This article originally appeared in our November 2012 issue.
If viewing the Orion Nebula (M42) and Pleiades star cluster (M45) through a telescope sends a shiver up your spine, you may be experiencing a deeper connection with the cosmos than you ever imagined. Astronomers have scoured the galaxy within approximately 6,500 light-years of Earth and have found no other objects that more closely resemble the first and second stages of the solar system’s evolution, which run from birth to roughly 100 million years later.
Although scientists have been piecing together the story of our system’s formation for centuries, the pace of discovery has quickened in the past decade. Detailed observations, better theories, and more robust computer simulations now point to the Sun forming in a nebula that held between 1,000 and 10,000 stars. One of them was a behemoth with a mass some 25 times that of the Sun that exploded within a light-year of our fledgling star. The Sun likely remained in this cluster for up to 100 million years, long enough for the dense clouds of gas and dust to dissipate. In other words, it was born in a stellar nursery like the Orion Nebula and nurtured during infancy in a cluster like the Pleiades.
Astronomers still need to work out many details and pin down some parameters in this birthing scenario, but here is the emerging picture of how the Sun and its planetary system came to be.
Night into day
All stars form out of the gas and dust that permeate space. By earthly standards, this interstellar medium is a nearly perfect vacuum. On average, one atom occupies each cubic centimeter. (The same volume of Earth’s atmosphere at sea level holds more than a billion billion atoms.) This material often clumps into clouds where densities climb significantly higher.
The story of the Sun’s formation begins in one of these clouds. The temperature in the interior likely registered between 10 and 20 kelvins (18° to 36° Fahrenheit above absolute zero), well below the interstellar medium’s average of around 100 K (–280° F), and particles packed together to the tune of a million atoms per cubic centimeter. Under these conditions, gas atoms join with one another to create molecules. Such molecular clouds can span dozens of light-years.
Star formation begins when the molecular cloud starts contracting. Astronomers have discovered several events that can initiate such a collapse — the shock wave from a close exploding star, a collision with another interstellar cloud, or strong winds from a nearby hot star. Regardless of the triggering mechanism, the cloud gets squeezed to the point where gravity’s attractive force overwhelms the internal pressure produced by gas motions.
Once the cloud starts to collapse, it fragments into smaller pieces, with each of these pockets splintering further. The process can continue for a million years or more until the initial cloud splits into thousands of individual clumps, each fated to become a star.
Although the event that started the Sun’s parent molecular cloud down the road to stardom some 4.6 billion years ago has been lost forever, scientists have identified many of the characteristics this nebula and the resulting cluster possessed. Researchers even have deduced some specific incidents that took place in the Sun’s natal environment.
Our star appears somewhat unusual but far from rare. First, the Sun lacks a binary companion, a status that only about one-third of solar-type stars can claim. But several other features make Sol stand out even more. Among the stars in our galaxy, it is relatively large, has a higher percentage of elements heavier than helium (what astronomers call “metals”), and hosts a well-ordered planetary system.
Of the 50 closest star systems to Earth, only Sirius, Procyon, and the brightest component of the Alpha Centauri system have more mass than the Sun. When astronomers cast a wider net, they find a slightly higher prevalence (around 12 percent) of heftier stars. The Sun’s abundance of heavy elements is also unusual — only about one-quarter of solar-type stars in our part of the galaxy have more metals.
Our planetary system comprises four large gaseous planets in its outer regions and four rocky worlds close to the Sun. Astronomers don’t yet have a good idea how common similar systems might be. Although the current count of known exoplanets is approaching 1,000, the methods used to discover them still have an easier time finding large planets in close orbits.
Worlds that lie far from their star require a long time to orbit, and careful observations extend back only about 20 years. Compare that with Saturn, the second closest of our system’s giants, which takes nearly 30 years to circle the Sun. Astronomers cautiously estimate that perhaps 20 percent of stars harbor giant planets. That number could rise to 50 percent as discoveries pour in during the next decade.
Observational techniques only recently have reached the stage where they can detect relatively small planets. Although most scientists expect to find lots of these terrestrial worlds in the next few years, the history of exoplanet discoveries is littered with shattered expectations.
If you combine these considerations — a single, relatively high-mass, and metal-rich star surrounded by giant planets — the probability of creating a Sun-like star hovers around 0.2 percent. That may seem like a tiny number, but the Milky Way Galaxy contains some 200 billion stars, so the Sun would share these characteristics with about 400 million others.
Life in a rich cluster
None of these four factors demands that the Sun formed in a molecular cloud, much less in a hefty one that gave birth to a cluster with at least 1,000 other stars. How do scientists know then that the Sun formed out of a giant cloud of gas and dust and not in isolation? Part of it is a numbers game — most stars form in groups or clusters. In the solar neighborhood, a region that extends about 6,500 light-years from the Sun, astronomers see many star-forming regions with the potential to make lots of suns but only a handful of isolated small clouds capable of producing a single luminary.
The Sun’s system of planets and smaller bodies makes a better case, however. Astronomers call the collapsing cloud that ultimately became the solar system the “solar nebula.” As the nebula contracted, it had to rotate faster to conserve its angular momentum (just like a figure skater spins more rapidly as she pulls her arms closer to her body). As a result, the cloud of gas and dust flattened into a disk that surrounded the spherical nascent Sun.
Innumerable collisions within this protoplanetary disk eventually built up the planets and other objects, a process that took roughly 10 million years. During this period, the Sun likely passed through a T Tauri stage (named for the prototype star in the constellation Taurus) marked by violent surface activity and strong winds.
The inner part of this disk developed into the eight major planets. A defining characteristic of these worlds is that they all orbit in nearly the same plane and follow close to circular paths — an orderly configuration not seen in most of the known exoplanet systems. This relatively serene environment extends to Neptune’s orbit, at a distance from the Sun of some 30 astronomical units (or AU; one AU is the average Earth-Sun distance, approximately 93 million miles [150 million kilometers]).
Beyond Neptune, things get a bit more chaotic. The Kuiper Belt extends from about 30 to 50 AU from the Sun and includes everyone’s favorite dwarf planet, Pluto. The 100,000 or so objects out there don’t have much mass (a total probably equal to between 1 and 10 percent of Earth’s mass), but their orbits tend to be more eccentric and angle quite steeply to the plane of the major planets.
Scoping out the birth cluster
The tidy orbits inside 30 AU argue that the solar system has suffered no close encounter with another star since the giant planets formed. Computer simulations show the magic distance to be about 225 AU; if a stellar system approached closer than that, the giant planets’ orbits would have significantly larger eccentricities. Examining the typical density of stars and their motions in clusters of different sizes, this separation implies that our system formed in a collection with no more than 10,000 stars.
The absence of large objects located beyond 30 AU speaks to possible close encounters during the solar nebula days. If another star ventured within 90 AU before the giant planets formed, it would have stripped off or captured some of the solar nebula inside 30 AU. The chance of such a rendezvous in a typical cluster is only about 1 percent, so our planet system had pretty good odds of surviving.
Some astronomers think that Sedna — an odd object in the solar system’s depths — constrains the Sun’s early years as well. This world follows a highly eccentric 12,000-year orbit that brings it within 75 AU of the Sun at closest approach but stretches out to nearly 1,000 AU at its most distant. Computer simulations show an encounter with another star could have altered Sedna’s orbit from a more sedate one. If so, the interloper would have passed between 400 and 800 AU of the Sun. To make such an encounter likely, the solar system would have to reside in a cluster containing at least 1,000 stars for approximately 100 million years.
The close passes the Sun and its neighbors must have experienced ejected a huge number of good-sized rocks into interstellar space, but at speeds low enough that they remained in the cluster where other stars could capture them. In fact, the solar system likely holds rocks that once belonged to other cluster members.
Being born in a big cluster also means sharing space with massive stars, which emit powerful ultraviolet radiation. In a complex with 10,000 or more stars, such high-energy light could have heated the solar nebula enough that gas near its outer edges would have started to evaporate and the distant planets would not have formed. The presence of giant planets argues for a smaller cluster with a less hostile environment during the solar system’s embryonic stages.
A blast heard round the world
The massive stars in the Sun’s birth cluster also provided a pyrotechnic display whose results still reverberate. Meteorites that date back to the solar system’s origin show the presence of a few isotopes that could have formed only through the decay of short-lived radioactive elements. Perhaps the best example is nickel-60, the so-called daughter product of iron-60. Iron-60 has a half-life of just 1.5 million years, so its daughter’s existence in meteorites means the iron had to be delivered from a nearby source during the solar system’s earliest days.
Massive stars generate iron-60 at the end of their brief lives and then spew it out when they explode as supernovae. The closest match to the meteorite abundances of iron and other radioactive elements (notably aluminum-26 and calcium-41) implies that the supernova destroyed a star that began its life with approximately 25 times the Sun’s mass. Calculations show that a cluster with about 1,000 members has a 50 percent chance of producing one such star. Larger clusters likely would possess more stars of this size as well as bigger ones.
The supernova blast had to be close enough to the solar nebula to supply the observed isotopes but far enough away so that the explosion wouldn’t strip disk material within 30 AU from the Sun. If the nebula’s disk lay face-on to the blast, the sweet spot would appear to lie between approximately 0.3 and 1 light-year. (A greater inclination would necessitate a closer explosion.) A supernova at this distance would have dazzled anyone around to see it. At its peak, the exploding star would have appeared brighter than the Sun with all of its light concentrated to a point.
The explosion’s timing had to be just right. A 25-solar-mass star lives about 7.5 million years before going supernova. Although that’s comfortably within the 10-million-year lifespan of the solar nebula, the massive star couldn’t spend its entire life in close proximity to the Sun because its ultraviolet radiation would strip the solar nebula’s outer parts. Most likely, the heavyweight star resided near the cluster’s center while the young solar system orbited along an elongated path that brought it close just as the star exploded.
Within a few million years of the supernova, the giant planets finished forming. Not too long after, the Sun started fusing hydrogen into helium in its core and officially became a star. But its life in the cluster likely lasted much longer. If Sedna’s odd orbit arose through an encounter, our system likely spent some 100 million years in this gravitationally bound cluster.
Eventually, however, the cluster dissipated and the Sun and its attendant planets charted a new course around the galaxy’s hub. And now, people on one of these worlds can look at the spectacular Pleiades and the Orion Nebula, and contemplate their similarities to our stellar nursery.
