Our Sun’s “Nemesis”

Our Sun’s “Nemesis”
By Judith E Braffman-Miller

Our Sun is a lonely Star, a blazing, glaring ball of roiling, searing-hot gas, with no stellar sibling to call its own. But did our solitary Star have a stellar twin when it was born 4.56 billion years ago? The answer is an almost certain “yes”–and so did every other sun-like star in the observable Universe. However, our Sun’s long-lost sibling was probably not an identical twin, according to new research conducted by a theoretical physicist from the University of California at Berkeley (UCB) and a radio astronomer from the Smithsonian Astrophysical Observatory (SAO) at Harvard University in Cambridge, Massachusetts. Astronomers have hunted for a solar stellar sibling for a very long time–an elusive, mysterious, and somewhat sinister star dubbed Nemesis. Nemesis got its ominous name because it is supposed to have been the culprit that hurled an asteroid into Earth’s orbit 65 million years ago, that blasted into our planet and exterminated the dinosaurs–but this mysterious and menacing companion to our Sun has never been found.

Many of the stars that dance around in the observable Universe have stellar siblings, including our nearest neighbor, Alpha Centauri, a triple star system. Astronomers have long sought a solution to this puzzle. Are binary and triplet stellar systems born that way? Or, alternatively, did one of the stars snare another? Do binary stars sometimes separate and go their separate ways –to become solitary stars, like our Sun?

Astronomers have long hunted for the hypothetical Nemesis because of a theory that ties it in with the mass extinction of the dinosaurs. This most recent proposal is based on a radio survey of a giant molecular cloud floating around in the constellation Perseus. The gigantic, dark cloud is heavily populated by newborn, fiery, baby stars that are cradled within its mysterious swirls. In addition, the new theory is also based on a mathematical model that can explain the Perseus observations only if all sun-like stars are born with a stellar sibling.

Giant, dark, and very frigid molecular clouds haunt our Milky Way Galaxy in huge numbers, and they serve as the bizarre nurseries of neonatal stars. Baby stars are born within especially dense blobs that are embedded within the swirling, whirling folds of these eerily beautiful clouds. The dense blobs collapse under their own gravitational pull, eventually forming new stars–igniting their nuclear-fusing furnaces that will be on raging fire for as long as the stars “live.”

“We are saying, yes, there probably was a Nemesis a long time ago,” noted study co-author Dr. Steven Stahler in a June 13, 2017 UCB Press Release. Dr. Stahler is a UCB research astronomer.

“We ran a series of statistical models to see if we could account for the relative populations of young single stars and binaries of all separations in the Perseus molecular cloud, and the only model that could reproduce the data was one in which all stars form initially as wide binaries. These systems then either shrink or break apart within a million years,” Dr. Stahler continued to explain.

In this study, “wide” means that the stellar duo of sibling stars are separated by more than 500 astronomical units (AU). One AU is the mean distance between our Sun and Earth, which is 93,000,000 miles. A wide binary companion to our Sun would have been 17 times farther from the Sun than its most distant major planet today–the beautiful, blue, banded ice-giant Neptune.

Our Sun is thought to have been born as a member of a dense open cluster with literally thousands of other sparkling sibling stars–as well as a stellar fraternal twin of its own. Astronomers think that the baby Sun was either rudely evicted from its birth cluster or it simply drifted away from its original home. The long-lost siblings of our Sun have since wandered off to more remote regions of our Galaxy. Based on the new model, our Sun’s binary twin probably escaped and went on to “live” its “life” far away, mixing with all the other stars in our region of the Milky Way Galaxy–never to be seen again.

“The idea that many stars form with a companion has been suggested before, but the question is: how many. Based on our simple model, we say that nearly all stars form with a companion. The Perseus cloud is generally considered to be a typical low-mass star-forming region, but our model needs to be checked in other clouds, ” explained first author Dr. Sarah Sadavoy in the June 13, 2017 UCB Press Release. Dr. Sadavoy is a NASA Hubble fellow at the SAO.

The theory that proposes all stars are born with siblings has important implications that go beyond star birth, including the very origins of galaxies, Dr. Stahler explained in the same UCB Press Release.

Dr. Stahler and Dr. Sadavoy posted their new findings in April 2017 on the arXiv server. Their paper has been accepted for publication in the Monthly Notices of the Royal Astronomical Society in London.

Something Wicked This Way Comes

Our Solar System emerged from the mixed fragments left over from the long-dead, nuclear-fusing furnaces of earlier generations of stars. Our own Sun–like other stars of its type–was born within a particularly dense blob embedded in a giant, frigid molecular cloud. Although it may seem counterintuitive, things have to get very cold for a searing-hot baby star to be born.

When the star-birthing blob finally collapsed under the relentless pull of its own gravity, fragile, delicate wisps of material gradually clumped together and merged–growing for hundreds of thousands of years. Finally, squeezed tightly together by the crush of gravity, hydrogen atoms within the clump rapidly, and dramatically fused, lighting a roiling, seething, fierce fire that would rage for as long as the new baby star lived–for that is how a star is born.

All of our Galaxy’s billions of stars were born this way–from the collapse of dense pockets tucked into the folds of diffuse, cold molecular clouds, composed mostly of gas, mixed up with a smattering of dust. Phantom-like dark molecular clouds are dispersed throughout our Galaxy in huge numbers, and they contain the remnants left over by older generations of ancient stars that had died long ago. These star-birthing clouds tend to combine and mix themselves up together, but stars of a kindred chemistry usually show up within the same clouds at about the same time.

Nemesis, our Sun’s hypothetical fraternal twin, is thought to be either a red dwarf or a brown dwarf. Red dwarfs are both the smallest and most numerous of true stars in our Milky Way, while brown dwarfs are stellar failures. Brown dwarfs are the runts of the stellar litter, and they are too puny for the process of nuclear-fusion to have ignited their fires. For this reason, brown dwarfs–which are really a purple-pink color called magenta–are often referred to as “failed stars.”

The Nemesis theory was originally proposed back in 1984. At that time, our Sun’s potentially troublesome sibling star was postulated to be in orbit around our Sun at the vast distance of about 95,000 AU–which amounts to 1.5 light-years. In 2011 Dr. Coryn Bailer-Jones studied craters pockmarking Earth’s surface and came to the conclusion that earlier findings of simple periodic patterns by a hypothetical Nemesis star were merely statistical artifacts, rather than the result of periodic showers of destructive crashing comets dislodged by a hypothetical Nemesis. However, in 2010, Dr. A.L. Melott and Dr. R.K. Bambach discovered evidence in the fossil record confirming the extinction event periodicity originally proposed by Dr. David Raup and Dr. Jack Sepkoski in 1984–but at a higher level of confidence and extending over a time period almost twice as long. The Infrared Astronomical Satellite (IRAS) did not detect any sign of Nemesis in the 1980s, and neither did the 2MASS astronomical survey, which lasted from 1997 to 2001.

Also, using newer and more powerful infrared telescope technology–capable of detecting brown dwarfs as cool as 150 Kelvins out to a distance of 10 light-years from our Sun–the Wide-field Infrared Survey Explorer (WISE) survey has likewise failed to detect this sinister fraternal twin of our own Star. In 2011, Dr. David Morrison, a senior scientist at NASA who is known for his research in risk assessment of potentially destructive near Earth objects, wrote a paper stating that there is no evidence of the existence of an object like Nemesis. This is because it should have been revealed in infrared sky surveys.

However, if Nemesis does indeed exist, its precise nature remains uncertain. Dr. Richard A. Muller has proposed that the most probable Nemesis star would be a red dwarf with an apparent magnitude between 7 and 12. Conversely, Dr. Daniel P. Whitmire and Dr. Albert A. Jackson suggest that Nemesis is a brown dwarf stellar failure. If a red dwarf Nemesis really does exist, it should be found in star catalogs, but it could only be confirmed by measuring its parallax. This is because, due to its orbit around our Sun, it would have a low proper motion and would, therefore, escape detection by older proper motion surveys. Dr. Muller has proposed that Nemesis will be discovered by the time parallax surveys that reach the 10th magnitude.

Dr. Muller, when referring to the date of a “recent” extinction, that occurred about 11 million years ago, said that Nemesis has a semi-major axis of approximately 1.5 light-years (95,000 AU). In addition, he suggests that Nemesis is located near Hydra, based on a hypothetical orbit derived from aphelions of many long-period comets (comets that enter the inner region of the Solar System every 200 years or longer). Dr. Muller’s most recent paper relevant to the Nemesis theory was published in 2002, where he suggests that Nemesis was jostled 400 million years ago by a passing star from a circular orbit into an orbit with an eccentricity.

Our Sun’s Nemesis

Astronomers have speculated about the origins of binary and multiple stellar systems (that contain three or more stars), for a very long time, and recently they have created computer simulations of collapsing masses of gas to help them understand how these systems condense under the powerful force of gravity to form baby stars. The scientists have also created computer simulations of the interaction of numerous baby stars, that have recently been liberated from their natal clouds of blanketing gas. Several years ago, one such simulation created by Dr. Pavel Kroupa of the University of Bonn in Germany led him to conclude that all stars are born in binaries–including our Sun.

However, there has been little direct evidence from observations to lend more credibility to this theory. As astronomers scan the skies in their search for ever younger and younger stars, they find a greater proportion of binaries. However, the reason for this observation remains a mystery.

“The key here is that no one looked before in a systematic way at the relation of real young stars to the clouds that spawn them. Our work is a step forward in understanding both how binaries form and also the role that binaries play in early stellar evolution. We now believe that most stars, which are quite similar to our own Sun, form as binaries. I think we have the strongest evidence to date for such an assertion,” Dr. Stahler noted in the June 13, 2017 UCB Press Release.

Dr. Stahler went on to explain that astronomers have known for decades that baby stars are born inside egg-shaped cocoons termed dense cores. Dense cores are scattered throughout these hauntingly beautiful giant, cold, dark clouds, that are primarily composed of molecular hydrogen. These clouds serve as the strange cradles of newborn stars. When viewed by astronomers using optical telescopes, molecular clouds appear as holes in the otherwise star-blazing sky. This is because the dust that lingers in the eerie clouds, along with the abundant molecular hydrogen gas, veils the light emanating from the stars being born inside, as well as the stars situated behind the cloud. The good news is that the clouds can be probed by radio telescopes. This is because the frigid motes of dust within them emit at these radio wavelenghts, and radio waves are not veiled by the dust.

The Perseus molecular cloud, like others of its kind, serves as a mysterious nursery for fiery newborn stars. The Perseus cloud is approximately 600 light-years from our planet and approximately 50 light-years long. In 2016, a team of astronomers completed a survey that used the Very Large Array (VLA) to peer at the stars being born within the cloud. The VLA is a collection of radio dishes located in New Mexico. The survey, named VANDAM, was the first complete survey of all youthful stars inhabiting a molecular cloud. The “young” stars were less than approximately 4 million years old–which means they are very young in star-years. The survey included both single and multiple young stars down to separations of approximately 15 AU. This means that the survey targeted multiple stars with a separation of more than the radius of Uranus’ orbit around our Sun, which is 19 AU.

The VANDAM survey produced a census of all Class 0 and Class 1 stars. Class 0 stars are less than about 500,000 years old, and Class 1 stars are those between about 500,000 and 1 million years old. Both types of stars are so young that that they haven’t had enough time to begin burning their supply of hydrogen to produce energy.

Dr. Sadavoy studied the results obtained from VANDAM and combined them with additional observations that detected the egg-shaped cocoons around the baby stars. These additional observations were derived from the Gould Belt Survey with SCUBA-2 on the James Clerk Maxwell Telescope in Hawaii. By combining both sets of data, Dr. Sadavoy was able to produce a robust census of the binary and single-star populations in Perseus–revealing 55 young stars in 24 multiple-star systems. All but five of the stars being observed were in binary systems, along with 45 single-star systems.

Dr. Sadavoy and Dr. Stahler found that all of the widely separated binary systems–those composed of a duo of stars separated by more than 500 AU–were young systems, containing two Class 0 stars. These systems also tended to be aligned with the long axis of the egg-shaped dense core. The slightly more mature Class 1 binary stars were closer together, many separated by approximately 200 AU, and displayed no tendency to align along the “egg’s” axis.

“This has not been seen before or tested, and is super interesting. We don’t yet know quite what it means, but it isn’t random and must say something about the way wide binaries form,” Dr. Sadavoy said in the June 13, 2017 UCB Press Release.

Dr. Stahler and Dr. Sadavoy mathematically modeled assorted scenarios in order to explain this distribution of stars, assuming typical formation, breakup and orbital shrinking times. They finally came to the conclusion that the only way their observations could be explained is to assume that all stars of masses similar to that of our Sun are born as wide Class 0 binaries in egg-shaped dense cores–after which about 60% go their separate ways over time. The rest shrink to create tight binaries.

“As the egg contracts, the densest part of the egg will be toward the middle, and that forms two concentrations of density along the middle axis. These centers of higher density at some point collapse in on themselves because of their self-gravity to form Class 0 stars,” Dr. Stahler said in the June 13, 2017 UCB Press Release.

“Within our picture, single low-mass, sun-like stars are not primordial. They are the result of the breakup of binaries,” Dr. Stahler added.

Their theory proposes that each dense core, which usually comprises a few solar-masses, converts twice as much material into stars than what had previously been thought.

Dr. Stahler has been asking radio astronomers to compare dense cores with their embedded young stars for over two decades, in order to test theories of binary star formation. The new data and model are a beginning, but more work needs to be done to understand the physics behind the rule, Dr. Stahler commented.

Such studies may come soon because the capabilities of an upgraded VLA and ALMA Telescopes in Chile, plus the SCUBA-2 survey in Hawaii, “are finally giving us the data and statistics we need. This is going to change our understanding of dense cores and the embedded stars within them,” explained Dr. Sadavoy.

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. 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 the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.

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