Astronomers Find Our Sun’s Twin Sister
Our Star, the Sun reigns majestically at the very heart of our Solar System–a roiling, broiling perfect sphere of searing-hot plasma. Despite its many sparkling attributes, our Sun is a lonely, solitary Star, as it shines with its beautiful, brilliant, and fierce light in Earth’s daytime sky. But our Sun was not always so alone without any other stellar kin nearby to provide it with delightful sparkling company. Indeed, our Sun was born a member of a dense open cluster with literally thousands of other sister stars that have tragically gone missing, wandering off to distant regions of our Milky Way Galaxy more than 4 billion years ago. In November 2018, an international team of astronomers, led by Portugal’s Instituto de Astrofisica e Ciencias do Espago (IA) researcher Dr. Vardan Adibekyan, announced that by using a new method to detect these long-lost solar-siblings, they have found our Star’s identical twin sister. The paper describing their research is published in the journal Astronomy & Astrophysics.
Our Sun’s sisters number in the thousands. These long-lost stars were all born in the same massive open stellar cluster as our own Star over 4 billion years ago. Since then, these sibling stars have drifted away–and some estimates propose that there may have been as many as 3,500 of these lost wanderers.
An open cluster is a cluster of up to a few thousand stars that all formed at about the same time from a particular region of a giant, cold, and dark molecular cloud, and are thus about the same age. Over 1,100 open clusters have been observed within our Milky Way Galaxy–but astronomers think that many more exist. They are only loosely bound together by their mutual gravitational attraction, and become readily disrupted by unfortunate close brushes with other clusters, as well as by clouds of gas, as they orbit around the Galactic center. This can trigger a migration to the main region of the Galaxy, as well as a loss of cluster siblings by way of internal close encounters. Open clusters generally last for a few hundred million years, with the most massive ones surviving for a few billion years. In dramatic contrast, the stars inhabiting a massive globular star cluster exert a more powerful gravitational pull on one another, and can survive intact for a much longer period of time. Open clusters have been detected only in spiral and irregular galaxies, in which active starbirth is ongoing.
Youthful open clusters may be cradled within the molecular cloud from which they were born, igniting it to create an H II region, which is a region of interstellar atomic hydrogen that is ionized. It is normally a cloud of only partially ionized gas within which starbirth has recently occurred. Sporting a size ranging from one to hundreds of light years, and a density from a few to a million particles per cubic centimeter, H II regions may be of any shape. This is because the distribution of stars and gas within them is irregular. As time goes by, radiation pressure from the cluster will shred the molecular cloud. Usually, approximately 10% of the mass of the frigid dark cloud will coalesce into stars before radiation pressure shoots the gas away.
Open clusters serve an important role in the study of stellar evolution. This is because the cluster stellar members are of about the same age and chemical composition, which makes their properties more easily determined than they are for isolated stars. Many open clusters are visible to the unaided human eye. These visible clusters include the Pleiades, Hydrae or the Alpha Persei Cluster. However, there are other clusters, such as the Double Cluster, that are barely visible without the aid of instruments, while many more can be observed with binoculars or telescopes. The Wild Duck Cluster (M11) is an example.
Like other open stellar clusters, our Sun’s natal cluster fell apart as time went by, and many of our Sun’s lost sisters are now so far away that it is very difficult for astronomers to find them.
«Since there isn’t much information about the Sun’s past, studying these stars can help us understand where in the Galaxy and under which conditions the Sun was formed,» Dr. Adibekyan explained in a November 16, 2018 IA Press Release.
Dr. Adibekyan, who is of the IA and the University of Porto in Portugal, continued to explain that «With the collaboration of Patrick de Laverny, we got a sample of 230,000 spectra from the AMBRE project.» AMBRE is a galactic archaeology project set up by the European Southern Observatory (ESO) and the Observatoire de la Cote d’ Azur (France), in order to determine the stellar atmospheric parameters for the archived spectra from ESO’s FEROS, HARPS, UVES and GIRAFFE spectrographs.
The team of astronomers then went on to use these very high quality spectral data obtained from the AMBRE project along with precise astrometric data derived from the second release of the European Space Agency’s (ESA’s) Gaia mission in order to «make a selection of stars with chemical compositions which best match the Sun’s composition, followed by an estimate of these stars age and kinematic properties,» Dr. Adibekyan added.
Solar Nursery
Our Solar System emerged from mixed fragments composed of lingering relics from the long-dead, nuclear-fusing furnaces of older generatons of stars. Our Sun (like its missing, sparkling sisters), was born tucked within a frigidly cold and dense blob, secreted within the ruffling, whirling folds of a giant, dark molecular cloud. Although it may seem counterintuitive, things have to get very cold before a new, fiery, searing-hot baby star can be born. The star-birthing, dense blob eventually collapsed under the intense pull of its own gravity–thus giving birth to a brand new baby star. In the hidden depths of these vast and dark molecular clouds composed mostly of gas, with much smaller quantities of dust, fragile and delicate threads of material gradually merge and then clump together–growing in size for hundreds of thousands of years. Finally, squeezed relentlessly by the merciless crush of gravity, hydrogen atoms tucked within this clump rapidly and dramatically fuse, lighting a raging stellar fire that will continue for as long as the new star «lives».
Our Star’s diameter is about 864,337.3 miles, which is approximately 109 times that of Earth, and its mass is about 330,000 times that of Earth. Our Sun accounts for approximately 99.86% of the mass of our entire Solar System. About three-quarters of the Sun’s mass is composed of hydrogen (about 73%), and the rest is mostly helium. (25%)–with significantly smaller quantities of heavier atomic elements, such as oxygen, carbon, and neon.
Our Sun is classified as a G-type main-sequence star, based on its spectral class. A star, like our Sun, is a still «living» star on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution, and it is informally (and somewhat inaccurately) frequently referred to as a yellow dwarf. However, our Star’s light is actually closer to white than it is to yellow. It was born about 4.6 billion years ago from the gravitational collapse of matter within its star-birthing molecular cloud. Most of the collapsing material collected at the center, while the rest flattened out into an orbiting disk that eventually became our Solar System. The central mass grew searing-hot and dense, and it ultimately initiated nuclear-fusion in its core. Our Sun was born this way–and all other stars also form as a result of this process.
Our Sun is currently enjoying an active, roiling midlife, and it has not changed much for more than four billion years. It will probably remain in this stable condition for another five billion years or so. Our Sun currently fuses approximately 600 million tons of hydrogen into helium every second, fusing about a million tons of matter into energy every second as a result. This energy, which can take between 10,000 and 170,000 years to escape from the searing-hot core, is the origin of our Star’s dazzling light and ferocious heat. But, in about 5 billion years, when hydrogen fusion in our Sun’s core has diminished to the point in which it can no longer remain in hydrostatic equilibrium, its looks will change. At this point, the core of our Star will undergo a dramatic increase in density and temperature, even as its outer layers balloon in size. Our bloated crimson Sun will eventually evolve into an enormous red giant star that will engulf Mercury and Venus–and possibly Earth as well. But even if our dying Sun, in its cannibalistic red giant phase, does not engulf our doomed planet, it will certainly render Earth uninhabitable.
When our dying Star finally reaches the end of that long stellar road, it will cast off its outer gaseous layers and evolve into a dense, cool stellar corpse called a white dwarf star, that can no longer produce energy by way of the process of nuclear fusion. Our Star will perish peacefully, as well as beautifully. The new and ghostly white dwarf will be surrounded by a beautiful shimmering, glimmering, multicolored shroud, called a planetary nebula, that is composed of what was once our Sun’s outer gaseous layers. Indeed, planetaries are so beautiful that astronomers frequently refer to them as the «butterflies of the Universe.»
An Identical Solar Twin
Even though only one sister of our Sun was discovered by Dr. Adibekyan and his team, the star itself–dubbed HD186302–is very special. That is because this G3-type main-sequence star is not only a solar sibling in both age and chemical composition, it is also our Sun’s twin.
Solar siblings might also be promising candidates in astronomers’ quest to discover life beyond our Solar System. This is because there is a possibility that life may have been transported between planets around sister stars dwelling in our Sun’s birth cluster. The transfer of life between exoplanetary systems is termed interstellar lithopanspermia.
Dr. Adibekyan is cautiously optimistic about this possibility.»Some theoretical calculations show that there is a non-negligible possibility that life spread from Earth to other planets or exoplanetary systems, during the period of the late heavy bombardment. If we are lucky, and our sibling candidate has a planet, and the planet is a rocky type, in the habitable zone, and finally if this planet was ‘contaminated’ by the life seeds from Earth, then we have what one could dream of–a second Earth orbiting a second Sun,» he commented in the November 16, 2018 IA Press Release.
The habitable zone surrounding a star is that Goldilocks region where the temperature is not too hot, not too cold, but just right for water to exist in its life-sustaining liquid phase. Life as we know it cannot exist without the presence of liquid water.
The team of astronomers at IA plan on beginning a new mission dedicated to hunt for planets around this solar twin using both HARPS and ESPRESSO spectrographs. ESPRESSO (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations) is a high resolution spectrograph, installed at ESO’s VLT. It was designed for the purpose of searching and discovering Earth-like planets, that are capable of hosting life, in orbit around distant stars.
Discovering and characterizing planetary systems around solar sisters could uncover some very important information about the outcome of planet formation in a common environment.
