Galaxy. Some of these widely scattered atoms now exist in a single strand of your genetic material (DNA), even though in the ancient Universe they were formed deep within alien stars lighting up our then very young Galaxy.
The Mysterious Magnetic Personality Of Our Star
The magnetic field of our Star has reversed every 11 years over the centuries. When these reversals happen, the solar south magnetic pole switches to the north and vice versa. This “flip” occurs during the peak of each solar cycle and it originates as a result of a process termed a dynamo. A dynamo generates magnetic fields, and this involves the rotation of the star as well as convection– the rising and falling of searing-hot gas within the star’s roiling interior.
Astronomers know that our Sun’s magnetic fields form in its turbulent outer layers, and that they have a complicated dependency upon how speedily our Star is rotating. Astronomers have also measured magnetic cycles for distant stars beyond our Sun, and they have shown fundamental properties that are similar to those of our own Star. By observing the characteristics of these magnetic properties, astronomers now have a promising new method that they can use to better understand the magnetic evolution of our Star that is associated with the dynamo process.
An international team of astronomers that includes scientists from the Harvard-Smithsonian Center for Astrophysics (CfA), the University of Montreal, the Commissariat a l’energie atomique et aux energies alternatives and the Universidade Federal do Rio Grande do Norte, conducted a set of 3D simulations of the mysterious, searing-hot turbulent interiors of Sun-like stars, in order to explain the origin of their magnetic field cycles. The astronomers found that the period of the magnetic cycle depends on the rotation rate of the spinning star. This revealed that more sluggishly spinning stars have magnetic cycles that repeat more frequently.
“The trend we found differs from theories developed in the past. This really opens new research avenues for our understanding of the magnetism of stars,” noted Dr. Antoine Strugarek in a July 26, 2017 CfA Press Release. Dr. Strugarek is of the Commissariat a l’energie atomique et aux energies alternatives, France, and the lead of of a paper describing this research published in the July 14, 2017 issue of the journal Science Magazine. The CfA is in Cambridge, Massachusetts.
One particularly important advance is that the astronomers’ new model can explain the cycle of both our Sun and stars that are similar to it–Sun-like stars, as astronomers categorize them. Previously, astronomers thought that our Sun’s magnetic cycle might differ in behavior from those of Sun-like stars, with a shorter magnetic cycle than predicted.
“Our work supports the idea that our Sun is an average, middle-aged yellow dwarf star, with a magnetic cycle compatible with cycles from its stellar cousins. In other words we confirm that the Sun really is a useful proxy for understanding other stars in many ways,” explained study co-author Dr. Jose-Dias Do Nascimento. Dr. Do Nascimento is of the CfA and the University of Rio G. do Norte (UFRN), in Brazil.
By carefully observing more and more stars and exploring stellar structures that are different from those of our Sun with numerical simulations, the team of astronomers hope to refine their new model for the origin of stellar magnetic cycles.
One goal for future work is to attain a better understanding of “space weather”, a term used to describe the wind of particles that rushes away from the Sun and other stars like it. The mechanism of acceleration for this blowing wind of particles is probably related to magnetic fields in the atmospheres of stars. In extreme cases, space weather can wreak havoc with electrical power on Earth, as well as creating a very dangerous environment for both satellites and astronauts.
Dr. Do Nascimento noted in the CfA Press Release that “The changes throughout a magnetic cycle have effects throughout the Solar System and other planetary systems thanks to the influence of space weather.”
