Is the sun an only child? Or was he born in a (very, very) big family?
The answer tells us more than just how awkward family gatherings over the holidays can be (if you think yours are bad, imagine how much worse it would be with a few thousand rival siblings). After all, the story of the origin of the sun is, ultimately, ours. We’ve seen huge advances in our understanding of star formation, but ironically we still have some pretty fundamental questions about our nearest, dearest. Such as, if the sun was born solo or with a huge passel of other stars.
Although the sun is close enough that we can almost touch it, the biggest problem with uncovering its origin story is that it is ancient. Born 4.6 billion years ago, our star has come of age well and drifted away from its ancestral home – an unnamed, now-defunct “stellar nursery” of gas that has long since dispersed or consolidated into stars.
We can’t find that nursery, but we can still learn about it. We have evidence of this, perhaps surprisingly, in the form of meteorites, some of which still bear clues to the gestational environment around them when the solar system was born. For example, isotopes of elements like potassium inside meteorites have told us where they formed in the presolar nebula, and variations between meteorites can be used to help determine nebula conditions long before the emergence of any planet.
With meteor data in hand and aided by state-of-the-art computer simulations, an international team of astronomers studied the likely home environment of the sun and recently published their findings in the Royal Astronomical Society Monthly Notices. Using clever reasoning, their research suggests that the sun not only had many siblings, but was born in a fairly metropolitan neighborhood.
Stars are born in cosmic clouds called nebulae, which form when their interiors collapse onto a central, stack-like point that becomes the birth star. Nebulae come in many shapes and sizes, from small dark globules to huge giant molecular clouds. How a star forms in a given nebula is much more a story of nature than of nurture.
For example, nearby Nebula Barnard 68 is a dark clot of cold gas and dust – tiny grains of silicates (rocky material) and complex carbon molecules similar to soot – relatively close to us in space, only a few hundred light-years away. It’s one of my favorite things; an eerie, pitch-black ghostly mass that completely blocks out all starlight behind it, like an opaque hole in the sky.
It is only half a light-year in diameter (about three trillion kilometers), with barely enough material to make a single star slightly heavier than the sun. It’s probably in the middle of that process now and could turn into a star in as little as 200,000 years.
At the other end of the scale we have the Orion B Molecular Cloud Complex, a truly enormous site of active star formation that lies over a thousand light-years away and several hundred years- diameter lumen. That’s big enough to make an impressive number of stars – at least 100,000 like the sun. The iconic Orion Nebula, visible to the naked eye and home to hundreds of stars, is just a small part of this huge stellar factory.
Giant clouds like this are relatively rare but produce stars on an industrial scale, while smaller clouds are less fertile but litter the galaxy. Just by looking at these numbers statistically, it is not possible to discern the origin of the sun: it could have come from either type of stellar nursery.
However, these nebular environments are very different, which affects the stars they create. The massive stars found in a nebula have a great influence on their unborn siblings. They can blast strong winds of subatomic particles, like the solar wind, but intensify well above 11. These winds can seed stars forming heavy elements like aluminum and magnesium. And later, when they explode as supernovae, they send a different mixture of these elements, like iron and cobalt, very far.
Massive stars, however, are rare. Maybe one in a hundred stars is massive enough to support that kind of grip, and small nebulae just don’t. This means, in principle, that examining the chemical makeup of the early solar system could tell us what kind of nursery the sun was born in.
This was the focus of the recently published research. Astronomers have focused on two elements in particular: aluminum-26 and iron-60. Aluminum 26 is created inside massive stars and blown away by their winds, while Iron 60 is forged in the thermonuclear hell of an exploding star. Both elements are radioactive, decaying into magnesium and cobalt, so carefully measuring the amounts of these daughter elements in pristine samples from the early days of the solar system – meteorites, that is – can tell us about the environment in which the sun was formed.
For their new analysis, the international team of scientists used the physics of nebulae and star formation to simulate the birth of a sun-like star in a variety of environments, from nebulae containing very few stars (a proxy for small clouds) up to huge ones with several thousand. . Next, they calculated the elemental composition of the proto-proxy-presolar disk that emerged in each, then compared those virtual yields to what is actually measured in meteorites.
Their findings indicate that as it formed in its natal disk, the early sun was likely hammered by powerful winds and supernova explosions, both from massive stars. This means that the Solar Nursery was more like Orion’s complex than Barnard 68.
In other words, the Sun was probably more of an inner-city kid than a rural, small-town star. Of course, with its nebular nursery gone, we can’t easily confirm that. After all, you can’t go home anymore.
And what about the sun’s siblings, the thousands of other stars in his extended family? Like the sun, they once nestled together like a litter of puppies, but likely wandered alone aeons ago and are now orphans, scattered across the galaxy. Still, astronomers are looking for them — ones that are the same age and composition as the sun — so we can learn more about our parent star.
A reunion is quite unlikely. If we want to see a family album, we’ll just have to put it together ourselves.
This is an opinion and analytical article, and the opinions expressed by the author or authors are not necessarily those of American scientist.
