Imagine a grain of fine beach sand. What’s its size? Classification scales vary, but let’s say less than a tenth (0.1) of a millmeter (mm). A grain of table salt. Same question. Maybe ~0.3 mm. In either grain there are a gazillion molecules.
Now imagine something a thousand (1000) times smaller – a micrometer-sized grain. That’s the context for scale in this post and the microscopic bits that tell a cosmic story.
Falling stars, Batman! Some grains at least, as noted this week in various science articles (below).
Do you watch shooting stars in the night sky? It always was fascinating as a kid to see something that fell from outer space. Those heavy pieces of meteorites in a science museum or other special exhibit. Well, they’re even cooler than in sci-fi tales of space invaders falling to Earth. Tiny solar relics in meteorites tell a cosmic story. A grand tale of meteoriticists, mass spectrometers, electron microscopes, theory, and lab research. A complex study in chemistry and more.
Presolar grains are the solid matter that was contained in the interstellar gas before the Sun formed. The stardust component can be identified in the laboratory by their abnormal isotopic abundances and consists of refractory minerals which survived the collapse of the solar nebula and the subsequent formation of planetesimals.
The stellar nucleosynthesis that took place within each presolar star gives to each granule an isotopic composition unique to that parent star, which differs from the isotopic composition of our solar system’s matter as well as from the galactic average. These isotopic signatures often fingerprint very specific astrophysical nuclear processes that took place within the parent star and prove their presolar origin.
• BBC News > “Oldest material on Earth discovered” by Paul Rincon, Science editor, BBC News website (13 January 2020) – Scientists analyzing a meteorite have discovered the oldest material known to exist on Earth.
They found dust grains within the space rock – which fell to Earth in the 1960s – that are as much as 7.5 billion years old.
The oldest of the dust grains were formed in stars that roared to life long before our Solar System was born.
A team of researchers has described the result in the journal Proceedings of the National Academy of Sciences.
When stars die, particles formed within them are flung out into space. These “pre-solar grains” then get incorporated into new stars, planets, moons and meteorites.
“They’re solid samples of stars, real stardust,” said lead author Philipp Heck, a curator at Chicago’s Field Museum and associate professor at the University of Chicago.
A team of researchers from the US and Switzerland analysed 40 pre-solar grains contained in a portion of the Murchison meteorite, that fell in Australia in 1969.
The complex (interstellar dust) model used for dating presolar grains depends on galactic cosmic rays – “the grains’ exposure to cosmic rays during their interstellar journey over billions of years” after stars end their lives – and cosmogenic gas contributions. In particular, assumptions about: the universality, constancy, and (average) flux of cosmic rays; dust production rates by stars; irradiation sources and profiles – as parts of larger grains or as grain aggregates; and theoretical lifetime estimates for interstellar dust.
So, including measurement errors, there’s a range in estimated ages. See the researchers’ published paper for more information on methodology, including notes about sample size and even the use of data from NASA’s Voyager 1 space probe. 
[While] “… the oldest yielded a date of around 7.5 billion years old.”] Dr Heck told BBC News: “Only 10% of the grains are older than 5.5 billion years, 60% of the grains are “young” (at) 4.6 to 4.9 billion years old, and the rest are in between the oldest and youngest ones.
• Space.com > “7 billion-year-old stardust is oldest material found on Earth” by Mindy Weisberger (January 13, 2020) – Some of these ancient grains are billions of years older than our sun.
Scientists recently identified the oldest material on Earth: stardust that’s 7 billion years old, tucked away in a massive, rocky meteorite that struck our planet half a century ago.
Though the universe abounds with floating stardust, no presolar grains have ever been found in Earth’s rocks. That’s because plate tectonics, volcanism and other planetary processes heated and transformed all the presolar dust that may have collected during Earth’s formation, said lead study author Philipp Heck, the Robert A. Pritzker Associate Curator of Meteoritics and Polar Studies at the Field Museum of Natural History in Chicago.
Most presolar grains measure about 1 micron in length, or are even smaller. But the grains the scientists analyzed for the study were much bigger, ranging from 2 to 30 microns in length.
“We call them ‘boulders,'” Heck said. “We can see them with an optical microscope.”
 Wiki recaps the history of finding and identifying presolar gains.
 So, I’ve always wondered about cosmic dust. Stuff floating through space other than individual gas molecules. Hubble space photos reveal vast expanses of cosmic dust. Star nurseries even. So, there’s dust and there’s stardust. And condensed stellar gas. Not all stellar gases are the same. A story much more complex than “the solar system began as a uniform hot gas” (as in some steady-state model).
Wiki: “Thousands of tons of cosmic dust are estimated to reach the Earth’s surface every year, …”
[Regarding stardust] Also important are their extreme isotopic compositions, which are expected to exist nowhere in the interstellar medium. This also suggests that the stardust condensed from the gases of individual stars before the isotopes could be diluted by mixing with the interstellar medium.
 Published paper excerpt:
Our results show that the majority of the cosmogenic 21Ne was acquired during presolar GCR exposure (SI Appendix, Fig. S7 and Calculation of He and Ne Exposure Ages). Specifically, at least 80% of the cosmogenic 21Ne for grains with 21Ne ages greater than 100 Ma was acquired by presolar GCR exposure. For these grains, the amount that might have been acquired during early Solar System formation is smaller than the uncertainty of the presolar exposure ages and, hence, not detectable. These findings only apply if the presolar grains were exposed to the early active sun at all. At most, the five grains with the lowest ages might have acquired all their cosmogenic 21Ne in the early Solar System (SI Appendix, Fig. S7). Early Solar System exposure does not significantly affect our interpretation of presolar ages, except, possibly, for these five grains.